Control Components

Control Components Presented By: Er. H.S. Dhaliwal Assistant Professor, Deptt. Of Electrical Engg BHSBIET, Lehragaga. Punjab Edusat Society

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. Punjab Edusat Society

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. Punjab Edusat Society

Control Components • There are various types of control components • Error Detectors • Potentiometers • Synchros • AC & DC Techogenerators • AC & DC Servomotors • Stepper Motors • Magnetic Amplifiers Punjab Edusat Society

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. Punjab Edusat Society

Error Detectors Punjab Edusat Society

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. Punjab Edusat Society

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 Punjab Edusat Society

Potentiometer Diagram Punjab Edusat Society

Potentiometer Symbol Punjab Edusat Society

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. Punjab Edusat Society

Potentiometer Construction Punjab Edusat Society

Potentiometer Construction • 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 Punjab Edusat Society

Potentiometer Construction Punjab Edusat Society

Theory of operation Punjab Edusat Society

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: Punjab Edusat Society

Potentiometer as an Error Detector Punjab Edusat Society

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. Punjab Edusat Society

Potentiometer as an Error Detector Punjab Edusat Society

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 Punjab Edusat Society

Synchros Punjab Edusat Society

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. Punjab Edusat Society

Synchro as Error Detector Punjab Edusat Society

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. Punjab Edusat Society

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. Punjab Edusat Society

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 Punjab Edusat Society

Tachogenerators • Tachometers are mainly of two types: • DC Tachometer • AC Tachometer Punjab Edusat Society

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. Punjab Edusat Society

DC Tachometer Punjab Edusat Society

DC Tachometer as Error Detector Punjab Edusat Society

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. Punjab Edusat Society

AC Tachometer Punjab Edusat Society

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 Punjab Edusat Society

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 can be Relative or Absolute. Punjab Edusat Society

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 Punjab Edusat Society

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 Punjab Edusat Society

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 Punjab Edusat Society

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 Punjab Edusat Society

Moving Coil Operation Punjab Edusat Society

DC Servo Motors Punjab Edusat Society

Torque-Speed Curve of a DC Servomotor Punjab Edusat Society

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 Punjab Edusat Society

Control Components