16. Mention two drawbacks of derivative action.
Ans:
The output of controller is zero
at constant error condition.
It will amplify the noise present
in the error signal.
17. What are decouplers?
Ans:
The special element introduced in
a system with two strongly interacting
loops to cancel the interaction effect between the two loops and thus
render two non-interacting control loops
is called decoupler.
18. Why is the electronic controller preferred to pneumatic controller?
Ans:
Electronic signals operate over
great distance without time lags. Electronic
signals can be made compatible with digital controllers. Electronic
devices can be designed to be essentially
maintenance free. Intrinsic safety techniques eliminate electrical hazards. Less expensive to
install. More energy efficient. Due to the
above said properties electronic controllers are preferred to pneumatic
controller.
19. Explain the function of controller.
The element in process control
loop that evaluates error of the controlled
variable and initiates corrective action by a signal to the controlling
variable.
20. Why derivative mode of control is not recommended for a noisy process?
The series capacitor in the
derivative controller will amplify the noise in the error signal.
21. Define cycling.
Oscillations of error about zero
is called cycling.
22. What is reaction curve.
In process controller, the
reaction curve is obtained by applying a step
change (either in load or in set point) and plotting the response of the
controlled variable with respect to
time.
23. Give the satisfactory control for gas pressure process, vapour pressure process, temperature process, composition process .
Proportional Control is the
satisfactory control for liquid level process.
PI Control is the satisfactory
control for vapour pressure process having
fast response.
PID Control is the satisfactory
control for temperature process.
PID Control is the satisfactory
control for composition process.
24. Define ratio control & cascade control.
Ratio control is a special type
of feed forward control where two disturbances
are measured and held in a ratio to each other.
Cascade control is defined as a
control system composed of two loops
where the set point of one loop (the inner loop) is the output of the
controller of the other loop (the outer
loop) .
25. What is limit switch?
In some cases it is necessary to change from the normal control action
and attempt to prevent a process
variable from exceeding an allowable upper or lower limit. This can be achieved by the use of
special type switches called limit
switches.
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