Different instruments produce sound due to different mechanisms. As regards to flute when the air is blown inside a hollow pipe Timbre, Pitch and Volume of a Synthesizer, which has openings around it, through which the air comes out, sound is produced. For a string instrument like violin or guitar the strings are oscillated to generate the sound.
In case of acoustic instruments something must be moved or resonated for sounds to be created. A synthesizer can imitate sound of any instrument. But they do not create direct sounds. They produce electric-signals. Hence, there is the need to attach headphones or speakers to hear the sound created; otherwise nothing will be audible. CD-players also work like this way; the sounds are inaudible unless there is an output device.
Synthesizers are different from the examples mentioned above; so are the components of synthesizer.
Listed below are the different components of a synthesizer:
Keyboard: A piano style keyboard is generally equipped in a synthesizer. The keys of a synthesizer are actually a type of switch. The main function of these switches is to on and off the electronic circuit. The user can choose to use mouthpieces, guitar like instruments, strings, drum pads or a computer to manage and organize the synthesizer. However, keyboards are one the most widely used input devices.
Amplifier: Amplifier is one of the important components of synthesizer. An amplifier circuit is used to control the volume of a synthesizer. The signal is passed through the circuit to command the amplification that would result in sound through the output device. A synthesizer amplifier, however, dictates on the waveform unlike the types used in, say, speakers. To connect loudspeaker from the output would result in attaching a power amplifier to it.
Software: There are lots of programs designed for the working of the synthesizer through the computer. The basic purpose is to record and edit MIDI sequences. It is in fact, better defined as music composition and production system. The name sequencer also has been given to them for this particular role; and is one of the integral parts of the synthesizer.
The track data is displayed in form of chart, waveform or diagram. There are a number of places that dish out free softwares, samplers and wave renderer for the synthesizer. Softwares are available for both platforms, MAC and Windows.
MIDI device: MIDI is the acronym of Musical Instrument Digital Interface. This is the worldwide standard in relating electronic musical instruments, computers and other devices among them. Sequencers are, typically, used to produce accurate, repeating eight- or sixteen-note bassline patterns.
When the microprocessor-controlled sequencers were created the task became even easier. These had the power to store a large number of notes, even a musical piece. Hence, MIDI soon became the standard in the world for communication between electronic musical instruments. MIDI sequencers record the note on or note off commands.
Drum Machines: Drum machines can also be included in the components of synthesizer. A drum machine is like a synthesizer only difference being that it is a dedicated drum and percussion sound instrument. Synthesizer is, however, generic. Instead of piano style keyboard, the drum machines have the touch sensitive pads, which are operated by fingers or drum sticks.

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A loudspeaker enclosure is a cabinet designed to transmit sound to the listener via mounted loudspeaker drive units. The major role of the loudspeaker enclosure is to prevent the out of phase sound waves of the rear of the speaker from combining with the in phase sound waves from the front of the speaker. This results in interface patterns and cancellation, causing the efficiency of the speakers to be reduced; particularly in the low frequencies where the wavelengths are so large that interference will affect the entire listening area.
Most loudspeaker enclosures use some sort of structure, more like a box to contain the out of phase sound energy. the box is characteristically made of wood or, more recently, plastic, both for the reasons of ease of construction and appearance. Loudspeaker cabinets are sometimes sealed and sometimes ported. Ported cabinets allow some of the sound energy inside the cabinet to be released, and if designed correctly with proper attention to phase relationships, both increase bass response and reduce driver excursion.
Many other engineering variations on the basic box design exist, such as acoustic transmission lines. Enclosures always play a significant role in sound production in addition to the intended design effects, adding unfortunate resonances, diffraction, and other unwanted phenomenons. Problems with resonance are usually reduced by increasing enclosure mass and rigidity, by hightened damping of enclosure walls, or by adding absorption internally.
Bass Reflex or vented loudspeaker enclosure
Vented or bass reflex enclosures require special constructions due to the large forces that can be developed by the drivers installed inside that act on them. Vented loudspeaker enclosures have two primary functions – the separation of vibrations from the front and rear of the loudspeakers, and the containment of air so that the air can act as a resonating elastic medium inside the enclosure.
Vented enclosure operation is analogous to the way a bottle will behave as a whistle. In a tuned system it is important to avoid air leaks, since the vent produces most of the sound at the frequency of resonance and the pressure inside the enclosure can be substantial.
Air leaks in the seams or walls of enclosure can cause the tuning of the system to shift in frequency, producing other undesirable effects as well. The material used for enclosure walls should be solid and dense and should be free of voids or warps. The ideal loudspeaker enclosure would have no wall resonance at frequencies that fall within the frequency range of loudspeakers mounted in it. 25 mm solid lead plate would make an excellent loudspeaker enclosure.
Woofer and subwoofer enclosures
Enclosures used for woofers and subwoofers can be adequately modelled in the low frequency region, approximately 100 to 200 Hz and below using acoustics and the lumped component model. Electrical filter theory has been used with considerable success for woofer and subwoofer enclosures.
Conclusion
Before the 1950s many manufacturers did not fully enclose their loudspeaker cabinets; the back of the cabinet was typically left open and early on, it was observed that the enclosure had a strong effect on the bass response of the speaker. Previously loudspeaker enclosures were invented to either wall off the out of phase sound from one side of the driver, or to modify it so that it could be used to enhance the sound produced from the other side.

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Often, our mobile phones declare the arrival of messages that read: ‘Company X is offering flat 50 per cent discount on its products or Bank Y is open with loans at a surprisingly low interest.’ We often feel disturbed but there is no way that we can deny that technology has given the brands an easy license to enter our bedrooms.The unprecedented growth in technology today has armed various companies with new and more sophisticated means to market their products and one of such means is mobile marketing. Mobile marketing is a practice by which companies can afford to reach out to probable consumers even when the latter is on the move. Mobile marketing can be of two types: a traditional one whereby flashy billboards or advertisements are being displayed on a moving vehicle and the other, a sharper and modern means, is the use of cell phones as a means for mobile marketing. It is the second type that has emerged the real eye-catcher in today’s world.Marketing through mobile phones can be done in several ways. First, the use of SMS or short message services. Often, our cell phones get bombarded with SMSs declaring the launch of new products or discount offers made available by a certain company. Mobile marketing through SMS has evolved as a popular channel of mobile marketing since the turn of this century. However, one mustn’t forget that there have been issues concerned with this mobile ‘Spam’ messages often causing discomfort to a subscriber and his privacy.Mobile gaming has shaped out into another major way of mobile marketing. Companies often attach promotional messages with games (advergaming) or even sponsor the entire game that users download through their sophisticated cellular devices. This leads to a situation where both parties gain: the user gets the game of his choice while the brand succeeds in promoting its value.A third means of mobile marketing is mobile web marketing. It means advertising on Internet web pages that are accessible through mobile phones. Several search engines have already engaged in advertising their products through mobile phones on a large scale.Perhaps the most popular and sophisticated means of mobile marketing is through Blue tooth. A lot of companies have brought out hotspot mechanisms that comprise content-management system equipped with a Blue tooth distribution system. Mobile marketing through Blue tooth has already started to make its impact felt in India.Multimedia Message Service (MMS) is another procedure of mobile marketing. This form of marketing is a process whereby a timed slideshow of images, video, audio or text is delivered to a mobile (that which is equipped with the facility) via MMS.The oldest means of mobile marketing is, however, Infrared waves, which were brought into practice by various European countries back in the 1990s.Mobile marketing, however, has a major difference from the traditional means of marketing methods. Here, the consumer will ultimately decide on whether the marketing campaign would continue or be terminated thus validating the conclusion that marketing through cell phone is largely a consumer-controlled marketing communication mechanism.

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What is a condenser microphone? This is the microphone where a capacitor is used; so, it is also known as capacitor microphone. All the microphones work on the same principle of a diaphragm receiving the sound or music and converting it into electrical signals. The diaphragm, in the case of condenser microphones, is positioned in a way where it acts as a capacitor and the distance between the plates is changed by the generated vibrations.
An electronic transducer is formed which is used for music creation. The audio, which is encoded in this form of transducers, can be extracted. Two different methods are used for this purpose. One is known as the DC based and the other one is known as RF or HF condenser microphones.
The condenser microphone needs power from a battery or any external power source. The audio signal that is extracted is stronger than that obtained by any dynamic process. The signal is also more likely to be more sensitive and receptive than dynamics.
External power source is provided with the professional microphones to suit the needs of the buyers. For useful output, power is one requirement that cannot be overlooked. In addition, the condenser microphone functions with two diaphragms. This type of microphone is much suitable in picking up feeble and distinct sound in and around it. For noisy works, however, they are not the favorites because of their over sensitiveness.
The plates of a DC based microphone are powered by a set charge; so, when vibrations occur, the voltage also changes. A mathematical equation is used to calculate it.
When the distance of a capacitor microphone changes, the capacitor charge also varies a little bit. For this reason, a constant charge is maintained for the capacitor. Maintaining the time frame of the change in the capacitance, the voltage regulating is instantaneous. It shows the alteration that has occurred in the capacitance. The series resistor then shows the voltage variation between the bias and the capacitor.
RF condenser microphone is just the opposite of the DC biased condenser microphone. RF microphones use a low RF voltage built up by a low noise oscillator. Frequency modulation produced by the sound waves is the guide force behind the movement of the oscillator.
The demodulation results in a very low source obstacle. It also produces low sound audio frequency signal. The RF biased condensers are used in damp conditions also.
Condenser microphones are used to make a number of appliances. It is capable of producing good quality music signals. It is also very popular in the field of studio recordings and laboratories.

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A force of one pound will accelerate a mass of one slug at one foot per second squared. The same relationship holds between the force, mass, time and distance units of the other measurement systems. Most people prefer to measure angles in degrees, and the common engineering practice of specifying mass in pounds or force in kilograms will not yield correct results in the formulas given here! Care must be taken to convert such irregular units to one of the standard systems outlined above before applying the formulas given here!

 

Statics

For a motor that turns S radians per step, the plot of torque versus angular position for the rotor relative to some initial equilibrium position will generally approximate a sinusoid. The actual shape of the curve depends on the pole geometry of both rotor and stator, and neither this curve nor the geometry information is given in the motor data sheets I’ve seen! For permanent magnet and hybrid motors, the actual curve usually looks sinusoidal, but looks can be misleading. For variable reluctance motors, the curve rarely even looks sinusoidal; trapezoidal and even assymetrical sawtooth curves are not uncommon.

 

For a three-winding variable reluctance or permanent magnet motors with S radians per step, the period of the torque versus position curve will be 3S; for a 5-phase permanent magnet motor, the period will be 5S. For a two-winding permanent magnet or hybrid motor, the most common type, the period will be 4S, as illustrated in Figure 2.1:

 

Figure 2.1

Again, for an ideal 2 winding permanent magnet motor, this can be mathematically expressed as:

T = -h sin( ((/2) / S)  )

Where:

T — torque

h — holding torque

S — step angle, in radians

 = shaft angle, in radians

But remember, subtle departures from the ideal sinusoid described here are very common.

The single-winding holding torque of a stepping motor is the peak value of the torque versus position curve when the maximum allowed current is flowing through one motor winding. If you attempt to apply a torque greater than this to the motor rotor while maintaining power to one winding, it will rotate freely.

 

It is sometimes useful to distinguish between the electrical shaft angle and the mechanical shaft angle. In the mechanical frame of reference, 2 radians is defined as one full revolution. In the electrical frame of reference, a revolution is defined as one period of the torque versus shaft angle curve. Throughout this tutorial,  refers to the mechanical shaft angle, and ((/2)/S) gives the electrical angle for a motor with 4 steps per cycle of the torque curve.

 

Assuming that the torque versus angular position curve is a good approximation of a sinusoid, as long as the torque remains below the holding torque of the motor, the rotor will remain within 1/4 period of the equilibrium position. For a two-winding permanent magnet or hybrid motor, this means the rotor will remain within one step of the equilibrium position.

 

With no power to any of the motor windings, the torque does not always fall to zero! In variable reluctance stepping motors, residual magnetization in the magnetic circuits of the motor may lead to a small residual torque, and in permanent magnet and hybrid stepping motors, the combination of pole geometry and the permanently magnetized rotor may lead to significant torque with no applied power.

