# My accelerometers aren't calibrated.

Joel Newkirk freerunner at newkirk.us
Wed Nov 26 04:03:37 CET 2008

```On Wed, 26 Nov 2008 00:12:22 +0100, Mateusz Kondej <mateuszkj at gmail.com>
wrote:
> Hi,
>
> I think that accelerometers in my FR aren't calibrated or driver doesn't
> work properly. For example, if I use script form
> http://wiki.openmoko.org/wiki/Accelerometer_data_retrieval, I got this
> data:
> X	Y	Z	SUM
> 54 -18 -1062 1063
> 54 -18 -1062 1063
> 36 -18 -1062 1062
> 54 -18 -1062 1063
> 54 -18 -1044 1045
> 36 -18 -1080 1080
> FR is laying on my table, screen to bottom. So it should return gravity,
> 981 cm/s^2. But It returns similar values but it's too big for me.
> How to get properly results from accelerometers? Eventualy How to
> calibrate them?
>
> Kondej Mateusz.

To calibrate you need a known standard to work from - you should have that
with the FR laying on a table.  Presuming the FR is somewhere on earth and
stationary with respect to the surface of the earth, the 'SUM' above is 1G.
The variations represent the uncertainty in the data.

If you're writing an application reading them directly and you need as much
accuracy as you can get then you probably want to calibrate by having the
user lay their FR on a table for 30 seconds and figuring out what the
average reading is and what the deviation is.

It's also worth considering using both accelerometers at the same time -
the orthogonal alignment difference can be useful.  You might also find one
is more steady that the other and be better-served by using it alone.

So for your FR it looks like the 'calibration factor' would be
1063(+-18)=1G.  For mine it's 918+-18=1G.  (SUM fluctuates between 900 and
935 for me)

j

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