Feedback Control

Question 1

Disadvantage of prop only control

Answer 1

An offset occurs after a setpoint change or sustained disturbance

Question 2

What is the disadvantage of P only control

Answer 2

An offset occurs after a setpoint change or sustained disturbance

Question 3

What is the condition for proportional bandwidth

Answer 3

Kc should be dimensionless

Question 4

What is the controller input equal to in prop only control

Answer 4

The error signal

Question 5

What is the aim of a FB controller?

Answer 5

To reduce the error signal

Question 6

Disadvantages of FF control

Answer 6

1. Dist. must be measured.
2. No corrective action for unmeasured disturbances.
3. A process model is required.

Question 7

Advantages of FF control

Answer 7

Corrective action is taken before a disturbance can upset the controller

Question 8

Advantages of FB control

Answer 8

1. Corrective action is taken regardless of the source of the disturbances.
2. Reduces the controllers sensitivity to unmeasured disturbances and process changes.

Question 9

Disadvantage of FB control

Answer 9

No corrective action is taken until after the disturbance has upset the system

Question 10

Why is negative feedback preferred over positive?

Answer 10

The corrective action taken by a negative feedback controller forces the controlled variable towards the setpoint

Question 11

What does the controller output in integral control depend on

Answer 11

It depends on the integral of the error signal over time

Question 12

Advantages of integral control

Answer 12

It eliminates offsets

Question 13

Why does a controller saturate?

Answer 13

It saturates when the disturb. or setpoint change is so large beyond the range of the manip. variable.

Question 14

Why is integral action used with proportional control?

Answer 14

Little control takes place until the error signal has persisted for sometime, whereas with prop. control takes action as soon as the error is detected.

Question 15

Disadvantages of integral action

Answer 15

1. Tends to produce oscillatory responses of the controlled variable thus reducing the stability of the feedback controller
2. Causes reset windup, where the integral terms become very large and the controller output saturates due to the occurrence of a sustained error

Question 16

An antireset method

Answer 16

Implement integral control action using a positive feedback loop

Question 17

How does derivative control work?

Answer 17

It anticipates future behavior pf the error signal by considering it’s rate of change

Question 18

Advantages of derivative control

Answer 18

1. By providing anticipatory control action, it stabilises the controlled process hence it’s often used to counteract the destabilising tendency of the integral mode.
2. It also tends to improve the dynamic response of the controlled variable by reducing settling time

Question 19

Downside of derivative control

Answer 19

If the process measurement is noisy(gas high frequencies and random fluctuations) then the d action of the measured variable will change wildy abd d action will amplify the noise unless it is filtered

Question 20

Disadvantages of PID controller

Answer 20

Derivative kick

Question 21

What is derivative kick?

Answer 21

When a sudden change in the setpoint and error causes the derivative term to momentarily become very large and provide a derivative kick to the final control element

Question 22

Reverse action

Answer 22

When kc>0, the controller output increases as the input signal ym decreases.
And direct action is the opposite. The definition is based on ym

Question 23

Disadvantages of on-off control

Answer 23

It results in continual cycling of the controlled variable and produces excessive wear on the control valve or any other final control element

Question 24

What are on-off controllers commonly used as

Answer 24

Thermostats in home heating systems and domestic refrigerators

Question 25

Why are on-off controllers less widely used than PID controllers

Answer 25

They are not as versatile or effective

Question 26

What do we do when we have two parallel t functions combining

Answer 26

We add them and simplify

Question 27

For a prop only control, the controller output is always proportional to the error signal

Answer 27

False, the controller output could saturate or the controller could be in manual mode

Question 28

A PID controller always eliminates offset after a sustained unmeasured disturbance. T/F

Answer 28

False, even with integral control action, offset can occur if the controller output saturates.