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non linear signal to linear signal

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chandu13

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Hai

Here aim measuring the vacuum in the chamber and displaying on the LCD using 89c51.

For this display aim using look up table method .It is taking a lot of memory for storing table values due to non linearity.

My vacuum range is 1.0E+3 to 1.0E-3.

How to convert non linear signal to linear signal.

Regards

chandu
 
There are really two methods, a lookup table, or a calculation - with a lookup table probably been the simplest, the most accurate, and by FAR the fastest. Can you derive a formula for the conversion?, and is the length of the table actually causing you problems? - if not why worry about it?.
 
If your PIC isn't fast enough then you could considere building the calculation in the form of analogue hardware but it won't be very accurate.
 
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hi chandu,

Depending upon the accuracy you require, you could consider a 'chord' error curve correction table in memory.

Rather than a correction table for every possible point on the curve.

That is, approximate the non-linear curve with 'chords' in a rom table.

Are you familair with this technique?

EricG
 
Eric, could you please elaborate?

I am really interested (and about to start doing something on that area).

Gracias
 
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I believe what Eric is talking about is piecewise linearization?
If you google on that, you should get lots.
Microchip has an app note on using a Programmable Gain Amplifier to do the same thing:
Thermistor Temperature Sensing with MCP6S2X PGA = An 897
**broken link removed**
 
hi chandu, etferrari

Attached a very simple use of chords. I hope it explaines the principle.

An easy way to generate your table values, is to write a VB [or equiv] program that evaluates the non linear formula values and prints them into a text box.

Copy/Paste and Edit into your coding.

If you are using a PIC with an 10 bit ADC value, most likely 255 chords, table length should be OK.

Makes the 'within range' comparisons easier if a byte is used.

If this does not explain well enough, let me know.

Eric

Yes BeeBop, thats the idea,
 
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Duly understood. Gracias!
 
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