LDmicro Forum - Advise on a capacitive transmitter

Advise on a capacitive transmitter (by Samukelo Shezi)

Hi Jonathan.

(This paragraph is supposed to be at the end of the message, but this site stops you from seeing your message at the bottom once you exceed a certain length, if YOU POST YOUR MESSAGE FROM A NOKIA 6210 CELL PHONE.) I would like to thank you once again for always availing yourself to us in this manner!

In a packaging industry, is it possible to get some kind of a capacitive transmitter, shaped like the letter "U", that sits on the edge of a long plastic belt that is being cut into bags. (Think of it as a normal conveyor belt.), but this capacitive transmitter must monitor the alligment of the plastic material. The allowed tolerance is about 30mm, and it gives 4-20mA accordingly as the plastic changes the position for whatever reason. I think the plastic can act as a dielectric.

Mon Dec 7 2009, 00:49:20

(no subject) (by Jonathan Westhues)

This may be practical, though depending on the thickness of the plastic, it won't be a very strong signal. The electrode design would need some thought, to avoid measuring thickness instead of alignment; perhaps by using multiple electrodes (one entirely within the plastic, one only partially) and working ratiometrically.

I don't know whether you can buy 4-20 mA instrumentation for this. At a board level, you would probably use something like the AD7746.

It seems like an optical sensor, perhaps with a linear CCD, would be the easy solution. Even if the plastic is transparent, I would guess that it would be simple to pick out the edge.

Mon Dec 7 2009, 01:12:17