Position estimation using tri-axis accelerometer with gyroscope in Static standing

Tim Wescott wrote:
> On Sun, 27 Dec 2009 23:06:45 -0600, haoxiang 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
> 
> Are you primarily interested in position or angle?  A 3-axis 
> accelerometer may not be too good with position, but in a gravity field 
> it'll be pretty good at telling you which way it's pointing with respect 
> to down, particularly if you've got 3-axes worth of gyro to back it up.  
> 
> If you could assume that the person has their feet planted and that they 
> stayed standing fairly straight I think it'd work as well (think of 
> making and observer for an inverted pendulum), but I don't think that the 
> "stayed fairly straight" assumption would be valid.
> 
> Or perhaps you care about the amount of sway around some point that you 
> can safely assume to be stationary or moving at a constant average 
> velocity.  In that case, once again, your 3-axis accelerometer may be 
> enough, and adding three axes of gyro would be a big help.
> 
> Getting a 6-axis IMU, even a cheap one, may well do what you want if you 
> work out the filtering well enough.
> 
> Think about what measurements you _really_ care about, and see if there's 
> enough information there to get what you want.  If not, you need more 
> sensors, if it's there then start figuring out how to extract it from 
> your data.
> 
> Using a camera system is definitely sexy, but it sounds like a 
> distraction from your main goal, to me.

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.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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:
>> 
>>> 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
>> 
>> Are you primarily interested in position or angle?  A 3-axis
>> accelerometer may not be too good with position, but in a gravity field
>> it'll be pretty good at telling you which way it's pointing with
>> respect to down, particularly if you've got 3-axes worth of gyro to
>> back it up.
>> 
>> If you could assume that the person has their feet planted and that
>> they stayed standing fairly straight I think it'd work as well (think
>> of making and observer for an inverted pendulum), but I don't think
>> that the "stayed fairly straight" assumption would be valid.
>> 
>> Or perhaps you care about the amount of sway around some point that you
>> can safely assume to be stationary or moving at a constant average
>> velocity.  In that case, once again, your 3-axis accelerometer may be
>> enough, and adding three axes of gyro would be a big help.
>> 
>> Getting a 6-axis IMU, even a cheap one, may well do what you want if
>> you work out the filtering well enough.
>> 
>> Think about what measurements you _really_ care about, and see if
>> there's enough information there to get what you want.  If not, you
>> need more sensors, if it's there then start figuring out how to extract
>> it from your data.
>> 
>> Using a camera system is definitely sexy, but it sounds like a
>> distraction from your main goal, to me.
> 
> 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.

Apparently there is a considerable amount that's just from poor muscle 
tone -- after years of ignoring the fact that spry older folk were always 
doing things and rarely falling down, researchers finally realized that 
exercise is _still_ a good thing _even when you have wrinkles_!

See, Rune -- there _are_ still valid PhD topics out there.

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.

-- 
www.wescottdesign.com

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:
>>>
>>>> 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
>>> Are you primarily interested in position or angle?  A 3-axis
>>> accelerometer may not be too good with position, but in a gravity field
>>> it'll be pretty good at telling you which way it's pointing with
>>> respect to down, particularly if you've got 3-axes worth of gyro to
>>> back it up.
>>>
>>> If you could assume that the person has their feet planted and that
>>> they stayed standing fairly straight I think it'd work as well (think
>>> of making and observer for an inverted pendulum), but I don't think
>>> that the "stayed fairly straight" assumption would be valid.
>>>
>>> Or perhaps you care about the amount of sway around some point that you
>>> can safely assume to be stationary or moving at a constant average
>>> velocity.  In that case, once again, your 3-axis accelerometer may be
>>> enough, and adding three axes of gyro would be a big help.
>>>
>>> Getting a 6-axis IMU, even a cheap one, may well do what you want if
>>> you work out the filtering well enough.
>>>
>>> Think about what measurements you _really_ care about, and see if
>>> there's enough information there to get what you want.  If not, you
>>> need more sensors, if it's there then start figuring out how to extract
>>> it from your data.
>>>
>>> Using a camera system is definitely sexy, but it sounds like a
>>> distraction from your main goal, to me.
>> 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.
> 
> Apparently there is a considerable amount that's just from poor muscle 
> tone -- after years of ignoring the fact that spry older folk were always 
> doing things and rarely falling down, researchers finally realized that 
> exercise is _still_ a good thing _even when you have wrinkles_!
> 
> See, Rune -- there _are_ still valid PhD topics out there.
> 
> 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?

