Instrumentation Interview questions and answers Control systems

Instrumentation Interview questions and answers Control systems

CONTROL SYSTEMS

 Explain the application of proportional integral and derivative action?

Proportional control only:

Proportional control only attempts to return a measurement to the set point after a load upset has occurred. How ever it is impossible for a proportional controller to return the measurement exactly to the set point.

Use: It is normally used for level controls. It reduces the effect of a load change but it cannot eliminate it.

Proportional plus reset (Integral) control:

Reset action is introduced to eliminate offset. It will integrate any difference between measurement and set point and cause the controller's output to change until the difference between the measurement and set point is zero. Reset will act as long as the error exists.

Use: Proportional + Reset controllers are by far the common types used in industrial process control and where predominate dead times occur.

Proportional plus reset plus derivative (Rate):

Derivative or rate action helps the controller overcome system inertia and result in faster, more precise control. Derivative action occurs whenever the measurement signal changes. Under study conditions the rate action does not act. Derivative allows the controller to inject more corrective action.

Use: On temperature controls.

67. What is difference gap control? (On off control)

Differential gap control is similar to on off control except that a band or gap exists around the control point.

Use: In industry differential gap control is often found in non-critical level control applications where it is desirable only to prevent a tank from flooding or drying. When a measured variable exceeds the upper gap the control valve will open fully or be closed fully. Similarly when it exceeds the lower gap it will open or close fully.

68. Where is on off control used?

On off control is used when

1.      Precise control is not needed.

2.      Processes that have sufficient capacity to allow the final operator to keep up with the measurement cycle.

3.      It is mainly used in refrigeration and is conditioning systems.

69. What is reset-wind up?     

When reset action is applied in controllers where the measurement is away from the set point for long periods the rest may drive the output to its maximum resulting in rest wind up. When the process starts again the output will no come off its maximum until the measurement crosses the so point causing large overshoots. This problem can be avoided by including anti-reset wind up circuit, which eliminates the problem of output saturation.

70. Why is reset called integral and Rate derivative?

Reset is called integral because of the mathematical relationship to the output.

Rate is called derivative because

   time                   

Oi = is the Output at any given time

O₀ = is the out put at time zero or zero error.

e   = is the error signal

t    = is time.

f    = is the reset rate in respects per minute.    

71. Explain tuning of controllers.

Tuning basically involves adjustment of proportional. Integral and derivative parameters to achieve good control. The gain, time constants, and dead times around the loop will dictate the settings of various parameters of the controller.

Tuning methods are broadly classified into two:

1. Closed Loop Method: e.g. Ultimate Gain Method.

2. Open Loop Method: e.g. process Reaction curve.

Ultimate gain method:

The term ultimate gain was attached to this method because their uses require the determination of the ultimate gain (sensitivity) and ultimate period. The ultimate sensitivity Ku is the maximum allowable value of gain (for a controller with only Proportional mode) for which the system is stable. The ultimate period is the period of the response with the gain set at its ultimate value.

Process reaction curve:

To deter mine the process reaction curve, the following steps are recommended. :

1.   Let the system come to steady state at the normal load level.

2.   Place the controller on manual.

3.   Manually set the output of the controller at the value at which it was operating in the automatic mode.

4.   Allow the system to reach the steady state.

5.   With controller on manual, impose a step changes in the output of controller, which is an signal to value.

6.   Record the response of controlled variable.

7.   Return the controller output to its previous value and return the controller to auto operation.

72. Explain the working of an electronic P.I.D. controller.

Input from the measurement transmitter is compared with the set point voltage to produce a deviation signal. The deviation signal is combined with a characterized feed back signal to provide the input for the function generator amplifier. This amplifiers output is delivered to the feed back network, and to the final output which is a 10-50m.a. do signal for actuation of final operators.