 

The residual torque in a permanent magnet or hybrid stepping motor is frequently referred to as the cogging torque or detent torque of the motor because a naive observer will frequently guess that there is a detent mechanism of some kind inside the motor. The most common motor designs yield a detent torque that varies sinusoidally with rotor angle, with an equilibrium position at every step and an amplitude of roughly 10% of the rated holding torque of the motor, but a quick survey of motors from one manufacturer (Phytron) shows values as high as 23% for one very small motor to a low of 2.6% for one mid-sized motor.

 

 

Half-Stepping and Micro stepping

 

So long as no part of the magnetic circuit saturates, powering two motor windings simultaneously will produce a torque versus position curve that is the sum of the torque versus position curves for the two motor windings taken in isolation. For a two-winding permanent magnet or hybrid motor, the two curves will be S radians out of phase, and if the currents in the two windings are equal, the peaks and valleys of the sum will be displaced S/2 radians from the peaks of the original curves, as shown in Figure 2.2:

 

Figure 2.2

This is the basis of half-stepping. The two-winding holding torque is the peak of the composite torque curve when two windings are carrying their maximum rated current. For common two-winding permanent magnet or hybrid stepping motors, the two-winding holding torque will be:

h2 = 20.5 h1

where:

h1 — single-winding holding torque

h2 — two-winding holding torque

 

This assumes that no part of the magnetic circuit is saturated and that the torque versus position curve for each winding is an ideal sinusoid.

 

Most permanent-magnet and variable-reluctance stepping motor data sheets quote the two-winding holding torque and not the single-winding figure; in part, this is because it is larger, and in part, it is because the most common full-step controllers always apply power to two windings at once.

 

If any part of the motor’s magnetic circuits is saturated, the two torque curves will not add linearly. As a result, the composite torque will be less than the sum of the component torques and the equilibrium position of the composite may not be exactly S/2 radians from the equilibria of the original.

 

Microstepping allows even smaller steps by using different currents through the two motor windings, as shown in Figure 2.3:

 

Figure 2.3

For a two-winding variable reluctance or permanent magnet motor, assuming nonsaturating magnetic circuits, and assuming perfectly sinusoidal torque versus position curves for each motor winding, the following formula gives the key characteristics of the composite torque curve:

h = ( a2 + b2 )0.5

x = ( S / (/2) ) arctan( b / a )

Where:

a — torque applied by winding with equilibrium at 0 radians.

b — torque applied by winding with equilibrium at S radians.

h — holding torque of composite.

x — equilibrium position, in radians.

S — step angle, in radians.

 

In the absence of saturation, the torques a and b are directly proportional to the currents through the corresponding windings. It is quite common to work with normalized currents and torques, so that the single-winding holding torque or the maximum current allowed in one motor winding is 1.0.

 

Friction and the Dead Zone

 

The torque versus position curve shown in Figure 2.1 does not take into account the torque the motor must exert to overcome friction! Note that frictional forces may be divided into two large categories, static or sliding friction, which requires a constant torque to overcome, regardless of velocity, and dynamic friction or viscous drag, which offers a resistance that varies with velocity. Here, we are concerned with the impact of static friction. Suppose the torque needed to overcome the static friction on the driven system is 1/2 the peak torque of the motor, as illustrated in Figure 2.4.

 

Figure 2.4

The dotted lines in Figure 2.4 show the torque needed to overcome friction; only that part of the torque curve outside the dotted lines is available to move the rotor. The curve showing the available torque as a function of shaft angle is the difference between these curves, as shown in Figure 2.5:

Figure 2.5

Note that the consequences of static friction are twofold. First, the total torque available to move the load is reduced, and second, there is a dead zone about each of the equilibria of the ideal motor. If the motor rotor is positioned anywhere within the dead zone for the current equilibrium position, the frictional torque will exceed the torque applied by the motor windings, and the rotor will not move. Assuming an ideal sinusoidal torque versus position curve in the absence of friction, the angular width of these dead zones will be:

d = 2 ( S / (/2) ) arcsin( f / h ) = ( S / (/4) ) arcsin( f / h )

where:

d — width of dead zone, in radians

S — step angle, in radians

f — torque needed to overcome static friction

h — holding torque

 

The important thing to note about the dead zone is that it limits the ultimate positioning accuracy! For the example, where the static friction is 1/2 the peak torque, a 90° per step motor will have dead-zones 60° wide! That means that successive steps may be as large as 150° and as small as 30°, depending on where in the dead zone the rotor stops after each step!

 

The presence of a dead zone has a significant impact on the utility of microstepping! If the dead zone is x° wide, then microstepping with a step size smaller than x° may not move the rotor at all. Thus, for systems intended to use high resolution microstepping, it is very important to minimize static friction.

 

Dynamics

Each time you step the motor, you electronically move the equilibrium position S radians. This moves the entire curve illustrated in Figure 2.1 a distance of S radians, as shown in Figure 2.6:

 

Figure 2.6

The first thing to note about the process of taking one step is that the maximum available torque is at a minimum when the rotor is halfway from one step to the next. This minimum determines the running torque, the maximum torque the motor can drive as it steps slowly forward. For common two-winding permanent magnet motors with ideal sinusoidal torque versus position curves and holding torque h, this will be h/(20.5). If the motor is stepped by powering two windings at a time, the running torque of an ideal two-winding permanent magnet motor will be the same as the single-winding holding torque.

It shoud be noted that at higher stepping speeds, the running torque is sometimes defined as the pull-out torque. That is, it is the maximum frictional torque the motor can overcome on a rotating load before the load is pulled out of step by the friction. Some motor data sheets define a second torque figure, the pull-in torque. This is the maximum frictional torque that the motor can overcome to accelerate a stopped load to synchronous speed. The pull-in torques documented on stepping motor data sheets are of questionable value because the pull-in torque depends on the moment of inertia of the load used when they were measured, and few motor data sheets document this!

 

In practice, there is always some friction, so after the equilibrium position moves one step, the rotor is likely to oscillate briefly about the new equilibrium position. The resulting trajectory may resemble the one shown in Figure 2.7:

 

 

 

 

Figure 2.7

 

Here, the trajectory of the equilibrium position is shown as a dotted line, while the solid curve shows the trajectory of the motor rotor.

 

Resonance

 

The resonant frequency of the motor rotor depends on the amplitude of the oscillation; but as the amplitude decreases, the resonant frequency rises to a well-defined small-amplitude frequency. This frequency depends on the step angle and on the ratio of the holding torque to the moment of inertia of the rotor. Either a higher torque or a lower moment will increase the frequency!

 

Formally, the small-amplitude resonance can be computed as follows: First, recall Newton’s law for angular acceleration:

 

T = µ A

Where:

T — torque applied to rotor

µ — moment of inertia of rotor and load

A — angular acceleration, in radians per second per second

We assume that, for small amplitudes, the torque on the rotor can be approximated as a linear function of the displacement from the equilibrium position. Therefore, Hooke’s law applies:

T = -k 

where:

k — the “spring constant” of the system, in torque units per radian

 – angular position of rotor, in radians

We can equate the two formulas for the torque to get:

µ A = -k 

Note that acceleration is the second derivitive of position with respect to time:

A = d2/dt2

so we can rewrite this the above in differential equation form:

d2/dt2 = -(k/µ) 

To solve this, recall that, for:

f( t ) = a sin bt

The derivitives are:

df( t )/dt = ab cos bt

d2f( t )/dt2 = -ab2 sin bt = -b2 f(t)

Note that, throughout this discussion, we assumed that the rotor is resonating. Therefore, it has an equation of motion something like:

 = a sin (2 f t)

a = angular amplitude of resonance

f = resonant frequency

This is an admissable solution to the above differential equation if we agree that:

b = 2 f

b2 = k/µ

Solving for the resonant frequency f as a function of k and µ, we get:

f = ( k/µ )0.5 / 2

 

It is crucial to note that it is the moment of inertia of the rotor plus any coupled load that matters. The moment of the rotor, in isolation, is irrelevant! Some motor data sheets include information on resonance, but if any load is coupled to the rotor, the resonant frequency will change!

In practice, this oscillation can cause significant problems when the stepping rate is anywhere near a resonant frequency of the system; the result frequently appears as random and uncontrollable motion.

 

Resonance and the Ideal Motor

 

Up to this point, we have dealt only with the small-angle spring constant k for the system. This can be measured experimentally, but if the motor’s torque versus position curve is sinusoidal, it is also a simple function of the motor’s holding torque. Recall that:

 

T = -h sin( ((/2)/S)  )

The small angle spring constant k is the negative derivitive of T at the origin.

k = -dT / d = – (- h ((/2)/S) cos( 0 ) ) = (/2)(h / S)

Substituting this into the formula for frequency, we get:

f = ( (/2)(h / S) / µ )0.5 / 2 = ( h / ( 8 µ S ) )0.5

Given that the holding torque and resonant frequency of the system are easily measured, the easiest way to determine the moment of inertia of the moving parts in a system driven by a stepping motor is indirectly from the above relationship!

µ = h / ( 8 f2 S )

For practical purposes, it is usually not the torque or the moment of inertia that matters, but rather, the maximum sustainable acceleration that matters! Conveniently, this is a simple function of the resonant frequency! Starting with the Newton’s law for angular acceleration:

A = T / µ

We can substitute the above formula for the moment of inertia as a function of resonant frequency, and then substitute the maximum sustainable running torque as a function of the holding torque to get:

A = ( h / ( 20.5 ) ) / ( h / ( 8 f2 S ) ) = 8 S f2 / (20.5)

Measuring acceleration in steps per second squared instead of in radians per second squared, this simplifies to:

Asteps = A / S = 8 f2 / (20.5)

Thus, for an ideal motor with a sinusoidal torque versus rotor position function, the maximum acceleration in steps per second squared is a trivial function of the resonant frequency of the motor and rigidly coupled load!

For a two-winding permanent-magnet or variable-reluctance motor, with an ideal sinusoidal torque-versus-position characteristic, the two-winding holding torque is a simple function of the single-winding holding torque:

 

h2 = 20.5 h1

Where:

h1 — single-winding holding torque

h2 — two-winding holding torque

Substituting this into the formula for resonant frequency, we can find the ratios of the resonant frequencies in these two operating modes:

f1 = ( h1 / … )0.5

f2 = ( h2 / … )0.5 = ( 20.5 h1 / … )0.5 = 20.25 ( h1 / … )0.5 = 20.25 f1 = 1.189… f1

This relationship only holds if the torque provided by the motor does not vary appreciably as the stepping rate varies between these two frequencies.

In general, as will be discussed later, the available torque will tend to remain relatively constant up until some cutoff stepping rate, and then it will fall. Therefore, this relationship only holds if the resonant frequencies are below this cutoff stepping rate. At stepping rates above the cutoff rate, the two frequencies will be closer to each other!

 

 

Living with Resonance

 

If a rigidly mounted stepping motor is rigidly coupled to a frictionless load and then stepped at a frequency near the resonant frequency, energy will be pumped into the resonant system, and the result of this is that the motor will literally lose control. There are three basic ways to deal with this problem:

 

Controlling resonance in the mechanism

 

Use of elastomeric motor mounts or elastomeric couplings between motor and load can drain energy out of the resonant system, preventing energy from accumulating to the extent that it allows the motor rotor to escape from control. Or, viscous damping can be used. Here, the damping will not only draw energy out of the resonant modes of the system, but it will also subtract from the total torque available at higher speeds. Magnetic eddy current damping is equivalent to viscous damping for our purposes.

 

Figure 2.8 illustrates the use of elastomeric couplings and viscous damping in two typical stepping motor applications, one using a lead screw to drive a load, and the other using a tendon drive:

 

Figure 2.8

In Figure 2.8, elastomeric moter mounts are shown at a and elastomeric couplings between the motor and load are shown at b and c. The end bearing for the lead screw or tendon, at d, offers an opportunity for viscous damping, as do the ways on which the load slides, at e. Even the friction found in sealed ball bearings or Teflon on steel ways can provide enough damping to prevent resonance problems.

 

Controlling resonance in the low-level drive circuitry

 

A resonating motor rotor will induce an alternating current voltage in the motor windings. If some motor winding is not currently being driven, shorting this winding will impose a drag on the motor rotor that is exactly equivalent to using a magnetic eddy current damper.

 

If some motor winding is currently being driven, the AC voltage induced by the resonance will tend to modulate the current through the winding. Clamping the motor current with an external inductor will counteract the resonance. Schemes based on this idea are incorporated into some of the drive circuits illustrated in later sections of this tutorial.

