Control Components

Control Components1Introduction to Control SystemsEvery activity in our day to day life is influenced by some sort of control system. Control systems are now integral parts of modern industrialization, industrial processes and home appliances. Hence the control systems and its components plays very important role in our daily life. In this lecture we will study the various control components. 2Introduction to Control SystemsA control system consisting of interconnected components is designed to achieve a desired purpose. To understand the purpose of a control system, it is useful to examine examples of control systems through the course of history. These early systems incorporated many of the same ideas of feedback that are in use today.

Modern control engineering practice includes the use of control design strategies for improving manufacturing processes, the efficiency of energy use, advanced automobile control, including rapid transit, among others.

We also discuss the notion of a design gap. The gap exists between the complex physical system under investigation and the model used in the control system synthesis.

The iterative nature of design allows us to handle the design gap effectively while accomplishing necessary tradeoffs in complexity, performance, and cost in order to meet the design specifications. 3Control ComponentsThere are various types of control componentsError DetectorsPotentiometersSynchrosAC & DC TechogeneratorsAC & DC ServomotorsStepper MotorsMagnetic Amplifiers4Error DetectorsAll feedback control systems operate from the error signal which is generated by a comparison of the reference and the output. Error detectors perform the crucial task of comparing the reference and output signals. In a purely electrical system where the reference and output are voltages, the error detector is a simple comparator.5Error Detectors

6Error Detectors

7Error Detectors

8Error Detectors

9PotentiometerApotentiometer, informally apot, is a three-terminalresistorwith a sliding contact that forms an adjustablevoltage divider.If only two terminals are used, one end and the wiper, it acts as avariable resistororrheostat.Apotentiometer measuring instrumentis essentially a voltage divider used for measuringelectric potential(voltage); the component is an implementation of the same principle, hence its name.

10PotentiometerPotentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as positiontransducers, for example, in ajoystick. Potentiometers are rarely used to directly control significant power (more than awatt), since the power dissipated in the potentiometer would be comparable to the power in the controlled load11Potentiometer Diagram

12Potentiometer Symbol13

Potentiometer ConstructionPotentiometers comprise a resistive element, a sliding contact (wiper) that moves along the element, making good electrical contact with one part of it, electrical terminals at each end of the element, a mechanism that moves the wiper from one end to the other, and a housing containing the element and wiper.14Potentiometer Construction

15Potentiometer ConstructionAnother type is the linear slider potentiometer, which has a wiper which slides along a linear element instead of rotating. Contamination can potentially enter anywhere along the slot the slider moves in, making effective sealing more difficult and compromising long-term reliability. An advantage of the slider potentiometer is that the slider position gives a visual indication of its setting. While the setting of a rotary potentiometer can be seen by the position of a marking on the knob, an array of sliders can give a visual impression of the effect of a multi-channel equalizer 16Potentiometer Construction

17Theory of operation

18Theory of operationThe potentiometer can be used as avoltage dividerto obtain a manually adjustable output voltage at the slider (wiper) from a fixed input voltage applied across the two ends of the potentiometer. This is their most common use.The voltage across can be calculated by:

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Potentiometer as an Error Detector20Potentiometer as an Error DetectorDC Motor control systems potentiometers can be used as position feedback as shown in the previous slide. The reference position of shaft is compared by a pair of two pots and reference input is fed to DC Amplifier, which is further amplifying the armature current of the DC Motor.21Potentiometer as an Error Detector

22SynchrosAsynchrois a type of rotary electricaltransformerthat is used for measuring the angle of a rotating machine such as anantennaplatform. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to therotor, is excited by analternating current, which byelectromagnetic induction, causes currents to flow in three star-connected secondary windings fixed at 120 degrees to each other on thestator. The relative magnitudes of secondary currents are measured and used to determine the angle of the rotor relative to the stator, or the currents can be used to directly drive a receiver synchro that will rotate in unison with the synchro transmitter23Pnjab Edusat SocietySynchros

24Synchro OperationOn a practical level, Synchros resemble motors, in that there is a rotor, stator, and a shaft. Ordinarily,slip ringsandbrushes connect the rotor to external power. A synchro transmitter's shaft is rotated by the mechanism that sends information, while the synchro receiver's shaft rotates a dial, or operates a light mechanical load. Single and three-phase units are common in use, and will follow the other's rotation when connected properly. One transmitter can turn several receivers; if torque is a factor, the transmitter must be physically larger to source the additional current. 25Synchro as Error Detector

