Included raw differential pressure field

src/drivers/meas_airspeed/meas_airspeed.cpp

@@ -206,20 +206,46 @@ MEASAirspeed::collect()
 	dT_raw = (0xFFE0 & dT_raw) >> 5;
 	float temperature = ((200.0f * dT_raw) / 2047) - 50;
 
-	/* calculate differential pressure. As its centered around 8000
-	 * and can go positive or negative, enforce absolute value
-	*/
+	// Calculate differential pressure. As its centered around 8000
+	// and can go positive or negative
 	const float P_min = -1.0f;
 	const float P_max = 1.0f;
-	float diff_press_pa = fabsf((((float)dp_raw - 0.1f * 16383.0f) * (P_max - P_min) / (0.8f * 16383.0f) + P_min) * 6894.8f) - _diff_pres_offset;
+	const float PSI_to_Pa = 6894.757f;
+	/*
+	  this equation is an inversion of the equation in the
+	  pressure transfer function figure on page 4 of the datasheet
 
-	if (diff_press_pa < 0.0f) {
-		diff_press_pa = 0.0f;
-	}
+	  We negate the result so that positive differential pressures
+	  are generated when the bottom port is used as the static
+	  port on the pitot and top port is used as the dynamic port
+	 */
+	float diff_press_PSI = -((dp_raw - 0.1f*16383) * (P_max-P_min)/(0.8f*16383) + P_min);
+	float diff_press_pa_raw = diff_press_PSI * PSI_to_Pa;
 
         // correct for 5V rail voltage if possible
-        voltage_correction(diff_press_pa, temperature);
+        voltage_correction(diff_press_pa_raw, temperature);
 
+	float diff_press_pa = fabsf(diff_press_pa_raw - _diff_pres_offset);
+	
+	/*
+	  note that we return both the absolute value with offset
+	  applied and a raw value without the offset applied. This
+	  makes it possible for higher level code to detect if the
+	  user has the tubes connected backwards, and also makes it
+	  possible to correctly use offsets calculated by a higher
+	  level airspeed driver.
+
+	  With the above calculation the MS4525 sensor will produce a
+	  positive number when the top port is used as a dynamic port
+	  and bottom port is used as the static port
+
+	  Also note that the _diff_pres_offset is applied before the
+	  fabsf() not afterwards. It needs to be done this way to
+	  prevent a bias at low speeds, but this also means that when
+	  setting a offset you must set it based on the raw value, not
+	  the offset value
+	 */
+	
 	struct differential_pressure_s report;
 
 	/* track maximum differential pressure measured (so we can work out top speed). */
@@ -232,6 +258,7 @@ MEASAirspeed::collect()
 	report.temperature = temperature;
 	report.differential_pressure_pa = diff_press_pa;
 	report.differential_pressure_filtered_pa =  _filter.apply(diff_press_pa);
+	report.differential_pressure_raw_pa = diff_press_pa_raw;
 	report.voltage = 0;
 	report.max_differential_pressure_pa = _max_differential_pressure_pa;
 
@@ -328,7 +355,7 @@ MEASAirspeed::voltage_correction(float &diff_press_pa, float &temperature)
 		return;
 	}
 
-	const float slope = 70.0f;
+	const float slope = 65.0f;
 	/*
 	  apply a piecewise linear correction, flattening at 0.5V from 5V
 	 */
@@ -478,7 +505,7 @@ test()
 	}
 
 	warnx("single read");
-	warnx("diff pressure: %8.4f pa", (double)report.differential_pressure_pa);
+	warnx("diff pressure: %d pa", (int)report.differential_pressure_pa);
 
 	/* start the sensor polling at 2Hz */
 	if (OK != ioctl(fd, SENSORIOCSPOLLRATE, 2)) {
@@ -506,7 +533,7 @@ test()
 		}
 
 		warnx("periodic read %u", i);
-		warnx("diff pressure: %8.4f pa", (double)report.differential_pressure_pa);
+		warnx("diff pressure: %d pa", (int)report.differential_pressure_pa);
 		warnx("temperature: %d C (0x%02x)", (int)report.temperature, (unsigned) report.temperature);
 	}
 

src/modules/uORB/topics/differential_pressure.h

@@ -52,13 +52,14 @@
  * Differential pressure.
  */
 struct differential_pressure_s {
-	uint64_t	timestamp;			/**< microseconds since system boot, needed to integrate */
-	uint64_t	error_count;
+	uint64_t	timestamp;			/**< Microseconds since system boot, needed to integrate */
+	uint64_t	error_count;			/**< Number of errors detected by driver */
 	float	differential_pressure_pa;		/**< Differential pressure reading */
+	float	differential_pressure_raw_pa;		/**< Raw differential pressure reading (may be negative) */
 	float	differential_pressure_filtered_pa;	/**< Low pass filtered differential pressure reading */
 	float	max_differential_pressure_pa;		/**< Maximum differential pressure reading */
-	float		voltage;			/**< Voltage from analog airspeed sensors (voltage divider already compensated) */
-	float		temperature;			/**< Temperature provided by sensor, -1000.0f if unknown */
+	float	voltage;				/**< Voltage from analog airspeed sensors (voltage divider already compensated) */
+	float	temperature;				/**< Temperature provided by sensor, -1000.0f if unknown */
 
 };