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  • PVDF force plates

    There seem to be a broad interest out there for more information on the
    PVDF force plate system we are putting together (given the responses I'm
    receiving to the initial post). I will try to give a few more details.

    Many of the commercial force plate systems available use piezoelectric
    transducers to provide the force data and since there is some expertize
    in our department in the field of piezoelectrics we decided to forego
    the expense of an off the shelf system by setting up a "homemade"
    version.

    I have seen such systems mentioned in biomechanical research literature
    on a few occasions. One reference that I have on hand is:
    Kim, Seung Kwon, A biomechanical analysis of the TaeKwondo front thrust
    kick, 1993, Ph. D. Thesis, University of Wisconsin, Vol. 54/08-A
    Dissertation Abstracts International, p. 2945.

    Assuming that you have access to a storage oscilloscope the set-up and
    calibration of the system is quite simple and cheap. For our system
    (which is still under construction) we use a 5cm by 6cm square of
    polyvinylidene fluoride (PVDF) piezoelectric film. The cost of this
    size of film is on the order of a few (less than ten) dollars and can be
    ordered from AMP sensors (who have a Web page at:
    http://www.amp.com/sensors/metfilm.html ).

    We mounted(ie glued) our piece of film on a piece of plexiglass and
    simply put one lead each from the oscilloscope onto each side of the
    piezo film (both tape and conductive epoxy have worked well). Care must
    be taken in the mounting system as the film is more sensitive to
    bendiing moments than to compressive forces so any bending of the film
    will swamp your results.

    The system was calibrated by taking masses at known velocities and
    colliding them with the detector. A simple photogate was used as the
    trigger for the scope. The voltage profile, V(t), as read on the
    oscilloscope will be directly proportional to the force profile, F(t) of
    the collision if all is right. Our system's readouts fit the expected
    profiles and the film's calibration ratio was approx. 800N force to 29.0
    V. The results are consistent with the expected values for the
    collisions we've looked at to better than 5%.

    Be aware that the collisions that interest us (and presumably others
    doing biomech research) are at least partially inelastic. When our
    device is struck by bare metal, for instance, the device output is
    similar to that of a damped harmonic oscillator and hence quite
    difficult to get a meaningful force reading from.

    Also, our interest lies in the force profile of collisions and that is
    why we use the storage scope. Those looking at peak voltages (forces)
    could use a simpler system, and indeed any device able to give voltage
    readings in the Mhz range will do for most purposes once the system is
    calibrated.

    The oscilloscope's voltage limitation of 30V means that in the simple
    configuration described we are limited to forces of about 800N. This
    can be dealt with by the use of voltage dividers.

    If there are any other questions feel free to send them on.

    Brad Waugh
    Physics Dept. RMC
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