26Uses of SynchrosSynchro systems were first used in the control system of thePanama Canalin the early 1900s to transmit lock gate and valve stem positions and water levels to the control desksFire-control systemdesigns developed duringWorld War IIused synchros extensively, to transmit angular information from guns and sights to an analog fire control computer, and to transmit the desired gun position back to the gun location. 27Uses of SynchrosSmaller synchros are still used to remotely drive indicator gauges and as rotary position sensors for aircraft control surfaces, where the reliability of these rugged devices is needed. Digital devices such as therotary encoderhave replaced synchros in most other applications.Selsyn motors were widely used inmotion pictureequipment to synchronizemovie camerasandsound recordingequipment.Large synchros were used on naval warships, such as destroyers, to operate the steering gear from the wheel on the bridge.28Tachogenerators (Tachometers)Tachometer is an electromechanical unit which generates an electrical output proportional to the speed of the shaft. In automatic control system tachometer performs two main functions:Stabilization of systemComputation of closed loops in a control system29TachogeneratorsTachometers are mainly of two types:DC TachometerAC Tachometer30DC TachometerThis is a small dc generator. It contains a permanent magnet and an iron core rotor. No external supply voltage is required. The winding on rotor are connected to commutator segments and the output voltage is taken across pair of brushes that ride on the commutator segments. DC tachometers provide visual speed readout of a rotating shaft. Such tachometers are directly coupled to a voltmeter which is calibrated in r.p.m.31DC Tachometer

32DC Tachometer

33DC Tachometer as Error Detector

34AC TachometerThe AC tachometer is a device, which is similar to a two phase induction motor, in which two stator windings are placed in quadrature with each other and rotor is short circuited. In AC Tachometer, a sinusoidal voltage of rated value is applied to the primary winding, which is known as reference winding, the secondary winding is placed 90 degrees apart from primary winding. The magnitude of sinusoidal output voltage is directly proportional to the speed of rotor.35AC Tachometer36

AC Tachometer

37AC Tachometer

38Servo SystemA closed-loop motion system using a current amplifier, servo controller, servomotor and a position feedback device such as an encoder to precisely control speed and position of a load.The MOTOR can be electric, hydraulic, pneumatic or even internal combustion.The FEEDBACK sensor can be mechanical or electronic, analog or digital and it can be rotary or linear39A Basic Servo SystemA servo controller with external inputMotor connected to a feedback deviceIs a closed loop systemHow might this system work?

The Feedback device canbe Relative or Absolute.4040ServomotorsThe servo system is the one, in which the output is some mechanical variable such as position, velocity or acceleration. The motors used in the servo systems are called servomotors. These motors are usually coupled to the output shaft for power matching. There are two types of servo motorsAC ServomotorsDC Servomotors41DC Servo MotorsDC servo motors are controlled by DC command signals applied directly to coilsThe magnetic fields that are formed interact with permanent magnets and cause the rotating member to turnOne type of PM uses a wound armature and brushes like a conventional DC motor, but uses magnets as pole piecesAnother type uses wound field coils and a permanent magnet rotor42Wound Armature PM MotorArmature contains wound coilsCurrent is supplied by brushesPole pieces are made of permanent magnetsTypically 2 or 4-pole structureSimilar characteristics to a DC shunt motor

43Moving Coil MotorThe stator field is provided with 8 pairs of permanent magnetsArranged to provide alternating magnetic fieldsThe armature is made of thin disc of fiberglass laminated with copper conductors

44Moving Coil Operation

45DC Servo Motors

46DC Servo Motors

47DC Servo Motors

48Torque-Speed Curve of a DC Servomotor

49AC Servo MotorsControlled by AC command signals applied to the coilsAC Brushless Servo MotorOperates on the same principle as single-phase induction motor50Two windingsMain windingAuxiliary windingElectronic drive provides the necessary phase shift for motor operation

AC Brushless Servo Motor

51Stepper MotorsConventional servo motors are classified as continuous rotation motorsStepper motors rotate through a specific number of degrees, or steps, then stopEach incoming pulse results in the shaft turning a specific angular distanceStepper motors can control velocity, distance, and direction of mechanical load52Permanent Magnet Stepper MotorPM stepper motors have rotor teeth made of permanent magnetsReaction of the rotor teeth to stator fields provides torque for the motorSignals are applied to the stator to determine direction and step rate of the rotor

5354Variable Reluctance Stepper MotorThe variable reluctance stepper motor uses electromagnetic stator polesThe soft iron core is un-magnetizedThe rotor is toothed, alternating N-SThe more teeth, the greater the resolution

