At 02.37 2012.06.18, you wrote:
>On Sat, Jun 16, 2012 at 4:10 AM, Electron <electron2k4@infinito.it> wrote:
>> On a 3-axial accelerator I compute the magnitude by simple=20
>Pythagorean theorem
>> in 3D, i.e. Mag=3Dsqrt(AccX*AccX+AccY*AccY+AccZ*AccZ)
>>
>> It is an analogue accelerometer with significant RC filters between=20
>its 3 analog
>> outputs and the PIC ADC.
>
>What is the range of your AccX, AccY and AccZ values?

0..1023

>What value gives you zero g?

~512


>What value gives you 1g?

~573

Vibration biases this a lot, around +100 at low RPM and up to +200 at high =
RPM.

I talk about average, i.e. not only there's a lot of AC noise depending on =
RPM,
but also this DC bias component which I was surprised to find, illuding mys=
elf
that the heavy vibrations would have cancelled out anyway (i.e. no DC, wron=
g).


>If you are using A/D counts, 0 to 1023, then you need to use an
>integer data type and subtract 512 so that for an acceleration of zero
>you have an input of zero.

Of course.

So far my deductions (but I hope any of You prove me wrong) is that, simply=
,
MEM accelerometers aren't compatible with high vibration environments, unle=
ss
you find one with a lot of KHz bandwidth and do the filtration in software.

In other words, that vibrations (those coming from the piston of a single
cylinder big bore 2 stroke for example) cannot be cancelled e.g. via RC fil=
ters,
as this will introduce a DC bias.

With kind regards,
Mario


>--
>Martin K.
>--=20
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