Chapters 1-4 Flashcards
What is a process/system?
Series of physical elements that interact to perform functions.
What is a dynamical system?
System exhibiting continuous motion in the face of internal or external forces
What are state variables?
The smallest possible subset of system variables that can represent the internal conditions of the process.
What are input variables?
The variables that independently induce change in the internal conditions of the process.
What are controlled variables?
The variables that are controlled; also called output variables
What are manipulated variables?
These input variables that are at our disposal to manipulate freely.
What are disturbance variables?
Input variables over which we have no control.
What is control
Act of adjusting the manipulated variable to keep the controlled variable at a desired value (while satisfying some constraints).
What is the set point?
The desired value referring to controlled variables
Which variable is measured in feedback control?
The Control variable
Which variable is not measured in feedback control?
Disturbance variable
What is a limitation of feedback control?
No action is taken until after the disturbance occurs
Are disturbance variables known in feedback control?
No
Which variable is measured in feedforward control?
Disturbance variable
Which variable is not measured in feedforward control?
Control variable
What is an important advantage of feedback control?
They reduce sensitivity of the controlled variable to
unmeasured disturbances and process changes.
What is an advantage of feedforward control?
Action is taken before
the controlled variable deviates from the set point.
What are three significant disadvantages of feedforward control?
(i) - the disturbance variable must be measured/modelled (or
accurately estimated)
(ii)- no corrective action is taken
for unmeasured/unmodelled disturbances
(iii) a process model
is required.
What is the difference between feedback and feedforward control in terms of corrective action?
FB: Corrective action is taken after process is affected (something goes wrong)
FF: Corrective action is taken before the process is affected
What does feedforward control not take corrective action of?
Unmeasured disturbance
For feedback control, does corrective action depend on the source of disturbance?
No. Corrective action occurs regardless of the source of disturbance.
How is the disturbance dealt with in a system with feedforward (FF) + feedback (FB) control?
FF deals with the most significant disturbance.
FB deals with the rest of disturbance.
For definining variables of the first letter for control methods, define what each letter stands for: T, L, F, P, and A
T -> temperature
L -> level
F -> flow
P -> pressure
A -> analyzer/concentration
For definining variables of the second letter for control methods, define what each letter stands for: T, C
T - transmitter
C - controller
Explain how feedback control works?
-CV is measured
-DV is not measured
-No action is taken until after the disturbance occurs.
Explain how feedforward control works?
-CV is not measured
-DV is measured
-Requires measuring and modelling DVs.
-No action is taken against DV’s that aren’t modelled.
When is feedforward control ideal to use?
It’s ideal in a theoretical sense that controller can take action via MV even while CV remains equal to its desired value.
Can laplace transform model a time delay?
Yes. This is known as the time delay property of the Laplace transform
Is time delay a linear operation or a non-linear one?
Time delay is a linear operation, not a non-linear one.
Explain why the following statement is false?: “The model for a system should be made as accurate as possible since a better model means better control.
“
The more accurate your model the more complex your design is and consequently the more expensive it is. Often a simpler model will do fine and cost less to implement
