Module IV LONG ANSWER QUESTIONS 1.EXPLAIN THE REQUIREMENTS OF CONTROL SYSTEMS. AND EXPLAIN ANY ONE TYPE OF CONTROL SYSTEM? Requirements of a control system Stability,accuracy&speed of response are the three requirements of a control system(a)STABILITY:A system is said to be stable if the output of the system after fluctuations, variation or oscillations if any, settles at a reasonable value for any change in input or change in disturbance

(b)ACCURACY: A system is said to be 100 percent accurate if the error is zero. An accurate system is costly. There is no point in going for 100 percent accurate systems when that much of accuracy is not really required.Eg, when a variation of say 0.2 degree centigrade cannot be sensed by a human being ,there is no need to have a home a home heating system of temperature variation equal to zero.

(c) SPEED OF RESPONSE: This refers to the time taken by the system to respond to given input & give that as output. Theoritically the speed of response should be infinity, ie the system should have an instantaneous response. This requirement is of prime concern with follow –up systems.

OPEN LOOP CONTROL SYSTEMS:

If in a physical system there is no automatic correction of the variation in its output,it is called an open loop control system.That is, in this type of system,sencing of the actual output and comparing of this with the desired input does not take place. The system on its own is not in apositon to give the desired output and it cannot take into account the disturbances.In these systems the changes in the output can be corrected only by changing the input manually.

Figure: Open Loop Control System

The system are simple in construction,stable and cost cheap.

But these systems are inaccurate and unreliable .Moreover these systems do not take account of external disturbances that affect the output and they do not initiate corrective actions automatically.

Examples of open loop control system(a) automatic washing machine(b) traffic signal system(c) home heating system

Any known feedback control system can be considered as afeedback control system.If it is under the provision of someone.Although openloop control system have economical components and are simple in design ,they are largely depend on human judgement.

2. CLASSIFY FEEDBACK CONTROL SYSTEMS AND EXPLAIN THEM?

The feedback control systems are classified as (a) regulator systems.(b) Follow up systems(c) Servo mechanisms(d) Continuous data systems(e) Sample or discrete data systems

(a) Regulator systems: This is a feedback control system where the input is not changed frequently.eg: home heating or cooling system.

(b) followup systems: This is a feedback control system where the input changes at every instant of time and hence the output should closely follow the input.Eg:tracer mechanism on a lathe,spacecrafts.

(c) Servo mechanism: This is a feedback control system with a mechanical quantity as the final output.Here the control variable is mechanical position or time derivatives of position.eg:movement of load over a certain distance.

(d)Continuous data feedback control systems: In this system the signals at various parts of the system are all functions of the continuous time variable.In general in an AC system the signals are modulated and in DC systems the signals are unmodulated.The DC provided is amplified and given as field excitation for the DC motor.Thus the speed of the motor is controlled.

Figure: DC Servo Motor

(e)Sampled or Discrete data control system: In this systems the signals involved are mostly in the form of pulses or numerical code.This systems receives data only at certain time intervals.Hence the system may receive only intermittently.The output of a sampler has a train of pulses whose amplitude is in direct proportion with analog signal.

Figure:

The techniques used in sampling are (a)periodic(after specific intervals of time)(b)cyclic(sampling points are cyclic)

(c)multi rate(rate of sampling varies) (d)Random(without order) (e)pulse width modulated(variation in width of pulse)

3.EXPLAIN ANY TWO TYPE OF RELAYS?

INDUCTION DISC RELAY:

Figure: Induction Disc Relay

It is similar in construction to the watt hour or energy meter. It has an electromagnetic core around which the exciting coil is wound. There is a disc mounted on the shaft and suspended b/w the gaps in the electromagnetic core on one end and is enclosed in the magnetic field on the other end. When a voltage is applied across the coil ,a magnetic field is produced .Now the disc is acted upon by two magnetic fields ,which produced an electro mechanical torque. This torque on the disc causes it to rotate and with suitable coupling. This rotary motion can be interpreted as the linear motion of a relay switch that can be used to activate the circuit breaker

INDUCTION CUP RELAY: The induction disc relay is comparatively slower in

operation as the electromechanical torque generated takes sometime to drive the disc.The induction disc relay has a factor of coupling of the shaft to the relay switch which may have some losses.Inorder to improve the response, the disc was replaced by an inverted cup and led to the induction cup relay.

The main advantages of an induction cup relay is (a)fast in operation(b)very quick response(c)higher torque(d)less contact chatter

Figure: Induction Cup Relay

Figure: Output of induction Cup Relay

4.EXPLAIN ANY ONE ELECTRONIC SYSTEMS?

Electronic systems are widely used in automatic control devices due to there easy fabrication assembling and most important easy compatibility to all other systems .Operational amplifiers are most widely used electronic control elements in any electronic systems .These are more flexible and versatile than and pneumatic system elements with ever increasing reliabity of integrated circuits the use of electronic control syste in industry has increased in recent times

Figure: Op-Amp Circuit

The OP AMP can be operated in two modes-inverting mode-non inverting mode In inverting mode the output is given to the inverting input terminal of the OP AMP

Figure: Inverting Mode of Op-Amp

In non inverting mode the input is given to the non inverting input terminal

Figure: Non Inverting Mode of Op-Amp

Important control appl: using OP AMPS are,-voltage regulators-phase locked loops-comparator circuits

VOLTAGE REGULATORS: A voltage regulator is a electronic circuit where a stable DC voltage ,independent of load current ,temperature and AC line voltage variations .The series pass transistor is in series with the input unregulated voltage V in and the output regulated voltage Vo .It absorbs the difference b/w the two when there is any fluctuation b/w both. The transistor is connected as an emitter follower and gives enough gain to drive the load. The output voltage Vo is sampled by R1- R2 divider and feedback to the inverting terminal of the control element. The reference voltage is compared by giving to the non inverting terminal using a zener diode. The output of the error amplifier drives the series pass transistor Q1.If V0 increases the sampled voltage Vo also increases

This reduces the output voltage V’o of error amplifier which is applied to base of Q1 which is an emitter follower. Thus Vo followers V’o ie, it reduces. Similar vice versa action takes place if Vo decreases.