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'[EE:] ADXL tilt sensor problems'
2003\12\06@143541 by D Yates

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I've been working with the ADXL202 in a robotics project to measure robot
tilt, but preliminary testing has shown that the acceleration of the robot
is skewing the ADXL's output.  The tilt output has to be accurate within
about +- 2 degrees from level, which is no problem with the ADXL when the
robot is still.  I've heard of accelerometers which don't detect static
acceleration fields like gravity, and was wondering if I could use one of
those to subtract off the bot's acceleration from the ADXL's output?  Has
anybody ever done something like this?  Couldn't find much on google...

Thanks,

D

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2003\12\06@150736 by SavanaPics

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I have never messed with the tilt sensors, but I may have a sugestion. Could
you put in a small delay at the onset of the ssensors reading ? Just enough to
overcome the acceleration.

Eddie, kc4awz

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2003\12\06@152406 by stanton54

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That will only work if you basically ignore the sensor while its speed
is changing.

There is no way for an accelerometer to tell the difference between
gravity and other forms of acceleration. In freefall all the internal
bits of the sensor will have the same acceleration, so it won't detect
gravity (it's more or less a very tiny pendulum). Unfortunately the
rather high acceleration at the bottom tends to be troublesome.

Perhaps you need to add something like a gyroscope?

SavanaPicsspamKILLspamAOL.COM wrote:
> I have never messed with the tilt sensors, but I may have a sugestion. Could
> you put in a small delay at the onset of the ssensors reading ? Just enough to
> overcome the acceleration.
>
> Eddie, kc4awz

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2003\12\06@164252 by Mike Harrison

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On Sat, 6 Dec 2003 15:23:26 -0500, you wrote:

>That will only work if you basically ignore the sensor while its speed
>is changing.
>
>There is no way for an accelerometer to tell the difference between
>gravity and other forms of acceleration. In freefall all the internal
>bits of the sensor will have the same acceleration, so it won't detect
>gravity (it's more or less a very tiny pendulum). Unfortunately the
>rather high acceleration at the bottom tends to be troublesome.
>
>Perhaps you need to add something like a gyroscope?
>
>EraseMESavanaPicsspam_OUTspamTakeThisOuTAOL.COM wrote:
>> I have never messed with the tilt sensors, but I may have a sugestion. Could
>> you put in a small delay at the onset of the ssensors reading ? Just enough to
>> overcome the acceleration.
>>
>> Eddie, kc4awz

You might want to look at electrolytic tilt sensors - these have some inherent damping, and you can
add more in software if necessary, or maybe combine with an accelerometer, e.g.use a weighted
avaraging scheme where the weight of the tilt data is reduced when acceleration is high.

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2003\12\06@170804 by D Yates

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I probably should have included the time response issue in the original
post - sorry!

I initially looked at some electrolytic tilt sensors, but the time response
was far too slow.  The tilt data will be used to control a mechanical system
designed with a 0.1s time response, and we'd like to match the electrical
system response to within roughly half that. This is the primary reason we
are using accelerometers instead of tilt sensors - all the electrolytic
companies I talked to recommended at least a 0.5 second sampling rate, some
as much as 2-3s (PLEASE correct me if you know of anything faster?!).
Apparently they're more designed for relatively smooth transitions, instead
of the jerky movements my project is working with.  We do have an onboard
schaevitz capacitive tilt sensor for "steady and level" output, but it
doesn't seem to work so well for moving readings.

> You might want to look at electrolytic tilt sensors - these have some
inherent damping, and you can
> add more in software if necessary, or maybe combine with an accelerometer,
e.g.use a weighted
> avaraging scheme where the weight of the tilt data is reduced when
acceleration is high.
>
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2003\12\06@221236 by Stephen D. Barnes

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{Quote hidden}

Take a look at the following link. The two wheeled balancing robot (inverted pendulum)
uses the tilt sensor you have, along with a gyro and software filtering to achieve a VERY
stable platform. There are even video clips of the thing operating under less than ideal
conditions without falling over!

Regards,
Stephen D. Barnes

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2003\12\06@234556 by Stephen D. Barnes

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SORRY! I forgot the link! Here it is.
http://www.geology.smu.edu/~dpa-www/robo/nbot/

Regards,
Stephen D. Barnes



{Quote hidden}

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2003\12\07@181420 by Olin Lathrop

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D Yates wrote:
> I've heard of accelerometers
> which don't detect static acceleration fields like gravity,

Let me guess, these were on the same web site selling free energy perpetual
motion machines?


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2003\12\07@183947 by Alexander JJ Rice

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On Sun, 7 Dec 2003 18:10:02 -0500, Olin Lathrop
<olin_piclistEraseMEspam.....EMBEDINC.COM> wrote:

> D Yates wrote:
>> I've heard of accelerometers
>> which don't detect static acceleration fields like gravity,
>
> Let me guess, these were on the same web site selling free energy
> perpetual
> motion machines?

Olin,

Although at first glance this seems silly it is not, sure the
accelerometer does detect static accelerations, but if you low pass filter
it you could always leave just the dynamic portion of the signal which
would effectively do the same thing. The other similar device would be a
rate gyro with an integrator. I am sure both of these systems have found
their way into a single black box that is simply marketed as perfrming the
above feat.

Alex Rice

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2003\12\07@184609 by stanton54

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Sounds a bit like a perpetual motion machine that requires a 9V battery
to run.

Alexander JJ Rice wrote:
{Quote hidden}

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2003\12\07@205025 by D Yates

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----- Original Message -----
From: "stanton54" <RemoveMEstanton54TakeThisOuTspamspamEARTHLINK.NET>
To: <EraseMEPICLISTspamspamspamBeGoneMITVMA.MIT.EDU>
Sent: Sunday, December 07, 2003 5:44 PM
Subject: Re: [EE:] ADXL tilt sensor problems

I looked into the problem some more today (staying away from keeley net) and
believe it or not, such things do exist.  For the skeptics among us,

http://www.gracey.com/downloads/accelerometers.pdf

Unfortunately it still won't work the way I suggested.

D

{Quote hidden}

filter
> > it you could always leave just the dynamic portion of the signal which
> > would effectively do the same thing. The other similar device would be a
> > rate gyro with an integrator. I am sure both of these systems have found
> > their way into a single black box that is simply marketed as perfrming
the
> > above feat.
> >
> > Alex Rice
>
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2003\12\08@025803 by hael Rigby-Jones

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{Quote hidden}

Surely you would use a high pass filter to remove the low frequency/DC
components?

Mike




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2003\12\08@071600 by Alexander JJ Rice

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> Surely you would use a high pass filter to remove the low frequency/DC
> components?
>
> Mike


Yeah, of course - sorry, i was tired.

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2003\12\08@165832 by Peter L. Peres

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>> I've heard of accelerometers
>> which don't detect static acceleration fields like gravity,
>
> Let me guess, these were on the same web site selling free energy
> perpetual motion machines?

No, the ones who sell rate gyros and rate (dynamic) accelerometers. Funny
you're asking that.

Peter

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