Feedback Control Flashcards
Disadvantage of prop only control
An offset occurs after a setpoint change or sustained disturbance
What is the disadvantage of P only control
An offset occurs after a setpoint change or sustained disturbance
What is the condition for proportional bandwidth
Kc should be dimensionless
What is the controller input equal to in prop only control
The error signal
What is the aim of a FB controller?
To reduce the error signal
Disadvantages of FF control
- Dist. must be measured.
- No corrective action for unmeasured disturbances.
- A process model is required.
Advantages of FF control
Corrective action is taken before a disturbance can upset the controller
Advantages of FB control
- Corrective action is taken regardless of the source of the disturbances.
- Reduces the controllers sensitivity to unmeasured disturbances and process changes.
Disadvantage of FB control
No corrective action is taken until after the disturbance has upset the system
Why is negative feedback preferred over positive?
The corrective action taken by a negative feedback controller forces the controlled variable towards the setpoint
What does the controller output in integral control depend on
It depends on the integral of the error signal over time
Advantages of integral control
It eliminates offsets
Why does a controller saturate?
It saturates when the disturb. or setpoint change is so large beyond the range of the manip. variable.
Why is integral action used with proportional control?
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.
Disadvantages of integral action
- Tends to produce oscillatory responses of the controlled variable thus reducing the stability of the feedback controller
- Causes reset windup, where the integral terms become very large and the controller output saturates due to the occurrence of a sustained error
An antireset method
Implement integral control action using a positive feedback loop
How does derivative control work?
It anticipates future behavior pf the error signal by considering it’s rate of change
Advantages of derivative control
- By providing anticipatory control action, it stabilises the controlled process hence it’s often used to counteract the destabilising tendency of the integral mode.
- It also tends to improve the dynamic response of the controlled variable by reducing settling time
Downside of derivative control
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
Disadvantages of PID controller
Derivative kick
What is derivative kick?
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
Reverse action
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
Disadvantages of on-off control
It results in continual cycling of the controlled variable and produces excessive wear on the control valve or any other final control element
What are on-off controllers commonly used as
Thermostats in home heating systems and domestic refrigerators
Why are on-off controllers less widely used than PID controllers
They are not as versatile or effective
What do we do when we have two parallel t functions combining
We add them and simplify
For a prop only control, the controller output is always proportional to the error signal
False, the controller output could saturate or the controller could be in manual mode
A PID controller always eliminates offset after a sustained unmeasured disturbance. T/F
False, even with integral control action, offset can occur if the controller output saturates.
