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SubjectRe: IBM HDAPS, I need a tip.
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On Mon, 1 Aug 2005, Vojtech Pavlik wrote:

> On Mon, Aug 01, 2005 at 08:55:53AM -0400, linux-os (Dick Johnson) wrote:
>
>>> Jan Engelhardt wrote:
>>>>> So in order to calibrate it you need a readily available source of
>>>>> constant acceleration, preferably with a known value.
>>>>>
>>>>> Hint: -9.8 m/sec^2.
>>>>
>>>> Drop it out of the window? :)
>>>
>>> No, no. Constant gravity (like having the laptop sitting on the desk)
>>> "feels like" constant acceleration.
>>>
>>> Dropping it out of the window should measure 0 m/sec^2, because the
>>> accelerometer is not working on an inertial referential (I hope this is
>>> the correct term in english...). For the accelerometer, this is just
>>> like the feeling of free falling inside an elevator: no gravity :)
>>>
>>> --
>>> Paulo Marques - www.grupopie.com
>>
>> You need a centrifuge or something that works like one. You can
>> make one and you can calibrate it using simple techniques.
>
> Not at all. It's enough to let the laptop lie on the table for [0,0]G
> calibration, then put sequentially it on all the four sides for [-1,0]G,
> [1,0]G, [0,1]G, [0,-1]G calibration.
>

Huh? A laptop on the table is subjected to 1G on earth. If turned over,
is still subjected to 1G, although the sensor may show -1G. Unless
the correct k (calibration factor) is known, for each load direction,
it cannot be derived from +/- 1, the only readings you have, because
of the 1G bias of the load-cell itself.
A pair of load-cells mounted orthagonally, will show the accelleration
in 4 axis. However, you can't assume that if you held the mass at
a 45 degree angle, for instance, that both sensors would read
0.707 (cos 45) or that if at 90, one pair would read 0.0

You need to calibrate using real values. With a sping-scale, and
some room to swing the device, you can readily obtain some accurate
load values.

>> From these five measurements you have both the zero point and the
> slopes, including a good error estimate.
>
> I've done that before when toying with IMUs.
>
> --

> Vojtech Pavlik
> SuSE Labs, SuSE CR
>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.12 on an i686 machine (5537.79 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
I apologize for the following. I tried to kill it with the above dot :

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