You work for an electrical services firm that was contacted for help to get a large parts washer working correctly again. Its purpose is to prepare large subassemblies for the enameling process. It removes any cutting oil, grime, and solvents, then dries the subassemblies using blowers.
Upon examining the system, you see it’s a lot like a car wash. Large assemblies enter at one end, get latched to the track, and move along toward the other end. After being rinsed, soaped, and rinsed, an assembly is dried.
Upon entering, an assembly is latched to the overhead track; a robotically inserts a hook into a slot that exists for this purpose. When the drying cycle times out, that hook is robotically removed, and another hook is robotically inserted to carry the assembly to the enameling room.
A series of photoeye switches tells the PLC where the assembly is. What’s been happening is the assemblies pile up in the drying area instead of going to the enameling area. What might be going on, and how can you get to the bottom of this?
Answer to quiz. Since it worked previously, we know the PLC programming is correct unless someone changed that. If it has not been changed, then perhaps the system has been blinded. Check that next.
You want to functionally test each photoeye to ensure it senses a suitably sized dummy object from the correct distance and at the right spot and switches output accordingly (put a meter on the leads locally, if they are accessible). A given eye may have been recalibrated during preventive maintenance because the technician thought it wasn’t working right due to improperly testing it without correctly simulating the distance.
Before you do the functional testing, check these things:
- Is the power light on?
- Is the ready light on when the system is online?
- Are any error lights on?
- Maybe it got knocked askew from its aiming orientation and is now looking at a spot other than where it needs to be looking. You can eyeball this, but using a laser level is better.
- Is the lens occluded? If it’s fogged up or dirty, find out from the manual or manufacturer’s website how to clean it. Generally, a microfiber cloth will work but if the lens is plastic you may scratch it.
If you check all of these things and it fails a functional test (won’t switch), you probably have a bad photoeye. Contact the manufacturer’s tech support to further verify.
Once each photoeye passes its functional test, then perform a meter check on the wiring between that eye and its corresponding terminals on the PLC input card terminal strip it goes to. You shouldn’t find any errors, but for the sake of thoroughness this wiring needs to be verified.
Since you’re at the terminal, simulate photoeye switching. The input card should have a light that changes state along with a change of state of the input. You should also view the PLC function (for example, watch the logic execute via laptop).
At this point, you have ruled out problems with the photoeye, wiring, and PLC. Before putting the whole system through a functional test, check the parts hooks to ensure they can actually snag a sample assembly. If, for example, a hook has been removed and reinstalled it may be 90° or 180° out of orientation.
If you now test the system with a couple of sample assemblies, it should work flawlessly. Let the operators have control over it again and watch what happens.