 

Controlling resonance in the high-level control system

 

The high level control system can avoid driving the motor at known resonant frequencies, accelerating and decelerating through these frequencies and never attempting sustained rotation at these speeds.

 

Recall that the resonant frequency of a motor in half-stepped mode will vary by up to 20% from one half-step to the next. As a result, half-stepping pumps energy into the resonant system less efficiently than full stepping. Furthermore, when operating near these resonant frequencies, the motor control system may preferentially use only the two-winding half steps when operating near the single-winding resonant frequency, and only the single-winding half steps when operating near the two-winding resonant frequency. Figure 2.9 illustrates this:

 

Figure 2.9

 

The darkened curve in Figure 2.9 shows the operating torque achieved by a simple control scheme that delivers useful torque over a wide range of speeds despite the fact that the available torque drops to zero at each resonance in the system. This solution is particularly effective if the resonant frequencies are sharply defined and well separated. This will be the case in minimally damped systems operating well below the cutoff speed defined in the next section.

 

Torque versus Speed

 

An important consideration in designing high-speed stepping motor controllers is the effect of the inductance of the motor windings. As with the torque versus angular position information, this is frequently poorly documented in motor data sheets, and indeed, for variable reluctance stepping motors, it is not a constant! The inductance of the motor winding determines the rise and fall time of the current through the windings. While we might hope for a square-wave plot of current versus time, the inductance forces an exponential, as illustrated in Figure 2.10:

 

Figure 2.10

               

The details of the current-versus-time function through each winding depend as much on the drive circuitry as they do on the motor itself! It is quite common for the time constants of these exponentials to differ. The rise time is determined by the drive voltage and drive circuitry, while the fall time depends on the circuitry used to dissipate the stored energy in the motor winding.

At low stepping rates, the rise and fall times of the current through the motor windings has little effect on the motor’s performance, but at higher speeds, the effect of the inductance of the motor windings is to reduce the available torque, as shown in Figure 2.11:

 

Figure 2.11

The motor’s maximum speed is defined as the speed at which the available torque falls to zero. Measuring maximum speed can be difficult when there are resonance problems, because these cause the torque to drop to zero prematurely. The cutoff speed is the speed above which the torque begins to fall. When the motor is operating below its cutoff speed, the rise and fall times of the current through the motor windings occupy an insignificant fraction of each step, while at the cutoff speed, the step duration is comparable to the sum of the rise and fall times. Note that a sharp cutoff is rare, and therefore, statements of a motor’s cutoff speed are, of necessity, approximate.

The details of the torque versus speed relationship depend on the details of the rise and fall times in the motor windings, and these depend on the motor control system as well as the motor. Therefore, the cutoff speed and maximum speed for any particular motor depend, in part, on the control system! The torque versus speed curves published in motor data sheets occasionally come with documentation of the motor controller used to obtain that curve, but this is far from universal practice!

 

Similarly, the resonant speed depends on the moment of inertia of the entire rotating system, not just the motor rotor, and the extent to which the torque drops at resonance depends on the presence of mechanical damping and on the nature of the control system. Some published torque versus speed curves show very clear resonances without documenting the moment of inertia of the hardware that may have been attached to the motor shaft in order to make torque measurements.

 

The torque versus speed curve shown in Figure 2.11 is typical of the simplest of control systems. More complex control systems sometimes introduce electronic resonances that act to increase the available torque above the motor’s low-speed torque. A common result of this is a peak in the available torque near the cutoff speed.

 

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Defensive Mechanism of S.S

CIRCUIT BREAKER:

I. Introduction

The primary functions of a circuit breaker are interrupting short circuit current, carrying normal currents, switching ON and OFF normal loads, and providing necessary insulating between live parts and earthed parts. The maintenance problems involved with bulk oil circuit breakers were immense. Minimum Oil technology had replaced bulk oil technology during 1950’s. Similarly the air -blast technology was developed for obtaining higher performance characteristics. However, the air -blast breakers are quite expensive, and their operation and maintenance cumbersome. Hence and need was felt during 1960’s for reduced maintenance.

SF6 was first obtained from Fluorine and Sulphur in 1900 by M/s. H.MOSSAN and PLEBEAU. Behavior of SF6 in Electrical field was studied by M/s. H.G. PQLLOCK and  P.S. COOPER in 4936 known for over two decades, perfection on commercial exploitation was attained during 1960’s. This development made it possible for SF6 gas at low pressure to be used in BIN circuit breakers for insulating and are’ quenching purposes, Some of the outstanding properties of SF 6 gas which make its use ideal in EHV circuit. breakers are:

            1. Inertness

            2. Non-toxicity

            3. Electro negative nature

            4. High dielectric strength

            5. Unique are quenching property

            6. Chemical and thermal stability

            7. Good Thermal conductivity

            8. Non corrosiveness

            9. Non-Flammability

            The combined electrical, physical, chemical and thermal properties of SF6 offer the following outstanding features when used  in power circuit breaker.

            1. Safety

            2. Size reduction

            3. Weight reduction

            4. Simplified design

            5. High degree of reliability

            6. Switching of  capacitive currents without restrike

            7. Very tow noise level

            8. Easy for handling

            9. Easy for installation

            10. Maintenance free service

2. Properties of Sulphur Hexafluoride (SF6 )

a) Physical properties:

                        SF6 is a colorless, odorless and non-flammable gas. The fluorine atoms are placed at the corners of a regular octa-hedran with the sulphur atom centrally placed at a distance of 1.58 angstrom units. The bonds are predominantly covalent and the dissociation equation is

                                    SF6  –à  SF5 + F __________

            The decomposition potential is 15.7 ev.  SF6 gas is a very heavy gas and its density is approximately 5.5 times that of air. It is highly stable. It is more compressible than air and follows the law of perfect gases.

b)Electrical properties:

                        The di-electric strength of SF6 gas is 3 times that of air at atmospheric pressure and is only marginally reduced by the presence of air as impurity. The dielectric strength increases with increasing pressure. At a pressure of three bars, the dielectric strength becomes equal to that transformer oil. The size and electro negative nature molecule explain this strength. The molecule provides a large electron collision diameter. This results in capture of electrons preventing them from attaining sufficient energy to create additional .current carrying particles. SF6moiecuie also has the ability to store energy in the vibrational and electronic’ levels of the molecule there by forming stable ions of low mobility.

            The dielectric strength of SF6 remains unaltered over a wide range of frequencies. since SF6 has no dipole moment, the dielectric constant does not vary with frequency. AT 27.30c and atmospheric pressure the dielectric constant is 1.00191 and loss angle is 2 x 10-7.

            The dielectric properties of SF6 remain unchanged even at low temperatures. Unlike solid insulation materials an electrical breakdown in SF 6 gas does not result in permanent deterioration of its properties. Break down in all filled equipment may result in enormous increased of pressure due to gas formation but such hazards do not exist in the case of SF6 filled equipment.

c)Arc quenching properties:

            The ability to quench arc is unique to SF 6. This results in the high dielectric strength of the gas and the very rapid recovery of dielectric strength after arcing occurs. SF6 is approximately 100 times more effective in this respect than air under similar conditions. The low arc-time constant and its capacity to absorb free electrons due to electro negative nature makes it an excellent medium for arc interruption. The complex molecular motion of SF6 enables it to absorb electric energy and form stable negative ions. Its tendency to form negative ion around current zero results in the fast disappearance of electrons liberated during arcing. Unlike oil, arcing in SF6 will produce no carbon deposits or carbon tracking.

            The electro-negative property of SF6 may be due to several factors, including its large collision diameter. If stray electron electric field can be absorbed before they attain sufficient energy to create additional current carrying particles though collision, the breakdown can be slowed or even stopped. The large collision diameter of SF6 molecule assists in capturing these electrons. energy can be stored in the vibration levels of the SF6 atom, forming stable negative ions of low mobility. Thus the gas is electronegative in nature and shows .great electron binding capacity. Hence SF6 gas displays splendid arc-extinguishing performance .

            The arc time constant is directly proportional to the radius of arc makes it possible to have large number of breakings at full capacity of the breaker. The characteristic curve of the arc is such that the extinction power b low. In a typical case where the extinction power was of the order of 20 KW for an SF6 breaker, the corresponding value of an air blast breaker was in hundreds of KW.

            Some ion formation process with SF6 are :

            Resonance capture                  :           SF6 + e  -à  (SF6) – SF5- + F

            Positive ion formation             :           SF6 + e  -à (SF6+) + 2e -SF5- + F + 2e-

            Excitation & dissociation                    :           SF6 + e  -à (SF6-) + e -SF5- + F + e

            Positive & negative ion formation:     SF + e -à  (SF6-) + e -SF5 + F -+ e

d) Heat Transfer characteristics:

            SF6 has excellent heat transfer characteristic, an important criterion for gaseous dielectric in power applications. The higher molecular weight together with low  gaseous viscosity of SF6 enables it to transfer heat by convention more effectively than the common gases. The co-efficient of heat transfer of SF6 is approximately 2.5 tip1es that of air under the same conditions. Hence when the breaker is energized, the temperature rise small.

e)Wide  temperature range :

            SF6 in the gaseous state follows the ideal gas laws fairly closely. Consequently the pressure change is only moderate for a considerable change in temperature. The low sublimation points of SF6 assures greater dielectric strength even at low temperature the liquification temperature is —270C at a pressure of 12 Kg / sq. cm. Hence no heater is      necessary.

 f)Toxity :

            SF6 is a non-toxic gas and produces no poisonous effect on human body. But the decomposition products produced by the discharge (SF4, SF2, S2, F2 etc.) are harmful. These products are minimized by controlling of moisture in the interrupter and by absorbing the decomposition products by synthetic zeolite.

g)Chemical and Thermal Stability:

            SF6 gas is inert and it is one of the least reactive substance known under normal operating conditions. It may be heated in quartz to 5000C without under going any decomposition. SF6 does not react with water, acids and alkalis. Tests conducted have shown          practically no corrosion for various metals exposed to SF6 

h) Various constants :

            Some of the outstanding properties of SF6 which makes it ideal for high voltage power applications are:

            Molecular weight                                                        ..          146.05

            Sublimation point at 1 atm                                          ..          63.9°C

            Density of gas at 21.19 C at 1 atm                             ..          6.139

            Viscosity liquid at 13.52°C                                         ..          0.305

            Gas at 31.16°C                                                                       ..           0.0157

            Critical temperature etc.                                              ..          318.80

            Critical pressure bars                                                    ..         37.772

            Critical volume cu.metre / g                                        ..          1.356

            Dielectric strength reI N2 = al at 50 Hs -1.2 Mhs      ..          2.3 -2.5

            Dielectric constant at 25°C 1atm                                ..          1.002049 ‘

            Thermal conductivity at 30°C, Cal / Sec. -on °C                   ..           3.36 x 10-5

3. Breakdown phenomenon in SF6 :

            Breakdown in gases takes place when the free electrons gain sufficient kinetic energy Under the influence of an electric field and collide with neutral gas molecules liberating electrons from their outer shells. A chain reaction like this results in an electron avalanche. In the case of electro-negative gases like SF6 this mechanism is slightly modified. The free electrons get attached to molecules forming negative ions. SF6 + e Z SF6 -e. This negative ions are too massive to produce collisional ionization. This attachment represents an effective way of removing electrons which would have otherwise contributed to an electron        avalanche. This particular behaviors gives rise to very high dielectric strength for electronegative gases.

            The breakdown voltage of an electro-negative gas in a uniform field is a simple function of the product of pressure and spacing. the breakdown characteristics in non-uniform fields will be different because ionization may be main aimed locally due to the presence of regions of high stress. This is the corona effect. This may be due to surface roughness, sharp comers, floating conducting or semi-conducting particles. In SF6 equipments special care is taken to ensure that such sharp points do not exist in the breaker so that a fairly uniform field distribution can be achieved.

4. Principles of interruption with SF6 :

            Techniques employed for interruption with SF6 can be classified into two :

            a)         Double pressure system.

            b)         Single pressure system.

