Hey there,
I am trying to measure air speed with this differential pressure sensor connected to a pitot tube: http://www.holybro.com/product/digital-air-speed-sensor/
And here is the datasheet of the sensor: http://www.holybro.com/manual/Air_Speed_Sensor_Datasheet_MS4525DO.pdf
The sensor communicates over i2c and I receive data, but have an issue with the sensitivity. The sensor only sends output data in steps of seven, so I get data like 8094, 8101, 8108. I suspected the problem was due to too short time between sensor polling and reading, but couldn't fix it with a delay after requesting data.
I think the calculation is not quite right, but the issue is also in the raw data, so first I need to get it right there.
Did somebody have the same issue and knows what to do?
#include <Wire.h> //I2C library 0x28H
#include <stdint.h> //Standard C, allows explicit data type decleration.
byte fetch_pressure(unsigned int *p_Pressure); //convert value to byte data type
//MS4525D sensor characteristic
const uint8_t MS4525DAddress = 0x28;
const int16_t MS4525FullScaleRange = 1; //1 psi
//const int16_t MS4525MinScaleCounts = 1638;
//const int16_t MS4525FullScaleCounts = 14746;
const int16_t MS4525MinScaleCounts = 1617; //Voff
const int16_t MS4525FullScaleCounts = 14569.2; //Vfso
const int16_t MS4525Span = MS4525FullScaleCounts - MS4525MinScaleCounts; //span
const int16_t MS4525ZeroCounts = (MS4525MinScaleCounts + MS4525FullScaleCounts)/2; //?
byte _status; // A two bit field indicating the status of the I2C read
uint16_t P_dat, T_dat; // 14 bit pressure data and 11 bit temperature data
float psi, Vms, Vkmh, psioff;
byte fetch_pressure(uint16_t &P_dat, uint16_t &T_dat)
{
byte _status, Press_H, Press_L, Temp_H, Temp_L;
Wire.beginTransmission(MS4525DAddress);
// Wire.endTransmission(); // Stop transmitting
delay(100);
Wire.requestFrom(MS4525DAddress, static_cast<uint8_t>(4), static_cast<uint8_t>(false)); // Request 4 bytes, 2 pressure/status and 2 temprature.
delay(100);
Press_H = Wire.read();
Press_L = Wire.read();
Temp_H = Wire.read();
Temp_L = Wire.read();
Wire.endTransmission();
_status = (Press_H >> 6) & 0x03;
Press_H = Press_H & 0x3f;
P_dat = (((uint16_t)Press_H) << 8 ) | Press_L;
Temp_L = (Temp_L >> 5);
T_dat = (((uint16_t)Temp_H) << 3) | Temp_L;
return _status;
}
void setup()
{
Serial.begin(9600);
Wire.begin();
while(! Serial)
{
delay(1);
}
Serial.println("MS4525D0 test");
_status = fetch_pressure(P_dat,T_dat);
Serial.print("P_raw Offset: ");
Serial.print(P_dat);
psioff = abs((static_cast<float>(static_cast<int16_t>(P_dat)-MS4525ZeroCounts))/static_cast<float>(MS4525Span)*static_cast<float>(MS4525FullScaleRange));
Serial.print("\t");
Serial.print("Psi Offset: ");
Serial.println(psioff,6);
}
void loop()
{
_status = fetch_pressure(P_dat,T_dat);
switch (_status)
{
case 0: //Serial.println("Ok ");
break;
case 1: Serial.println("Busy");
break;
case 2: Serial.println("Slate");
break;
default: Serial.println("Error");
break;
}
//Berechnung Druck
psi =(static_cast<float>(static_cast<int16_t>(P_dat)-MS4525ZeroCounts))/static_cast<float>(MS4525Span)*static_cast<float>(MS4525FullScaleRange)-psioff;
psi = abs(psi);
//Berechnung Geschwindigkeit
Vms = sqrt((psi*13789.5144)/1.225);
Vkmh = Vms*3.6;
//Ausgabe im seriellen Monitor
Serial.print("psioff: ");Serial.print(psioff,6);
Serial.print(" , ");
Serial.print("raw Pressure: ");Serial.print(P_dat);
Serial.print(" , ");
Serial.print("pressure psi: ");Serial.print(psi,6);
Serial.print(" , ");
Serial.print("speed m/s: ");Serial.print(Vms,2);
Serial.print(" , ");
Serial.print("speed km/h: ");Serial.println(Vkmh,2);
delay(1000);
}
