use serde::{Deserialize, Serialize}; use super::{abs, copysign, macc, max, min}; use core::f32; /// IIR state and coefficients type. /// /// To represent the IIR state (input and output memory) during the filter update /// this contains the three inputs (x0, x1, x2) and the two outputs (y1, y2) /// concatenated. Lower indices correspond to more recent samples. /// To represent the IIR coefficients, this contains the feed-forward /// coefficients (b0, b1, b2) followd by the negated feed-back coefficients /// (-a1, -a2), all five normalized such that a0 = 1. #[derive(Copy, Clone, Default, Deserialize, Serialize)] pub struct Vec5(pub [f32; 5]); /// IIR configuration. /// /// Contains the coeeficients `ba`, the output offset `y_offset`, and the /// output limits `y_min` and `y_max`. /// /// This implementation achieves several important properties: /// /// * Its transfer function is universal in the sense that any biquadratic /// transfer function can be implemented (high-passes, gain limits, second /// order integrators with inherent anti-windup, notches etc) without code /// changes preserving all features. /// * It inherits a universal implementation of "integrator anti-windup", also /// and especially in the presence of set-point changes and in the presence /// of proportional or derivative gain without any back-off that would reduce /// steady-state output range. /// * It has universal derivative-kick (undesired, unlimited, and un-physical /// amplification of set-point changes by the derivative term) avoidance. /// * An offset at the input of an IIR filter (a.k.a. "set-point") is /// equivalent to an offset at the output. They are related by the /// overall (DC feed-forward) gain of the filter. /// * It stores only previous outputs and inputs. These have direct and /// invariant interpretation (independent of gains and offsets). /// Therefore it can trivially implement bump-less transfer. /// * Cascading multiple IIR filters allows stable and robust /// implementation of transfer functions beyond bequadratic terms. #[derive(Copy, Clone, Default, Deserialize, Serialize)] pub struct IIR { pub ba: Vec5, pub y_offset: f32, pub y_min: f32, pub y_max: f32, } impl IIR { pub const fn new(gain: f32, y_min: f32, y_max: f32) -> Self { Self { ba: Vec5([gain, 0., 0., 0., 0.]), y_offset: 0., y_min, y_max, } } /// Configures IIR filter coefficients for proportional-integral behavior /// with gain limit. /// /// # Arguments /// /// * `kp` - Proportional gain. Also defines gain sign. /// * `ki` - Integral gain at Nyquist. Sign taken from `kp`. /// * `g` - Gain limit. pub fn set_pi(&mut self, kp: f32, ki: f32, g: f32) -> Result<(), &str> { let ki = copysign(ki, kp); let g = copysign(g, kp); let (a1, b0, b1) = if abs(ki) < f32::EPSILON { (0., kp, 0.) } else { let c = if abs(g) < f32::EPSILON { 1. } else { 1. / (1. + ki / g) }; let a1 = 2. * c - 1.; let b0 = ki * c + kp; let b1 = ki * c - a1 * kp; if abs(b0 + b1) < f32::EPSILON { return Err("low integrator gain and/or gain limit"); } (a1, b0, b1) }; self.ba.0.copy_from_slice(&[b0, b1, 0., a1, 0.]); Ok(()) } /// Compute the overall (DC feed-forward) gain. pub fn get_k(&self) -> f32 { self.ba.0[..3].iter().sum() } /// Compute input-referred (`x`) offset from output (`y`) offset. pub fn get_x_offset(&self) -> Result { let k = self.get_k(); if abs(k) < f32::EPSILON { Err("k is zero") } else { Ok(self.y_offset / k) } } /// Convert input (`x`) offset to equivalent output (`y`) offset and apply. /// /// # Arguments /// * `xo`: Input (`x`) offset. pub fn set_x_offset(&mut self, xo: f32) { self.y_offset = xo * self.get_k(); } /// Feed a new input value into the filter, update the filter state, and /// return the new output. Only the state `xy` is modified. /// /// # Arguments /// * `xy` - Current filter state. /// * `x0` - New input. pub fn update(&self, xy: &mut Vec5, x0: f32) -> f32 { let n = self.ba.0.len(); debug_assert!(xy.0.len() == n); // `xy` contains x0 x1 y0 y1 y2 // Increment time x1 x2 y1 y2 y3 // Shift x1 x1 x2 y1 y2 // This unrolls better than xy.rotate_right(1) xy.0.copy_within(0..n - 1, 1); // Store x0 x0 x1 x2 y1 y2 xy.0[0] = x0; // Compute y0 by multiply-accumulate let y0 = macc(self.y_offset, &xy.0, &self.ba.0); // Limit y0 let y0 = max(self.y_min, min(self.y_max, y0)); // Store y0 x0 x1 y0 y1 y2 xy.0[n / 2] = y0; y0 } }