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.