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Control Engineering Basics

Which of the following correctly describes integral windup?

  1. AA phenomenon in which the derivative term amplifies noise and the manipulated variable keeps oscillating finely; it is prevented by adding a low-pass filter to the derivative
  2. BA phenomenon in which a steady-state error persists because the sampling period in the integral calculation was made too long
  3. CA phenomenon in which the integral term keeps accumulating the error while the manipulated variable is stuck at its upper limit, so the manipulated variable does not come down for a while even after the error changes sign, causing a large overshoot
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Answer: C. A phenomenon in which the integral term keeps accumulating the error while the manipulated variable is stuck at its upper limit, so the manipulated variable does not come down for a while even after the error changes sign, causing a large overshoot

Manipulated variables such as a motor voltage or a valve opening have upper and lower limits. When a large error continues and the manipulated variable saturates at its upper limit, the manipulated variable cannot increase any further, yet the integral term keeps accumulating the error and grows large. Even after the controlled variable passes the setpoint and the error changes sign, the manipulated variable stays at its upper limit until the accumulated integral comes down, which causes a large overshoot and a long oscillation. As a countermeasure, anti-windup is used, which stops integrating or limits the integral while the output is saturated.

  • A. This describes noise amplification by the derivative term.
  • B. It has nothing to do with the length of the sampling period. With the period multiplied in correctly, the integral works as intended.
  • C. Correct. It arises from the combination of actuator saturation and integration.
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More Control Engineering Basics questions

  1. Positional PI control is implemented on a microcontroller, with the duty cycle of a PWM signal (0 to 100%) as the manipulated variable. Which of the following is a correct countermeasure against integral windup?
  2. A digital PI controller with a sampling period Ts = 1 ms calculates the integral with the rectangular approximation Σe·Ts. With an integral gain Ki = 10 s⁻¹, which of the following is the increase in the integral term when an error e = 0.2 lasts for 50 samples?
  3. In digital control with a sampling period Ts = 1 ms, the zero-order hold of the D/A conversion acts as a time delay of about Ts/2. With a gain crossover frequency of 100 Hz, which of the following is closest to the phase lag caused by this delay?
  4. A continuous-time first-order system with its pole at s = −100 s⁻¹ is discretized with a sampling period Ts = 1 ms. Which of the following is closest to the discrete-time pole given by the relation z = e^(sTs)?
  5. A motor is controlled with cascade control, with a current (torque) loop inside and a speed loop outside. Which of the following is the correct design approach?

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