Control Engineering Basics
Which of the following is the correct pair of the main purpose of adding integral (I) action in PI control and its side effect?
- AThe purpose is to remove high-frequency noise, and the side effect is a larger steady-state error
- BThe purpose is to remove the steady-state error to a constant setpoint or disturbance, and the side effect is a phase lag that makes the loop more prone to oscillation
- CThe purpose is to anticipate changes in the error, speeding up the response and suppressing overshoot, and the side effect is strong amplification of high-frequency noise
Show answer and explanation
Answer: B. The purpose is to remove the steady-state error to a constant setpoint or disturbance, and the side effect is a phase lag that makes the loop more prone to oscillation
Integral action accumulates the error over time into the manipulated variable, so it keeps changing the manipulated variable as long as any error remains and finally brings the steady-state error to zero. On the other hand, an integrator delays the phase by 90°, which reduces the phase margin and makes overshoot and oscillation more likely. The integral time Ti is therefore tuned so that it is not too short. Speeding up the response, suppressing overshoot and tending to amplify noise are features of derivative action.
- A. Removing noise is the job of a low-pass filter. Integral action is used to remove the steady-state error.
- B. Correct. The steady-state error disappears, at the cost of stability margin.
- C. This is the effect and side effect of derivative (D) action.