AP_Airspeed: use raw (signed) airspeed

this allows us to detect when the user has the ports the wrong way
around, and mark the sensor unhealthy

libraries/AP_Airspeed/AP_Airspeed.cpp

@@ -180,8 +180,19 @@ void AP_Airspeed::read(void)
     if (!_enable) {
         return;
     }
-    airspeed_pressure       = get_pressure();
-    airspeed_pressure       = max(airspeed_pressure - _offset, 0);
+    /*
+      when we get negative pressures it means the tubes are probably
+      connected the wrong way around. We don't just use the absolute
+      value as otherwise we could get bad readings in some flight
+      attitudes (eg. a spin) due to pressure on the static port
+     */
+    airspeed_pressure = get_pressure() - _offset;
+    if (airspeed_pressure < -32) {
+        // we're reading more than about -8m/s. The user probably has
+        // the ports the wrong way around
+        _healthy = false;
+    }
+    airspeed_pressure       = max(airspeed_pressure, 0);
     _last_pressure          = airspeed_pressure;
     _raw_airspeed           = sqrtf(airspeed_pressure * _ratio);
     _airspeed               = 0.7f * _airspeed  +  0.3f * _raw_airspeed;

libraries/AP_Airspeed/AP_Airspeed.h

@@ -88,7 +88,7 @@ public:
 
     // return true if airspeed is enabled, and airspeed use is set
     bool        use(void) const {
-        return _enable && _use && _offset > 0 && _healthy;
+        return _enable && _use && fabsf(_offset) > 0 && _healthy;
     }
 
     // return true if airspeed is enabled

libraries/AP_Airspeed/AP_Airspeed_I2C.cpp

@@ -82,8 +82,22 @@ void AP_Airspeed_I2C::_collect(void)
 	dT_raw = (data[2] << 8) + data[3];
 	dT_raw = (0xFFE0 & dT_raw) >> 5;
 
-	_temperature = ((200 * dT_raw) / 2047) - 50;
-	_pressure = fabs(dp_raw - (16384 / 2.0f));
+	const float P_min = -1.0f;
+	const float P_max = 1.0f;
+	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
+
+	  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);
+
+	_pressure = diff_press_PSI * PSI_to_Pa;
+	_temperature = ((200.0f * dT_raw) / 2047) - 50;
+
     _last_sample_time_ms = hal.scheduler->millis();
 }
 

libraries/AP_Airspeed/AP_Airspeed_PX4.cpp

@@ -60,7 +60,7 @@ bool AP_Airspeed_PX4::get_differential_pressure(float &pressure)
     
     while (::read(_fd, &report, sizeof(report)) == sizeof(report) &&
            report.timestamp != _last_timestamp) {
-        psum += report.differential_pressure_pa;
+        psum += report.differential_pressure_raw_pa;
         tsum += report.temperature;
         count++;
         _last_timestamp = report.timestamp;