non-contact distance measurement


Thread Starter

Gantt Robert

I have a similar need as the one posted by Ludovic
DEMEULENAERE yesterday for crane positioning, but for a short distance application in an extrusion machine. I need to measure the position of a moving carriage with a 1.5 meter travel; the position sensor's most viable mounting position is about 30 cm from the carriage at rest or zero position (therefore, 1.8 m from the carriage when fully extended).

I have been looking at several options, from cable or reel type position sensors/string pots (like Unimeasure) to ultrasonic analog output sensors to laser type devices (like Sick). The most cost effective solution we've seen so far is the cable or string pot, at about $500. However, a non-contact method is much more desirable in this app. Does anybody know of a non-contact (laser or LED or ultrasonic) device with analog output in that same price range? All laser options investigated so far are too expensive (and overfeatured) for our needs. We have no limitations on fixing any required type of
reflective surface to the carriage, so that is not a problem. Also, resolution/accuracy is not very critical(up to 5mm or so could be ok). Any
suggestions welcome.

We haven't looked much into ultrasonic sensors,
because since they have a much wider "aperture" or
"window" than lasers, we suspect that we may have
problems with them, since the line of sight from the sensor to the carriage in its furthest position is pretty narrow (possibly a square foot), as it runs inside a metal cage. However, we are ignorant in this matter, and any feedback will be welcome.

Thanks and best regards, Bob.

Gene Wilkins

You might check on the Wenglor analog sensors. They have several that are very cost effective($600) range and would do a fine job. Also, check the Magnetostrictive absolute linear transducers from Balluff. They have a variety of units with captive and noncaptive magnets that are extremely
accurate and also in this price range. Unlike a string pot., there is nothing to wear out.
Gene Wilkins
Why dont you give magnetostrictive transducers a try? they are in that price range ( about U$S 798 for a 72 inches model, it could be lower though
because the price list is a bit old), non-contact and have good accuracy (better than any ultrasonic transducer you could ever find). We have been using Schaevitz Magnerule line for a while now (sorry but i dont have the URL handy) to control the positioning in Tensile Strenght Test machines without a problem. There are a couple of manufacturers other than Schaevitz
so i guess the best thing you can do is go to a search engine and do a search using the keyword 'magnetostrictive'
Hope it helps,
Hugo Voras
MetroStar S.R.L.
We use some Banner ultrasonics at Honda that cost us less than $200 each. They can be programmed to give discrete output within a set window
or used to measure with the analog. They have a fairly tight aperture, but the surface must be pretty close to a right angle to the sensor. I bet they have a photoelectric that can do the job very well and meet your specs.
I personally am sold on them, mainly because of customer service. I once had a stray noise problem and they sent out a PHD engineer to solve the problem. They redesigned the eye to work in my environment, but in the short term they replaced 60 of them with a different version (which cost twice as much) at no cost to me.

Check them out.