The latter can be further classified as double flow fixed nozzle and single flow series piston breakers.

a)Double pressure system:

            The functions of insulation and interruption are performed in separate chambers. SF6 at a pressure of 14 Kg/sq. cm. is stored in a high pressure chamber. This is used for quenching the are SF6 at low pressure (2.5 to 3.5 Kg/sq. cm.) provides the insulation. When the contacts separate under fault, gas at high pressure is forced into the arcing region and then it follows in to the low pressure region. The gas thus exhausted in to the low pressure region is compressed again and returned to the high pressure reservoir. The arcing takes place between the arcing tip and arcing ring thus relieving the contact area from the stresses of arc. A filter with actual alumna is kept at the intake of the compressor so that all the decomposition products of gas can be absorbed before re-circulating in to the system. A thermostatically controlled heating system will be provided in the high pressure reservoir to prevent condensation of gas at low temperature.

b) Single pressure system :

            In this case SF6 at low pressure (3 to 6.5 Kg/sq.cm.) provides the insulation and the energy for interruption. The breaker chamber consists of the fixed and moving contacts, and the piston arrangement in the puffer type fixed contact. As the moving contact separates under fault, the piston moves forward with high speed. This compresses the SF 6 inside the hallow fixed contact and forces the gas into the arc resulting in quenching. The force with which the gas could be blast depends on the design of the piston arrangement and the energy of the control mechanism.

            A further improvement is the Magnetic puffer type breakers where the operating force on the moving contact rod is increased, by magnetic repulsive force. The short circuit current is passed through a set of coils fixed on the support of the moving contact fed. A secondary short circuit ring is positioned and magnetically coupled with primary winding. This ring acts as piston as well. This interaction between the. two fields produces a repulsive force and it pushes the moving contact rod forward. The addition of this simple magnetic drive mechanism improves the interrupting capabilities of the breaker.

            The single pressure system has an inherent advantage of simplicity in construction. It needs no additional compressor as required in double pressure system. The manufacturing cost of puffer type equipment is lower.

5. Construction:

            The arc extinguishing system employs a synchronized double flow single pressure puffer type design. This leads to a simple construction.

            The SF 6 circuit breaker mainly comprises of the following:

            1.         Breaker poles it.

            2.         Base tube and mechanism box

            3.         Control unit

4.                  Air compressor electro-hydraulic operating mechanism

           

1.Movable Cylinder(Puffer cylinder)    2.Moving Contact 

3.Fixed Contct 4.Insulating Nozzle

5.Fixed Piston  6.Gas Trapped in before compression 

7.Compressed gas between 1 & 5

8.The arc-being extinguished by puffer action

5.1.Breaker Pole:

            The primary functions of a circuit breaker are carried out of breaker pole. The breaker pole consists of interrupter unit and support insulator.

            The interrupter unit consists of fixed contact tube, guide tube, moving contact tube, puffer or blast cylinder and piston. The fixed contact tube is connected to the top terminal via. Contact support.

The guide tube is fastened to the lower terminal. The other ends of the fixed contact tube and guide tube which are subjected to arcing during the arc interruption are provided with arc quenching nozzles. the nozzles are made up of graphite materials which keeps the contact wear to minimum. The moving contact tube consists of spring loaded finger contacts arranged in the form of a ring. The front end of the moving contact tube is provided with an arc resistance insulating ring and arcing ring of high arc resistant materials

            The blast cylinder which is made up of high arc resistant insulating material and the moving contact tube are rigidly coupled to each other and connected to the operating rod in the supporting insulator.  The blast piston which is made up of aluminum is fastened to the lower terminal pad. The fixed contact tube, guide tube, moving contact tube, blast cylinder and blast piston are “all housed inside a porcelain ,insulator. When the circuit breaker is in close position current flows from top terminal to bottom terminal through contact support, fixed contact tube, moving contact tube and guide tube.

            The support insulator apart from supporting the interrupter unit provide insulation between live parts and earthed parts. It houses the operating rod (insulated), one end of which is connected to the interrupter unit and the other end is connected to the mechanism.

5.2. Base Tube mechanism box:

            The base tube which supports the breaker pole and the mechanism box acts as a local air reservoirs. The mechanism box enclosed electromagnetic valve, closing coil, trip coil and operating cylinder. Lower mechanism case encloses the complete lever system to transmit the operation force from the mechanism box to the breaker pole.

 5.3.Control Unit :

            This accommodates the gas pressure switches, gas density detector, gas pressure gauge, air pressure gauge, air valve heater, auxiliary relays, terminal blocks, etc. for electrical and pneumatic control and monitoring of the breaker. The control devices of the air and SF6 gas systems are common for 3 poles of the breaker.

5.4.      Compress

            Since the operating energy requirement is greater the MOCBS either air compressor or electro-hydraulic operating mechanism is used.

6. The principle of Arc extinction:

            When the circuit breaker is in closed position the moving contact assembly bridges the fixed contact tube and the guide tube. When an opening operation is initiated, the blast cylinder moves towards  the stationary blast piston so that the SF6 gas in the blast cylinder is compressed to a pressure required to quench the arc. The gas compressed during the above process is released only when the contacts are separated with moving contact assembly acting as a slide valve. At the instant of contact separation, arc strikes between the front end of the arc quenching nozzle of the fixed contact tube and the arcing ring of the moving contact tube. The compressed gas in the blast cylinder is released in the break radically as the contacts are separated. As the moving contact assembly moves further, the arc between the front end of the fixed contact nozzle and the arcing ring of the moving contact is transferred from the arcing ring of the moving contacts of nozzle of the guide tube , by gas jet and its own electrodynamics forces. the arc is further elongated by the gas flow axially into the nozzles and safety extinguished. While the arc is being interrupted, the blast cylinder which is made up of arc resistant insulating material enclosed the arc quenching assembly, there by protecting the porcelain insulator from arcing effects. After arc extinction, the moving contact assembly and blast is free of any parts of the chamber which may have a bridging effect or influence the electric field distributor.

7. Operation principles:

7.1. Opening operation:

            When the trip coil is energized, the space of pilot valve is filled with compressed air and the charging valve moves to right. The space in the operating cylinder is filled with compressed air from the air received and the operating piston is rapidly driven to the left. the operating rod connected to the operating piston is pulled in the opening direction to drive the puffer cylinder at the high speed through the insulated operating rod in the supporting insulator. the SF6 gas in the puffer cylinder is compressed and the SF6 gas blast extinguishes the arc generated between the moving and stationary contacts.

            Simultaneous with the opening operation, the cam rotates and causes the electromagnet valve to return to its original position. As a result, compressed air in the space of pilot valve is exhausted into atmosphere and the charging valve is reset to the original piston. As the open state is retained by the link mechanism attached to the end of the operating piston.

7.2. Closing operation:

            When the closing coil is energized, the arc nature is made to rotate causing the hook to be disengaged. Thus the sector line rotates to release the roller and the operating piston is driven in the closing direction by the force of the closing spring, upon completion of closing, the link mechanism is held in a state to be ready for the subsequent opening operation.

8. Caution :

            When operating the breaker observes the following:

I)Keep correct SF6 gas pressure and operating air pressure as specified.

2)Operate the stop valves properly.

3)Do not allow ingress of moisture and dust into the SF6 gas supplying point.

4)Do not pump the gas piping and air piping with any object.

5)Do not damage the gasket and seal face on the leakage tight joint in the gas and air system.

6)When opening the circuit breaker by the manual handle. ‘

                        a) confirm that the main circuit is not energized.

                        b) Be sure to turn off the control power supply.

                        c) Confirm that compressed air in receivers is released.

                        d) Confirm that manual operating rod and handle are removed before                                             changing the receiver with compressed air.

7)Do not operate any part other than the manual operating handle before filling SF6 gas at the rated pressure. Do not fill compressed air before filling SF6 gas.

8)When checking interior parts of interrupter, blow air into the system for   sufficiently long time and confirm that sufficient supply of air is available before starting any work.

9.Gas Leak Detection:

            If the gas leaks through any point, this can result in reduction of pressure and consequent loss of insulation properties Gas Leak detection is done with the help of a halogen torch type detector. The detector works on the principle that SF6 absorbs a certain number of electron when passed through an atmosphere where free electrons flow. The free electrons are generated with in the sector by a small radio active source in the presence of a carrier gas. these electrons are collected at the detector anode and give a small base line current which is amplified. When the probe of the detector is kept near the joints of the SF6 filled equipment and if SF6 leaks out there will be variation in amplified valve of current due to electron absorption by SF6. The variation can be directly calibrated to indicate the magnitude of the leak.

9.2. Detention of presence of conducting particles:

            This is done by conducting a dielectric test when the test voltage is applied there will be an internal corona if metallic particle or sharp comers are present. The presence of internal discharges is located with the help of an ultrasonic detector which is very sensitive in detecting noise due to internal corona. The sector translates the ultrasonic vibrations into audible frequencies and directly indicates the intensity of sound in decibels. The probe is pressed firmly against the grounded enclosure tube while the conductor is energized at varying AC I DC voltage. If the noise disappears at low voltage, appears at some intermediate voltage and the intensity continues to increase, it is certain that the noise is due to internal corona. It has also been observed that in some cases the small sharp potty branched in areas of high dielectric stress get burnt or the particles driven to low stress areas. The effect of conducting particles on the break down strength of SF6 is more serious for power frequency voltage test than for impulses voltage.

10. Performance of SF6 Breaker:

            SF6 gas circuit breaker combines the advantageous features minimum oil and air blast breakers and exhibits a number of additional advantages over both.

            1)It is possible to have large number of breaking operations near full breaking                                    capacity with out any undue wear.

            2)Because of the fast recovery of dielectric strength across the parting contacts                                  during interruption.

                        a) These breakers are restrict free while switching of capacitive currents.

                        b) These breakers are incentive to short time faults and are capable of                                                    breaking at every high values of RRRV and

                        c) These breakers are suitable for multi-short re closing with out any reduction                                in breaking capacity

            3)There is no necessity to change any parts in the breaking chamber even after                                   a period often years of service in the actual system. This means that there are                                       practically no problem of maintenance for SF6 breakers.

            4)The operation is noiseless since the gas is used in a closed circuit. There will                                   be no discharge of arc products into atmosphere.

            5)Puffer type breakers are autonomous and independent because no auxiliary                                 equipment is required.

            6)Fire hazards are eliminated.

RELAY

A relay is an electrical switch that opens and closes under the control of another electric circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts.

Operation

When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.

If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This “shading ring” creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.

By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light – emitting diode (LED) coupled with a photo transistor.

Types of relay

The following types of relays are commonly encountered:

SPST – Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology “SPNO” and “SPNC” is sometimes used to resolve the ambiguity.

SPDT – Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.

DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. It is ambiguous whether the poles are normally open, normally closed, or one of each.

DPDT – Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil.

QPDT – Quadruple Pole Double Throw. Often referred to as Quad Pole Double Throw, or 4PDT. These have four rows of change-over terminals. Equivalent to four SPDT switches or relays actuated by a single coil, or two DPDT relays. In total, fourteen terminals including the coil.

SURGE ARRESTERS AND INSULATION CO-ORDINATION

I.Introduction:

            Electrical systems by nature involve two forms of protection over current and over voltage since over current protection of electrical equipment’s are well known to all, it is not elaborated here. Over voltage protection on the other hand, remains a relatively new subject to many engineers. Both types of protection equally necessary for safe system operation.

            The importance of over voltage protection for a power system can not be over emphasized. Major equipment failures, expensive repairs, personnel safety and plant down time are certain consequences of inadequate protection from voltage surges.

            Surge arresters are designed to limit dangerous system over voltages. Whether lighting-or System- produced-to safe values when they occur on power systems. An arresters is a voltage limiting device. The functions are to discharge energy associated with a system over voltage condition, limit and interruption the power fellow current that follows the transient current through the arresters and return to an insulating state prepared for the next over voltage occurrence.

            In performing its voltage limiting function, certain protective characteristics of the arrester must be coordinated with the prevailing insulation levels on the system being protected. Insulation is a basic factor that must be considered in the application of arresters on a system. Insulation co-ordination is only a small part of the over all subject of arrester application. Several other factors must also be considered by the engineer when selecting surge protection. The location of the arresters, the inter-connection of ground leads, the insulation level of the protected equipment and the rating of the surge arresters are important in protecting equipment from harmful over voltage.

II.Surge Arrester operation:

            The basic operation of a surge arrester is single. In its noffi1al state, an arrester must act as an insulator. When a high voltage surge occurs. The arrester must cease to be an insulator and must turn into a short to-ground-in million thus of a second. The operation of the most widely used type of surge arresters the value, type of arrester is dealt with. Other types of arresters, such as expulsion arresters and line Oxide arresters (Gapless arresters) are either on the decline or too new for a general discussion at this time. The active elements of a valve type arrester are the spark gap and the valve block. these are housed in a porcelain shell for atmospheric protection and external insulation.

