accelerometer jitter

[Joel Newkirk]

Thanks - I was just reading through
http://www.freescale.com/files/sensors/doc/app_note/AN3107.pdf about tilt
measurements, which is linked from the wiki. (I'd missed it before)  The
one you linked is mostly about position calculations, double integration of
the acceleration data, although it discusses noise and some management
thereof. (which is much more important with the double-integral)

In this usage, any decrease on one axis should be accompanied by a matching
increase on a second axis, since I'm dealing specifically with vertical or
horizontal orientations. The third axis only becomes significant in the
"lying flat, face up" scenario, where decreases in 'z' will have matching
increases in 'x+y' - noisier summing them, but probably workable. (since
the common use-cases wouldn't demand as frequent updates) Which is actually
the one that got me thinking about this in the first place - I was leveling
my clothes washer and while looking for one of my levels my eyes drifted
toward the Freerunner, and my brain said "hmmmmmmmmm - if I just laid it on
top..."

Which doesn't even consider the second sensor - I'm just looking at the one
with all three axes aligned with the housing, not the one rotated 45
degrees about the Z-axis.  Utilizing both would let me be more certain of
small orientation changes. (I'm assuming noise isn't induced from outside,
and so they wouldn't be in sync)  This is turning out more calculationally
involved than I'd first anticipated, but still looks workable.

j

On Sat, 20 Sep 2008 20:05:19 -0700, Steve Mosher <steve at openmoko.com>
wrote:
> Have a look at this might help.
> 
> Note, we dont use a freescale accel, so some things may be different but
> you'll get the basic idea.
> 
>
http://www.freescale.com/files/sensors/doc/app_note/AN3397.pdf?fpsp=1&WT_TYPE=Application%20Notes&WT_VENDOR=FREESCALE&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation
> 
> 
> Joel Newkirk wrote:
>> On Sat, 20 Sep 2008 17:36:00 -0400, "Michael Fisher"
> <desnotes at gmail.com>
>> wrote:
>>> On Sat, Sep 20, 2008 at 4:07 PM, Joel Newkirk <freerunner at newkirk.us>
>>> wrote:
>>>
>>>> I wrote a short shell script to repeatedly take the raw output from
> one
>>> of
>>>> the accelerometers
>>> (/sys/devices/platform/spi_s3c24xx_gpio.1/spi0.1/dump)
>>>> and massage the data to output X,Y,and Z-axis readings in decimal,
> -128
>>> to
>>>> 128.
>>
>>> As a bit OT but since you have gone to the trouble of developing the
>>> script,
>>> could you run a test while driving/riding in the car and post what the
>>> readings might be? I was thinking of the possibility of having the FR
>>> determine if it was in a moving car or not.
>>>
>>> Mike
>>>
>>> --
>>> Michael Fisher
>>> desNotes at gmail.com
>>
>> No problem, I'll alter the script to intersperse timestamps and take it
> for
>> a ride tomorrow.  It's dumping samples about once per second ATM - I'd
>> initially included a 'sleep 1' command then commented it out since the
> loop
>> execution ended up taking about 1 sec anyway.  (chains of
> cat/grep/sed/cut
>> piped)
>>
>> The one caveat is that I don't have a good mount for it in the car - for
>> the requested test I'll just lay it on the floor on the passenger side
> and
>> note the physical orientation.  (more OT than this request, I found out
>> that the rubberized surface on the back of the Freerunner is essentially
>> non-stick if you try to lay the Freerunner on a Handstands StickyPad -
> I've
>> had one of those on my dashboard for years, always kept my cellphone and
>> Zaurus there even on a sloped dashboard, but the Freerunner won't stick
> to
>> it even on the flat dash in my new car)
>>
>> j
>>
>>
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