Components of Control System Automatic

POTENTIOMETERS — COMPONENTS CONTROL SYSTEM

A potentiometer is a device that can be used to convert a linear or angular displacement into a voltage. A potentiometer is a variable resistance whose value varies according to the angular/linear displacement of the wiper contact (movable contact)

APPLICATION OF POTENTIOMETERS

Potentiometer can be used either to convert a mechanical motion to proportional voltage or as an error signal. A single potentiometer excited by dc or ac voltage is used to produce an output voltage proportional to displacement of the input shaft.

CHARACTERISTICS OF POTENTIOMETER

  1. The ideal characteristics of a potentiometer is linear variation of resistance with displacement. This is best realized by having very large radius, more number of turns and high resistance elements.
  2. The device which measures the output voltage of potentiometer should have high input impedance to avoid loading error. If necessary an isolation amplifier with high input impedance may be used.
  3. When the wiper slides over resistance it makes simultaneous contact with adjacent turns to avoid discontinuity in output. Consequently the output is in the form of staircase steps.

SPECIFICATIONS OF POTENTIOMETER

The specification of potentiometer used in control system are the following,

  1. Turns per unit length is in the range of 6 to 30 turns per mm.
  2. Torque required for wiper movement is the range of 1x10–3Kg-m to 1x10–2Kg-m.
  3. The total resistance of the potentiometer is in the range of 25 ohms to 1 mega-ohms.
  4. Power rating is 1 to 10 watts.
  5. Heat dissipation is 1/2 watts per cm2.
  6. Excitation voltage is 4 to 20 volts.
  7. Voltage gradient is 0.01 to 0.05 volt per degree.

AC POTENTIOMETER — CONTROL SYSTEM

In potentiometer excited by ac supply the output will be a modulated voltage. The carrier is the excitation voltage. The envelope of the carrier is modulated by the movement of the wiper arm. Hence, the information is available in the envelope of the carrier. The ac potentiometer will have inductive effect in addition to resistance which leads to difficulty in balancing the potentiometers used as error detectors.

SYNCHROS

The term synchro is a generic name for a family of inductive devices which works on the principle of a rotating transformer (Induction motor). The trade names for synchros are Selsyn, Autosyn and Telesyn. Basically they are electro-mechanical devices or electromagnetic transducers which produces an output voltage depending upon angular position of the rotor.

  1. To control the angular position of load from a remote place/long distance.
  2. For automatic correction of changes due to disturbance in the angular position of the load.

SYNCHRO TRANSMITTER

Construction

The constructional features, electrical circuit and a schematic symbol of synchro transmitter . The two major parts of synchro transmitter are stator and rotor. The stator is identical to the stator of three phase alternator. It is made of laminated silicon steel and slotted on the inner periphery to accommodate a balanced three phase winding. The stator winding is concentric type with the axis of three coils 1200 apart. The stator winding is star connected (Y-connection).

Working principle — Components of Control System

When the rotor is excited by ac voltage, the rotor current flows, and a magnetic field is produced. The rotor magnetic field induces an emf in the stator coils by transformer action. The effective voltage induced in any stator coil depends upon the angular position of the coils axis with respect to rotor axis.

STEPPER MOTORS

A stepper motor transforms electrical pulses into equal increment of rotary shaft motion called steps. Components of Control System A one-to-one correspondence exists between the electrical pulses and the motor steps. They control windings of the stepper motor with a frequency and sequence corresponding to the command issued. It has a wound stator and a non excited rotor. They are classified as variable reluctance, permanent magnet or hybrid, depending on the type of rotor. They are also classified as 2-phase, 3-phase or 4-phase depending on the number of windings (called control winding) on the stator.

APPLICATIONS OF STEPPER MOTOR

Stepper motors are used in computer peripherals, X-Y plotters, scientific instruments, robots and machine tools. Stepper motors are used in quartz-crystal watches. The one used in wrist watch has a diameter of 3mm and takes a few microamperes at 1.5V. Stepper motors are also used for colour registration in printing. Stepper motors can upgrade mechanical systems by replacing cams, complex linkages, and similar mechanisms to give greater precision and production rate. Stepper motors are available with torques in precision and production rate. Stepper motors are available with torques in the range from 0.5µN-m to 100N-m, outputs in the range from millwatts to several kilowatts and pulse rates of 1200 or more per second.

ACTIVE ROTOR OR PERMANENT MAGNET ROTOR STEPPER MOTOR

The stator of this type of stepper motor has salient poles carrying control windings. Each pole carries a control winding. A pair of control winding are connected in series and called a phase, Components of Control System.

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