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more fixes for the airspeed readout

This commit is contained in:
Thomas Gubler 2013-02-24 16:01:08 +01:00
parent 2c2c65d446
commit 2707d2c1dd
4 changed files with 77 additions and 61 deletions
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23
apps/sensors/sensors.cpp
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@ -1025,8 +1025,7 @@ Sensors::adc_poll(struct sensor_combined_s &raw)
} else if (ADC_AIRSPEED_VOLTAGE_CHANNEL == buf_adc[i].am_channel) {
/* calculate airspeed, raw is the difference from */
float voltage = buf_adc[i].am_data / 4096.0f;
float voltage = (float)(buf_adc[i].am_data ) * 3.3f / 4096.0f * 2.0f; //V_ref/4096 * (voltage divider factor)
/**
* The voltage divider pulls the signal down, only act on
@ -1034,21 +1033,23 @@ Sensors::adc_poll(struct sensor_combined_s &raw)
*/
if (voltage > 0.4f) {
float pres_raw = fabsf(voltage - (3.3f / 2.0f));
float pres_mbar = pres_raw * (3.3f / 5.0f) * 10.0f;
// float pres_raw = fabsf(voltage - (3.3f / 2.0f));
// float pres_mbar = pres_raw * (3.3f / 5.0f) * 10.0f;
//XXX depends on sensor used..., where are the above numbers from?
float diff_pres_pa = fabsf(voltage - 2.5f) * 1000.0f; //for MPXV7002DP //xxx: need an offset calibration
float airspeed_true = calc_true_airspeed(pres_mbar + _barometer.pressure,
_barometer.pressure, _barometer.temperature - 5.0f);
float airspeed_true = calc_true_airspeed(diff_pres_pa + _barometer.pressure*1e2f,
_barometer.pressure*1e2f, _barometer.temperature - 5.0f); //factor 1e2 for conversion from mBar to Pa
// XXX HACK - true temperature is much less than indicated temperature in baro,
// subtract 5 degrees in an attempt to account for the electrical upheating of the PCB
float airspeed_indicated = calc_indicated_airspeed(pres_mbar + _barometer.pressure,
_barometer.pressure, _barometer.temperature - 5.0f);
// XXX HACK
float airspeed_indicated = calc_indicated_airspeed(diff_pres_pa);
//printf("voltage: %.4f, diff_pres_pa %.4f, v_ind %.4f, v_true %.4f\n", (double)voltage, (double)diff_pres_pa, (double)airspeed_indicated, (double)airspeed_true);
_differential_pressure.timestamp = hrt_absolute_time();
_differential_pressure.static_pressure_mbar = _barometer.pressure;
_differential_pressure.differential_pressure_mbar = pres_mbar;
_differential_pressure.differential_pressure_mbar = diff_pres_pa*1e-2f;
_differential_pressure.temperature_celcius = _barometer.temperature;
_differential_pressure.indicated_airspeed_m_s = airspeed_indicated;
_differential_pressure.true_airspeed_m_s = airspeed_true;
@ -1064,7 +1065,7 @@ Sensors::adc_poll(struct sensor_combined_s &raw)
}
_last_adc = hrt_absolute_time();
break;
//break;
}
}
}

