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Position "Sensing"

Anyone know of any reference that discusses the methods of measuring distances both small and large and the electronics behind them? I want a method to measure with mil to sub mill accuracy over a few inches. My initial thought was to use high freq sound pulses since this would be a cheap way but after doing some calculations it seems that it might be impossible, or at least very difficult without precise sensors(piezo's might work but still a problem of determining when the sound is "heard"). Speed in measuring is not really an issue either(can take a few seconds if necessary).

I'm a bit afraid of using light because it seems it would require relatively expensive devices and techniques. BTW, I need a wireless method but the device itself can be wired(so it can compare the speed with that of a wired EM signal).

Any ideas where I can look for more info on this? The main reason for this is to get around having to manufacture very precise positioning parts. Obviously if the cost of "remote" position sensing is more than that of just manufacturing the parts then its not worth it.

Also note that I suppose that I could have many sensors(I was thinking 4 to

6) that have super-mil accuracy but take some statistical average to get a better result.

In fact I don't care much about the relative position but only the change in position.

I just found this site:

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Which seems to say that the ultrasonic method might work... It also gives two other methods. I'm going to read more about it but a book would probably be much nicer.

Thanks, Jon

Reply to
Jon Slaughter
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Laser interferometry?

Reply to
Charles

I'll look into it but my guess was that it would be too complicated for my setup. Maybe for the accuracy I need it can be done easier than I think. I'll see what I can learn about it. (if it can be done easy and cheaply then it probably would be the best bet. I really know nothing about it except for the basic physics behind it so I guess its time to learn more.

Thanks, Jon

Reply to
Jon Slaughter

Yes, it probably is too expensive but I am amazed at the positioning accuracy that it can achieve. I had some experience with it, 12 years ago, and hoped that the cost has come down. Good luck.

Reply to
Charles

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As Paul Hovnanian says, you will have to be a lot more specific about the items whose separation you want to measure. Heidenhain has a lot of postion-measuring products

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but it is by no means obvious that you can fix a moving sensor to the object whose position you want to monitor, and have it travel along a fixed scale, reading off where it is as it goes.

High frequency sound pulses may not be a good idea - the speed of sound in air does vary.

For precision interferometry, the speed of light in air can vary enough to disturb very accurate measurements.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

Non contact position sensing, I assume (otherwise the problem becomes trivial). Can you attach a target to the object? What are its optical and acoustic properties? Also the characteristics of the environment in which the measurement will be performed.

How many degrees of freedom must be monitored? X, Y, and Z position? Orientation of the object as well?

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--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
It\'s easier said than done.
... and if you don\'t believe it, try proving that it\'s easier done than
said, and you\'ll see that it\'s easier said that `it\'s easier done than
said\' than it is done, which really proves that it\'s easier said than
done.
Reply to
Paul Hovnanian P.E.

Take a look at linear incremental optical encoders made by Gurley Precision instruments. See

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Scale length to 125 inches, resolution to 0.5 micrometers with incremental interpolators and non-contacting friction free.

If you do a Google search on "linear optical encoder" you will find other companies also make them.

Howard

Reply to
hrh1818

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Reply to
a7yvm109gf5d1

Also take a look at linear optical encoders made by Renishaw. See

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and also click on their non-contact position encoder brochure.

Reply to
hrh1818

1) The sensors and encoder strips used in ink jet printers are economical & hi-res, made by one of HP's orphans. 2) For small sale use, Chinese digital calipers / (dremel + cutoff wheel) = cheap digital scale.

Cheers, James Arthur

Reply to
James Arthur

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Reply to
ChairmanOfTheBored

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Not enough details to say whether this is appropriate, but Omron, Sick Optik, Sunx, and others offer laser ranging devices that use optical parallax to measure range. This keeps the cost down considerably. Can be calibrated to work quite well if sensed surfaces are consistent and not highly polished. Paul Mathews

Reply to
Paul Mathews

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