ADC question again

[Bobbo_ZA]

Hi Guys, and Lady, :-)

Im no good with maths, :-(

I have a differential pressure sensor and am looking to use it to get an airspeed, once I have the airspeed I can work with it, :-)

Vref = 3.0V
ADC = 10-bit = 1023 steps
Air density @ sea Level = 1.225
Sensor Range = 0.6V per kpa

So, the formula that Im currently trying to get right is as follows:

VelocityKPH VAR WORD
A VAR WORD

ADCIN 4, A ‘will do a loop of 10 samples later

VelocityKPH = (A-186)*36667
VelocityKPH = SQR VelocityKPH
VelocityKPH = VelocityKPH / 18

As I say, my maths is shocking, :-( I kinda understand the principles but dont get how to string it all together, any help will be very much appreciated.

Kind Regards
Rob

[mackrackit]

First thing I see is you will want to use LONGs.

VelocityKPH VAR LONG

You will over run the WORD size variable
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[Bobbo_ZA]

Strange, does not look like we can edit our previous posts in the new forum, :-(

Slight correction was, the sensor range is not 0.6v, :-), rather, sensitivity.

The Zero (min Vout) value is 0.66V, thus I need to remove the first 207 / 208 counts of the AD value in order to get a 0 value.

The sensor range is rather: 0 to 3.92Kpa

Dave, I tried the long Var and PBP complains on compile, help only lists BIT, BYTE and WORD as valid variable types?

Any ideas on what the PBP equivalent equation / formula would be to extrapolate the Kpa to a speed?

In a C / Arduino example using the same sensor (although 5V sensor) is as follows:

;#define Pressure_MTS(x) sqrt((((x*(INTPUT_VOLTAGE/1024.0))/1.0)*2.0)/1.225) //Converts to AirSpeed in meters per second
;#define Pressure_KMH(x) Pressure_MTS(x)*3.6 //converts to KPH

I've been able to get the RAW ad reading as well as the sensor output voltage displaying on the LCD, however I'm lost on the conversion from a ratiometric voltage to a speed, :-(

[Bobbo_ZA]

Well, managed to get a result in excel, :-) but my conversion to "integer" math for PBP does not work, I've tried probably 40 different variations of the code below and am still not getting a result, :-(

The excel formula works as follows:

rho: 1.225 Air Density constant
Zo: 0.75 Zero Offset
Vs: 3260 Voltage supply
Vo: 1.2 Voltage out from sensor (Output from ADC)

Formula1 Dp=(Vs*(Vo/Vs)-Zo)
Formula 2 Mps=SQRT((2*Dp*1000)/rho)

However, translated into PBP I tried the following:

  rho = 1225        'Constant for pressure calc
  Zo = 74           'Zero Offset
  Vs1 = 3260        'Vref
    
  AD_in = 0
  Pres = 0
  volt = 0  
    for samples = 1 to 4      '4 Samples from AD - Sensor is low pass filtered
            ADCIN 4, ad_in
         volt = volt + ad_in
            Pause 25
     Next Samples
     volt = volt / 4          'Average AD
     
volt = volt * 3186            'convert to voltage out
Vind = DIV32 10000            'DIV32

'Excel Formula:
'Dp=(Vs*(Vo/Vs)-Zo)

dy_p = vind * 1000
Dy_p = (vind / Vs1)
dy_p = (dy_p * vind) / 1000
dy_p = dy_p - Zo

LCDOUT $FE, $80+6, dec2 dy_p, "    " 'output to first line

'Excel Formula:
'Mps=SQRT((2*Dp*1000)/rho)

Vmps = dy_p / 1000
vmps = vmps * 2 * 1000
vmps = vmps / (rho/1000)
vmps = sqr vmps

lcdout $FE, $C0+5, dec2 vind        'output to second line

Can anyone help steer me in the right direction? I tried using the DIV32 after the multipys but also then got some unexpected values.

Many Thanks
Rob

[mackrackit]

Sorry for not following up sooner.

LONGs are only available in PBP 2.5 and newer and for 18F parts only. LONGs might make this easier but should not be needed.

First off, do you have

DEFINE ADC_BITS 10
In your code?

Next I have a question about your senso to get started.
The sensor has an output from
0.66 to 3 volts ?
0 to 3.92Kpa ?

[Bobbo_ZA]

Hi Dave,

Appreciate the help, still struggling to get PBP to understand my math, :-)

yip, i do,

;----- NEW LCD Configuration for Testing-------------
DEFINE OSC 20	'20 MHz Osc
DEFINE ADC_BITS 10

yes, the input from the regulator is acutally 3.26v which is just under the sensors "Max", output @ rest should be "Min" = 0.45v Typ = "0.60" and "Max" = 0.75 i assme this is due to the barometric pressure varying.

Range of the sensor is: 0 to 3.92kpa

in excel the maths seems to give me a realistic number based on changing the Vo cell where i would expect the reading from the sensor to be, however the testing has been blowing into the tube so not really very scientific, :-)

Kind Regards
Rob