vin1391:
Hi all...I wanted to read the pressure values and convert it to Pressure in kPa. We are supposed to use the pressure sensor MPXV7002DP ( data sheet is available here --> www.freescale.com/files/sensors/doc/data_sheet/MPXV7002.pdf )
Ranges are
-2 to 2 kPa (-0.3 to 0.3 psi)
0.5 to 4.5 V OutputWe were told to use the transfer function given in the data sheet. This is the transfer function..
Transfer Function:
Vout = VS × (0.2 × P(kPa)+0.5) ± 6.25% VFSS
VS = 5.0 VdcNow my confusion is
- what is the need for transfer function?
It tells you how the output voltage depends on the pressure. Its a linear relation fortunately.
- can't we use map function to directly map from the bit values to Pressure values?
Because its linear, yes.
- and if we are to use the transfer function what value should I tale for Vfss?
Vfss is full-scale voltage - its just telling you the error is +/- 6.25% of full-scale, you only need that to calculate the error-bar.
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By my assumption I thought that
- We get the pressure analog value in bits
- Convert it to voltage using map function
- Use that Voltage value in the transfer function after rearranging the function to Pressure using the Voltage value
Yup
To get the correct Vfss value we are trying out random values in the range of 3.5 to 4.5 and getting pressure value for this from a sensor as well as a manometer. By comparing which Vfss value gives the similar Pressure reading from Sensor and manometer we try to get the correct value of vfss.
You only need that for error calculations, not to derive the nominal result value.
We tried this but the thing is for whatever air speed we give, the voltage output after bit to voltage conversion remains at 2 volt. Is this how it is supposed to be or?
Don't know if this is what needs to be done. Also we should find the velocity if the air but I did not get to that yet. I am assuming Bernoulli's equation will be enough.
First check that the thing is responding to known static pressure differences, then worry about Bernoilli readings.