            The gap assembly consists of a number of in-series air gaps with sufficient dielectric strength to withstand the highest power frequency on the system. During severe over voltage conditions, the gap must always, breakdown at a voltage level some what below the insulation withstand voltage level of the equipment it is protecting, other wise equipment damage and or plant down time will result. the gap therefore serves as the switch which turns on the arrester. the voltage level at which the arrester goes from the passive (insulating) to the active (conducting) state, is called the spark over voltage.

            The valve block controls what happens after the arrester has been turned on. If only a gap is used, once a surge has been diverted to ground, a dead short circuit exists between line and ground and the 50 hertz-system energy tries to flow to ground causing a fuse, re-closer or breaker to operate to interrupt the system fault current.

            The valve element does exactly as its name implies. It conducts when surge current is flowing and it ceases to conduct when 50 Hz line current begins to flow. the valve block is able to do this because It is made of a non-linear resistance material, silicon carbide. The valve block offers a very high resistance to 50 Hz current while displaying a low resistance to surge current. In addition, it also consumes the surge energy passes through it.

            Spark over and discharge voltage are the two protective characteristics of an arrester which are used in calculating margins of protection when studying insulation co-ordination. These protective characteristics are published by arrester manufacturers.

III. Arrester Classification :

            There are three classifications of surge arresters used for over voltage protection in a system.

1.Distribution Type:

            The arresters are generally used in distribution system for equipment protection. Standards distribution arresters are used for protecting oil. Insulated distribution transformers, these arresters are also used as line entrance arresters, for 11KV and 22KV lines. They are the lowest in cost.

2.Intermediate Type :

            These units cost approximately two or three times as much as equivalent distribution units. For this, the arrester offers lower maximum spark over and discharge voltage characteristics that afford a greater margin of protection plus the capability of discharging large surge levels. These arresters also have a pressure relief system to safely vent internal pressure if the unit falls before the porcelains shell has a chance to rupture. These arresters are used for the L.V. protection of Power transformers in sub-transmission sub-station i.e.110/33/22/11KV and 66/22/11KV sub-station.

3.Station Type:

            These arresters offer the best protective characteristics and the highest thermal capability but they cost about twice as much as equivalent intermediate units. Like intermediate arresters, station arresters have a pressure-relief system to safely vent internal pressure if the unit fails before a porcelain shell has a chance to rupture. These arresters are generally used in 230KV, 110KV and 66KV systems.

4.Basic insulation level:

            Basic Impulse Insulation Level (BIL) is the voltage level that equipment insulation is capable of withstanding without sustaining damage. The voltage withstand of insulation is function of time. Inorder to establish volt-time impulse insulation levels of transformers standard impulse tests standard voltage withstand tests are conducted on selected units as type test. Transformers are subjected to impulse voltage tests (at rated BIL) and a chopped wave test (15% above BIL). A steep front – of wave test (65% above BIL) is also performed on some units. A curve plotted through these three points defines the minimum insulation withstand curve for insulation co-ordination (Fig.3) The true withstand level for the transformer lies above the plotted curve.

5. Surge arrester application:

            With an understanding of how an arrester performs its functions and a knowledge of equipment insulation, we can now move into the application area and consider the several factors that comprise surge arrester application as it relates to over voltage protection of transformers, The selection of surge arresters merit are carefully considered. Various factors have to be taken into account in order to arrive at a reliable and at the same time economical means of protection. The important points are:

            i)Selection of rated voltage.

            ii)Selection according to the standards, codes, recommendations for insulation coordination.

i)Arrester rating :

            The voltage rating of an arrester is defined as the highest 50 Hz voltage at which the arrester is  designed to operate and reseal effectively after a surge has passed. Because of the system grounding and connection, this, voltage is typically higher than the phase to ground voltage / on the healthy phases will increase temporarily and it depends upon the earthing factor or the system. The selection of an arrester voltage rating for station depends upon grounding system connection and system voltage rating.

            Also the voltage impressed across an arrester during a surge discharge is directly proportional to the arrester voltage rating that is, a 10,000 Amps surge produces a higher discharge voltage if it is flowed through a 10KV arrester than it does flowed through a 9KV arrester generally it is desirable from the stand  point of equipment protection to select the lowest voltage rating for the application.

ii)Arrester location:

            Surge arresters should always be located as close as possible to the terminals of the equipment protected. In the case of transformer protection, mounting the arresters directly on the transformer is the best of insurance. An appreciable distance between the surge arrester, and the protected equipment reduces protection, afforded by the arresters and also increases the voltage impressed upon the transformer at time of surge discharge. Also because of the extra travel distance between the equipment and its arrester, surge wave could rise above the equipment damage point before the arrester comes to its rescue.

            n addition, the arrester connecting leads should be kept as short as possible because of their voltage contribution to discharge the voltage. During current flow to ground through an arrester, the interconnecting leads provide a voltage contribution because of current passing through an impedance. Depending on surge magnitude, rate of rise type of conductor, a typical value of voltage contribution to discharge voltage by interconnecting leads is i.e. 1.6 KV / foot.

            In practice, the protection range is given by the following simple formula.

                        L          =          U – Ua  x V     Where

                                                2 X S

                        L          =          Protection range of arrester in meters

                                                (measured along the line)

                        U         =          Impulse withstand voltage of protected equipment in KV.                                                   (BIL of equipment)

                        Ua       =          Spark over voltage of an arrester in K. V. (Peak) of the system.                                                       During earth fault conditions, the voltage

                        V         =          Velocity of wave progression with

                                                V line              =          300 meters /micro  sec.

                                                V cable            =          150 meters /micro  sec.

                        S          =          Steepness of incoming wave front in KV /  sec.

                       

            (The protection range of an arrester increases with the difference between the impulse voltage IV’ and the spark over voltage Va. Therefore, an arrester with protective level tends to extend the protective range)

iii)Interconnection of Grounds:

            It is essential that the arrester ground terminal be interconnected with the transformer tank and secondary neutral to provide reliable surge protection for the transformers.

Iv)Insulation coordination: .

            Now let us consider the selection of an arrester according to standards, codes or recommendations for insulation coordination. Calculating the margin of protection is the  major part of an. insulation co-ordination study. Insulation coordination is the process of comparing the impulse strength of insulation with the voltage that can occur across the arrester for the severity of surge discharge for which the protection is desired. For a transformer, this means a comparison of the volt-time insulation withstand curve with the impulse and switching surge spark over and discharge voltage curve of the arrester.

            After determining the rated voltage of an arrester, the protective level has to be carefully selected. For complete protection of the equipment, the “protective level” viz. the level to which the over voltages are omitted by the arrester, must be lower than the withstand level by a factor of at least 1.2 for lightning surges and 15 for switching surges. The value thus selected must be checked against that given in I.S.S. or the technical details furnished by the arrester manufactures.

            To arrive at the discharge voltage of an arrester for these calculations discharge voltage for a 10,000 Amps. surge is normally used. The following formula define these two margins of protection calculations:

                                    CWW -FOW SO                    BIL -DV + IX)

            MP1 =                         CWW      x 100% MP2           =           BIL      x 100%

Where

CWW              = Chopped -waved withstand voltage of transformer winding = 1.15 BIL

FOW SO         = Front of wave spark over of surge arrester in KV (Crest)

BIL                 = Basic Impulse Insulation level of the transformer.

DV                  = Discharge voltage of the arrester at 10 KA surge.

IX                    =  Voltage contribution of connecting leads at the rate of 1.6 KV / ft.

MP                  =  Margin of Protection

            Insulation co-ordination in an important aspect to be considered when surge protective is to be afforded to transformers with reduced BILS

vi Protection against direct strokes:

i)          Protection against direct strokes can be handled by shielding the station equipment’s by                the provision of either

            a)         Mast or rods or

            b)         a net work of overhead ground wires in such a way that equipment’s and switches                                     of all lie in the protected zone.

ii)         The protected zone for a rod mast is generally assumed as a cone with a base radius                       equal to the height of the rod or mast above ground.

iii)        For small sub-stations it may be sufficient to run one or GI wires across the station                        from adjacent line towers. Extra wires may be run from the tower to the structure and                over the station.

iv)        The grounds of the station shield should be solidly tied to the station ground bus to                       prevent difference of surge potential between the shield and other g-rounded parts of                    the Station.

SAFETY IN SUB-STATION

            Prevention of damages to equipment’ s and men working on then due to any accidents is an essential aspect in any establishment. Prevention of accident which is an unforeseen one is more essential aspect of any establishment / organisation.

            As accidents occur mainly due to unsafe execution, actions and circumstances, these accidents can be avoided by adopting safety precautions, implementing safety procedures and following safety rules.

General safety methods:

I.          While execution of any work, that part of equipment or line is to be isolated from the                    supply.

2.         Using discharge rods, charging, current if any is to be discharged.

3.         Using Earth rods, all phases/conducting path are to be property earthed by securing                       good Earthing.

4.         When even opening an AB switch or removing of fuse, it is also advisable and                   preferable to wear rubber gloves.

5.         Use of belt rope is another safety method to be adopted to work on elevated places.

Safety methods to be adopted in Sub-Stations :

            In any work is to be attended to any line, first and fore most item of work is to get proper approval from the competent controlling authority for execution of the work specifying the date, time, duration,  place of work, affected parties etc. .

            For Grid feeders and Stations, the authorized officer for issue of approval is S.E.               (L.D. Centre), Madras, For 110 KV, 66 KV, radial feeders Superintending Engineer /                        Distribution is the approving authority. Similarly for 33 KV Divisional Engineer incharge of distribution is the approving authority.

             Above details with the list of authorised officers is enclosed herewith (enclosure I)

             Without obtaining proper approval from the competent authority, no L.C. should be issued nor availed by anybody. If the above procedure is not followed, it is nothing but a suicidal. Further it also amounts to murder of others.

            So, after getting proper approval, line clear is to be  issued to the requested party. But the issue and receiver should be aware/have full knowledge about the SS equipment’s, control room panel details etc.,

The line clear issuing person should clearly record the following:

            a) Which breaker have been tripped

            b) Which A.B. switches were opened

            c) Where Earthing was done

            d) What is the Safer place / Line to carry on the execution of work

Safety arrangements in control room:

1)         Key Board should be in open condition so that the keys could be taken out quickly                       during any urgency.

            Line clear keyboard should be in locked up condition to prevent other persons from           using the keys inside, before the cancellation of the Line clear permit.

The keys should be placed in the key board in an orderly manner according to their numbers. Otherwise, the required lock could not be opened in time and the possibility of opening a wrong lock may happen.

2)         Rubber mat should be provided on the floor in front of the panel board.

3)         The following details should be clearly displayed in the control room.

                        Approved operating instructions for all equipment’s.

                        Break down instructions.

Operating instructions including for the emergency operations to be carried out in the event of operation of buchholz relay. Differential relay, Group control trip, total supply failure, grid failure. The operator should be fully conversant with the above instructions and   the must be able to act quickly and effectively.

4)         The Board containing D.C. cable layout. A cable layout panel wiring diagram and Earthing layout should be displayed in the control room. This is necessary to attend the faults immediately after their occurrence.

5)         D.C. Earth leakage test system should be available.

6)         There should not be any defective power plugs, switches and bulb holders in the    control room wiring.

7)         One artificial respirator should be available in ready condition.

8)         Stools made of insulating material should be used for operating high tension          communication equipment’s (Telephones).

9)         Adequate number of rubber gloves, belt ropes, discharge rods, and earth rods in     good condition should be available in the control room.

Battery room:

1.         Battery room should be in locked up condition.

“Naked flame is prohibited inside of the battery room” and “Smoking prohibited” warnings should be kept written on the battery room door.

2.         One exhaust fan should be functioning.

3.         Accurate D.C. cell testing volt meters, hydro meters and thermometers should be               available in the battery room.

4.         Pilot cell voltage, specific gravity and temperature should be taken every week.

5.         The specific gravity should not be maintained below 1195 at 15.6°C and below 1183                    at 32. 20°C. The battery should not be allowed to discharge below 1160.

6.         Cell voltage should be maintained between 1.95 V to 2.05 V. The battery should not                     be allowed to discharge below 1.85 V.

