Calculate current using linear fit from measurements

scripts/current_fit.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+# coding: utf-8
+
+import pylab as p
+
+data = [
+        # SolarLader, Multimeter
+        [0, 0],
+        [0.412, 0.388],
+        [0.421, 0.392],
+        [0.64,  0.591],
+        [0.72,  0.655],
+        [0.78,  0.705],
+        [1,     0.903],
+        [1.05,  0.928],
+        [1.2,   1.04],
+        [1.36,  1.17],
+        [1.4,   1.205],
+        [1.706, 1.470],
+        [1.93,  1.58],
+        [2,     1.661],
+        [2.01,  1.683],
+        [2.08,  1.69],
+        [2.22,  1.78],
+        [2.27,  1.815],
+    ]
+
+data_array = p.transpose(p.array(data))
+
+Ichg = data_array[0]
+Imulti = data_array[1]
+
+adc = Ichg * 4096/8.6 + 89
+
+fit = p.polyfit(adc, Imulti, 1)
+
+poly = p.poly1d(fit)
+
+#Ifit = adc**2 * fit[0] + adc * fit[1] + fit[2]
+Ifit = poly(adc)
+Ierror = Ifit - Imulti
+
+adc_expanded = p.arange(0, 4095, 1)
+Ifit_expanded = poly(adc_expanded)
+
+#print("adc² • {:.3f} + adc • {:.3f} + {:.3f}".format(fit[0], fit[1], fit[2]))
+print("adc • {:.3g} + {:.3g}".format(fit[0], fit[1]))
+
+p.figure()
+p.plot(adc, Imulti, 'rx')
+p.plot(adc, Ifit, 'r-')
+p.figure()
+p.plot(adc, Imulti, 'rx')
+p.plot(adc_expanded, Ifit_expanded, 'b-')
+p.figure()
+p.plot(adc, Ierror)
+p.show()

src/main.c

@@ -26,7 +26,7 @@
 #define MAX_SLEEP_TIME 3600
 #define MAX_SLEEP_TIME_LOW_VOLTAGE 300
 
-#define CURRENT_ADC_OFFSET 89
+#define ADC_VALUE_AT_ZERO_CURRENT 90
 
 enum OperState {
 	Bootstrap,
@@ -562,12 +562,14 @@ int main(void)
 	// Calculated values
 	//fxp_t VIN_SCALE = fxp_from_float(3.3f * (100 + 10.0f) / 10.0f / 4095.0f);
 	//fxp_t VOUT_SCALE = fxp_from_float(3.3f * (100 + 12.0f) / 12.0f / 4095.0f);
-	//fxp_t CURRENT_SCALE = fxp_from_float(9.7f / 4095.0f);
 
 	// Calibrated from measurements
 	fxp_t VIN_SCALE = fxp_from_float(36.41f / 4096.0f);
 	fxp_t VOUT_SCALE = fxp_from_float(30.87f / 4096.0f);
-	fxp_t CURRENT_SCALE = fxp_from_float(8.60f / 4096.0f);
+
+	// current = adc • 0.00166 + -0.0725 = adc • m + t
+	fxp_t ADC2CURRENT_M = fxp_from_float( 0.00166f);
+	fxp_t ADC2CURRENT_T = fxp_from_float(-0.07250f);
 
 	/* if power changes by more than this factor, MPP is tested again */
 	MPP_MAX_POWER_CHANGE_FACTOR = fxp_from_float(0.2f);
@@ -664,7 +666,13 @@ int main(void)
 		// convert read values
 		power_state.vin     = fxp_mult(fxp_from_int(adc_values[0]), VIN_SCALE);
 		power_state.vout    = fxp_mult(fxp_from_int(adc_values[1]), VOUT_SCALE);
-		power_state.current = fxp_mult(fxp_from_int(adc_values[2] - CURRENT_ADC_OFFSET), CURRENT_SCALE);
+
+		if(adc_values[2] <= ADC_VALUE_AT_ZERO_CURRENT+3) {
+			power_state.current = 0;
+		} else {
+			// current = adc • m + t
+			power_state.current = fxp_add(fxp_mult(fxp_from_int(adc_values[2]), ADC2CURRENT_M), ADC2CURRENT_T);
+		}
 
 		power_state.vin_avg     = fxp_add(fxp_mult(power_state.vin, AVG_FACT),     fxp_mult(power_state.vin_avg, AVG_FACT_INV));
 		power_state.vout_avg    = fxp_add(fxp_mult(power_state.vout, AVG_FACT),    fxp_mult(power_state.vout_avg, AVG_FACT_INV));