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Safety PLC logic validation

Safety PLC logic validation

Safety PLC logic validation

Simple question ... How?

If you have one robot, and one light curtain, and one gate, and one E-stop button, all the logic pretty much fits on a couple of rungs that will all fit on the screen of your laptop, and it's simple. But that's not what I'm talking about.

Now try a cell with 15 robots and 6 operator stations, with multiple robots going into each operator station (some welding, some material handling), multiple zone rings and/or Fanuc DCS zones (depending on age of robot) to define various safety zones, plus area scanners to detect if someone reaches past the operator station, etc.

We had an operator station pass routine function testing of the safety logic. Break the outer light curtain while the robot was on the zone switches indicating that it was potentially accessing the station and through an area scanner that also detected if the robot was accessing the station, and the robot stopped. All seemed well, until the operator found a particular spot where the robot was accessing the station but wasn't through the area scanner, and the robot jumped while the operator was in there. In retrospect, now that the needle in the haystack had been pinpointed, it was apparent that the two bits (area scanner interrupted, and robot on the zone ring switches) should have been in series rather than parallel. But how do you know where to look for that needle in the haystack before someone else finds it the hard way?

If you ask the system (A-B GuardLogix) to produce a configuration report for the complete system, including all the DCS zones, it takes several hours to generate and it's several thousand pages. Debugging the logic "on paper" is a non-starter.

What are others doing? How do you handle this?

RE: Safety PLC logic validation

Once you have the system installed and in production I would suggest running a trend of safety I/O, and checking it daily for any errors, you might have seen the scanner clear while the ring switches were indicating the robot was in an unsafe position. As far as the base limits and scanner in series, I have had many systems where parallel was the norm, the base limits muting the scanner. While I am talking about base limits we need to be careful how they are configured. Generally speaking the base limits should be actuated while the robot should be muting the scanner, if a wire opens or the switch fall off no muting will occur. I have had equipment arrive on my floor where this was not the case, and the switch bracket was bent, fortunately no injuries.

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