7.         Battery should be allowed neither to over charge not to undercharge. It should not also                 be kept idle.

8.         Electrolyte level must be checked in every shift. It must be ensured that the level is                       10mm above the top of the plates.

9.         Weak cells should be rectified then and there.

10.       While taking specific gravity readings, care must be taken not to allow the acid to                         come in contact with the eyes.

Safety adopted for transformers:

1.         Transformers are to be maintained periodically as per schedule. Switches on HV side                     and LV side are to be isolated after reducing the Load by tripping the breakers.

2.         Kiosks and OCB : All the Live parts of the kiosk should have H. T. insulation tape. To be protected by wiremesh. It should be vermin proof. Keys are to be kept with interlock. When ever to open the door of the kiosk, kiosk should be tripped link should be opened by the interlock key. The opening of the links are to be verified physically. After doing all the above precautions, the tank should be lowered down. Proper care is to be taken and it should be kept in mind that supply is available at the roofing.

            Oil leak should be arrested. Back feeding is avoided.

            Cotton waste should not be used for cleaning purpose.

3.         AB switches:

Handle of the AB switch is to be earthed properly. Blades should be kept at opening position. It should not be closed automatically, proper maintenance is to be done for this. AB switch blades are to be opened fully. AB switches are to be kept locked on both            conditions. AB switches are to be opened only after tripping the breakers.

4.         Lightning arresters :

            Lightning arresters are used to bypass the sudden lightning surges and thereby to protect the equipment’s.Only after proper discharging is done on lightning arresters, it should be attempted to attend to maintenance.Fencing is to be provided around lightning arresters. Door arrangements with lock is to be provided. Separate earth connections are to be provided for lightning arresters.

 5.        Current transformers:

            Current transformer secondary side is to be short circuited during maintenance and testing. Before doing any testing, the current transformers are to be discharged.

6.         Potential transformers:

            Potential transformers primary side is to be Earthed during maintenance and testing. Secondary side is to be earthed at only one place. Whenever giving connection, or removing meters on the secondary side of die potential transformer, the fuses are to be removed and renewed.

7.         Capacitors and H. T. Coupling capacitor:

            Capacitors should be provided inside fencing. Before attempting to do any work, proper discharging is to be done. They only it should be attempted for maintenance work. Proper Earthing should be provided during the execution of the work. After completion of the work, Earthing is to be removed.

8.         Earth pits:

            Sub-station earth connections should be properly maintained so that the earth            resistance is minimum. Water should be poured in the earth pits daily. Earth connections, must be capable of protecting the persons working in the electrical equipment’s and protect in the equipment’s during heavy fault current. Earth resistance should not exceed the following limits.

            Grid stations: I Ohm Other sub-stations ..2 Ohm.

            Distribution transformers ..5 Ohm.

            They must be a clearance of 5 feet, between the sub-station fence and the electrical equipment’s / live points. The fence should be earthed at every 200 feet, separately. Generally the fence Earthing should not be linked with the sub-station Earthing. But if the clearance  is less than 5 ft. feet fence Earthing must be linked with the sub-stations Earthing. The iron gates in the sub-station fence should also be earthed separately.

9.         Fire fighting equipments:

            These equipment’s are to be kept on good and working condition. Proper schedule of maintenance is to be done for keeping them in good conditions. These equipment’s should be kept at an easily accessible place so as to use them immediately under emergency. Dry sand heaps are to be available wherever necessary. Empty buckets are to be provided.

 10.      S.S. Yard:

            1.         S.S. yard should be provided with fencing.

            2.         Unauthorised persons should not enter into the yard

            3.         Cable ducks are to be provided with slabs.

            4.         Best illumination is to be provided for the yard.

            5.         A warning board with a display that “Umbrella” stick Dogs should not be brought                         inside the  yard” is to be provided at the entrance of the yard.

            6.         A separate room is to- be provided for keeping the empty drums. At the                                         entrance of the room “No smoking” Board is to be provided.

General

1.         The territory of the work spot which was declared safety to work is to be clearly identified by tying a rope. Inside this boundary is to be further identified by hanging a green flag. Outside this boundary where it is unsafe to work is to be identified by a red flag.

2.         Wherever necessary caution boards like “Men on working” “Don’t Switch on“ Safe for work” etc., are to be provided.

3.         If any unauthorized, unskilled staff happen to go near the equipment’s he can do so with the assistance and under the vigil of an experienced, authorised staff.

4.         Conversation is strictly prohibited wile execution of any work. It should be                        totally avoided especially when work is being carried out on any bus bars.

5.         Placing the materials, tools and plants and men are to be at a safety clearance from the Live. parts.

            6.         T & Ps like spanners etc. are to be lifted and brought down only by means of                                 ropes and not by throwing and catching.

7.         Study and safe ladder with steps at convenient intervals is to be used. To avoid slippage of the ladder, necessary precaution is to be taken at the bottom of the ladder by providing empty gunnies.

            8.         Lifting of any ladder or rods (Earth) are to be done only horizontally. Vertical

                         lifting may cause damages by interrupting with the safe clearances.

            9          The bus and line links art’; to be kept opened while doing work on OCB and

       

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A button microphone is also known as carbon microphone or carbon transmitter. The design of a carbon microphone is quite a simple one. It consists of two metal plates separated by small pieces of carbon. One of the pieces between the two faces outwards. Its function is to work like a diaphragm.
The mechanism by which a button mic works is quite complex. The sound wave, which is caused by the displacement of the air particles, strikes the plate, which acts like a diaphragm. The original pressure that was there in the mic is changed, which results in changing of the electrical resistance between the plates. Difference in the wave pressure changes the speed of the plate movement. The step comes next is the passing of direct current from one plate to another. The change in current can then be used to pass, though an electric system transforming it into a signal.
Advantages and disadvantages of a Button Microphone
One of the basic advantages of a carbon microphone was that it has high output level, as the impedance or the obstacle amount is low. The high level audio signal output comes from very low DC voltages; and as it is itself an amplifier so there is no need for any external amplifiers or batteries.
Various areas, where physical engagement is high, the use of carbon microphones is growing. For example, in opera theatre, where the singers are busy with movements, there they use carbon microphones for smooth flexibility along with easy grasp of the music. In more modern disco clubs, DJs most of the time use button microphones for creating musical blenders.
But the carbon microphones were discarded by the radio broadcasting system after the 1920s due to their low quality sound output, which more often than not had high noise or hiss level. Also, the frequency response for the carbon mic was limited.
History of Button Microphone
The year 1978 saw T. A. Edison and Emile Berliner both claiming for the invention of the carbon transmitter. The two sides had a tough battle over the patent rights; which was later sorted by a Federal court giving Edison the right over the invention.
Other Various Uses of Carbon Microphone
Use of Carbon Microphone as an Amplifier
Surprisingly carbon microphones can also be used as an amplifier. In earlier telephone repeaters this attribute was put into use. In this arrangement, a telephone receiver was mechanically coupled with a carbon mic to boost weak signals and send them down the telephone line. In the 1930s it was also used in some audio equipments, especially in hearing aids. In the 1950s, however, the transistors replaced their earlier counterparts.
Early Amplitude Modulation or AM based radio also depended on the carbon mic for voice modulation of the radio signal.
Modern day use of the Button Microphone
In some developing countries the legacy telephone installations still uses them to a greater extent. Some third world countries use them in some niche applications. One such example is the Shure 104c, which is quite compatible with some of the existing equipments.

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This definition implies that a device can only be called a “robot” if it contains a movable mechanism, influenced by sensing, planning, and actuation and control components. It does not imply that a minimum number of these components must be implemented in software, or be changeable by the “consumer” who uses the device; for example, the motion behavior can have been hard-wired into the device by the manufacturer.

 

So, the presented definition, as well as the rest of the material in this part of the Book, covers not just “pure” robotics or only “intelligent” robots, but rather the somewhat broader domain of robotics and automation. This includes “dumb” robots such as: metal and woodworking machines, “intelligent” washing machines, dish washers and pool cleaning robots, etc. These examples all have sensing, planning and control, but often not in individually separated components. For example, the sensing and planning behavior of the pool cleaning robot have been integrated into the mechanical design of the device, by the intelligence of the human developer.

 

Robotics is, to a very large extent, all about system integration, achieving a task by an actuated mechanical device, via an “intelligent” integration of components, many of which it shares with other domains, such as systems and control, computer science, character animation, machine design, computer vision, artificial intelligence, cognitive science, biomechanics, etc. In addition, the boundaries of robotics cannot be clearly defined, since also its “core” ideas, concepts and algorithms are being applied in an ever increasing number of “external” applications, and, vice versa, core technology from other domains (vision, biology, cognitive science or biomechanics, for example) are becoming crucial components in more and more modern robotic systems.

 

This part of the WEBook makes an effort to define what exactly is that above-mentioned core material of the robotics domain, and to describe it in a consistent and motivated structure. Nevertheless, this chosen structure is only one of the many possible “views” that one can want to have on the robotics domain.

 

In the same vein, the above-mentioned “definition” of robotics is not meant to be definitive or final, and it is only used as a rough framework to structure the various chapters 

 

Components of robotic systems

 

 

 

 

 

 

 

 

This figure depicts the components that are part of all robotic systems. The purpose of this Section is to describe the semantics of the terminology used to classify the chapters in the WEBook: “sensing”, “planning”, “modeling”, “control”, etc.

 

The real robot is some mechanical device (“mechanism”) that moves around in the environment, and, in doing so, physically interacts with this environment. This interaction involves the exchange of physical energy, in some form or another. Both the robot mechanism and the environment can be the “cause” of the physical interaction through “Actuation”, or experience the “effect” of the interaction, which can be measured through “Sensing”.

 

Robotics as an integrated system of control interacting with the physical world.

 

Sensing and actuation are the physical ports through which the “Controller” of the robot determines the interaction of its mechanical body with the physical world. As mentioned already before, the controller can, in one extreme, consist of software only, but in the other extreme everything can also be implemented in hardware.

 

Within the Controller component, several sub-activities are often identified:

 

Modelling. The input-output relationships of all control components can (but need not) be derived from information that is stored in a model. This model can have many forms: analytical formulas, empirical look-up tables, fuzzy rules, neural networks, etc.

 

The name “model” often gives rise to heated discussions among different research “schools”, and the WEBook is not interested in taking a stance in this debate: within the WEBook, “model” is to be understood with its minimal semantics: “any information that is used to determine or influence the input-output relationships of components in the Controller.”

 

The other components discussed below can all have models inside. A “System model” can be used to tie multiple components together, but it is clear that not all robots use a System model. The “Sensing model” and “Actuation model” contain the information with which to transform raw physical data into task-dependent information for the controller, and vice versa.

 

Planning. This is the activity that predicts the outcome of potential actions, and selects the “best” one. Almost by definition, planning can only be done on the basis of some sort of model.

 

Regulation. This component processes the outputs of the sensing and planning components, to generate an actuation setpoint. Again, this regulation activity could or could not rely on some sort of (system) model.

 

The term “control” is often used instead of “regulation”, but it is impossible to clearly identify the domains that use one term or the other. The meaning used in the WEBook will be clear from the context.

 

Scales in robotic systems

 

The above-mentioned “components” description of a robotic system is to be complemented by a “scale” description, i.e., the following system scales have a large influence on the specific content of the planning, sensing, modelling and control components at one particular scale, and hence also on the corresponding sections of the WEBook.

 

Mechanical scale. The physical volume of the robot determines to a large extent the limites of what can be done with it. Roughly speaking, a large-scale robot (such as an autonomous container crane or a space shuttle) has different capabilities and control problems than a macro robot (such as an industrial robot arm), a desktop robot (such as those “sumo” robots popular with hobbyists), or milli micro or nano robots.

Spatial scale. There are large differences between robots that act in 1D, 2D, 3D, or 6D (three positions and three orientations).

 

Time scale. There are large differences between robots that must react within hours, seconds, milliseconds, or microseconds.

 

Power density scale. A robot must be actuated in order to move, but actuators need space as well as energy, so the ratio between both determines some capabilities of the robot.

 

System complexity scale. The complexity of a robot system increases with the number of interactions between independent sub-systems, and the control components must adapt to this complexity.

 

Computational complexity scale. Robot controllers are inevitably running on real-world computing hardware, so they are constrained by the available number of computations, the available communication bandwidth, and the available memory storage.

