steinhart_hart: fix calculation, rename parameters

src/steinhart_hart.rs

@@ -3,28 +3,39 @@ use lexical_core::Float;
 /// Steinhart-Hart equation parameters
 #[derive(Clone, Debug)]
 pub struct Parameters {
-    pub a: f64,
-    pub b: f64,
-    pub c: f64,
-    /// Parallel resistance
-    ///
-    /// Not truly part of the equation but required to calculate
-    /// resistance from voltage.
-    pub parallel_r: f64,
+    pub t0: f64,
+    pub r: f64,
+    pub r0: f64,
+
+    r_fact: f64,
 }
 
 impl Parameters {
+    /// Update the cached r_fact
+    pub fn update(&mut self) {
+        self.r_fact = (self.r / self.r0).ln();
+    }
+
     /// Perform the voltage to temperature conversion.
     ///
     /// Result unit: Kelvin
     ///
     /// TODO: verify
-    pub fn get_temperature(&self, voltage: f64) -> f64 {
-        let r = self.parallel_r * voltage;
-        let ln_r = r.abs().ln();
-        let inv_temp = self.a +
-            self.b * ln_r +
-            self.c * ln_r * ln_r * ln_r;
+    pub fn get_temperature(&self, b: f64) -> f64 {
+        let inv_temp = 1.0 / self.t0 + self.r_fact / b;
         1.0 / inv_temp
     }
 }
+
+impl Default for Parameters {
+    fn default() -> Self {
+        let mut p = Parameters {
+            t0: 0.001_4,
+            r: 0.000_000_099,
+            r0: 5_110.0,
+            r_fact: 0.0,
+        };
+        p.update();
+        p
+    }
+}