Wieslaw said:
I would suggest to use ultrasonic interferometry. We have made such
devices (see here:
http://www.optel.pl/manual/english/vibration.htm) for
vibration measurement, but distance measurement with such precision is
possible too. Accuracy is achievable, measurement can be continuous,
300$ if mass produced no problem.
If interested, we can develop such device.
IIRR Polaroid used an ultrasonic distance measurement in the autofocus
system for their Swinger camera. More recently, you could buy
"ultrasonic tape measures" that were useful for getting rough room
dimensions quickly.
As far as I know these depended on sending out a brief pulse and
detecting the echo. The speed of sound in air at 20C is 343.4 m/sec, so
for a 50mm maximum range the go and return time is 290usec. If you use
separate transmit and receive elements, you could send out a suitably
long pseudo-random binary sequence and track the correlation between
the transmitted and received signal, which could let you sample faster
- probably beyond the once every 100usec being asked for.