Lecture 7 - Feedforward Control

Question 1

Feedforward Control

Answer 1

A control strategy that aims to proactively adjust a manipulated variable by measuring the disturbance variable before they can cause deviations in the process variable
KEY - anticipate the effect of the disturbance and take corrective action before the process is affected
– not a subsitute for feedback control as it is only an approximation

Question 2

Feedback Control

Answer 2

A control strategy that reacts to deviatipns in the process variable after they have occurred - operates in a closed loop, continously monitoring the PV and making corrections to maintain the desired set point, the error value is used to calculate the necessary adjustment

Question 3

Feedback vs Feedforward

Answer 3

Feedforward control aims to prevent PV changes by responding to disturbance variables, feedback control reacts after the PV has already been affected

Question 4

Feedforward Decoupler

Answer 4

A decoupler is a component that combines the outputs of the feedforward controller and feedback controller when both are used to adjust the same manipulated variable
- merges the adjustments trying to be made by feed forward and feedback control into a single signal
Common:
- Ratio
- Bias

Question 5

Ratio vs Bias Decouplers

Answer 5

Ratio - when the feedforward and feedback actions are related proportionally
- Output of algorithm is product of output and ratio
Bias - decoupler adds a bias value, suitable when actions from each strategy are additive
- Output of bias algorithm is the sum of the output and the bias

Question 6

Three Element Level Control (Steam Drums)

Answer 6

Level - the primary controlled variable
Feedwater Flow - the flowarate of water entering the drum
Steam Flow - the flowrate of steam exiting the drum
By measuring the inflow and outflow, the controller can anticipate and compensate for disturbances more effectively than with simple level control

Question 7

Dynamic Compensation

Answer 7

Dynamic compensation is crucial in feedforward control when the processs respinse to MV changes differ significantly from its response to disturbance variable changes.
- ensures that feedforward control actions occur at the correct time and with the appropriate magnitue to counteract the effect of the disturbance
– Introduce deadtime and lag