Detect safety motion; don't try to work around it
A sound system requires operator compliance with the safety protocol
Machine safeguarding is typically accomplished with a combination of hard guarding, protective doors, e-stops, and possibly safety light curtains. The idea is to prohibit an operator from getting too close to the hazardous motion, and should that ever occur to disable that motion as quickly as possible.
But what if maintaining the system requires access to the machine? Wouldn't it be advantageous if the safety system would allow a machine to move at creep speeds while the operator is nearby? Is creeping the machine all that is necessary to assure personal safety? What if it could provide data that safely determines the difference between running clockwise vs. counter-clockwise?
Consider a rolling press. When all the guarding is in place the rolls run such that they draw in sheet metal, reducing its thickness as needed. Clearly no operator must ever be allowed close to the rolls, as they could easily be pulled in along with the sheet metal. At regular intervals maintenance personnel need to enter the hazardous area to clean the rolls. Historically this operation demanded the rolls to stop, allowing personnel to enter and clean a small, exposed section of the rolls.
After exiting the protected area and closing all doors the machine is restarted, advanced a bit and stopped again, allowing another small section to be cleaned. This certainly works, but because it is time consuming the rolls probably won't be cleaned as often as they should causing undesirable variations in the sheet metal.
Any good safety system accomplishes two goals simultaneously that are “invisible” to the operator: Protecting the worker while not making her job unduly cumbersome.
Using a safety encoder to safely determine the speed and direction of rotation is an alternative worth considering. Safety encoders makes it possible to assure that the rolls creep and only move in the direction as to “expel” rather that draw in. Maintenance can then enter the protected area, clean the rolls while they are moving slowly, and remain safe. This setup considerably reduces the time it takes to maintain the system, resulting in higher machine productivity.
Using such an approach makes cleaning the rolls convenient, which may even have a positive effect on the quality and surface constancy of the sheet metal parts. Since this safety solution works with the operator, it also reduces the likelihood of operators manipulating the safety system. After all, the best safety system does no good if an operator is constantly trying to override it (or get around it).
Helge Hornis, PhD, is manager of the Intelligent Systems Group for Pepperl+Fuchs. www.pepperl-fuchs.us
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