Lecture 6 - Control Systems

Question 1

Define in one sentence a control system?

Answer 1

A control system is a network of devices and components that regulates and influences the behavior of a system to achieve or maintain a desired state

Question 2

What is the role of sensors in a control system?

Answer 2

They act as the eyes and ears of the system since they feel and see what is happening

Question 3

What is a controller in a control system?

Answer 3

A controller in a control system plays the role of the brain. When the sensor tells it a piece of information, the controller makes a decision for the future steps of the system

Question 4

In a control system, what would be the role of the actuators?

Answer 4

These play the role of muscles, they follow whatever command is fed to them by the controller

Question 5

What does PV stand for?

Answer 5

Process variable

Question 6

What does SP stand for?

Answer 6

Set point

Question 7

What are the two classes of control systems?

Answer 7

1) Open-loop systems
2) Closed-loop systems

Question 8

Define an open-loop control system

Answer 8

Control system where the output is determined solely by the input command, dosen’t take any feedback mechanism to monitor or adjust the output based on the actual performance. (Control action is totally independent from the output)

Question 9

What do open-loop control systems often get used for?

Answer 9

They get used for mainly simple applications since they have well defined input-output behavior

Question 10

What is another term used to refer to closed-loop control systems?

Answer 10

Feedback control

Question 11

Define a closed-loop control system

Answer 11

1) Output is continuously compared with the desired result
2) The control output (which is the system’s input) is modified and adjusted to reduce deviation
3) Forces the response to follow the reference

Question 12

What would be an example of a simple open-loop controller?

Answer 12

ON-OFF controller; switches the output on and off according to a set timer

Question 13

Name 3 pros to a simple open-loop controller

Answer 13

1) Simple and economical
2) Easy to maintain
3) Stable

Question 14

Name 3 cons to a simple open-loop controller

Answer 14

1) Inaccurate
2) Unreliable (don’t know if the goal output is ever reached)
3) Change due to disturbance cannot be corrected (lack of feedback)

Question 15

What is an example of a simple closed-loop controller?

Answer 15

ON-OFF controller; controls temperature while using a temperature sensor and a switch.

ON-OFF controller will switch the output when the output crosses a setpoint

Question 16

Define what is a Process Variable (PV)

Answer 16

The system parameter that needs to be controlled, such as a temperature, pressure, flow rate, etc

Question 17

Define what is a sensor

Answer 17

Used to measure the process variable and provide feedback to the control system

Question 18

Define what is the set point (SP)

Answer 18

The desired or command value for the PV

Question 19

Define what may be an ‘Error e’

Answer 19

The difference between the SP and the PV; it’s used by the control system to determine the action to get/reach that desired input.

Question 20

Define the term ‘rise time’

Answer 20

The amount of time the system takes to go from 10-90% of the steady-state or final value (difference in time between those two values)

Question 21

Define the term ‘percent overshoot’

Answer 21

The amount that the process variable overshoots the final value, expressed as a percentage of the final value

Question 22

Define the terme ‘settling time’

Answer 22

The time required for the process variable to settle whithin a certain percentage (commonly 5%) of the final value

Question 23

Define the term ‘steady-state error’

Answer 23

The final difference between the process variable and set point

Question 24

What is a PID controller?

Answer 24

Controller which calculates an error value as the difference between SP and PV and then applies a correction based on the following three:
1) Proportional
2) Integral
3) Derivative Terms

Question 25

What does Kp represent?

Answer 25

Kp represents the proportional constant that accounts for the present error value

Question 26

Whats does Ki represent?

Answer 26

Integral constant that accounts for historical error values

Question 27

What does Kd represent?

Answer 27

Derivative constant that accounts for future error values

Question 28

What does SSE stand for?

Answer 28

SSE: Steady-state error

Question 29

What is SSE?

Answer 29

SSE is the difference between the desired final output and the actual one

Question 30

How can SSE be mitigated?

Answer 30

1) Adding a compensating term to the setpoint and the outpoint 2) Dynamically: adding an integral term

Question 31

What does Kp account for?

Answer 31

Kp accounts for present error

Question 32

What does Kd account for?

Answer 32

Kd accounts for future error

Question 33

What does Ki account for?

Answer 33

Ki accounts for historical error value

Question 34

What happens in the case where Kp is high?

Answer 34

Large change in output + reduces rise time (will reduce but never rid of SSE)

Question 35

What happens when Kp is too high?

Answer 35

Introduces oscillations; repeatedly goes above and under SP

Question 36

What happens when Kp is low?

Answer 36

Delays system response (reduce Kp for sensitive systems that overshoot easily)

Question 37

What happens when Kd is too low?

Answer 37

Too overshot

Question 38

What happens when Kd is too high?

Answer 38

Much under the SP line

Question 39

What does Kd do?

Answer 39

1) Increases system stability 2) Reduces overshoot 3) Improves transient response

Question 40

What effect does Kd have?

Answer 40

Has a damping effect by reducing oscillations that may have been caused by a large Kp.

Question 41

What are the 3 categories of loop responses?

Answer 41

1) Under Damped 2) Critically Damped 3) Over Damped

Question 42

Define 'under damped'

Answer 42

PID tuned to prevent the oscillation of the graph but results in an overshoot of the PV and somewhat continuous oscillation which eventually settles at the Setpoint.

Question 43

Define 'critically damped'

Answer 43

This is the ideal situation where the PV may approximately equal to the SP; overshoot does not occur and the process responds in a non-oscillatory and controlled manner

Question 44

Define 'over damped'

Answer 44

The system responds in a very slow rise time manner which causes non ideal and unnecessarily slow

Question 45

What is tuning in the context of PID controllers?

Answer 45

Tuning is the adjusting of each Kp, Ki and Kd until a desired overall response is obtained

Question 46

What are the steps to designing a PID controller?

Answer 46

1) Obtain an open-loop response and determine what needs to be improved 2) Add proportional control to improve rise time 3) Add derivative control to improve overshoot 4) Add an integral control to eliminate SSE 5) Adjust/tune all the parameters until one obtains the desired overall response

Question 47

What is the effect of the proportional control over the 4 main elements?

Answer 47

Rise Time: Decreases Overshoot: Increases Settline Time: Small change SSE: Decreases

Question 48

What is the effect of derivative control over the 4 main elements?

Answer 48

Rise Time: Small change Overshoot: Decreases Settline Time: Decreases SSE: Small change

Question 49

What is the effect of integral control over the 4 main elements?

Answer 49

Rise Time: Decreases Overshoot: Increases Settline Time: Increases SSE: Eliminate