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Stanislaw Flatto electronics forum addict
Joined: 10 Sep 2005
Posts: 74

Posted: Wed Jul 19, 2006 12:06 am Post subject:
Re: Pressure Transducer Linear algorithm needed



William Hightower wrote:
Quote:  I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.

What IS the accuracy of your pressure standard? As I remember from doing
those things this part has to be at least 510 times more accurate than
the calibrated transducer. So in my case it was ordered from
manufacturer for placing at precisely measured gravitional force in the
test group building (67 digits). This was THE standard from which other
standards (mobile) were compared.
Also the precise zeroing of output at start of calibration and its
stability over time is very important.
Then comes the transducers full scale and its +/ FS error promissed by
manufacturer. Readings <30% FS are considered "gossip".
BTW is your transducer absolute or atmospheric as I havn't seen 0
pressure reading in your numbers.
And last, from experience, "on a plane you can draw a straight line
through any three points if the line is thick enough".
The rest is statistics.
Have fun
Stanislaw
Slack user from Ulladulla 

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Bill Sloman electronics forum Guru
Joined: 12 May 2005
Posts: 1080

Posted: Tue Jul 18, 2006 11:25 pm Post subject:
Re: Pressure Transducer Linear algorithm needed



Tim Wescott wrote:
Quote:  Spehro Pefhany wrote:
On Tue, 18 Jul 2006 12:24:06 GMT, the renowned "William Hightower"
william.hightower@earthlink.net> wrote:
I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.
Fitting a simple quadratic polynomial to your data yields:
P = 5.621655 * Vin  0.00047733 * Vin^2 56.02
...and gives an answer that's within +/12 psi over the range.
(+/ 0.35% FS)
Your data looks a mite "noisy" to me. Perhaps better measurements
could yield a better fit.
If not better, then perhaps a whole bunch of them. Assuming that the
noise is zero mean (or at least consistent mean) then you'll effectively
average it out when you do a curve fit.

Noise decreases as the square root of the number of observations being
averaged  better measurements are always the place to start.
One of the traps for young players in curve fitting is trying to fit
too many parameters for the data available. Good curvefitting programs
give error estimates on the parameters allowing for covariance in the
parameters, and this usually makes it obvious when you are trying to go
too far.

Bill Sloman, Nijmegen

Bill Sloman, Nijmegen 

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Tim Wescott electronics forum Guru
Joined: 04 May 2005
Posts: 707

Posted: Tue Jul 18, 2006 4:18 pm Post subject:
Re: Pressure Transducer Linear algorithm needed



Spehro Pefhany wrote:
Quote:  On Tue, 18 Jul 2006 12:24:06 GMT, the renowned "William Hightower"
william.hightower@earthlink.net> wrote:
I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.
Fitting a simple quadratic polynomial to your data yields:
P = 5.621655 * Vin  0.00047733 * Vin^2 56.02
...and gives an answer that's within +/12 psi over the range.
(+/ 0.35% FS)
Your data looks a mite "noisy" to me. Perhaps better measurements
could yield a better fit.
If not better, then perhaps a whole bunch of them. Assuming that the 
noise is zero mean (or at least consistent mean) then you'll effectively
average it out when you do a curve fit.

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html 

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Spehro Pefhany electronics forum Guru
Joined: 01 May 2005
Posts: 2326

Posted: Tue Jul 18, 2006 4:17 pm Post subject:
Re: Pressure Transducer Linear algorithm needed



On Tue, 18 Jul 2006 12:24:06 GMT, the renowned "William Hightower"
<william.hightower@earthlink.net> wrote:
Quote:  I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.

Fitting a simple quadratic polynomial to your data yields:
P = 5.621655 * Vin  0.00047733 * Vin^2 56.02
....and gives an answer that's within +/12 psi over the range.
(+/ 0.35% FS)
Your data looks a mite "noisy" to me. Perhaps better measurements
could yield a better fit.
Best regards,
Spehro Pefhany

"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com 

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John Fields electronics forum Guru
Joined: 24 Mar 2005
Posts: 3260

Posted: Tue Jul 18, 2006 3:22 pm Post subject:
Re: Pressure Transducer Linear algorithm needed



On Tue, 18 Jul 2006 12:24:06 GMT, "William Hightower"
<william.hightower@earthlink.net> wrote:
Quote:  I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.


View in Courier:
++ ++ ++ ++ ++ ++
 XDCR  GAIN  ADC  LUT  DAC  DISPLAY 
++ ++ ++ ++ ++ ++

John Fields
Professional Circuit Designer 

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HKJ electronics forum beginner
Joined: 15 Feb 2005
Posts: 33

Posted: Tue Jul 18, 2006 2:52 pm Post subject:
Re: Pressure Transducer Linear algorithm needed



William Hightower wrote:
Quote:  I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.

Your can do i with a polynom:
MiscEl can calculate the coefficeints:
http://www.miscel.dk/FitTest.png
or a table (MiscEl can also generate a table).
Link to program:
http://www.miscel.dk/MiscEl/miscel.html 

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Mochuelo electronics forum Guru Wannabe
Joined: 10 May 2005
Posts: 128

Posted: Tue Jul 18, 2006 12:57 pm Post subject:
Re: Pressure Transducer Linear algorithm needed



On Tue, 18 Jul 2006 12:24:06 GMT, "William Hightower"
<william.hightower@earthlink.net> wrote:
Quote:  I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.
PSI Output (m volts)
3400 649
3300 629
[...]
22 15.7
20 15.6

http://80.35.195.131/ng/ng_20060718A00.png
That is quite linear, isn't it? 

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William Hightower electronics forum beginner
Joined: 06 Jan 2006
Posts: 8

Posted: Tue Jul 18, 2006 12:24 pm Post subject:
Pressure Transducer Linear algorithm needed



I have a pressure transducer that I am trying to make its output linear to
drive a digital readout.
The digital readout takes a voltage of 0 to 2 Volts. The problem I am having
is Cumming up with an algorithm to crunch the following numbers to feed the
digital display. Any and all help will be greatly appreciated.
PSI Output (m volts)
3400 649
3300 629
3200 610
3100 591
3000 570
2900 550
2800 532
2700 514
2600 495
2500 476
2400 456
2300 437
2200 417
2100 398
2000 380
1900 360
1800 341
1700 322
1600 302
1500 284
1400 265
1300 246
1200 227
1100 208
1000 190
900 171
800 152
700 134
600 115
500 98
400 79
300 60
200 42
100 27.5
98 27.2
96 26.8
94 26.5
92 26.2
90 25.8
88 25.5
86 25.2
84 24.8
82 24.4
80 24.1
78 23.8
76 23.5
74 23.1
72 22.8
70 22.5
68 22.1
66 21.8
64 21.5
62 21.2
60 20.9
58 20.6
56 20.2
54 19.9
52 19.6
50 19.3
48 19
46 18.7
44 18.4
42 18.2
40 17.8
38 17.5
36 17.2
34 16.9
32 16.7
30 16.4
28 16.3
26 16
24 15.8
22 15.7
20 15.6 

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