px4_autopilot/apps/systemlib/airspeed.c

/****************************************************************************
 *
 *   Copyright (C) 2012-2013 PX4 Development Team. All rights reserved.
 *   Author: Lorenz Meier <lm@inf.ethz.ch>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name PX4 nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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/**
 * @file airspeed.c
 * Airspeed estimation
 *
 * @author Lorenz Meier <lm@inf.ethz.ch>
 *
 */

#include <stdio.h>
#include <math.h>
#include "conversions.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(float differential_pressure)
{

	if (differential_pressure > 0) {
		return sqrtf((2.0f*differential_pressure) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
	} else {
		return -sqrtf((2.0f*fabs(differential_pressure)) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
	}

}
 
/**
 * Calculate true airspeed from indicated airspeed.
 *
 * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
 *
 * @param speed_indicated current indicated airspeed
 * @param pressure_ambient 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_from_indicated(float speed_indicated, float pressure_ambient, float temperature_celsius)
{
	return speed_indicated * sqrtf(CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C / get_air_density(pressure_ambient, temperature_celsius));
}
 
/**
 * Directly calculate true airspeed
 *
 * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
 *
 * @param total_pressure pressure inside the pitot/prandtl 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 total_pressure, float static_pressure, float temperature_celsius)
{
	float density = get_air_density(static_pressure, temperature_celsius);
	if (density < 0.0001f || !isfinite(density)) {
	 density = CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C;
	 printf("[airspeed] Invalid air density, using density at sea level\n");
	}

	float pressure_difference = total_pressure - static_pressure;

	if(pressure_difference > 0) {
		return sqrtf((2.0f*(pressure_difference)) / density);
	} else
	{
		return -sqrtf((2.0f*fabs(pressure_difference)) / density);
	}
}