Wednesday, October 12, 2011

Vision systems and PLC’s

Vision systems rarely run in isolation. Usually they are interfaced with a PLC that handles part presentation and disposition of the inspected items, and in my experience, this is where a lot of integration issues arise.

At the heart of the problem, I believe, is the issue that machine vision people want to deal with optics, light, image acquisition and processing, and many of the breed have no “controls” expertise. On the other side of the divide sits the controls guru. He can make a PLC sing and dance, but vision systems are not his “thing”. The result is that, far too often, “What we’ve got here is a failure to communicate,” as Paul Newman said in Cool Hand Luke.

To help address this, the September 2011 edition of Vision Systems Design has an excellent article titled, “Programmable Logic Keeps Machine-Vision Systems Moving”. This describes the basic principles of PLC – vision system communication before going on to propose an additional concept, the “Clear-to-Send” PLC output.

The idea is to improve handshaking between PLC and vision system by having the PLC tell vision when it’s ready to receive data. (I think that’s the gist of it.) This is an excellent idea, except I’m not sure it’s new.

In my on-the-job training years ago I was introduced to the idea of a Busy signal. As far as I can tell, Clear-to-Send is the same thing as NOT Busy. Now it may be that I’m missing some subtleties of inverting the logic, so if I am, please send me a comment or email to explain.

I’m not trying to be Mister Negative; I think this is a great idea, and a great article, (Andy – we need more like this.) I’m just suggesting that perhaps those skilled in the art of PLC-vision system integration already know.

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