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georpo
Joined: 18 Nov 2008
Posts: 281
Location: Athens, Greece.
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| table results |
 Posted: Tue Feb 23, 2010 3:35 pm |
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Hello!
I am using a KTY84 temperature sensor to make a thermometer.
This changes its resistance according to the temperature.
I have some given values from the datasheet for 0,10,20,25,30.....290,300 degrees celcius in 10 degrees steps except for the 25 degrees that I have a given value also.
So, I did this:
ADC reading---temperature to display
| Code: |
if (value==339) return(0);
else if (value==357) return(10);
else if (value==375) return(20);
else if (value==384) return(25);
else if (value==393) return(30);
else if (value==411) return(40);
else if (value==429) return(50);
else if (value==445) return(60);
else if (value==462) return(70);
else if (value==478) return(80);
else if (value==495) return(90);
else if (value==511) return(100);
else if (value==526) return(110);
else if (value==540) return(120);
else if (value==556) return(130);
else if (value==571) return(140);
else if (value==584) return(150);
else if (value==598) return(160);
else if (value==609) return(170);
else if (value==623) return(180);
else if (value==635) return(190);
else if (value==647) return(200);
else if (value==658) return(210);
else if (value==667) return(220);
else if (value==680) return(230);
else if (value==687) return(240);
else if (value==698) return(250);
else if (value==708) return(260);
else if (value==716) return(270);
else if (value==726) return(280);
else if (value==734) return(290);
else if (value==740) return(300);
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Of course this is very simple and it works but how can I calculate the values between these? I want to have a result for every single degree not for every ten degrees... 
Thanks. |
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jaimechacoff
Joined: 14 Feb 2010
Posts: 24
Location: Santiago, Chile
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| Posted: Tue Feb 23, 2010 3:48 pm |
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you can convert all those points in a polynomial function.
I use to use Cubic Spline, that means that you will find a function and it will cross each one of the point of your table, and of course, between each point as well.
read about:
http://en.wikipedia.org/wiki/Spline_interpolation
its pretty much easy.
good luck. |
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jaimechacoff
Joined: 14 Feb 2010
Posts: 24
Location: Santiago, Chile
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| Posted: Tue Feb 23, 2010 3:52 pm |
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ok, I did it very faster and i believe is an exponential function, of course, with the Spline you will find a polynomial, and insted of all those "else if" you put the function : )))))))))))
Last edited by jaimechacoff on Tue Feb 23, 2010 3:54 pm; edited 1 time in total |
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georpo
Joined: 18 Nov 2008
Posts: 281
Location: Athens, Greece.
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| Posted: Tue Feb 23, 2010 3:53 pm |
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Thanks but I think that this it way too "math" for me.
I will study this anyway. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Tue Feb 23, 2010 3:54 pm |
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It would be far better to do it with an equation. But if you don't want to
do that (or you can't), then you can use the "brute force" method of one
big look-up table. Example:
| Code: |
#include <16F877.H>
#fuses XT, NOWDT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)
const int16 temp_table[] =
{
0, // 339 // Lowest input temperature
1, // 340
2, // 341
2, // 342
3, // 343
3, // 344
4, // 345
4, // 346
5, // 347
5, // 348
6, // 349
6, // 350
7, // 351
7, // 352
8, // 353
8, // 354
9, // 355
9, // 356
10, // 357
// Add more table entries, from 358 up to 740.
};
int16 adjust_temp(int16 temperature)
{
int16 retval;
// Force temperature to be within limits of lookup table.
if(temperature < 339)
temperature = 339;
if(temperature > 740)
temperature = 740;
// Remove offset
temperature -= 339;
// Get adjusted value from table.
retval = temp_table[temperature];
return(retval);
}
//-------------------------
// Substitute your actual sensor driver code
// for this routine.
int16 get_temp(void)
{
return(348);
}
//======================================
void main()
{
int16 raw_temperature;
int16 final_temperature;
while(1)
{
raw_temperature = get_temp(); // Read the sensor
final_temperature = adjust_temp(raw_temperature);
printf("Temperature = %lu \n\r", final_temperature);
}
}
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jaimechacoff
Joined: 14 Feb 2010
Posts: 24
Location: Santiago, Chile
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| Posted: Tue Feb 23, 2010 3:57 pm |
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PCM already did your work ; )))
but really, try fitting a equation, even you can do it with excel : ) |
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georpo
Joined: 18 Nov 2008
Posts: 281
Location: Athens, Greece.
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| Posted: Tue Feb 23, 2010 3:58 pm |
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Thanks PCM programmer.
I want to avoid this method of a huge table with 300 values.
I prefer to use a function which takes the 2 values and gives me the values between them. |
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jaimechacoff
Joined: 14 Feb 2010
Posts: 24
Location: Santiago, Chile
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| Posted: Tue Feb 23, 2010 4:02 pm |
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I got this equation:
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Return = 0.0008value**2 - 0.1224value - 42.736
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Its not actually correct but fits the point you gave us in the original post, now its up to you do it better ; ) |
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georpo
Joined: 18 Nov 2008
Posts: 281
Location: Athens, Greece.
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| Posted: Tue Feb 23, 2010 4:09 pm |
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I am not sure of how to use this. I am sorry. 
As for the table option the PIC16F877A can not hold 300 values in ram. |
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jaimechacoff
Joined: 14 Feb 2010
Posts: 24
Location: Santiago, Chile
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| Posted: Tue Feb 23, 2010 4:17 pm |
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first that all, you should do a kind of simulation, Excel can help you a lot.
Make a graph of the equation, test if all your point are in the curve, or at least close (your know which is your better accuracy), then you will know what you will have and just then you will be able to programm it ... just be careful ; ) |
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John P
Joined: 17 Sep 2003
Posts: 331
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| Posted: Tue Feb 23, 2010 11:23 pm |
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| You can't get 300 values in RAM, but you can in ROM. |
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mutthunaveen
Joined: 08 Apr 2009
Posts: 100
Location: Chennai, India
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| interpolation method is very useful for u |
| Posted: Wed Feb 24, 2010 5:13 am |
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first of all
use MSEXCEL and plot the graph with your tabel values
then, graph will be displayed. split the graph by 4 or 5 parts, according to the slopes.
extract the X and Y cordinates of 5 parts
apply them in the interpolation formula. derieve the formula and load into ur chip.....
this will do all the work....................... 
--------------
hard work makes all |
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georpo
Joined: 18 Nov 2008
Posts: 281
Location: Athens, Greece.
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| Posted: Wed Feb 24, 2010 8:49 am |
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Thank you all for the support.
I do not know how to do this in excel so I have to follow PCM programmer's idea to write all 300 values manually. 
Thanks! |
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