Sure. Four cells might make analysis a bit simpler than three, but 
without adding new information.

> You almost wouldn't need the accelerometers at all.

If I had to choose one or the other, I'd go with the load cells. Still, 
both kinds of information allow correlations that might reveal the 
unexpected.

I believe that an experiment using accelerometers to measure position of 
an object supported on a floor is poorly designed. I would use, say, 
strings to ball-bearing potentiometers supported by walls or posts. (Or, 
to be high tech, video.)

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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:
>>>
>>>> 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
>>> Are you primarily interested in position or angle?  A 3-axis
>>> accelerometer may not be too good with position, but in a gravity field
>>> it'll be pretty good at telling you which way it's pointing with
>>> respect to down, particularly if you've got 3-axes worth of gyro to
>>> back it up.
>>>
>>> If you could assume that the person has their feet planted and that
>>> they stayed standing fairly straight I think it'd work as well (think
>>> of making and observer for an inverted pendulum), but I don't think
>>> that the "stayed fairly straight" assumption would be valid.
>>>
>>> Or perhaps you care about the amount of sway around some point that you
>>> can safely assume to be stationary or moving at a constant average
>>> velocity.  In that case, once again, your 3-axis accelerometer may be
>>> enough, and adding three axes of gyro would be a big help.
>>>
>>> Getting a 6-axis IMU, even a cheap one, may well do what you want if
>>> you work out the filtering well enough.
>>>
>>> Think about what measurements you _really_ care about, and see if
>>> there's enough information there to get what you want.  If not, you
>>> need more sensors, if it's there then start figuring out how to extract
>>> it from your data.
>>>
>>> Using a camera system is definitely sexy, but it sounds like a
>>> distraction from your main goal, to me.
>> 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.
> 
> Apparently there is a considerable amount that's just from poor muscle 
> tone -- after years of ignoring the fact that spry older folk were always 
> doing things and rarely falling down, researchers finally realized that 
> exercise is _still_ a good thing _even when you have wrinkles_!
> 
> See, Rune -- there _are_ still valid PhD topics out there.
> 
> 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.

As to other sensors in use - the rehab unit uses games built 
around the Wii Balance Board in the pediatric unit.

In my case, balance issues are related to decreased muscle tone 
and sensory(proprioception) problems. Reaction time seems to be a 
secondary issue.

On Tue, 29 Dec 2009 14:57:16 -0600, Richard Owlett <rowlett@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)

Frnak McKenney wrote:
> On Tue, 29 Dec 2009 14:57:16 -0600, Richard Owlett <rowlett@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

I couldn't visualize how that would be implemented physically. 
I've never used/seen a load cell as a component. I didn't find 
anything that would seem to fit mounting constraints/methods I 
could visualize.

However I came across a slightly different approach after 
following some rabbit trails. Tekscan has what it refers to as an 
"In-Shoe Pressure Measurement System".

http://www.tekscan.com/medical/system-fscan1.html
http://www.tekscan.com/medical/general-foot-function.html

I don't know, but I suspect that some "position" sensor attached 
to the torso might be more informative concerning resulting 
stability.

As I sit here, I'm wondering that if the goal is stability 
evaluation, wouldn't the appropriate output be velocity vs time 
rather than position vs time? After all the standard test used on 
me was a Berg Balance Test. 
(www.aahf.info/pdf/Berg_Balance_Scale.pdf)
As can be seen several of the tests observe the _process_ of 
changing position.

How did the project come out?


haoxiang 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
> 
> Regards,
> Hao Xiang 
> 
>