Interfacing FSR's

In 1997, Arie van Schutterhoef, one of the founders of the Schreck Ensemble, decided to build an FSR-based MIDI instrument, inspired on the old Kraakdoos from STEIM. Arie called his new instrument the Stratifier (photograph on the right). The instrument, made from bamboo, was built by visual artist Hans van Koolwijk.

A solution is presented here concerning the 'simultaneous' but independant pressure and position-measurement with Force Sensitive Resistor-strips manufactured by Interlink electronics. A simple chopper-circuit containing just one single CMOS-switch produces not quite independant pressure and postion-outputs. I developed a 'chopper circuit' by adding some sample & hold switches (cicuit diagram below).

I had was some trouble with interfacing FSR strips to STEIM's SensorLab. The simple 'one-dimensional' pressure-pad FSR's were easy, but the 'more-dimensional' FSR's gave problems: using only one CMOS-switch (a 4053 for instance) yielded postion-measurements that depend on the applied pressure. And the other way around: pressure measurements were influenced by position.

Research with different types of OPAMPS and CMOS-switches pointed out that most problems occur at the transition time of the CMOS switches. I think the capacitive properties of the FSR-stips play a role in this when operating at rather high clock-frequencies.
Unfortunately, the Sensorlab could not provide a software-solution (there was no way to let the Sensorlab control an external muliplexer at high speed), so some additional hardware had to be developed for Arie's new instrument.

To reduce pressure-influence on position-measurement, I introduced sample & hold switches in the chopper circuitry. The actual measuring-time was limited to a short period in between the input-switching. This improved things a lot, however, a small amount of crosstalk is still present.