Scaling?

Mike15

·I am using an ADC to read an accelerometer and I'm trying to scale the input.

P75······· CON $00C0 ' 0.75
P50······· CON $0080 ' 0.50
P25······· CON $0040 ' 0.25
SCLK······ CON 6
CS········ CON 7
SerData_in CON 8
adc······· VAR Word
sValue···· VAR Word
main:
GOSUB convert
GOSUB print
GOTO main

print:
DEBUG HOME
DEBUG "Raw value... ", DEC adc, " ", CR
DEBUG "Filtered.... ", DEC sValue, " "
sValue = adc
RETURN

convert:
LOW CS
SHIFTIN Serdata_in,SCLK,MSBPOST,[noparse][[/noparse]adc\12]
HIGH CS
sValue = (adc */ P25) + (sValue */ P75)
adc = adc */ (?)
sValue = sValue */ (?)
RETURN

I know this my ADC has 4096 counts each count = 1mV and my accelerometer out puts 27 mV per G and has an at rest reading of 2.5V or 2500 counts.

My question is how can I scale this so I get an at rest reading and +/- G's?

·

Jon Williams

[noparse][[/noparse]edit] PJ has a better solution -- here's my version that gets around the PBASIC limitation of no division of negative values:

· IF counts.BIT15 THEN
··· gForce = 0 - ((ABS(counts) - 2500) / 27)
· ELSE
··· gForce = (counts - 2500) / 27
· ENDIF

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Jon Williams
Applications Engineer, Parallax

Post Edited (Jon Williams (Parallax)) : 4/10/2006 11:22:01 PM GMT

PJAllen

0G = 2500 counts

+G [noparse][[/noparse]if counts > 2500] =·counts - 2500 / 27

-G [noparse][[/noparse]if counts < 2500] = 2500 - counts / -27

???



Post Edit --

Jon Williams said...
...no division of negative values

D'Oh!

Post Edited (PJ Allen) : 4/11/2006 1:07:50 AM GMT

Mike15

instead of using 2500 would it be better to take a few at rest samples and average together?
gForce = 0 - ((ABS(counts) - atrest) / 27)

Is it possable to average numbers?

PJAllen

Mike15 said...
Is it possible to average numbers?

Yes.· You could, say,·stash 3 readings into 3 variables, add those up, and divide by 3.
But, remember, with acceleration, time is a big deal.· So, mind what you are averaging.

Post Edited (PJ Allen) : 4/11/2006 1:48:08 PM GMT