Control Components Presented By: Er. H.S. Dhaliwal Assistant Professor, Deptt. Of Electrical Engg...

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Control Components

Presented By:Er. H.S. DhaliwalAssistant Professor,Deptt. Of Electrical EnggBHSBIET, Lehragaga.


Introduction to Control Systems

• Every 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.


Introduction to Control Systems• A 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.


Control Components

• There are various types of control components1. Error Detectors2. Potentiometers3. Synchros4. AC & DC Techogenerators5. AC & DC Servomotors6. Stepper Motors7. Magnetic Amplifiers


Error Detectors

• All 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.


Error Detectors


Potentiometer

• A potentiometer, informally a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.

• A potentiometer measuring instrument is essentially a voltage divider used for measuring electric potential (voltage); the component is an implementation of the same principle, hence its name.


Potentiometer

• Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick. Potentiometers are rarely used to directly control significant power (more than a watt), since the power dissipated in the potentiometer would be comparable to the power in the controlled load


Potentiometer Diagram

http://en.wikipedia.org/wiki/File:Potentiometer.jpg


Potentiometer Symbol


Potentiometer Construction

• Potentiometers 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.



• Another 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



http://en.wikipedia.org/wiki/File:Faders.jpg


Theory of operation

http://en.wikipedia.org/wiki/File:Potentiometer_with_load.svg


Theory of operation

• The potentiometer can be used as a voltage divider to 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:

http://en.wikipedia.org/wiki/File:Potentiometer_with_load.svg


Potentiometer as an Error Detector



• DC 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.


Synchros• A synchro is a type of rotary electrical transformer that is

used for measuring the angle of a rotating machine such as an antenna platform. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to the rotor, is excited by an alternating current, which by electromagnetic induction, causes currents to flow in three star-connected secondary windings fixed at 120 degrees to each other on the stator. 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 transmitter


Synchros


Synchro Operation

• On a practical level, Synchros resemble motors, in that there is a rotor, stator, and a shaft. Ordinarily, slip rings and brushes 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.


Synchro as Error Detector


Uses of Synchros

• Synchro systems were first used in the control system of the Panama Canal in the early 1900s to transmit lock gate and valve stem positions and water levels to the control desks

• Fire-control system designs developed during World War II used 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.


Uses of Synchros

• Smaller 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 the rotary encoder have replaced synchros in most other applications.

• Selsyn motors were widely used in motion picture equipment to synchronize movie cameras and sound recording equipment.

• Large synchros were used on naval warships, such as destroyers, to operate the steering gear from the wheel on the bridge.


Tachogenerators (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 system• Computation of closed loops in a control

system


Tachogenerators

• Tachometers are mainly of two types:– DC Tachometer– AC Tachometer


DC Tachometer• This 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.


DC Tachometer


DC Tachometer as Error Detector


AC Tachometer• The 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.


AC Tachometer


Servo System

• A 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 linear

A Basic Servo System

• A servo controller with external input

• Motor connected to a feedback device

• Is a closed loop system

• How might this system work?

The Feedback device canbe Relative or Absolute.

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Servomotors

• The 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 motors– AC Servomotors– DC Servomotors


DC Servo Motors

• DC servo motors are controlled by DC command signals applied directly to coils

• The magnetic fields that are formed interact with permanent magnets and cause the rotating member to turn

• One type of PM uses a wound armature and brushes like a conventional DC motor, but uses magnets as pole pieces

• Another type uses wound field coils and a permanent magnet rotor


Wound Armature PM Motor• Armature contains wound coils• Current is supplied by brushes• Pole pieces are made of permanent magnets• Typically 2 or 4-pole structure• Similar characteristics to a DC shunt motor


Moving Coil Motor• The stator field is provided with 8 pairs of

permanent magnets• Arranged to provide alternating magnetic

fields• The armature is made of thin disc of fiberglass

laminated with copper conductors


Moving Coil Operation


DC Servo Motors


Torque-Speed Curve of a DC Servomotor


AC Servo Motors

• Controlled by AC command signals applied to the coils

• AC Brushless Servo Motor– Operates on the same principle as single-phase

induction motor


• Two windings– Main winding– Auxiliary winding

• Electronic drive provides the necessary phase shift for motor operation

AC Brushless Servo Motor


Stepper Motors

• Conventional servo motors are classified as continuous rotation motors

• Stepper motors rotate through a specific number of degrees, or steps, then stop

