Capacitive sensor with a mind of its own!

Ned_Flanders

Member
Join Date
Oct 2008
Location
In hiding, England
Posts
535
I have a strange fault on a capacitive sensor which follows.
On a packaging machine, a capacitive sensor detects a moving belt that has metal rollers (I was rubber belts with metal rollers until 6 years ago). In the last day or so the sensor started to switch on when the belt had passed. The sensor was changed for a new one(Some damage to the face) but it still does it.

Suspecting a damaged cable somewhere, or floating 24v voltage levels it was supplied direct to/from the plc(Siemens S7) but it still randomly switches "On" after the belt has passed and sometimes before the belt has arrived!

You can run the belt in manual all day without the rest of the machine and it will not false trigger.

To prove a point we removed the sensor from the machine and had it dangling outside the machine in free air.

When the machine runs in auto the sensor output led switches on and off randomly - and the output lead is disconnected.

I wonder if anyone has experienced anything similar?

My thoughts are it should be an inductive sensor but the old one has worked correctly for the last 6 years!!!
 
I saw a problem like this once, but it appeared after an inverter was fitted to the machine.
The machine had been relocated to a new position and some electrical upgrading had been done in the building. a new clean earth connection fixed the problem.
 
Like Liam said, check your grounding. Somewhere there is stray voltage floating around or static electricity is building up.
 
I had my arse kicked for over a week due to static in a rubber belt. I ended up fixing it by replacing a bearing with a composite housing to one with a steel housing to "drain" the static voltage to ground.
 
You can run the belt in manual all day without the rest of the machine and it will not false trigger.
When the machine runs in auto the sensor output led switches on and off randomly - and the output lead is disconnected.
I think you have to take a closer look to what is really different in these situations. Like suggested before, maybe interference from some motor cables.
Can't you switch other parts of the machine also on in manual mode to see if that is triggering?
The output lead disconnected has nothing to do with the led on the sensor, that should operate already with a +/- supplied.
Most capacitive sensors I come across are adjustable. Is yours? (correctly?)
 

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