[sdiy] Velocity/Position sensing

[Magnus Danielson]

harrybissell at wowway.com wrote:
> Piezos sense high frequencies best... to do an X-Y position with them
> would probably require a hard surface like metal, or glass etc which
> would transmit a surface wave well. Fabrics would damp the high frequencies
> and probably not work well.  You need to damp out the previous note hit before
> the next one... quirky trade-off there.  It you hit a hard surface with drum
> sticks it would be easy... soft surface with hands is really tough ! (imho)

Actually, all you need to do is to measure the impact impulse in X-Y by 
setting up a pair of contact mikes in each direction so you get X and Y 
response. You want some damping so the pad you hit does not ring badly, 
ringing should die of fairly quickly.

What you now do is amplify the gain to get a nice clean signal, detect 
the rising edge of the impulse and let that trigger a SR-flip-flop.
For a X or Y pair you then use the Q outputs to control the charge 
current of an integrator. The Q outputs is also anded together to create 
the a sample-and-hold signal which is delayed with and RC chain to form 
the reset-signal for the SR-flip-flops and integrators. Manual trimming 
of the reset may be neccessary to discriminate additional triggers.
An op-amp sums the output of the integrators prior to sample and hold.

The time-difference between the pair of transducers now produces a 
linear CV signal. The trouble of doing this on a X-Y pad is that 
single-point transducers will create a distored patter which will be 
hyperbolical. If this is important compensation can be done using 
non-linear processing. For linear responses it would be trivial to 
achieve fairly linear responses. A CV scaling and offsetting may be 
needed. The full range depends on the speed of sound in the hit media.
If hitting hard, some medias become non-linear which also fucks up the 
speed of sound, which cause position skewing related to how hard you 
hit. Naturally that could be used to aid in expressiveness for some 
odd-jobbs.

Cheers,
Magnus