slope intercept

J^3

Good Evening all,· I am trying to write this equation y= (-50/41)X + (193700/41), and am having a little trouble.· So far this is what I have come up with, and it is close but I would like my results to be within +- .1 degree.· Any help would be appreciated.· Thank you.


' File:AD592.bsp
' Author:
' Date:2-29-08
' {$STAMP BS2p}
' {$PBASIC 2.5}
'----Program Description

---

'Display temperature in degrees celsius
'----PIN Declaration

---

'----Constants

---

Slope··· CON $0138
Offset·· CON 4724
'----Variables

---

rawvalue VAR Word
adjvalue VAR Word
celsius· VAR Byte
'----EEPROM DATA

---

'----Initialization

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DEBUG CLS
'----Program Code

---

Main:
· DEBUG "Enter rawvalue:"
· DEBUGIN DEC rawvalue
· IF ((rawvalue >=3792) AND (rawvalue <=3874)) THEN
··· adjvalue = rawvalue */ Slope
··· celsius = (Offset - adjvalue) - 3
··· DEBUG "Celsius = ",DEC celsius," Degrees",CR
· ELSE
··· DEBUG "Out of range",CR
· ENDIF
GOTO Main
'----Subroutines

---

skylight

Im not a maths person and what you are asking is way over my head but i wondered if the stamp plot software would do what you want? One of the graphs allows formulas to be input.

Stamp plot software page scroll down for download

phil kenny

I've been able to deduce most of what your code does. From the
file name, I assume you're using an AD592 to measure the temperature
which yields 1 ua per degree K.

Then running the 592 current thru a resistor (value unknown) and a
12-bit ADC generates the variable, "rawvalue".

The constant, $0138, you found by evaluating the fraction 50/41 or
1.219512.

0.219612 x 256 = 56.1951219512

56 decimal = 38 hex

The problem is that the Stamp works with only integers and using 0138 hex
is only a close approximation of the slope. Using 0138 hex introduces a
error in the calculated slope of roughly 0.06 percent.

The error introduce by using 4724 for the offset is less and can be
ignored.

My first thought would be to slightly change the value of the resistor
that converted the AD592 current to voltage. Change this resistor
such that the constant, $0138 gives the slope exactly and not
approximately.

phil

Post Edited (phil kenny) : 3/2/2008 5:23:52 PM GMT

Tracy Allen

Write the equation as ten times:
y= (-500/41)X + (1937000/41)
y = -12.19512 + 47244

Then the multiplier for */ is 12.19512 * 256 = 3122. In PBASIC,

celsius = 47244 - (rawvalue */ 3122) -3 ' not sure why -3
    DEBUG "Celsius = ",REP "-"\celsius.bit15,DEC ABS celsius/10,".",dec1 ABS celsius," Degrees",CR 
   ' in units of xx.x

The display routine works for both + and - temperature, displays to tenths.

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Tracy Allen
www.emesystems.com

J^3

Thank you all for your posts.· The -3; when I first tried my code values were off by 3 consistently so I just said -3.· Don't know if that was the right way to deal with the problem, but it worked,· I will try the times ten tonight and let you all know how it gos.· Once agian thank you very much.

J^3

Mr. Allen, how do you post your code in the little box, instead of copy and paste.

Tracy Allen

The little box, see in the editor window that has all the annoying little emoticons to the left, along the top there are some formatting buttons. Click on the one labled CODE before the stuff you want in the little box and click on it again at the end. Or, you can just enclose the word in square brackets like this [noparse][[/noparse] code]stuff to be in the box[noparse][[/noparse] /code]. In the previous sentence I put a space after the left square bracket, and that prevented it from making the box, but without the space it would be

stuff to be in the box

.

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Tracy Allen
www.emesystems.com

J^3

Thank you

J^3

Got it