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Pidloop improvements #2922

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Pidloop improvements #2922

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e4fa8f5
Use forward Euler method for PIDLoop integration.
ayzenquwe Mar 9, 2021
c1f4dff
Add unit tests for PIDLoop
ayzenquwe Mar 10, 2021
00d84d7
Add more anti windup modes to PIDLoop
ayzenquwe Mar 10, 2021
a67d707
Derivative part of PID should work measure the error, not input
ayzenquwe Mar 31, 2021
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11 changes: 9 additions & 2 deletions src/kOS.Safe/Encapsulation/PIDLoop.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -80,6 +80,7 @@ public static PIDLoop DeepCopy(PIDLoop source)
DTerm = source.DTerm,
ExtraUnwind = source.ExtraUnwind,
ChangeRate = source.ChangeRate,
lastIError = source.lastIError,
unWinding = source.unWinding
};
return newLoop;
Expand Down Expand Up @@ -119,6 +120,7 @@ public static PIDLoop DeepCopy(PIDLoop source)

public double ChangeRate { get; set; }

private double lastIError;
private bool unWinding;

public PIDLoop()
Expand Down Expand Up @@ -198,7 +200,11 @@ public ScalarValue Update(ScalarValue sampleTime, ScalarValue input)
double pTerm = error * Kp;
double iTerm = 0;
double dTerm = 0;
if (LastSampleTime < sampleTime)
if (LastSampleTime == double.MaxValue)
{
lastIError = error * Ki;
}
else if (LastSampleTime < sampleTime)
{
double dt = sampleTime - LastSampleTime;
if (Ki != 0)
Expand All @@ -219,7 +225,8 @@ public ScalarValue Update(ScalarValue sampleTime, ScalarValue input)
unWinding = false;
}
}
iTerm = ITerm + error * dt * Ki;
iTerm = ITerm + lastIError * dt;
lastIError = error * Ki;
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Can you explain why is it supposed to be better to time-shift the I term one iteration like this so it's using the previous error value instead of the current one from this iteration? The issue #2919 only has one sentence in the description which doesn't explain why it's desired to delay it like this.

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I have added some explanations to PR comments, but I shouldn't have changed the integration method, so I'll revert this.

}
ChangeRate = (input - Input) / dt;
if (Kd != 0)
Expand Down