I'm having a problem with reed switches sticking on a couple of pneumatic cylinders stalling out my systems. I'm running Allen Bradley SLC500 PLC's with 1746-IA16 input modules. I normally use Hoerbiger-Origa KC3356 reed switches, but have also used Festo reed switches. Both present the same problems. I have 5 systems all with the same equipment but only have this problem on 2 of them. Several times a day the reed switch will stay made even after the cylinder has shifted the other way, and will stop the process. All you have to do is tap on the switch to make it go off. Which leads me to think the leaves inside the switch are sticking. I have replaced the reed switches themselves as well as the input modules. I have checked the wiring best to my ability and found no bad or chafed spots. No voltage problems that I have found. The reed switch is rated for .2A but when closed is only pulling 12.5mA so I do not think the switch could be welding closed. I have run out of ideas. Why else could these switches be sticking.

 

A photograph and a supplier drawing of the reed switch location is a good start

 

If this is a cylinder with a short stroke, make sure the reed switches is properly adjusted so it will reliably turn off. Otherwise I would be getting someone to check the cylinder to see if the magnet is damaged or replace the cylinder altogether.

 

Consider checking environmental conditions, mechanical alignment, and actuation force for the reed switches. Ensure proper vibration control and try a different switch model. Implement debouncing in your PLC program for stability.

 

Consider checking environmental conditions, mechanical alignment, and actuation force for the reed switches. Ensure proper vibration control and try a different switch model. Implement debouncing in your PLC program for stability.

Definitely check this. We had a slew of Festo reed switches fail in the machine and then work perfectly on the bench. The machines were ultrasonic washers and the switches were in a very hot very humid environment, beyond their ratings. We replaced them with properly rated models and the problems went away.

 

Consider checking environmental conditions, mechanical alignment, and actuation force for the reed switches. Ensure proper vibration control and try a different switch model. Implement debouncing in your PLC program for stability.

Actuation force has been an issue for me, more so with older switches, but the other thing that happened recently was a single lead grounding out caused a false high input of a limit switch. It gave no other indications of degradation: switch operating properly, no continuity between process leads when lifted. It would throw high when just the negative lead was attached, positive lifted.

 

Thank you for all of your suggestions. I had considered vibration to be a problem but wasn't sure what to do about it. I have since gone with a different manufacture reed switch with a higher current rating and have not had the problem repeat since.