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unknown user
09-12-2000, 02:00 AM
Hi all,

here the summary to my answer regarding the investigation of the resonance
frequency of force plates. Basically, it turned out that what is called
resonance frequency are the oscillations of the system as a whole not the
vibrations within the plates. An important point was raised by Robert Riener
by mentioning that the lowest oscillation frequency of the whole system
depends on the actual load on the plate.
However, this can hardly be simulated for calibration experiments.

Thanks to everybody for their response,

best regards,
Matthias



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Answers:

--------------------------------------------
Robert Riener:

Dear Matthias:

Concerning your question about the resonance frequency, I
think that you should consider the system as a whole and
take into account all possible oscillation frequencies that
can occur during a typical experimental set-up. This includes
also that the plate should be loaded with a weight that
represents the human body. This will reduce the resonance
frequency compared to the unloaded case, and is, thus,
important since you are searching for the lowest possible
resonance frequencies.Hammering onto the plate and measuring the forces is
ok,
but
take care that you don't strike close to the region of a potential
oscillation node, where it is difficult or impossible to induce any
oscillation. Note that the bearings of the plate, where the sensors
are integrated, represent such oscillation nodes.
Take also care that the sampling frequency is as high as possible
and that you apply aliasing filtering to avoid an erroneous ac-
quisition.

We have published a short paper about the assessment of a
self-made 6 component sensor step (built into a staircase)
that may help you in your work. We also compared some
of the data with Kistler force plates.

--------------------------------------------
Judith Bower

Matthias
I also found lack of information regarding the accuracy of force platforms
but did find two other refernces.

Fleming and Hall warn of the dangers of striking with a hammer as the
frequencies may approach one of the natural frequencies and cause damage.

--------------------------------------------
Silvia Conforto:

Dear Matthias,
the approach we take for the determination of the resonance frequency is to
consider it as the resonance frequency of the whole system (the plate
together with its base of support), thus the way estimate this value is by
"hammering" it, and by analyzing the force responses.

--------------------------------------------
For those who know German ;)

Hallo Matthias,

ich weiss nicht genau, ob ich Deine Frage richtig verstanden habe und ich
kenne
mich auch nicht so gut in diesem Bereich aus....
zu 1) ich denke man sollte die Platte zusammen mit der Basis in Schwingung
versetzen und dann messen.
zu 2) Wenn man die Platte mit einem kurzen Stoß in Schwingung versetzt,
diese
Schwingung aufzeichnet und eine FFT darüber macht dann erkennt man in der
Frequenzanalyse die Resonanzschwingung oder Eigenfrequenz.

Eric

********************************************
References:

Riener, R., Rabuffetti, M., Frigo, C., Quintern, J., Schmidt, G.
(1999) Instrumented staircase for ground reaction measurement.
Medical & Biological Engineering & Computing 37, p. 526-529.

Fleming, H.E & Hall, M.G.et al (1997) Quality framework for force plate
testing, Journal of Engineering in Medicine, July Vol 211 (3) p213-219

Hall & Fleming et al. (1996) Static in situ calibration of force plates. J
Biomechanics 29 (5) 659-665

M.M:Gola "Mechanical design, constructional details and calibration of a
new force plate" J. of Biomech. Vol.13, pp.113-123, 1980;

G.Bizzo, N. Guillet et al. "Specifications for building a vertical force
platform designed for clinical stabilometry" Med. Biol. Eng. Comput. Vol.
23, pp.474-476, 1985;

R.S.Lakes, K. Korttila et al. "Instrumentd force platform for postural
sway studies" IEEE Trans. Biomed. Eng. Vol. bme-28, pp 725-729, 1981.


************************************************** *******************
*** Matthias Schablowski
*** Abt. Forschung 2: Laufbandlokomotion
*** Schlierbacher Landstrasse 200a
*** 69118 Heidelberg
***
*** Tel: ++49-6221-969284
*** Fax: ++49-6221-969234
*** Email: Matthias.Schablowski@ok.uni-heidelberg.de
************************************************** *******************

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