Wednesday, May 23, 2018

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The PLC Never Reprograms Itself

The other week, our product autostrapping system went haywire.  One of its sequences got mixed up and and was doing it 'before' instead of 'after'. The strap bayonet (a channel that extends underneath the product  to provide a pathway for the strap and then feeds before tightening it, retracts then position itself unto another side for another strap) was extending all the while it is moving to another position resulting in the bayonet hitting the product and getting bent and shearing its bolts.

Since the the system was run by a programmable logic controller, a group of maintenance personnel decided to disable one output bit as they are concluding that there was something wrong with the program. It made the machine run like normal though. But one colleague and I was thinking otherwise (me and that guy is not assigned for that problem, we are working somewhere else).

Several points came through my mind. First, the PLC can only be reprogrammed through its programmer or its corresponding software. It will never change by its own. Second, It was working perfect for a very long time until the recent events. Third, permissives are there. What I mean is that to have an output of the system, the inputs should be satisfied.

So one thing is for sure. The output bit was being energized due to an early actuation of the input bits. That is the thing that 'they' should look into. Some sensors could be getting actuated very early on or it might be already damaged that it might be shorted or something.

Well, If I will be assigned there, that will be the first thing I will be looking at. The output bit is still bypassed though up to now.

Comments (4)Add Comment
written by James Sanders, March 06, 2012
You're right. The PLC code is stored in a ROM and can be modified only with the use of a proper programmer and software. I believe with your post that one or some of the inputs have been improperly energized to generate such errors.

Keep posting! nice one!
written by Nisha, December 07, 2017
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Thanks for the great information about PLC reprogramming.
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