40
apps/systemlib/airspeed.c
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@ -40,14 +40,25 @@
*
*/
#include "math.h"
#include <stdio.h>
#include <math.h>
#include "conversions.h"
#include "airspeed.h"
float calc_indicated_airspeed(float pressure_front, float pressure_ambient, float temperature)
/**
* 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(float differential_pressure)
{
return sqrtf((2.0f*(pressure_front - pressure_ambient)) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
return sqrtf((2.0f*differential_pressure) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
}
/**
@ -55,14 +66,14 @@ float calc_indicated_airspeed(float pressure_front, float pressure_ambient, floa
*
* Note that the true airspeed is NOT the groundspeed, because of the effects of wind
*
* @param speed current indicated airspeed
* @param speed_indicated current indicated airspeed
* @param pressure_ambient pressure at the side of the tube/airplane
* @param temperature air temperature in degrees celcius
* @param temperature_celsius air temperature in degrees celcius
* @return true airspeed in m/s
*/
float calc_true_airspeed_from_indicated(float speed, float pressure_ambient, float temperature)
float calc_true_airspeed_from_indicated(float speed_indicated, float pressure_ambient, float temperature_celsius)
{
return speed * sqrtf(CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C / get_air_density(pressure_ambient, temperature));
return speed_indicated * sqrtf(CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C / get_air_density(pressure_ambient, temperature_celsius));
}
/**
@ -70,12 +81,17 @@ float calc_true_airspeed_from_indicated(float speed, float pressure_ambient, flo
*
* Note that the true airspeed is NOT the groundspeed, because of the effects of wind
*
* @param pressure_front pressure inside the pitot/prandl tube
* @param pressure_ambient pressure at the side of the tube/airplane
* @param temperature air temperature in degrees celcius
* @param total_pressure pressure inside the pitot/prandl tube
* @param static_pressure pressure at the side of the tube/airplane
* @param temperature_celsius air temperature in degrees celcius
* @return true airspeed in m/s
*/
float calc_true_airspeed(float pressure_front, float pressure_ambient, float temperature)
float calc_true_airspeed(float total_pressure, float static_pressure, float temperature_celsius)
{
return sqrtf((2.0f*(pressure_front - pressure_ambient)) / get_air_density(pressure_ambient, temperature));
float density = get_air_density(static_pressure, temperature_celsius);
if (density < 0.0001f || isnan(density)) {
density = CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C;
printf("Invalid air density, using density at sea level\n");
}
return sqrtf((2.0f*(total_pressure - static_pressure)) / density);
}

27
apps/systemlib/airspeed.h
View File

@ -48,43 +48,42 @@
__BEGIN_DECLS
/**
/**
* 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 pressure_front pressure inside the pitot/prandl tube
* @param pressure_ambient pressure at the side of the tube/airplane
* @param temperature air temperature in degrees celcius
* @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(float pressure_front, float pressure_ambient, float temperature);
__EXPORT float calc_indicated_airspeed(float differential_pressure);
/**
/**
* Calculate true airspeed from indicated airspeed.
*
* Note that the true airspeed is NOT the groundspeed, because of the effects of wind
*
* @param speed current indicated airspeed
* @param speed_indicated current indicated airspeed
* @param pressure_ambient pressure at the side of the tube/airplane
* @param temperature air temperature in degrees celcius
* @param temperature_celsius air temperature in degrees celcius
* @return true airspeed in m/s
*/
__EXPORT float calc_true_airspeed_from_indicated(float speed, float pressure_ambient, float temperature);
__EXPORT float calc_true_airspeed_from_indicated(float speed_indicated, float pressure_ambient, float temperature_celsius);
/**
/**
* Directly calculate true airspeed
*
* Note that the true airspeed is NOT the groundspeed, because of the effects of wind
*
* @param pressure_front pressure inside the pitot/prandl tube
* @param pressure_ambient pressure at the side of the tube/airplane
* @param temperature air temperature in degrees celcius
* @param total_pressure pressure inside the pitot/prandl tube
* @param static_pressure pressure at the side of the tube/airplane
* @param temperature_celsius air temperature in degrees celcius
* @return true airspeed in m/s
*/
__EXPORT float calc_true_airspeed(float pressure_front, float pressure_ambient, float temperature);
__EXPORT float calc_true_airspeed(float total_pressure, float static_pressure, float temperature_celsius);
__END_DECLS

2
apps/uORB/topics/differential_pressure.h
View File

@ -49,7 +49,7 @@
*/
/**
* Battery voltages and status
* Differential pressure and airspeed
*/
struct differential_pressure_s {
uint64_t timestamp; /**< microseconds since system boot, needed to integrate */