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[e-sam]

For a company project we need to measure a gap of 100 microns in real time. One surface is metallic and moving the other is plastic. We tried a proximity sensor (inductive) with an analog output (it is not a displacement sensor), and custom-wrote a program to make it linear. It is working fine as long the temperature is constant. Perhaps we need a real displacement sensor, but we have budget limis. Is there is an inexpensive displacement sensor 0 to 5 mm range and of 5 to 8 mm diameter? We need only the sensor (not a system). Also:

1. The gap we must measure is filled with some water-based substance. The composition of this gel-like substance changes in time. One presumes that capacitive sensors are not appropriate for this application.

2. For the same reason we can't use the ultrasonic sensors.
The sound velocity is different. Perhaps the precision of the ultrasonic sensors is limited by the wave length. We need a range of 5 mm (20 mils) with an accuracy of max 5 microns (0.2 mils).

[Geoff]

Check out Banner Engineering, Automation Direct, Gem Sensors, etc. Did the proximity switch not meet accuracy specs within your temperature range? They should perform well. Is the output current or voltage? Maybe an ultrasonic or laser sensor would work. Ultrasonics are not too expensive and may be used for distance. In hydraulic cylinders (6"stroke) they are uses for detecting an objects distance from another (accuracy depends on range-span; 15 mils accuracy with distance between objects of 7-12" costs $300 USD). Lasers are quit a bit more expensive but accuracy is superb.

[F-A-B]

Ultrasonic works by changing the oscillation frequency of the sensor. But it would not work well with substance between them. If the frequency could be lowered it may work (similar to the way transmitting through water allows lower frequencies to travel farther distances). Nevertheless this would have a big impact on accuracy at the distances your talking about.
How about using a sliding potentiometer -- attach this between the two pieces? Is this possible in your case? You could put voltage across the pot and measure the voltage across the wiper to determine the position.
Cylinders have this option but they may not make sensors this small.

[daqmaster]

What is the temperature factor in your current setup? Can you modify the results based on temperature? If so, all you may have to do is put a thermocouple on your sensor and modify your results based on the temperature registered at the sensor. You may get lucky; the relationship might be linear.

[e-sam]

1. Not lucky! The relationship is not linear. The temperature is cycling the characteristic somewhere in the middle. We could use this point for the calibration, but this means a very fine tuning. One could compensate using some math, but there is no guarantee that the next sensor will behave similar.
2. The gel is water-based and not transparent. The gap is 2 mils. The gap error should be 0.1 max 0.2 mils. The gap is between a stainless steel plate (the moving one) and a plastic surface (the one with the sensor) Using a uP with ADC we can take the readings within 0.1 mils. Things become ugly when temperature changes (and it is changing). We are using a proximity sensor and not a displacement sensor (expensive). The proximity sensor in some degree has a analog output (using slope).
3. The sensor has to be non-contact.
4. For the ultrasonic sensor there is a relation between the minimum length and the wavelength (1/4?). At 1000 ft/sec (in the air) the frequency should be too high in order to have the accuracy of 0.1 mils

We think the use of inductive displacement sensor is a must (at least is designed for this and is compensated). We currently see no other option.