pios_mpxv5004.c

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/* 
 * This program is free software; you can redistribute it and/or modify 
 * it under the terms of the GNU General Public License as published by 
 * the Free Software Foundation; either version 3 of the License, or 
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful, but 
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 
 * for more details.
 * 
 * You should have received a copy of the GNU General Public License along 
 * with this program; if not, see <http://www.gnu.org/licenses/>
 */

/* Project Includes */
#include "pios.h"
#include "physical_constants.h"

#define VCC 5.0f           //Supply voltage in V
#define POWER (2.0f/7.0f)

#if defined(PIOS_INCLUDE_MPXV5004)

#include "pios_mpxv5004.h"

static uint32_t calibrationSum = 0; //static?
static int16_t calibrationOffset; //static?


/*
 * Reads ADC.
 */
uint16_t PIOS_MPXV5004_Measure(uint8_t airspeedADCPin)
{
        return PIOS_ADC_GetChannelRaw(airspeedADCPin);
}

/*
 *Returns zeroPoint so that the user can inspect the calibration vs. the sensor value
 */
uint16_t PIOS_MPXV5004_Calibrate(uint8_t airspeedADCPin, uint16_t calibrationCount){
        calibrationSum +=  PIOS_MPXV5004_Measure(airspeedADCPin);
        uint16_t zeroPoint = (uint16_t)(((float)calibrationSum) / calibrationCount + 0.5f);     

        calibrationOffset = zeroPoint - (int16_t)(1.0f/3.3f*4096.0f+0.5f); //The offset should set the zero point to 1.0V
        
        return zeroPoint;
}


/*
 * Updates the calibration when zero point is manually set by user.
 */
void PIOS_MPXV5004_UpdateCalibration(uint16_t zeroPoint){
        calibrationOffset = zeroPoint - (int16_t)(1.0f/3.3f*4096.0f+0.5f); //The offset should set the zero point to 1.0V
}


/*
 * Reads the airspeed and returns CAS (calibrated airspeed) in the case of success. 
 * In the case of a failed read, returns -1.
 */
float PIOS_MPXV5004_ReadAirspeed(uint8_t airspeedADCPin)
{
        float sensorVal = PIOS_MPXV5004_Measure(airspeedADCPin);
        
        //Calculate dynamic pressure, as per docs
        float Qc = 5.0f*(((sensorVal - calibrationOffset)/4096.0f*3.3f)/VCC - 0.2f);

        //Saturate Qc on the lower bound, in order to make sure we don't have negative airspeeds. No need
        // to saturate on the upper bound, we'll handle that later with calibratedAirspeed.
        if (Qc < 0) {
                Qc=0;
        }
        
        //Compute calibraterd airspeed, as per http://en.wikipedia.org/wiki/Calibrated_airspeed
        float calibratedAirspeed = STANDARD_AIR_MACH_SPEED*sqrtf(5.0f*(powf(Qc/(STANDARD_AIR_SEA_LEVEL_PRESSURE/1000)+1.0f,POWER)-1.0f));
        
        //Upper bound airspeed. No need to lower bound it, that comes from Qc
        if (calibratedAirspeed > 80) { //in [m/s]
                calibratedAirspeed=80;
        }
        
        
        return calibratedAirspeed;
}

#endif /* PIOS_INCLUDE_MPXV5004 */