Airspeed from MPX5500DP differential pressure sensor using a PIC16F88

**marcusvm** · - 9th November 2010, 20:33

Hey All,
I am working on a project to calculate the airspeed from a pitot tube that is connected to a differential pressure sensor (MPX5500DP) and then output it to an LCD screen (44780 Driver) using a PIC16F88. I have very little background in picbasic and I am having a hard time writing the code.

To calculate the velocity from the pressure, I want to use the equation

Airspeed = sqrt(2P/1.2) where 1.2 is the density of air

But, the problem is I need P from my sensor. How would I go about getting this value?

The basic setup is I have the Vout from the sensor going through a non-inverting amplifier (LM741C). So the amplified voltage values range from something like 9V to 13V, depending on what the pressure is on the sensor. I know that I need to turn on the A/D conversion and right justify to get 10-bit results, but I am stuck after that. How do I write the code to take the voltage coming in to the pic and turn that into a value for pressure. I've read several other threads involving pressure sensors but none of them were differential pressure sensors.

My current code is very short, but its all I could manage to get together from my research.

'Identify and set the internal oscillator clock speed (required for PIC16F88)
Define OSC8
OSCCON.4=1
OSCCON.5=1
OSCCON.6=1

'Set Up A/D Conversion
TRISA = %11000000 'sets RA6 and RA7 as inputs and the rest as outputs
ADCON1.7 = 1 'Right justify for 10-bit results
Pause 500 'Wait .5 seconds for warm up

I've attached a wiring diagram of how I would like to wire this set up.

Any help would be greatly appreciated. Let me know if I need to provide more information. Again, I am not very familiar with picbasic so the more simple it is, the better.

Thanks!

-Marcus

**mackrackit** · - 9th November 2010, 20:51

Welcome to the forum.
Here is a similar one. See if it helps.
http://www.picbasic.co.uk/forum/showthread.php?t=13141

**marcusvm** · - 9th November 2010, 22:18

Dave,

Thanks for your help. That makes a huge difference. Few questions for you though. Since this sensor will never hit it's maximum pressure reading at any point, is there a way to cut the max pressure to a tenth of what it is? i.e. the MPX5500DP has a Pmax of 500 kpa, if I cut that down to 50 kpa (500 / 10), would that make any major difference in my calculations?

So, from the copying from the other thread and plugging in my numbers.

"Using the old formula of
y = mx-b
y = output 'Volts
m = slope
x = input units
b = offset
wanting to read form 0 to 500kpa.
Output is 10 to 14 volts.

Start off by looking at the output in volts. I have a 4 volt span. Giving a span of 500 / 4 = 125
m = 125

The offset. Starting at 10 volts. 10 volts = 0 kpa. The offset is 10 * 125 = 1250
1250

Plug all this into the formula and lets say the input is 14 volts.(full scale)
y = (125 * 14) - 1250
y = 500

Now convert this to an 10 bit PIC resolution.

10 bits ADC = 0 to 1023, 1024 steps.
At 14 volts one volt of input will will read 73.
Spanning 4 volts or 292 steps (4 * 73).

New value for m. 500 / 292 = 1.7123
m = 1.7123

The new offset starting at 730, (10 * 73).
730 * 1.7123 = 1250
b = 1250

y = (ADC * 1.7123) - 1250
ADC = 1023
y = 500kpa

ADC is from the sensor."

Correct?

But, then in the code you multiple m by 10,000 and b by 100, what does this do for the values? Is that simple to prevent truncation in the program?

"Code:

' m = 0.00479
' b = 0.98
' y = (ADC * 0.00479) - 0.98
' ADC = 1023
' y = 3.92kpa
M CON 479 'm = 0.00479 * 10,000
B CON 98 'b = 0.98 * 100
ADC VAR WORD 'INPUT FROM SENSOR
Y VAR WORD 'KPA
Z VAR WORD 'DUMMY VAR
Z1 VAR WORD 'DUMMY VAR
'V = SQRT((2 * (Y/1000)) / 1.225)
V VAR WORD 'METERS/SECOND
D CON 122 'AIR DENSITY

START:
ADC = 1023 'FULL SCALE HARD CODED, NO SENSOR
Z = ADC * M
Z1 = DIV32 1000
Y = Z1 - B
Z = (2 * Y * 10000)
Z1 = DIV32 1000
V = SQR(Z1 / D *100)
LCDOUT $FE,1,"KPA= ",DEC Y/100,".",DEC Y//100
LCDOUT $FE,$C0,"M/S= ",DEC V
PAUSE 250
GOTO START"

I really appreciate the help.

-Marcus

**mackrackit** · - 9th November 2010, 23:02

Output is 10 to 14 volts.

Remember that the MCU can not handle 14 volts. 5 is the max. You can use a voltage divider to bring the voltage down though. Do a 3 to 1.

To tighten the readings up you can add a VREF value to the MCU via voltage dividers.

But, then in the code you multiple m by 10,000 and b by 100, what does this do for the values? Is that simple to prevent truncation in the program?

Yes, the decimal constants are converted to whole numbers for calculating.

I will suggest setting up a test board with some pots and play with the ADC some to get a feel for it.

**marcusvm** · - 9th November 2010, 23:16

Oh jeeze. I should have realized that... okay. Well I can fix that by bumping down the gain on the op amp by swapping resistors so that's not a problem. I'll work on this and see if I can't get something working. Thanks for your help! I'll probably be back with more questions later.