• Each incoming pulse results in the shaft turning a specific angular distance

• Stepper motors can control velocity, distance, and direction of mechanical load


Permanent Magnet Stepper Motor• PM stepper motors have rotor teeth made of

permanent magnets• Reaction of the rotor teeth to stator fields provides

torque for the motor• Signals are applied to the stator to determine

direction and step rate of the rotor


Variable Reluctance Stepper Motor

• The variable reluctance stepper motor uses electromagnetic stator poles

• The soft iron core is un-magnetized

• The rotor is toothed, alternating N-S– The more teeth, the

greater the resolution


Stepper Motor Terminology

• Stepping Rate - maximum number of steps the motor can make in one second

• Step Angle - number of degrees per arc the motor moves per step– Step angle is determined by the number of rotor

teeth and stator poles used


Stepper Motor Speed

• Stepper motor speed depends upon the step angle and stepping rate

n = Y x S

6

n = Speed in RPM

Y = Step angles in degrees

S = Steps per second

6 = Formula constant


Microstepping

• Stepper motors tend to jerk at low speeds• Stepper motors have limited resolution• Microstepping overcomes these problems

– Uses simulated sine waves that increment or decrement in small steps called microsteps


Microstepper Operation


Rotor

Stator

Coils

2

1

S

N

1

2

Outside Casing

Stator

Rotor

Internal components of a Stepper Motor


2 2

1

N

S

1

S

N

Stators

Rotor

Cross Section of a Stepper Motor


2 2

1

1

S

N

S

N

NN

S S

1

a b

Winding number 1

2

a b

Winding number 2

One step6 pole rotor


Rotor

Stator coils

CNC Stepping MotorCNC Stepping Motor


Advantages:-

Low cost for control achieved

Ruggedness

Simplicity of construction

Can operate in an open loop control system

Low maintenance

Less likely to stall or slip

Will work in any environment

Disadvantages:-

Require a dedicated control circuit

Use more current than D.C. motors

High torque output achieved at low speeds

Advantages / Disadvantages


Magnetic Amplifiers• The magnetic amplifier (known as a "mag amp") is

an electromagnetic device for amplifying electrical signals. The magnetic amplifier was invented early in the 20th century, and was used as an alternative to vacuum tube amplifiers where robustness and high current capacity were required. World War II Germany perfected this type of amplifier, and it was used in the V-2 rocket The magnetic amplifier has now been largely superseded by the transistor-based amplifier, except in a few safety critical, high reliability or extremely demanding applications. Combinations of transistor and mag-amp techniques are still used.


Strength of Magnetic Amplifiers• The 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 the flux gate compass.


Limitations of Magnetic Amplifiers• The 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 than utility 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.


Principle of operation• Visually a mag amp device may resemble

a transformer but the operating principle is quite different from a transformer - essentially the mag amp is a saturable reactor. It makes use of magnetic saturation of 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 specific B-H curve shape that is highly rectangular, in contrast to the slowly-tapering B-H curve of softly saturating core materials that are often used in normal transformers


Principle of operation• The typical magnetic amplifier consists of two physically

separate but similar transformer magnetic 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".


Principle of operation

http://en.wikipedia.org/wiki/File:Magnetic_amplifier.svg


Magnetic Amplifier


Applications of Magnetic Amplifier

• Magnetic 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, for stage lighting and 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


Applications of Magnetic Amplifier

• Magnetic amplifiers can be used for measuring high DC-voltages without direct connection to the high voltage and are therefore still used in the HVDC-technique

• Magnetic amplifiers were used by locomotives to detect wheel slip

• Magnetic amplifiers are also still used in instrumentation for measuring current

• Such instrumentation mag. amps. are commonly found on space craft where a clean electromagnetic environment is highly desirable.


THANKSPresented By:

Er. H.S. [email protected] 098150-74800

mailto:[email protected]

Control Components Presented By: Er. H.S. Dhaliwal Assistant Professor, Deptt. Of Electrical Engg...

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