Add preliminary pressure transducer implementation with INA219 (#30)

* Add preliminary function to read from INA219

* Change INA219 base address and calibration value

* Add pressure transducer abstraction

* Add pressure transducer abstraction

* Extract PT demo to new function

* Extract calibration statement to new()

* Extract calibration statement to new()

* Reformatting

* Remove ina219 module, simplify PressureTransducer

* Add mA to PSI scaling factor

* Adjust terminology

* Fix mA to PSI conversion

* Remove unnecessary return statement

* Make read_current private method

* Use From instead of as to convert to f32

pod-operation/Cargo.toml

@@ -13,4 +13,5 @@ serde = { version = "1.0.192", features = ["derive"] }
 serde_json = "1.0.108"
 tracing = "0.1.40"
 tracing-subscriber = { version = "0.3.17", features = ["env-filter"] }
-rppal = "0.17.1"
+rppal = { version = "0.17.1", features = ["hal"] }
+ina219 = "0.1.0"

pod-operation/src/components/mod.rs

@@ -1 +1,2 @@
+pub mod pressure_transducer;
 pub mod signal_light;

pod-operation/src/components/pressure_transducer.rs

@@ -0,0 +1,57 @@
+use ina219::INA219;
+use rppal::i2c::I2c;
+use tracing::debug;
+
+pub struct PressureTransducer {
+	ina: INA219<I2c>,
+}
+
+// The calibration value is used to adjust the maximum current measurement
+// and precision of measurements.
+const INA219_CALIBRATION_VALUE: u16 = 0xffff;
+
+// Even with the calibration values, the readings from the INA219 are not in
+// mA. A scaling factor is needed in order to convert the raw reading to mA and
+// this is not provided in the INA219 library that we are using. Note that this
+// value changes according to the calibration value. The exact formula can be
+// found in the INA219 datasheet.
+const INA219_SCALING_VALUE: f32 = 160.0;
+
+// The pressure transducer outputs a current between 4 mA and 20 mA with 0 PSI
+// and 300 PSI respectively. Assuming a linear interpolation, a 1 mA increase
+// results in a 18.75 PSI increase.
+const REF_CURRENT_LOW: f32 = 4.0;
+const REF_CURRENT_HIGH: f32 = 20.0;
+const REF_PRESSURE_LOW: f32 = 0.0;
+const REF_PRESSURE_HIGH: f32 = 300.0;
+
+const REF_CURRENT_SPAN: f32 = REF_CURRENT_HIGH - REF_CURRENT_LOW;
+const REF_PRESSURE_SPAN: f32 = REF_PRESSURE_HIGH - REF_PRESSURE_LOW;
+
+impl PressureTransducer {
+	pub fn new(ina219_addr: u8) -> Self {
+		let device = I2c::new().unwrap();
+
+		let mut ina219 = INA219::new(device, ina219_addr);
+		debug!("Initialized I2C and INA219");
+
+		ina219.calibrate(INA219_CALIBRATION_VALUE).unwrap();
+		debug!("Calibrating INA219");
+
+		PressureTransducer { ina: ina219 }
+	}
+
+	// Read current from the INA219 and apply a scaling factor to translate
+	// the current reading to PSI.
+	pub fn read(&mut self) -> f32 {
+		let current = self.read_current();
+
+		REF_PRESSURE_LOW + REF_PRESSURE_SPAN * (current - REF_CURRENT_LOW) / REF_CURRENT_SPAN
+	}
+
+	// Read from the INA219 and divide the reading by a scalar factor to
+	// convert the reading to mA.
+	fn read_current(&mut self) -> f32 {
+		f32::from(self.ina.current().unwrap()) / INA219_SCALING_VALUE
+	}
+}

pod-operation/src/demo.rs

@@ -1,5 +1,6 @@
 use tracing::info;
 
+use crate::components::pressure_transducer::PressureTransducer;
 use crate::components::signal_light::SignalLight;
 
 pub async fn blink(mut signal_light: SignalLight) {
@@ -17,3 +18,12 @@ pub async fn blink(mut signal_light: SignalLight) {
 		i += 1;
 	}
 }
+
+pub async fn read_pressure_transducer(mut pressure_transducer: PressureTransducer) {
+	info!("Starting pressure transducer demo.");
+
+	loop {
+		tokio::time::sleep(std::time::Duration::new(1, 0)).await;
+		println!("{:?}", pressure_transducer.read());
+	}
+}

pod-operation/src/main.rs

@@ -7,6 +7,7 @@ mod components;
 mod demo;
 mod handlers;
 
+use crate::components::pressure_transducer::PressureTransducer;
 use crate::components::signal_light::SignalLight;
 
 #[tokio::main]
@@ -23,6 +24,9 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 	let signal_light = SignalLight::new();
 	tokio::spawn(demo::blink(signal_light));
 
+	let pressure_transducer = PressureTransducer::new(0x40);
+	tokio::spawn(demo::read_pressure_transducer(pressure_transducer));
+
 	let app = axum::Router::new().layer(layer);
 
 	info!("Starting server on port 5000");