I have a project in which we want to place a proximity sensor inside/ on the bottom of a tapered spring in order to track the compression of a plastic material on top of the spring. The distance range would only be 1-10 cm away from the sensor and the plastic would be compressed 80-200 times/minute. We were thinking ultrasound (but the waves bouncing off of the spring may be a problem) or an IR emitter/detector system. Are there any other suggestions or examples of similar devices that can accurately measure the distance from the sensor to the plastic and the rate of movement (compression/minute). Thanks.
You could wind a coil around a former and then mount a plunger to the spring so that as the spring went up and down the plunger would move in and out of the coil, changing its inductance.
Build an oscillator around the coil, use the zero-crossings to drive a one-shot, integrate the one-shot's output pulses and voilà!, frequency-to-voltage converter.