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mwaters66
06-16-2004, 09:27 PM
Dear All

Many thanks for your replies regarding a non zero start level for the Kistler force plate. In combining all comments I've constructed a solution: I hope, however offset gain may pose a problem. In the newest version of Bioware software when you click acquire data, (say your acquiring by clicking a button or pressing a key) Bioware has two stages. Initially it reads ofset voltages and resets the plates to zero and uses this value, then it asks you to click ok. So in theory and in the little time I've had since this has worked, you set up the situation, click acquire data then position your subject. It is at this point that the system already has a zero point and thus when you position your pariticipant they are at the correct force and thus when you click ok (second stage) it records the correct reading. (hope this makes sense). The only worry is that of force plate gain through warming etc... I have just received results back this afternoon from leaving the force plate running continuously recording for an hour and only shower a 8 N change in force, so in theory if you don't leave it too long between clicking acquire and clicking ok, it should work fine.

A summary of responses is posted below:

1.by doubleclicking the operate-button the sensors are resettet with the
result of 0V/0N - even with 1000N on the plate. You've to do this
reset before the participant steps on the plate if you want to mesure
it. Depending on the software used, it may be possible to calculate
this in the software ...

2.Kistler force plates need a OPERATE / RESET signal. All forces applied
during the "Reset" phase (1-3 seconds) are considered as 0 (zero).
This signal is used to set all channels to 0 V in order to measure correct
values for the following trial(s). You should reset the signals as often as
possible.
Make sure that nobody is on the force plate when the "reset" signal is sent
because his body weight will be "zeroed" (subtracted from the measured
values) until the next "reset" is sent.

External amplifiers have a corresponding switch, force plates with internal
amplifiers need a signal to be sent by the controlling software.
All software packages should have the option to send the "reset" signal
automatically just before data acquisition or at any time on user request.

3.I have plenty of experience with similar KISTLER Force-Plates and know
your pain.

Two recommendations:

a) You might want to try resetting the force platform before the subject
loads the force plate, but if this technique increases the time between
resetting and data collection excessively and drift is a concern, it may not
be suitable for your application.
b) In post processing, you can just take the minimum value from your data;
assume it to be zero, and subtract the negative value from the whole data
set, thus "shifting" your data up. This can be done easily in Matlab or
Excel.

I don't know of a way to actually set the KISTLER force-plate itself to a
specific value before data collection. I don't think that this is possible
due to the inherent drift in a piezoelectric force platform. I think that
you'll probably end up having to correct the data in post processing.

4. ..encountered the same thing, - I suppose theres no way you can start
capturing with the plate unloaded ie zero then get your subject partial
weight bearing for a few sample then conduct your trial? Alternatively, even
if your plate goes negative after stepping off, you can use this value as
your zero or offset, add this your data series and you should be fine....in
other words shift all your data up by that amount

Kind Regards

Mark


Mark Waters
Research Assistant (Biomechanics)
Teesside Centre for Rehabilitation Sciences
The James Cook University Hospital
Marton Road
Middlesbrough
TS4 3BW

Tel. 01642 854324
Mob. 07739013621

Email. M.Waters@tees.ac.uk
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