Airspeed measurement: Add accurate models for dynamic pressure

This allows to get very accurate readings from the SDP3x sensor series from Sensirion using a complete sensor model.
2026-10-16 04:33:34 +08:00 · 2017-09-19 09:01:11 -07:00
parent 9cd915949a
commit b59aefc989
6 changed files with 164 additions and 6 deletions
@@ -45,6 +45,93 @@
 #include <geo/geo.h>
 #include "airspeed.h"

+/**
+ * Calculate indicated airspeed.
+ *
+ * Note that the indicated airspeed is not the true airspeed because it
+ * lacks the air density compensation. Use the calc_true_airspeed functions to get
+ * the true airspeed.
+ *
+ * @param differential_pressure total_ pressure - static pressure
+ * @return indicated airspeed in m/s
+ */
+float calc_indicated_airspeed_corrected(enum AIRSPEED_PITOT_MODEL pmodel, enum AIRSPEED_SENSOR_MODEL smodel,
+					float tube_len, float differential_pressure, float pressure_ambient, float temperature_celsius)
+{
+
+	// air density in kg/m3
+	double rho_air = get_air_density(pressure_ambient, temperature_celsius);
+
+	double dp = fabsf(differential_pressure);
+	// additional dp through pitot tube
+	float dp_pitot;
+	float dp_tube;
+	float dv;
+
+	switch (smodel) {
+
+	case AIRSPEED_SENSOR_MODEL_MEMBRANE: {
+			dp_pitot = 0.0f;
+			dp_tube = 0.0f;
+			dv = 0.0f;
+		}
+		break;
+
+	case AIRSPEED_SENSOR_MODEL_SDP3X: {
+			//
+			double flow_SDP33 = (300.805 - 300.878 / (0.00344205 * dp * 0.68698 * 0.68698 + 1)) * 1.29 / rho_air;
+
+			switch (pmodel) {
+			case AIRSPEED_PITOT_MODEL_HB:
+				dp_pitot = 28557670.0 - 28557670.0 / (1 + (flow_SDP33 / 5027611.0) * 1.227924 * 1.227924);
+				break;
+
+			default:
+				dp_pitot = 0.0f;
+				break;
+			}
+
+			// pressure drop through tube
+			dp_tube = flow_SDP33 * 0.000746124 * (double)tube_len * rho_air;
+
+			// speed at pitot-tube tip due to flow through sensor
+			dv = 0.0331582 * flow_SDP33;
+		}
+		break;
+
+	default: {
+			dp_pitot = 0.0f;
+			dp_tube = 0.0f;
+			dv = 0.0f;
+		}
+		break;
+	}
+
+	// if (!PX4_ISFINITE(dp_tube)) {
+	// 	dp_tube = 0.0f;
+	// }
+
+	// if (!PX4_ISFINITE(dp_pitot)) {
+	// 	dp_pitot = 0.0f;
+	// }
+
+	// if (!PX4_ISFINITE(dv)) {
+	// 	dv = 0.0f;
+	// }
+
+	// sum of all pressure drops
+	float dp_tot = (float)dp + dp_tube + dp_pitot;
+
+	// computed airspeed without correction for inflow-speed at tip of pitot-tube
+	float airspeed_uncorrected = sqrtf(2 * dp_tot / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
+
+	// corrected airspeed
+	float airspeed_corrected = airspeed_uncorrected + dv;
+
+	// return result with correct sign
+	return (differential_pressure > 0.0f) ? airspeed_corrected : -airspeed_corrected;
+}
+

 /**
 * Calculate indicated airspeed.
@@ -59,6 +146,7 @@
 float calc_indicated_airspeed(float differential_pressure)
 {

+
 	if (differential_pressure > 0.0f) {
 		return sqrtf((2.0f * differential_pressure) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);

@@ -1,7 +1,6 @@
 /****************************************************************************
 *
- *   Copyright (C) 2012-2013 PX4 Development Team. All rights reserved.
- *   Author: Lorenz Meier <lm@inf.ethz.ch>
+ *   Copyright (c) 2012-2013, 2017 PX4 Development Team. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
@@ -36,7 +35,7 @@
 * @file airspeed.h
 * Airspeed estimation declarations
 *
- * @author Lorenz Meier <lm@inf.ethz.ch>
+ * @author Lorenz Meier <lorenz@px4.io>
 *
 */

@@ -48,6 +47,29 @@

 __BEGIN_DECLS

+enum AIRSPEED_SENSOR_MODEL {
+	AIRSPEED_SENSOR_MODEL_MEMBRANE = 0,
+	AIRSPEED_SENSOR_MODEL_SDP3X,
+};
+
+enum AIRSPEED_PITOT_MODEL {
+	AIRSPEED_PITOT_MODEL_HB = 0
+};
+
+/**
+ * Calculate indicated airspeed.
+ *
+ * Note that the indicated airspeed is not the true airspeed because it
+ * lacks the air density compensation. Use the calc_true_airspeed functions to get
+ * the true airspeed.
+ *
+ * @param total_pressure pressure inside the pitot/prandtl tube
+ * @param static_pressure pressure at the side of the tube/airplane
+ * @return indicated airspeed in m/s
+ */
+__EXPORT float calc_indicated_airspeed_corrected(enum AIRSPEED_PITOT_MODEL pmodel, enum AIRSPEED_SENSOR_MODEL smodel,
+		float tube_len, float differential_pressure, float pressure_ambient, float temperature_celsius);
+
 /**
 * Calculate indicated airspeed.
 *