PID controller integral term causing extreme instability

Question

I have a PID controller running on a robot that is designed to make the robot steer onto a compass heading. The PID correction is recalculated/applied at a rate of 20Hz.

Answer 1

The integral term is already accumulated over time, multiplying by deltaTime will make it accumulate at a rate of time-squared. In fact since SteadyError is already erroneously calculated by multiplying error by deltaTime, that is time-cubed!

In SteadyError, if you are trying to compensate for an aperiodic update, it would be better to fix the aperiodicity. However, the calculation is flawed in any case. You have calculated in units of error/time whereas you want just error units. The arithmentiaclly correct way to compensate for timing jitter if really necessary would be:

SteadyError += (error * 50.0f/deltaTime);

if deltaTime remains in milliseconds and the nominal update rate is 20Hz. However deltaTime would be better calculated as a float or not converted to milliseconds at all if it is timing jitter you are trying to detect; you are needlessly discarding precision. Either way what you need is to modify the error value by the ratio of nominal time to actual time.

A good read is PID without a PhD