Micrologix power supply, hardware Q

[a12bee]

Hi guys,
I'm neck deep in other projects so I don't have the time to do the full research at the moment.
I have a machine running a micrologix 1000 and recently had a prox sensor start detecting things further out then it should. Normally it looks out about 1" and started looking out about 6".
I replaced the sensor which worked correctly for about 2 weeks then it started looking out to far like the first one did. For the time being I've just done the electrical tape diffuser hack.
When I check the dc output from the plc I'm only getting 22 volts instead of the expected 24.

Could this be why I'm having problems?
Can I just fix that bit of it or are these disposable units?

Any help would be appreciated, thanks fellers.

 

[Ken Roach]

Most proximity sensors that use a 24V supply can tolerate a pretty wide supply voltage, often 18 to 30 volts. It would make proximity sensors very hard to use if they changed sensing range based on the supply voltage.

But that's just a guess.

My suggestion is to change to a separate power supply instead of the user supply on the MicroLogix to diagnose if that's the problem.

You can't service or replace anything inside a MicroLogix 1000- if they're broken you replace the whole controller.

I think it's more likely that the failing component is the proximity sensor itself.

 

[a12bee]

Thanks for that.
As mentioned the one that's in there now has only been running two weeks and has 'failed' in the same way. I'd hate to keep throwing sensors at it if the p/s is what's causing the failure.

I kind of thought these were a disposable unit, the whole plc I mean.

The 22 v vs. 24 v doesn't seem out of hand to me but figured if anybody knew it would be the group that gathers here.

 

[mordred]

might be worth looking into a diffrent brand of proximity sensor sound like this particular model loses its caibration easily

 

[Clay B.]

Sensor Question

Is this a Capacitive Prox or an Inductive? Also is this prox sheilded or non sheilded sensor.

I have never seen 2 volts have an effect on the range. Most 24 VDC sensors will operate form 18 vdc to 30 vdc. And the range is pretty much a constant thoughtout that voltage range.

 

[Lancie1]

You could be unlucky enough to have the 2nd prox sensor fail that quick, but I would look around for something else that has changed recently. Has the equipment been changed, speeded up, slowed down? Does it vibrate more than it did previously? Have any electro-magnetic radiating devices been added in the area (high frequency devices, big electromagnet, large transformerss, X-ray machine, microwave device)?

EDIT: Wait, you said prox sensor, but is it really a "proximity" type or a photo sensor (light activated) type? Your statement about the tape makes me bring this up.

For the time being I've just done the electrical tape diffuser hack.

If it is really a photo sensor, then disregard my previous advice above. A photo switch is not affected by magnetic radiation, but it can be fooled by light reflections from shiny objects nearby.

 

[a12bee]

Ahhh, you're right Lancie, it is an infared photo. Duh.
I'll have to ask my helpers if they've swapped any of the lighting over there recently. One of those things you don't really notice when you're busy looking down. I have several of the same sensors on the machine and they're not acting funny but this one is exposed out in the light.
Heh, I knew y'all would point me right.

 

[Gil47]

If its a photo sensor then some of them have a teach function, by pushing and holding a pushbutton built in to them, will change the sensing distances.

Looks like someone has taught these sensors new settings

 

[a12bee]

I'm having a slightly less hectic day. So far.
Nothings changed in the lighting, darn it, I can completely shadow it with the same results. Oh, and it's not the trainable type btw.
Weirder things have happened I suppose.
I'll just shove another one in there and see what I have in a week or two.

Thanks guys!

 

[Lancie1]

For an infared detector, it can be fooled by a new shiny label on a package, or a change in the packaging material, or simply if the package is hotter (giving off more radiation) than it used to be.

 

[PhilipD]

a12bee,

I don't know if this will help or not but here goes anyway. Infrared light is not the same as visible light so shading it might not help. A few years ago I had a problem that used a photo switch with infrared. It started to "act up" and would detect items when none were present. I switched out the switch a couple of times but I still had the problem. I put one together in the shop and tested it for two days with no errors and put it on. Still didn't work right. Then I started looking around for something that might have changed and what I found was on the machine next to the one I was having the problem. A new system was being tested and evaluated for inclusion on some of the production equipment. This system detected the presence of glue on a box before the box was folded. If no glue a light went off to alert the operator. You know what? The glue detector used infrared to scan each box as it went by. The infrared from that sensor was being picked up by the one I was using and sending a false signal. I told the vendor running the test and they were able to adjust their system and no more problem.

By the way. The glue detect system didn't work out to well. There were too many problems with it. Ended up going with another system for glue detector.

 

[ScottTheEngineer]

We had camera systems that mysteriously started failing at around 9pm every day though it ran without issue for monthes. Sunlight from the truck dock got it. Maybe look for a window? sunlight does funny things. also if the medium thats being read changes slightley, 2nd shift guy puts paint marks where 1st shift guy doesn't etc..

 

[HJTRBO]

Check your ripple voltage...

 

[a12bee]

Check your ripple voltage...

Sorry, I'm unfamiliar with the term ripple voltage, would you mind a brief description?
Scratch that, quick trip around the 'net answered it.

I had a helper install another eye and oddly this one reads at 10-12 inches despite being the exact model/number/specs as the first one I took out.

This is a case erector and the box manufacturer may have changed the ink on the packaging which could be reflecting more back then the previous ink. Seems to detect the same no matter what you put in front of it though. My hand hasn't left my body so I'd have to guess that it hasn't changed.

Sure, there are other ways around the problem but the initial WTH?!?!? is tough to shake.