Control Engineering Basics
Which of the following is the correct reason for combining feedforward control with feedback control?
- AFeedforward can respond to measurable disturbances and setpoint changes before an error appears, but it cannot correct unmeasured disturbances or model errors, so feedback takes care of the rest
- BAdding feedforward changes the gain and phase of the loop transfer function and increases the phase margin, so the loop stays stable even with a larger feedback gain
- CWith feedback alone a steady-state error always remains even with integral action, so feedforward is essential to bring the steady-state error to a constant setpoint or disturbance to zero
Show answer and explanation
Answer: A. Feedforward can respond to measurable disturbances and setpoint changes before an error appears, but it cannot correct unmeasured disturbances or model errors, so feedback takes care of the rest
Feedforward control measures disturbances or setpoint changes and, before they affect the controlled variable, applies a manipulated variable calculated from a model. It does not wait for an error to appear, so it responds quickly, but it assumes an accurate model and measurable disturbances. A common combination is therefore to let feedforward handle the main disturbances and setpoint tracking and to let feedback correct the remaining deviation. The feedforward path lies outside the loop, so it does not change the stability of the loop (the phase margin and so on).
- A. Correct. Feedforward provides speed and feedback provides certainty.
- B. The feedforward signal does not go around the loop, so neither the loop transfer function nor the phase margin changes.
- C. A steady-state error to a constant setpoint or disturbance can be brought to zero by feedback with integral action.