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Control Engineering Basics

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?

  1. AWhile the output is saturated, increase the proportional gain to get out of saturation sooner
  2. BIf the integral term is calculated in floating point no overflow occurs, so no special measure is needed
  3. CWhile the output is saturated, stop integrating if the error would deepen the saturation, or feed the difference from the limited value back into the integrator, to hold down the integral
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Answer: C. While the output is saturated, stop integrating if the error would deepen the saturation, or feed the difference from the limited value back into the integrator, to hold down the integral

The first method is conditional integration (clamping): while the output is saturated and the error would deepen the saturation, the integral is not updated. The second is back-calculation: the difference between the value calculated before limiting and the value actually output is multiplied by a coefficient and fed back into the integrator to pull the integral back. In the velocity form of PID control, which calculates the change in the manipulated variable, simply limiting the output makes windup unlikely. When calculating in integer fixed point, overflow of the integral variable itself must also be guarded against, for example with saturating arithmetic.

  • A. Raising the proportional gain does not reduce the accumulated integral, so it is not a real fix.
  • B. Windup is a different phenomenon from overflow; even in floating point the integral grows too large and causes overshoot.
  • C. Correct. Conditional integration and back-calculation are the typical methods.
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