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92 lines
3.1 KiB
C
92 lines
3.1 KiB
C
/**
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******************************************************************************
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* @addtogroup PIOS PIOS Core hardware abstraction layer
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* @{
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* @addtogroup PIOS_MPXV Functions
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* @brief Hardware functions to deal with the DIYDrones airspeed kit, using MPXV*.
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* This is a differential sensor, so the value returned is first converted into
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* calibrated airspeed, using http://en.wikipedia.org/wiki/Calibrated_airspeed
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* @{
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*
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* @file pios_mpxv.c
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* @author The OpenPilot Team, http://www.openpilot.org Copyright (C) 2012.
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* @brief ETASV3 Airspeed Sensor Driver
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* @see The GNU Public License (GPL) Version 3
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*
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******************************************************************************/
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/*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with this program; if not, write to the Free Software Foundation, Inc.,
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* 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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*/
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/* Project Includes */
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#include "pios.h"
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#if defined(PIOS_INCLUDE_MPXV)
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#define A0 340.27f //speed of sound at standard sea level in [m/s]
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#define P0 101.325f //static air pressure at standard sea level in kPa
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#define POWER (2.0f/7.0f)
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#include "pios_mpxv.h"
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/*
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* Reads ADC.
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*/
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uint16_t PIOS_MPXV_Measure(PIOS_MPXV_descriptor *desc)
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{
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if (desc)
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return PIOS_ADC_PinGet(desc->airspeedADCPin);
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return 0;
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}
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/*
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*Returns zeroPoint so that the user can inspect the calibration vs. the sensor value
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*/
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uint16_t PIOS_MPXV_Calibrate(PIOS_MPXV_descriptor *desc,uint16_t measurement){
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desc->calibrationSum += measurement;
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desc->calibrationCount++;
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desc->zeroPoint = (uint16_t)(((float)desc->calibrationSum) / desc->calibrationCount);
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return desc->zeroPoint;
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}
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/*
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* Reads the airspeed and returns CAS (calibrated airspeed) in the case of success.
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* In the case of a failed read, returns -1.
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*/
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float PIOS_MPXV_CalcAirspeed(PIOS_MPXV_descriptor *desc,uint16_t measurement)
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{
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//Calculate dynamic pressure, as per docs
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float Qc = 3.3f/4096.0f * (float)(measurement - desc->zeroPoint);
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//Saturate Qc on the lower bound, in order to make sure we don't have negative airspeeds. No need
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// to saturate on the upper bound, we'll handle that later with calibratedAirspeed.
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if (Qc < 0) {
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Qc=0;
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}
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//Compute calibrated airspeed, as per http://en.wikipedia.org/wiki/Calibrated_airspeed
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float calibratedAirspeed = A0*sqrt(5.0f*(pow(Qc/P0+1.0f,POWER)-1.0f));
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//Upper bound airspeed. No need to lower bound it, that comes from Qc
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if (calibratedAirspeed > desc->maxSpeed) { //in [m/s]
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calibratedAirspeed=desc->maxSpeed;
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}
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return calibratedAirspeed;
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}
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#endif /* PIOS_INCLUDE_MPXV */
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