 

Obviously, these scale parameters never apply completely independently to the same system. For example, a system that must react at microseconds time scale can not be of macro mechanical scale or involve a high number of communication interactions with subsystems.

 

Background sensitivity

 

Finally, no description of even scientific material is ever fully objective or context-free, in the sense that it is very difficult for contributors to the WEBook to “forget” their background when writing their contribution. In this respect, robotics has, roughly speaking, two faces: (i) the mathematical and engineering face, which is quite “standardized” in the sense that a large consensus exists about the tools and theories to use (“systems theory”), and (ii) the AI face, which is rather poorly standardized, not because of a lack of interest or research efforts, but because of the inherent complexity of “intelligent behaviour.” The terminology and systems-thinking of both backgrounds are significantly different, hence the WEBook will accomodate sections on the same material but written from various perspectives. This is not a “bug”, but a “feature”: having the different views in the context of the same WEBook can only lead to a better mutual understanding and respect.

 

Research in engineering robotics follows the bottom-up approach: existing and working systems are extended and made more versatile. Research in artificial intelligence robotics is top-down: assuming that a set of low-level primitives is available, how could one apply them in order to increase the “intelligence” of a system. The border between both approaches shifts continuously, as more and more “intelligence” is cast into algorithmic, system-theoretic form. For example, the response of a robot to sensor input was considered “intelligent behaviour” in the late seventies and even early eighties. Hence, it belonged to A.I. Later it was shown that many sensor-based tasks such as surface following or visual tracking could be formulated as control problems with algorithmic solutions. From then on, they did not belong to A.I. any more.

 

 

 

Robotics Technology

 

Most industrial robots have at least the following five parts:

 

Sensors, Effectors, Actuators, Controllers, and common effectors known as Arms.

 

Many other robots also have Artificial Intelligence and effectors that help it achieve Mobility.

 

This section discusses the basic technologies of a robot. Click one of the links above or use the navigation bar menu on the far right.

 

Robotics Technology – Sensors

 

Most robots of today are nearly deaf and blind.  Sensors can provide some limited feedback to the robot so it can do its job.  Compared to the senses and abilities of even the simplest living things, robots have a very long way to go.

 

The sensor sends information, in the form of electronic signals back to the cfontroller.  Sensors also give the robot controller information about its surroundings and lets it know the exact position of the arm, or the state of the world around it.

Sight, sound, touch, taste, and smell are the kinds of information we get from our world.  Robots can be designed and programmed to get specific information that is beyond what our 5 senses can tell us. For instance, a robot sensor might “see” in the dark, detect tiny amounts of invisible radiation or measure movement that is too small or fast for the human eye to see.

 

Here are some things sensors are used for:

 

Physical Property

 Technology

 

Contact Bump, Switch

Distance Ultrasound, Radar, Infra Red

Light Level Photo Cells, Cameras

Sound Level microphones

Strain Strain Gauges

Rotation Encoders

Magnetism Compasses

Smell Chemical

Temperature Thermal, Infra Red

Inclination Inclinometers, Gyroscope

Pressure Pressure Gauges

Altitude Altimeters

 

    Sensors can be made simple and complex, depending on how much information needs to be stored.  A switch is a simple on/off sensor used for turning the robot on and off.  A human retina is a complex sensor that uses more than a hundred million photosensitive elements (rods and cones).  Sensors provide information to the robots brain, which can be treated in various ways.  For example, we can simply react to the sensor output: if the switch is open, if the switch is closed, go. 

 

Levels of Processing

 

    To figure out if the switch is open or closed, you will need to measure the voltage going through the circuit, that’s electronics.  Now lets say that you have a microphone and you want to recognize a voice and separate it from noise; that’s signal processing.  Now you have a camera, and you want to take the pre-processed image and now you need to figure out what those objects are, perhaps by comparing them to a large library of drawings; that’s computation.  Sensory data processing is a very complex thing to try and do but the robot needs this in order to have a “brain”.  The brain has to have analog or digital processing capabilities, wires to connect everything, support electronics to go with the computer, and batteries to provide power for the whole thing, in order to process the sensory data.  Perception requires the robot to have sensors (power and electronics), computation (more power and electronics, and connectors (to connect it all). 

 

Switch Sensors

 

 Switches are the simplest sensors of all.  They work without processing, at the electronics (circuit) level.  Their general underlying principle is that of an open vs. closed circuit.  If a switch is open, no current can flow; if it is closed, current can flow and be detected.  This simple principle can (and is) used in a wide variety of ways.

 

Switch sensors can be used in a variety of ways:

 

contact sensors: detect when the sensor has contacted another object (e.g., triggers when a robot hits a wall or grabs an object; these can even be whiskers)

 

limit sensors: detect when a mechanism has moved to the end of its range

 

shaft encoder sensors: detects how many times a shaft turns by having a switch click (open/close) every time the shaft turns (e.g., triggers for each turn, allowing for counting rotations)

 

   There are many common switches: button switches, mouse switches, key board keys, phone keys, and others.  Depending on how a switch is wired, it can be normally open or normally closed.  This would of course depend on your robot’s electronics, mechanics, and its task.  The simplest yet extremely useful sensor for a robot is a “bump switch” that tells it when it’s bumped into something, so it can back up and turn away. Even for such a simple idea, there are many different ways of implementation.

 

Light Sensors

 

Switches measure physical contact and light sensors measure the amount of light impacting a photocell, which is basically a resistive sensor.  The resistance of a photocell is low when it is brightly illuminated, i.e., when it is very light; it is high when it is dark.  In that sense, a light sensor is really a “dark” sensor.  In setting up a photocell sensor, you will end up using the equations we learned above, because you will need to deal with the relationship of the photocell resistance photo, and the resistance and voltage in your electronics sensor circuit.  Of course since you will be building the electronics and writing the program to measure and use the output of the light sensor, you can always manipulate it to make it simpler and more intuitive.  What surrounds a light sensor affects its properties.  The sensor can be  shielded and positioned in various ways.  Multiple sensors can be arranged in useful configurations and isolate them from each other with shields.

 

Just like switches, light sensors can be used in many different ways:

 

Light sensors can measure:

 

light intensity (how light/dark it is)

 

differential intensity (difference between photocells)

 

break-beam (change/drop in intensity)

 

Light sensors can be shielded and focused in different ways

 

Their position and directionality on a robot can make a great deal of difference and impact

 

Polarized light

 

“Normal” light emanating from a source is non-polarized, which means it travels at all orientations with respect to the horizon.  However, if there is a polarizing filter in front of a light source, only the light waves of a given orientation of the filter will pass through.  This is useful because now we can manipulate this remaining light with other filters; if we put it through another filter with the same characteristic plane, almost all of it will get through.  But, if we use a perpendicular filter (one with a 90-degree relative characteristic angle), we will block all of the light.  Polarized light can be used to make specialized sensors out of simple photocells; if you put a filter in front of a light source and the same or a different filter in front of a photocell, you can cleverly manipulate what and how much light you detect. 

 

Resistive Position Sensors

 

    We said earlier that a photocell is a resistive device.  We can also sense resistance in response to other physical properties, such as bending.  The resistance of the device increases with the amount it is bent.  These bend sensors were originally developed for video game control (for example, Nintendo Powerglove), and are generally quite useful.  Notice that repeated bending will wear out the sensor.  Not surprisingly, a bend sensor is much less robust than light sensors, although they use the same underlying resistive principle.

 

Potentiometers

 

    These devices are very common for manual tuning; you have probably seen them in some controls (such as volume and tone on stereos).  Typically called pots, they allow the user to manually adjust the resistance.  The general idea is that the device consists of a movable tap along two fixed ends.  As the tap is moved, the resistance changes.  As you can imagine, the resistance between the two ends is fixed, but the resistance between the movable part and either end varies as the part is moved.  In robotics, pots are commonly used to sense and tune position for sliding and rotating mechanisms.

 

Biological Analogs

 

All of the sensors we described exist in biological systems

 

Touch/contact sensors with much more precision and complexity in all species

 

Bend/resistance receptors in muscles 

 

Reflective Optosensors

 

    We mentioned that if we use a light bulb in combination with a photocell, we can make a break-beam sensor. This idea is the underlying principle in reflective optosensors: the sensor consists of an emitter and a detector. Depending of the arrangement of those two relative to each other, we can get two types of sensors:

 

reflectance sensors (the emitter and the detector are next to each other, separated by a barrier; objects are detected when the light is reflected off them and back into the detector)

 

break-beam sensors (the emitter and the detector face each other; objects are detected if they interrupt the beam of light between the emitter and the detector)

 

    The emitter is usually made out of a light-emitting diode (an LED), and the detector is usually a photodiode/phototransistor.

 

    Note that these are not the same technology as resistive photocells. Resistive photocells are nice and simple, but their resistive properties make them slow; photodiodes and photo-transistors are much faster and therefore the preferred type of technology.

 

What can you do with this simple idea of light reflectivity? Quite a lot of useful things:

 

object presence detection

 

object distance detection

 

surface feature detection (finding/following markers/tape)

 

wall/boundary tracking

 

rotational shaft encoding (using encoder wheels with ridges or black & white color)

 

bar code decoding

 

    Note, however, that light reflectivity depends on the color (and other properties) of a surface. A light surface will reflect light better than a dark one, and a black surface may not reflect it at all, thus appearing invisible to a light sensor. Therefore, it may be harder (less reliable) to detect darker objects this way than lighter ones. In the case of object distance, lighter objects that are farther away will seem closer than darker objects that are not as far away. This gives you an idea of how the physical world is partially-observable. Even though we have useful sensors, we do not have complete and completely accurate information.

 

    Another source of noise in light sensors is ambient light. The best thing to do is subtract the ambient light level out of the sensor reading, in order to detect the actual change in the reflected light, not the ambient light. How is that done? By taking two (or more, for higher accuracy) readings of the detector, one with the emitter on, and one with it off, and subtracting the two values from each other. The result is the ambient light level, which can then be subtracted from future readings. This process is called sensor calibration. Of course, remember that ambient light levels can change, so the sensors may need to be calibrated repeatedly.

 

Break-beam Sensors

 

    We already talked about the idea of break-beam sensors. In general, any pair of compatible emitter-detector devices can be used to produce such a sensors:

 

an incandescent flashlight bulb and a photocell

 

red LEDs and visible-light-sensitive photo-transistors

 

or infra-red IR emitters and detectors

 

Shaft Encoding

 

Shaft encoders measure the angular rotation of an axle providing position and/or velocity info. For example, a speedometer measures how fast the wheels of a vehicle are turning, while an odometer measures the number of rotations of the wheels.

 

In order to detect a complete or partial rotation, we have to somehow mark the turning element. This is usually done by attaching a round disk to the shaft, and cutting notches into it. A light emitter and detector are placed on each side of the disk, so that as the notch passes between them, the light passes, and is detected; where there is no notch in the disk, no light passes.

 

If there is only one notch in the disk, then a rotation is detected as it happens. This is not a very good idea, since it allows only a low level of resolution for measuring speed: the smallest unit that can be measured is a full rotation. Besides, some rotations might be missed due to noise.

 

Usually, many notches are cut into the disk, and the light hits impacting the detector are counted. (You can see that it is important to have a fast sensor here, if the shaft turns very quickly.)

 

An alternative to cutting notches in the disk is to paint the disk with black (absorbing, non-reflecting) and white (highly reflecting) wedges, and measure the reflectance. In this case, the emitter and the detector are on the same side of the disk.

 

In either case, the output of the sensor is going to be a wave function of the light intensity. This can then be processes to produce the speed, by counting the peaks of the waves.

 

Note that shaft encoding measures both position and rotational velocity, by subtracting the difference in the position readings after each time interval. Velocity, on the other hand, tells us how fast a robot is moving, or if it is moving at all. There are multiple ways to use this measure:

 

measure the speed of a driven (active) wheel

 

use a passive wheel that is dragged by the robot (measure forward progress)

 

We can combine the position and velocity information to do more sophisticated things:

 

move in a straight line

 

rotate by an exact amount

 

Note, however, that doing such things is quite difficult, because wheels tend to slip (effector noise and error) and slide and there is usually some slop and backlash in the gearing mechanism. Shaft encoders can provide feedback to correct the errors, but having some error is unavoidable.

 

Quadrature Shaft Encoding

 

So far, we’ve talked about detecting position and velocity, but did not talk about direction of rotation. Suppose the wheel suddenly changes the direction of rotation; it would be useful for the robot to detect that.

