Product changeovers require manual modifications to the blanking patterns for safety light curtains at the product feed and takeoff stations of our presses. We think programmable light curtains can speed this up. We're unclear if we can just include them as automatic changes inside the stored part profiles we provide, or do they require some sort of redundant check before first use?
—From July '10 Control Design
Answers
Yes, anytime you change configurations in a safety system, the system must be checked for proper installation and operation before it is used in production. In the case of safety light curtains, one of the items that you'll need to verify is that the resolution is correct for your application. Typically, this is accomplished through a resolution check or trip test, in which an appropriate test piece is manually passed through the sensing field in several locations to ensure detection.
This is very important if the resolution is being selected by some machine logic automatically. If it's not being checked every time the resolution changes, at a minimum, the means of selection would need to be accomplished at the same level of safety performance as the safety light curtain (i.e., control reliability, Category 4/PLe, SIL3, etc.). Even then, it would still be required to verify the operation periodically.
Another issue is that each time the resolution increases, the location of the sensing field in respect to the hazard (safety distance) also must increase. So, for each programmed resolution, the mounting of the emitter and receiver must be moved. This might eliminate any efficiencies gained due to the automatic programming of the light curtain.
A simple option is to configure the safety light curtain for the worst-case resolution (i.e., the biggest "hole") and install the sensing field at the specified safety distance for that resolution. Placing the safety light curtain at the worst- case distance sufficiently protects operators in scenarios where the largest products must be fed into the machine. The same arrangement can be used for smaller products without adjustment—the operator is granted a larger safety distance than is required for the small object, but it does not compromise the operator or the production process in any way.
Of course, if the resolution is so large (a very big hole) that safety distance becomes an issue, a vertically mounted safety light curtain might not be the correct safeguarding choice.
Mike Carlson, safety products marketing manager,
Banner Engineering, www.bannerengineering.com
