Control Engineering Basics
Which of the following is the correct pair of a property of derivative (D) action in PID control and a measure taken when implementing it?
- AIt accumulates the error over time to remove the steady-state error, and to prevent integral windup the upper and lower limits on the manipulated variable are removed
- BIts gain becomes smaller at higher frequencies, so it is robust to noise, and no low-pass filter is used because a filter would add phase lag and cause instability, so it is used as it is
- CIt suppresses oscillation with a manipulated variable that follows the rate of change of the error, but it strongly amplifies high-frequency noise, so it is combined with a low-pass filter as a filtered derivative
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Answer: C. It suppresses oscillation with a manipulated variable that follows the rate of change of the error, but it strongly amplifies high-frequency noise, so it is combined with a low-pass filter as a filtered derivative
Derivative action produces a manipulated variable proportional to the rate of change of the error and responds in advance, before the error grows, so it suppresses oscillation and increases damping. The gain of a differentiator s grows in proportion to frequency, so it strongly amplifies high-frequency components such as sensor noise. A first-order low-pass filter is therefore combined with it, as in Td·s/(1 + Td·s/N) (a filtered derivative), limiting the gain at high frequencies to about N times. N is often chosen around 8 to 20.
- A. This describes integral action. Removing the limits also does not remove the saturation of the real actuator.
- B. The gain of a derivative grows at higher frequencies, which makes it sensitive to noise.
- C. Correct. A filtered derivative limits the gain at high frequencies.