pid: integrate time_delta to free gain parameters from sampling period

Fixes Gitea issue #22

README.md

@@ -110,8 +110,8 @@ formatted as line-delimited JSON.
 | `pid`                            | Show PID configuration                                               |
 | `pid <0/1> target <deg_celsius>` | Set the PID controller target temperature                            |
 | `pid <0/1> kp <value>`           | Set proportional gain                                                |
-| `pid <0/1> ki <value>`           | Set integral gain (unit: 10 Hz)                                      |
+| `pid <0/1> ki <value>`           | Set integral gain                                                    |
-| `pid <0/1> kd <value>`           | Set differential gain (unit: 0.1 seconds)                            |
+| `pid <0/1> kd <value>`           | Set differential gain                                                |
 | `pid <0/1> output_min <amp>`     | Set mininum output                                                   |
 | `pid <0/1> output_max <amp>`     | Set maximum output                                                   |
 | `pid <0/1> integral_min <value>` | Set integral lower bound                                             |

src/channel_state.rs

@@ -1,13 +1,15 @@
-use smoltcp::time::Instant;
+use smoltcp::time::{Duration, Instant};
 use uom::si::{
     f64::{
         ElectricPotential,
         ElectricalResistance,
         ThermodynamicTemperature,
+        Time,
     },
     electric_potential::volt,
     electrical_resistance::ohm,
     thermodynamic_temperature::degree_celsius,
+    time::millisecond,
 };
 use crate::{
     ad7172,
@@ -23,6 +25,7 @@ pub struct ChannelState {
     pub adc_data: Option<u32>,
     pub adc_calibration: ad7172::ChannelCalibration,
     pub adc_time: Instant,
+    pub adc_interval: Duration,
     /// VREF for the TEC (1.5V)
     pub vref: ElectricPotential,
     /// i_set 0A center point
@@ -39,6 +42,8 @@ impl ChannelState {
             adc_data: None,
             adc_calibration,
             adc_time: Instant::from_secs(0),
+            // default: 10 Hz
+            adc_interval: Duration::from_millis(100),
             // updated later with Channels.read_vref()
             vref: ElectricPotential::new::<volt>(1.5),
             center: CenterPoint::Vref,
@@ -56,6 +61,7 @@ impl ChannelState {
         } else {
             Some(adc_data)
         };
+        self.adc_interval = now - self.adc_time;
         self.adc_time = now;
     }
 
@@ -63,10 +69,18 @@ impl ChannelState {
     pub fn update_pid(&mut self) -> Option<f64> {
         let temperature = self.get_temperature()?
             .get::<degree_celsius>();
-        let pid_output = self.pid.update(temperature);
+        let pid_output = self.pid.update(temperature, self.get_adc_interval());
         Some(pid_output)
     }
 
+    pub fn get_adc_time(&self) -> Time {
+        Time::new::<millisecond>(self.adc_time.total_millis() as f64)
+    }
+
+    pub fn get_adc_interval(&self) -> Time {
+        Time::new::<millisecond>(self.adc_interval.total_millis() as f64)
+    }
+
     pub fn get_adc(&self) -> Option<ElectricPotential> {
         Some(self.adc_calibration.convert_data(self.adc_data?))
     }

src/channels.rs

@@ -3,7 +3,7 @@ use serde::{Serialize, Serializer};
 use smoltcp::time::Instant;
 use stm32f4xx_hal::hal;
 use uom::si::{
-    f64::{ElectricCurrent, ElectricPotential, ElectricalResistance},
+    f64::{ElectricCurrent, ElectricPotential, ElectricalResistance, Time},
     electric_potential::{millivolt, volt},
     electric_current::ampere,
     electrical_resistance::ohm,
@@ -425,7 +425,8 @@ impl Channels {
         );
         Report {
             channel,
-            time: state.adc_time.total_millis(),
+            time: state.get_adc_time(),
+            interval: state.get_adc_interval(),
             adc: state.get_adc(),
             sens: state.get_sens(),
             temperature: state.get_temperature()
@@ -510,7 +511,8 @@ type JsonBuffer = Vec<u8, U1024>;
 #[derive(Serialize)]
 pub struct Report {
     channel: usize,
-    time: i64,
+    time: Time,
+    interval: Time,
     adc: Option<ElectricPotential>,
     sens: Option<ElectricalResistance>,
     temperature: Option<f64>,

src/pid.rs

@@ -1,4 +1,8 @@
 use serde::{Serialize, Deserialize};
+use uom::si::{
+    f64::Time,
+    time::second,
+};
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Parameters {
@@ -45,7 +49,9 @@ impl Controller {
         }
     }
 
-    pub fn update(&mut self, input: f64) -> f64 {
+    pub fn update(&mut self, input: f64, time_delta: Time) -> f64 {
+        let time_delta = time_delta.get::<second>();
+
         // error
         let error = input - self.target;
 
@@ -53,7 +59,7 @@ impl Controller {
         let p = f64::from(self.parameters.kp) * error;
 
         // integral
-        self.integral += f64::from(self.parameters.ki) * error;
+        self.integral += f64::from(self.parameters.ki) * error * time_delta;
         if self.integral < self.parameters.integral_min.into() {
             self.integral = self.parameters.integral_min.into();
         }
@@ -64,8 +70,10 @@ impl Controller {
 
         // derivative
         let d = match self.last_input {
-            None => 0.0,
+            None =>
-            Some(last_input) => f64::from(self.parameters.kd) * (input - last_input),
+                0.0,
+            Some(last_input) =>
+                f64::from(self.parameters.kd) * (input - last_input) / time_delta,
         };
         self.last_input = Some(input);