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pid: add comments on gain/limit signs and fix period/rate mixup #25

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Feb 9, 2024
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pid: add comments on gain/limit signs and fix period/rate mixup #25

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4 changes: 4 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

* `Dsm`: Delta sigma modulator/noise shaper in MASH-(1)^K architecture.

### Changed

* `Pid`: Fixed period/rate mixup.

## [0.14.1](https://github.com/quartiq/idsp/compare/v0.14.0..v0.14.1) - 2024-01-15

* Fixed changelog
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14 changes: 10 additions & 4 deletions src/iir/pid.rs
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Expand Up @@ -85,6 +85,8 @@ impl<T: Float> Pid<T> {
/// Gains are accurate in the low frequency limit. Towards Nyquist, the
/// frequency response is warped.
///
/// Note that limit signs and gain signs should match.
///
/// ```
/// # use idsp::iir::*;
/// let tau = 1e-3;
Expand All @@ -97,7 +99,7 @@ impl<T: Float> Pid<T> {
/// .into();
/// let x0 = 5.0;
/// let y0 = i.update(&mut [0.0; 4], x0);
/// assert!((y0 / (x0 * ki / tau) - 1.0).abs() < 2.0 * f32::EPSILON);
/// assert!((y0 / (x0 * tau * ki) - 1.0).abs() < 2.0 * f32::EPSILON);
/// ```
///
/// # Arguments
Expand All @@ -113,6 +115,9 @@ impl<T: Float> Pid<T> {
/// Gain limit units are `output/input`. See also [`Pid::gain()`].
/// Multiple gains and limits may interact and lead to peaking.
///
/// Note that limit signs and gain signs should match and that the
/// default limits are positive infinity.
///
/// ```
/// # use idsp::iir::*;
/// let ki_limit = 1e3;
Expand Down Expand Up @@ -175,7 +180,8 @@ impl<T: Float> Pid<T> {
}

// Scale gains, compute limits
let mut zi = self.period.powi(low as i32 - KP as i32);
let mut zi = self.period.powi(KP as i32 - low as i32);
let p = self.period.recip();
let mut gl = [[T::zero(); 2]; 3];
for (gli, (i, (ggi, lli))) in gl.iter_mut().zip(
self.gains
Expand All @@ -186,7 +192,7 @@ impl<T: Float> Pid<T> {
) {
gli[0] = *ggi * zi;
gli[1] = if i == KP { T::one() } else { gli[0] / *lli };
zi = zi * self.period;
zi = zi * p;
}
let a0i = T::one() / (gl[0][1] + gl[1][1] + gl[2][1]);

Expand Down Expand Up @@ -271,7 +277,7 @@ mod test {
let mut xy = [0.0; 4];
for i in 1..10 {
let y_have = b.update(&mut xy, 1.0);
let y_want = (i as f32) * (ki / tau);
let y_want = (i as f32) * tau * ki;
assert!(
(y_have / y_want - 1.0).abs() < 3.0 * f32::EPSILON,
"{i}: have {y_have} != {y_want}"
Expand Down
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