Teach-in 2000 : Time-dependant behavior of inshoe force sensors
by Chris Kirtley, The Catholic University of America, Washington DC

In 3D computerized gait analysis, Ground Reaction Force (GRF) data is normally recorded using a six-channel force platform, which measures the forces applied, and moments about the three orthogonal axes. 

Force platforms have some basic limitations, however:

For these reasons, several attempts have been made at developing force transducers which can be mounted in the shoe itself. Basically, all sensors utilize one of the following techniques to sense the force: Unfortunately, all these sensors suffer to some extent from the following problems: In this experiment, two types of FSR sensor (Interlink and TekScan Flexiforce A101-100) were cyclically loaded by an Instron materials testing machine.

The FSR was incorporated in a simple operational amplifier circuit:
The voltages from the FSRs were plotted against the voltage from the Instron machine (LVDT) at a range of maximum loads and loading rates, with the sensors sandwiched between two 5mm layers of Pelite foam.

Results

Here are the calibration curves for both sensors at a loading rate of 200 mm/min:

Here are the time series (same trial as above) of both sensor outputs:

Notice that the loading is not sinusoidal - this Instron machine was not designed for cyclical loading, and was modified to enable automatic control via LabView. The LabView software simply switches the machine between compression and decompression when the force falls below and exceeds preset thresholds, respectively. The load cell of the Instron machine is situated in the head (top G-clamp in the picture above), which moves. The lower clamp is stationary.

Here is the same experiment, but this time a much lighter top clamp was used:

Notice that the strange loop in the middle of the Flexiforce calibration curve has now disappeared.

Finally, here are the curves plotted again - this time on a log scale:

     
Notice that the Interlink curve is now linearized.


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Bibliography

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