Hello everybody I'm having a little bit of a problem with some pneumatic cylinder reed switches and need your help.

We have a pretty simple machine consisting of a slide and press. These machines are all over our plant and consist of essentially the same bill of materials.

One machine constantly stops during autocycle.

When looking at the reed switches on the cylinders during one of these instances the switch is stuck on (according to the LED) while it should be off. We are running a slc 500 brick that we've changed as well as 8 Numatics reed switches and three cylinders.

Do any of you know what could be causing these reed switches to be staying on? Thanks.

 

Check out the mechanics on the inside of the cylinder. Sounds like there might be some broken parts wandering around inside that cause the reed to think that the piston is still there. Also check the current load on the reed... if you are just feeding into a PLC, it should be OK. If there is anything else on the same line...it could weld the contacts closed. Those little guys don’t take much current. Also check for mechanical damage to the reed. I have seen some that were cracked and still worked for quite a while until the contacts became misaligned and stayed closed.

Gerald Beaudoin

 

Two things that will do that are residual magnetism and switching excessive or capacitive loads. The reed switches work by mutual attraction when temporarily magnetized, but if they actually get magnetized, they no longer need the external magnet. Try degaussing one and see if that fixes it. If, not you are probably welding them. Oh, and check if the cylinder has been magnetized. Most are non-magnetic
stainless, but you never know.

Regards

cww

 

There could be something in this machine that pulls much higher current through the reed switch contacts and welds them closed.

 

The MRS switches may not be mechanical magnetic reed switches. They may be two wire hall effect switches. If this is the case, there may be enough leakage current through the switch to turn on the PLC input. The two wire hall effect switches are not necessarily compatible with a PLC.

If they are two wire hall effect switches, try changing them to actual reed switches, or to three wire hall effect switches.

 

Yes I know - I'm pretty sure the reed has been burnt. Quality reeds are good for millions of operations if connected correctly. The reed has experienced a load surge and the contacts have most likely burnt shut. Even a low voltage relay will induce an arc - only for a mili second - yet high enough to burn the plating off the switch. You can check this with an ohm meter - do not use a light to check.

Don't take this for granted - the reeds are usually rated a 0.5A resistive. You can add a snubber circuit to prevent this with a varistor or resister - capacitor circuit if it's AC or a diode if it's DC.

For circuit explanation and instructions look at Hookup Rules at http://www.almegcontrols.com/manual_level_control.htm

Thanks,
Bob

 

I suggest replacing the reed relay to a mechanical relay. That's from my quality product development experience. Reed relays tend to stick in the on position when there is a high current going through the main line.

 

I hate to discourage someone from giving suggestions, but in this case I can't see how a mechanical relay could replace the reed switches. These are cylinder switches, meaning that a magnet inside the cylinder activates the reed switch that sits on the outside of the cylinder.

Of course, it could be ME misunderstanding your answer. If you mean a mechanical limit switch, that would make some sense. Of course, that wouldn't be a drop-in replacement. You'd have to physically mount them, and the reason for using reed switches in the first place is often lack of space. Also, if you can mount a mechanical limit switch I usually recommend an inductive prox switch instead.

In modern times, there are solid state cylinder proxes that are better than the old mechanical reed switches, but even these I've seen problems with.

-James Ingraham
Sage Automation, Inc.

 

How about some stray mag field in the area sufficient to hold the switch on once activated by the piston magnet?

Or, if the wire from the switch cable is terminated at a terminal rather than run directly to the SLC wiring arm, then the wire from the the other side of the terminal to the SLC doesn't sound like it's one of the items you've replaced. Maybe there's a spot worn through the insulation and grounded. This would create a lot of load on the switch when on, and might weld the contacts.

 

My sentiments, as well, James Ingraham.

And, this is for a thread that was started in 2005 and didn't see any activity until 2011....

 

It helps to limit the current through the reed switch and suppress any arcing. And degaussing the reed capsule with a tape head demagnetizer can sometimes work wonders.

Regards
cww

 

But Curt,

there's something special about this particular unit, and it's in a piece he hasn't changed yet. He's got lots of other identical units that don't do this. Or at least he did back in 05 when the question was originally asked. Who knows why we're talking about it today unless for the sake of someone scanning the archive with a similar problem.

 

Yeah, I didn't look at the dates:^)
But, the general tips may be of use. This is a common enough problem.

Regards
cww

 

I saw the date. I decided to post anyway and not mention it. One could argue that this is because I couldn't pass up an opportunity to hear myself "talk." I'm not sure I have a good defense against that accusation.

-James Ingraham
Sage Automation, Inc.