 

An example of a common system that needs to measure position, velocity, and direction is a computer mouse. Without a measure of direction, a mouse is pretty useless. How is direction of rotation measured?

 

Quadrature shaft encoding is an elaboration of the basic break-beam idea; instead of using only one sensor, two are needed. The encoders are aligned so that their two data streams coming from the detector and one quarter cycle (90-degrees) out of phase, thus the name “quadrature”. By comparing the output of the two encoders at each time step with the output of the previous time step, we can tell if there is a direction change. When the two are sampled at each time step, only one of them will change its state (i.e., go from on to off) at a time, because they are out of phase. Which one does it determines which direction the shaft is rotating. Whenever a shaft is moving in one direction, a counter is incremented, and when it turns in the opposite direction, the counter is decremented, thus keeping track of the overall position.

 

Other uses of quadrature shaft encoding are in robot arms with complex joints (such as rotary/ball joints; think of your knee or shoulder), Cartesian robots (and large printers) where an arm/rack moves back and forth along an axis/gear.

 

Modulation and Demodulation of Light

 

We mentioned that ambient light is a problem because it interferes with the emitted light from a light sensor. One way to get around this problem is to emit modulated light, i.e., to rapidly turn the emitter on and off. Such a signal is much easier and more reliably detected by a demodulator, which is tuned to the particular frequency of the modulated light. Not surprisingly, a detector needs to sense several on-flashes in a row in order to detect a signal, i.e., to detect its frequency. This is a small point, but it is important in writing demodulator code.

 

The idea of modulated IR light is commonly used; for example in household remote controls.

 

Modulated light sensors are generally more reliable than basic light sensors. They can be used for the same purposes: detecting the presence of an object measuring the distance to a nearby object (clever electronics required, see your course notes)

 

Infra Red (IR) Sensors

 

Infra red sensors are a type of light sensors, which function in the infra red part of the frequency spectrum.  IR sensors consist are active sensors: they consist of an emitter and a receiver.  IR sensors are used in the same ways that visible light sensors are that we have discussed so far: as break-beams and as reflectance sensors.  IR is preferable to visible light in robotics (and other) applications because it suffers a bit less from ambient interference, because it can be easily modulated, and simply because it is not visible.

 

IR Communication

 

Modulated infra red can be used as a serial line for transmitting messages. This is is fact how IR modems work. Two basic methods exist:

 

bit frames (sampled in the middle of each bit; assumes all bits take the same amount of time to transmit)

 

bit intervals (more common in commercial use; sampled at the falling edge, duration of interval between sampling determines whether it’s a 0 or 1)

 

Ultrasonic Distance Sensing

 

As we mentioned before, ultrasound sensing is based on the time-of-flight principle. The emitter produces a sonar “chirp” of sound, which travels away from the source, and, if it encounters barriers, reflects from them and returns to the receiver (microphone). The amount of time it takes for the sound beam to come back is tracked (by starting a timer when the “chirp” is produced, and stopping it when the reflected sound returns), and is used to compute the distance the sound traveled. This is possible (and quite easy) because we know how fast sound travels; this is a constant, which varies slightly based on ambient temperature.

 

At room temperature, sound travels at 1.12 feet per millisecond. Another way to put it that sound travels at 0.89 milliseconds per foot. This is a useful constant to remember.

 

The process of finding one’s location based on sonar is called echolocation. The inspiration for ultrasound sensing comes from nature; bats use ultrasound instead of vision (this makes sense; they live in very dark caves where vision would be largely useless). Bat sonars are extremely sophisticated compared to artificial sonars; they involve numerous different frequencies, used for finding even the tiniest fast-flying prey, and for avoiding hundreds of other bats, and communicating for finding mates.

                                                         

Specular Reflection

 

A major disadvantage of ultrasound sensing is its susceptibility to specular reflection (specular reflection means reflection from the outer surface of the object). While the sonar sensing principle is based on the sound wave reflecting from surfaces and returning to the receiver, it is important to remember that the sound wave will not necessarily bounce off the surface and “come right back.” In fact, the direction of reflection depends on the incident angle of the sound beam and the surface. The smaller the angle, the higher the probability that the sound will merely “graze” the surface and bounce off, thus not returning to the emitter, in turn generating a false long/far-away reading. This is often called specular reflection, because smooth surfaces, with specular properties, tend to aggravate this reflection problem. Coarse surfaces produce more irregular reflections, some of which are more likely to return to the emitter. (For example, in our robotics lab on campus, we use sonar sensors, and we have lined one part of the test area with cardboard, because it has much better sonar reflectance properties than the very smooth wall behind it.)

 

In summary, long sonar readings can be very inaccurate, as they may result from false rather than accurate reflections. This must be taken into account when programming robots, or a robot may produce very undesirable and unsafe behavior. For example, a robot approaching a wall at a steep angle may not see the wall at all, and collide with it!

 

Nonetheless, sonar sensors have been successfully used for very sophisticated robotics applications, including terrain and indoor mapping, and remain a very popular sensor choice in mobile robotics.

 

The first commercial ultrasonic sensor was produced by Polaroid, and used to automatically measure the distance to the nearest object (presumably which is being photographed). These simple Polaroid sensors still remain the most popular off-the-shelf sonars (they come with a processor board that deals with the analog electronics). Their standard properties include:

 

32-foot range

 

30-degree beam width

 

sensitivity to specular reflection

 

shortest distance return

 

Polaroid sensors can be combined into phased arrays to create more sophisticated and more accurate sensors.

 

One can find ultrasound used in a variety of other applications; the best known one is ranging in submarines. The sonars there have much more focused and have longer-range beams. Simpler and more mundane applications involve automated “tape-measures”, height measures, burglar alarms, etc.

 

Machine Vision

 

So far, we have talked about relatively simple sensors. They were simple in terms of processing of the information they returned. Now we turn to machine vision, i.e., to cameras as sensors.

 

Cameras, of course, model biological eyes. Needless to say, all biological eyes are more complex than any camera we know today, but, as you will see, the cameras and machine vision systems that process their perceptual information, are not simple at all! In fact, machine vision is such a challenging topic that it has historically been a separate branch of Artificial Intelligence.

 

The general principle of a camera is that of light, scattered from objects in the environment (those are called the scene), goes through an opening (”iris”, in the simplest case a pin hole, in the more sophisticated case a lens), and impinging on what is called the image plane. In biological systems, the image plane is the retina, which is attached to numerous rods and cones (photosensitive elements) which, in turn, are attached to nerves which perform so-called “early vision”, and then pass information on throughout the brain to do “higher-level” vision processing. As we mentioned before, a very large percentage of the human (and other animal) brain is dedicated to visual processing, so this is a highly complex endeavor.

 

In cameras, instead of having photosensitive rhodopsin and rods and cones, we use silver halides on photographic film, or silicon circuits in charge-coupled devices (CCD) cameras. In all cases, some information about the incoming light (e.g., intensity, color) is detected by these photosensitive elements on the image plane.

 

In machine vision, the computer must make sense out of the information it gets on the image plane. If the camera is very simple, and uses a tiny pin hole, then some computation is required to compute the projection of the objects from the environment onto the image plane (note, they will be inverted). If a lens is involved (as in vertebrate eyes and real cameras), then more light can get in, but at the price of being focused; only objects a particular range of distances from the lens will be in focus. This range of distances is called the camera’s depth of field.

 

The image plane is usually subdivided into equal parts, called pixels, typically arranged in a rectangular grid. In a typical camera there are 512 by 512 pixels on the image plane (for comparison, there are 120 x 10^6 rods and 6 x 10^6 cones in the eye, arranged hexagonally). Let’s call the projection on the image plane the image.

 

The brightness of each pixel in the image is proportional to the amount of light directed toward the camera by the surface patch of the object that projects to that pixel. (This of course depends on the reflectance properties of the surface patch, the position and distribution of the light sources in the environment, and the amount of light reflected from other objects in the scene onto the surface patch.) As it turns out, brightness of a patch depends on two kinds of reflections, one being specular (off the surface, as we saw before), and the other being diffuse (light that penetrates into the object, is absorbed, and then re-emitted). To correctly model light reflection, as well as reconstruct the scene, all these properties are necessary.

 

Let us suppose that we are dealing with a black and white camera with a 512 x 512 pixel image plane. Now we have an image, which is a collection of those pixels, each of which is an intensity between white and black. To find an object in that image (if there is one, we of course don’t know a priori), the typical first step (”early vision”) is to do edge detection, i.e., find all the edges. How do we recognize them? We define edges as curves in the image plane across which there is significant change in the brightness.

 

A simple approach would be to look for sharp brightness changes by differentiating the image and look for areas where the magnitude of the derivative is large. This almost works, but unfortunately it produces all sorts of spurious peaks, i.e., noise. Also, we cannot inherently distinguish changes in intensities due to shadows from those due to physical objects. But let’s forget that for now and think about noise. How do we deal with noise?

 

We do smoothing, i.e., we apply a mathematical procedure called convolution, which finds and eliminates the isolated peaks. Convolution, in effect, applies a filter to the image. In fact, in order to find arbitrary edges in the image, we need to convolve the image with many filters with different orientations. Fortunately, the relatively complicated mathematics involved in edge detection has been well studied, and by now there are standard and preferred approaches to edge detection.

 

Once we have edges, the next thing to do is try to find objects among all those edges. Segmentation is the process of dividing up or organizing the image into parts that correspond to continuous objects. But how do we know which lines correspond to which objects, and what makes an object? There are several cues we can use to detect objects:

 

We can have stored models of line-drawings of objects (from many possible angles, and at many different possible scales!), and then compare those with all possible combinations of edges in the image. Notice that this is a very computationally intensive and expensive process. This general approach, which has been studied extensively, is called model-based vision.

 

We can take advantage of motion. If we look at an image at two consecutive time-steps, and we move the camera in between, each continuous solid objects (which obeys physical laws) will move as one, i.e., its brightness properties will be conserved. This hives us a hint for finding objects, by subtracting two images from each other. But notice that this also depends on knowing well how we moved the camera relative to the scene (direction, distance), and that nothing was moving in the scene at the time. This general approach, which has also been studied extensively, is called motion vision.

 

We can use stereo (i.e., binocular stereopsis, two eyes/cameras/points of view). Just like with motion vision above, but without having to actually move, we get two images, which we can subtract from each other, if we know what the disparity between them should be, i.e., if we know how the two cameras are organized/positioned relative to each other.

 

We can use texture. Patches that have uniform texture are consistent, and have almost identical brightness, so we can assume they come from the same object. By extracting those we can get a hint about what parts may belong to the same object in the scene.

 

We can also use shading and contours in a similar fashion. And there are many other methods, involving object shape and projective invariants, etc.

 

Note that all of the above strategies are employed in biological vision. It’s hard to recognize unexpected objects or totally novel ones (because we don’t have the models at all, or not at the ready). Movement helps catch our attention. Stereo, i.e., two eyes, is critical, and all carnivores use it (they have two eyes pointing in the same direction, unlike herbivores). The brain does an excellent job of quickly extracting the information we need for the scene.

 

Machine vision has the same task of doing real-time vision. But this is, as we have seen, a very difficult task. Often, an alternative to trying to do all of the steps above in order to do object recognition, it is possible to simplify the vision problem in various ways:

 

Use color; look for specifically and uniquely colored objects, and recognize them that way (such as stop signs, for example)

 

Use a small image plane; instead of a full 512 x 512 pixel array, we can reduce our view to much less, for example just a line (that’s called a linear CCD). Of course there is much less information in the image, but if we are clever, and know what to expect, we can process what we see quickly and usefully.

 

Use other, simpler and faster, sensors, and combine those with vision. For example, IR cameras isolate people by body-temperature. Grippers allow us to touch and move objects, after which we can be sure they exist.

 

Use information about the environment; if you know you will be driving on the road which has white lines, look specifically for those lines at the right places in the image. This is how first and still fastest road and highway robotic driving is done.

 

Those and many other clever techniques have to be employed when we consider how important it is to “see” in real-time. Consider highway driving as an important and growing application of robotics and AI. Everything is moving so quickly, that the system must perceive and act in time to react protectively and safely, as well as intelligently.

 

Now that you know how complex vision is, you can see why it was not used on the first robots, and it is still not used for all applications, and definitely not on simple robots. A robot can be extremely useful without vision, but some tasks demand it. As always, it is critical to think about the proper match between the robot’s sensors and the task.

 

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