Small distance measure

I would like to measure short distances up to 3" (7.5 cm) with perhaps an accuracy of 0.1" (2.5 mm). I noticed that commercial inductive sensors that I have found only measure up to a few tenths of an inch (about 0.5 cm).

Is this possible using any sort of inductive or capacitive measurement method? There could be opaque obstacles (not electrically conductive) in the path to the "target" so ultrasound and light based methods would not work.

Thanks,
Dale

Don't know if you can get to 3", but an LVDT can get you incredible resolution.

PB,
Thanks for the suggestion, but unfortunately there is a barrier between the two points to be measured. However, I will keep this in mind for other applications.
Thanks,
Dale

material types?

To know whether or not an inductive, capacitive or other method of detection is possible, you might have to know more about both the detected object and the barrier types. I'm not sure how you could ever do it otherwise.

Consider the case of trying to measure the distance to a balloon through lead foil. You'd be pretty screwed.

ericwertz said:

To know whether or not an inductive, capacitive or other method of detection is possible, you might have to know more about both the detected object and the barrier types.

Click to expand...

Eric,
I am not trying to detect the distance to an object, just the distance between two moving points. One point can be a metal target for an inductive measurement for example or an electronic "transmitter" of some type, the other point can be some sort of sensor or detector.

Sometimes there is an object between the two points. At this point I cannot disclose the object being measured through for proprietary reasons. All I can say is that ultrasonic will not work and a light-based system will not work, hence the suggestion for inductive or capacitive solutions.

How much variation is there in the distance between them? How fast are they moving?

One thought is to use the capacitance between the two objects. The capacitance could be the C in an LC oscillator. The frequency would be quite sensitive to changes in the capacitance and thus the object distance.

Carl,
Speed is slow and I can stop the motion briefly while taking a quick measurement. I would like to measure a distance within the range of perhaps 0.5" to 3" (1 to 7.5 cm). Thinking more carefully about this, I am now concerned about using capacitance because the user's hand may interfere with the capacitance. Any ideas about using inductance? Is it possible to measure the distance to a metal "target" (or perhaps even a magnet) up to 3 inches away somehow?
Thanks,
Dale

Well, you could use a strong magnet (you can find strong neobydium magnets in the head drive mechanism of old hard drives) and an IC Hall Effect sensor to detect the magnetic field. Don't know off hand what type of sensitivity or resolution you could obtain that way, but it should get you in the ballpark.

This method shouldn't be much affected by a nearby hand.