Stepper Motor TerminologyStepping Rate - maximum number of steps the motor can make in one secondStep Angle - number of degrees per arc the motor moves per stepStep angle is determined by the number of rotor teeth and stator poles used5556Stepper Motor SpeedStepper motor speed depends upon the step angle and stepping rate

n = Speed in RPMY = Step angles in degreesS = Steps per second6 = Formula constant57MicrosteppingStepper motors tend to jerk at low speedsStepper motors have limited resolutionMicrostepping overcomes these problemsUses simulated sine waves that increment or decrement in small steps called microsteps 58Microstepper Operation

RotorStatorCoils

Outside CasingStatorRotorInternal components of a Stepper Motor5959

StatorsRotorCross Section of a Stepper Motor6060

Winding number 1

Winding number 2One step6 pole rotor6161

RotorStator coilsCNC Stepping Motor6262

Advantages:-Low cost for control achievedRuggednessSimplicity of constructionCan operate in an open loop control systemLow maintenanceLess likely to stall or slipWill work in any environmentDisadvantages:-Require a dedicated control circuitUse more current than D.C. motorsHigh torque output achieved at low speeds Advantages / Disadvantages6363Magnetic AmplifiersThemagnetic amplifier(known as a "mag amp") is anelectromagneticdevice for amplifying electrical signals. The magnetic amplifier was invented early in the 20th century, and was used as an alternative tovacuum tubeamplifiers where robustness and high current capacity were required.World War II Germanyperfected this type of amplifier, and it was used in theV-2 rocketThe magnetic amplifier has now been largely superseded by thetransistor-based amplifier, except in a few safety critical, high reliability or extremely demanding applications. Combinations of transistor and mag-amp techniques are still used.64Strength of Magnetic AmplifiersThe magnetic amplifier is a static device with no moving parts. It has no wear-out mechanism and has a good tolerance to mechanical shock and vibration. It requires no warm-up time.Multiple isolated signals may be summed by additional control windings on the magnetic cores. The windings of a magnetic amplifier have a higher tolerance to momentary overloads than comparable solid-state devices. The magnetic amplifier is also used as a transducer in applications such as current measurement and theflux gate compass.

65Limitations of Magnetic AmplifiersThe gain available from a single stage is limited and low compared to electronic amplifiers. Frequency response of a high gain amplifier is limited to about one-tenth the excitation frequency, although this is often mitigated by exciting magnetic amplifiers with currents at higher thanutility frequency Solid-state amplifiers can be more compact and efficient than magnetic amplifiers. The bias and feedback windings are not unilateral, and may couple energy back from the controlled circuit into the control circuit. This complicates the design of multistage amplifiers when compared with electronic devices.66Principle of operationVisually a mag amp device may resemble atransformerbut the operating principle is quite different from a transformer - essentially the mag amp is a saturable reactor. It makes use ofmagnetic saturationof the core, a non-linear property of a certain class of transformer cores. For controlled saturation characteristics the magnetic amplifier employs core materials that have been designed to have a specificB-H curveshape that is highly rectangular, in contrast to the slowly-tapering B-H curve of softly saturating core materials that are often used in normal transformers67Principle of operationThe typical magnetic amplifier consists of two physically separate but similar transformermagnetic cores, each of which has two windings - a control winding and an AC winding. A small DC current from a low impedance source is fed into the series-connected control windings. The AC windings may be connected either in series or in parallel, the configurations resulting in different types of mag amps. The amount of control current fed into the control winding sets the point in the AC winding waveform at which either core will saturate. In saturation, the AC winding on the saturated core will go from a high impedance state ("off") into a very low impedance state ("on") - that is, the control current controls at which voltage the mag amp switches "on".68Principle of operation

69Magnetic Amplifier

70Magnetic Amplifier

71Applications of Magnetic AmplifierMagnetic amplifiers were important as modulation and control amplifiers in the early development of voice transmission by radio.The ability to control large currents with small control power made magnetic amplifiers useful for control of lighting circuits, forstage lightingand for advertising signs.Magnetic amplifiers were used extensively as the switching element in early switched-mode (SMPS) power supplies.Magnetic amplifiers are still used in some arc welders

72Applications of Magnetic AmplifierMagnetic amplifiers can be used for measuring high DC-voltages without direct connection to the high voltage and are therefore still used in theHVDC-techniqueMagnetic amplifiers were used by locomotives to detect wheel slipMagnetic amplifiers are also still used in instrumentation for measuring currentSuch instrumentation mag. amps. are commonly found on space craft where a clean electromagnetic environment is highly desirable.

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