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From: Richard Owlett on 29 Dec 2009 12:25
The OP's replies snipped in the wrong places causing some of my
comments to be attributed to others. May have been artifact of
posting thru DSPRelated.com usenet HTML frontend.

I've reconstructed relevant portions of the exchange to put my
comments in context.

Richard Owlett wrote:
> Rune Allnor wrote:
>> On 28 Des, 06:06, "haoxiang" <haoxiang1...(a)hotmail.com> wrote:
>>> Hi all,
>>>
>>> I am working on a project on human static standing and i am measuring
>>> the angular body sway of a human when he/she is standing still. I am
>>> using
>>> a tri-axis accelerometer
>>> (http://www.sparkfun.com/commerce/product_info.php?products_id=8563) and
>>> obtaining angular displacement estimate based on double integration of
>>> acceleration reading and some least-square estimates. Unfortunately my
>>> experiment duration need to be at least 1 minute and the drift error
>>> causes
>>> the estimate to be way off the mark... It doesn't help that i can't
>>> exactly
>>> pinpoint my initial acceleration.
>>>
>>> Can anyone suggest any method to reduce the error in my
>>> measurement? My
>>> prof suggested calibrating the result which another camera system which
>>> simultaneously track position but as time is tight, i would hope to find
>>> solution based solely on accelerometer and parameters i have.... Hope to
>>> hear good news soon
>>
>> Don't expect any fast/simple/easy/convenient solutions...
>>
>> I would *guess* that part of the problem is that humans
>> tend to rotate when swaying. So if you fix the sensor
>> to the human body, the device which is designed to
>> measure purely translational movements, will also be
>> subjected to rotations. Which would severely mess up
>> the data if not accounted for.
>>
>> And no, I have no idea how to correct for such effects
>> based on the data you have.
>>
>> Rune
>
> I would suggest he run his experiment with an idealized subject - a
> pendulum.
>
> A Google search for his accelerometer would also prove informative (I
> tried it)
>

To which haoxiang replied:

> I have tried placing the accelerometer on a flat ground and
> measure the accelerations and double integrate to obtain
> position estimate. Since initial acceleration is unknown i
> assume as 0.


Bad assumption ;)
Hint - Think about how the accelerometer is built.


> With this setup, i still obtain a significant
> error in position estimate after elapsed time of 1 > min.
>
> Even with a pendulum, i would assume the drift error would be
> the same? I need some method to calibrate the measurement i
> guess.
>

Think about what is common about your professor's suggestion and
my pendulum suggestion.


> For eg, since when accelerometer is placed on flat ground there
> should be no position displacement hence i should calibrate the
> error result with 0 or something along that line?
>


Richard Owlett commented:

> ... the chip manufacturer has an app note for a related
> application. I'll have to wait to get to a high speed
> connection to download and read it.

To which Hao Xiang replied:

> Can you attach the link of this application? I can't seem
> to find it. It'll be a great help to me.
>

I could, but for pedagogical reasons I will not.
What were your Google search terms?
Remember the only thing I had to go on was your original post.

Did you use http://www.google.com/ ?
I have found that occasionally European servers give a different
pattern of results. The useful links I found were in the first 20
or so hits.

HTH



From: Frnak McKenney on 30 Dec 2009 09:44
On Tue, 29 Dec 2009 14:57:16 -0600, Richard Owlett <rowlett(a)pcnetinc.com> wrote:
> Tim Wescott wrote:
>> On Tue, 29 Dec 2009 14:50:13 -0500, Jerry Avins wrote:
>>
>>> Tim Wescott wrote:
>>>> On Sun, 27 Dec 2009 23:06:45 -0600, haoxiang wrote:
>>>>> I am working on a project on human static standing and i am
>>>>> measuring
>>>>> the angular body sway of a human when he/she is standing still. I am
>>>>> using a tri-axis accelerometer
>>>>> (http://www.sparkfun.com/commerce/product_info.php?products_id=8563)
>>>>> and obtaining angular displacement estimate based on double
>>>>> integration of acceleration reading and some least-square estimates.
>>>>> Unfortunately my experiment duration need to be at least 1 minute and
>>>>> the drift error causes the estimate to be way off the mark... It
--snip--
>>> If I were designing the experiment ab ovo, I would include load-cell
>>> information from the heel and ball of each foot. I think that the way
>>> sway is corrected and the lags involved would be most informative. I
>>> suspect (without direct evidence) that the larger sways observed in
>>> older people is due at least in part to more delay in correcting the
>>> inevitable result of non-rigidity.
--snip--
>> And yes -- load cells under the feet is probably a good way to go.
>> Perhaps little platforms, with three load cells each, to get total
>> weight, fore and aft as well as side to side torque, ditto for the other
>> foot?
>>
>> You almost wouldn't need the accelerometers at all.
>
> Speaking as a spinal injury case now out of therapy ;)
>
> Current technology does use load cells for both evaluation and
> training. The implementation in the unit I've used had what I
> considered usability problems. You had to initially place your
> feet in the exactly right place to start. It also can only work
> in a nominally static environment. Having the sensor attached to
> the patient as he maneuvers would be useful. The manufacture of
> the chip involved has an app note for using it in evaluating
> stability prior to a fall.

Some of the chips I've seen from TI recently (I assume others exist)
make it clear that very small short-distance RF links can be set up.
What if one were to build heel-and-toe load cells into a pair of
shoes and pass the data on to a nearby host?

Of course, that assumes that one can obtain small load cells. How
small do these get? (And are they available for less than an arm
_and_ a leg?)


Frank Mckenney
--
The thing which keeps life romantic and full of fiery possibilities
is the existence of these great plain limitations which force all
of use to meet the things we do not like or do not expect.
-- G.K. Chesterton: On the Institution of the Family (1905)
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)
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