Reed switch not always triggering

[rguimond]

I'm using Aurora ARS302 reed switches to sense position of a pneumatic cylinder piston (it's equipped with magnetic ring). We started having trouble a month ago whereby the reed switch would sometimes not pick up the piston. It could usually be remedied by someone moving the reed switch. If not, changing the reed switch usually helped. We installed a new cylinder, thinking the magnet was weak, but it didn't help.

The reed switches are connected to a ML1100 (24VDC input. Since changing the cylinder didn't help, I've begun to question whether or not the internal resistance of the inputs is sufficient to always trigger the reed switch.

Can anyone recommend a suitable resistor size I can try in series with the reed switch?

 

[bce123]

We started having trouble a month ago

sufficient to always trigger the reed switch

Was there any trouble before this how long have they been in service?
have you checked/replaced wiring to the switch?
have you tried moving inputs you could have a bad one
can you consistently trigger the switch/input with a magnet?

 

[rguimond]

It's been in service 20+ years, but ML1100 was installed about 2 years ago, replacing SLC500 fixed I/O. No trouble until about a month ago.

Wiring has been changed.

Didn't have spare input, but tried swapping with another and symptoms returned.

Switch will not trigger with magnet when it's in the state where it should detect the piston.

 

[The Plc Kid]

Did someone change switch model or type. There are different types of switches that use different magnets. mix them up and it won't work at al or be very unreliable, Also waer on the pistion seal will make it not set concentric and can caues pickup problems. try a new cylinder and check the switch type.

 

[jrwb4gbm]

Just a blind guess here, and some ideas. Maybe the orientation of the switch could be changed. Here are some suggestions, turn it left, right, up, down, rotate the reed switch, swap the reed switches from one end to the other. Maybe remove some support material from the swich to be closer to the cylinder piston. Maybe someone substituted the wrong type reed switch for that particular model cylinder. It could have been anyone from the manufacturer on down thru the supply chain. If possible, add some external limit switches that sense what the cylinder is moving.
I never have liked reed switches on cylinders mainly because of linkage failure between the cylinder and what it was actuating.

 

[rguimond]

Just a blind guess here, and some ideas. Maybe the orientation of the switch could be changed. Here are some suggestions, turn it left, right, up, down, rotate the reed switch, swap the reed switches from one end to the other. Maybe remove some support material from the swich to be closer to the cylinder piston. Maybe someone substituted the wrong type reed switch for that particular model cylinder. It could have been anyone from the manufacturer on down thru the supply chain. If possible, add some external limit switches that sense what the cylinder is moving.
I never have liked reed switches on cylinders mainly because of linkage failure between the cylinder and what it was actuating.

Switch model is identical to the originals, which have lasted over a decade.

Orientation can only be connector-up or connector-down, due to the way the switch clamps to the cylinder.

I hate reed switches, too, but it's the only alternative for this piece of equipment.

 

[jrwb4gbm]

I'm sorry none of the suggestions helped. I had a few more crazy ideas since my earlier post to possibly check out. There may be something mechanical on the machine stopping the cylinder before end of travel or a foreign object inside the cylinder. Perhaps you could remove the cylinder temporarily to test this. Another idea is that somehow the reed switches are polarity sensitive due to the magnetism. Try swapping the leads going to the switch. Another idea is to put an external magnet at the proper polarity near the reed switch to assist the cylinder magnet. Just trying to think outside the box. Good luck.

 

[keithkyll]

1) High resistance contacts (corrosion). Monitor voltage at PLC module.
2) Module or other high current load causing reed to get arced. Cut apart a failing unit. Reed should be glass and easy to see inside.
3) Add another circuit temporarily, with reed driving a relay. Relay in turn signals PLC. Get a relay with an LED. This gives you a bit of a load on the reed. Should be better at revealing intermittents, etc.

 

[iant]

Have you tried to set up a directional latch
so as the cylinder moves past the switch it's input signal is latched ON
it is one way to confirm that the switch is read.
you Unlatch when you move the opposite direction.

 

[geoplc]

Not sure what type of machinery is using the switch but I have seen guarding or guard covers on weld fixtures that will cause reed switches to turn off when they should be on and turn on when they should be off. I am not sure if the metal becomes magnetized over time from the weld current but we started to ensure that our guarding was not too close in proximity to the switches or we used a non-metallic type material. At first thought it was some type of voodoo but were able to duplicate and verify this was happening. This particular switch was made by Turck.

 

[Lancie1]

To summarize your problem:

Didn't have spare input, but tried swapping with another and symptoms returned.

Switch will not trigger with magnet when it's in the state where it should detect the piston.

Switch model is identical to the originals, which have lasted over a decade.

... changing the cylinder didn't help...

It's been in service 20+ years, but ML1100 was installed about 2 years ago, replacing SLC500 fixed I/O. No trouble until about a month ago.

Wiring has been changed.

It seems that the wiring for your switches (or for some other equipment in the area) was changed, added, or rerouted so that your switch circuits are picking up stray induced noise from the environment. Try changing the wires to shielded cables. Connect the cable shields to a ground point at the PLC end, and leave the other shield end at the switch ungrounded.