Static is causing Proximity switch to short

Thread Starter

Jstevensroyal

Joined Mar 27, 2018
1
I have a client with a food and bev plant. they have a prox switch that continually shorts out due to static. They can't avoid the static from occuring, but also does not want to keep replacing the switches. The switches have been grounded, but that has not fixed the issue, it has only shortly prolonged the life of the switches. they are currently replacing them twice a year. the switches they are using is an Allen Bradley 875C-F10N30-R3. does anybody know of a static suppressing prox switch, or a solution to eliminate static interference? thanks for any help you can offer.
 

LesJones

Joined Jan 8, 2017
2,317
From Googling the part number they are capacity type proximety switches. If you have a schematic of the internal circuit of the proximeyy switches it may be possible to add some diodes to short the static to ground. It is unlikely you will be able to obtain a schematic and as they are from a reputable manufacturer I thing some effort would have already have been made to reduce the effects of static. Some more information on the way they are being used may be helpful. For example what sort of item are they detecting, Are the items on a conveyer belt ? Some pictures may also be helpfull. I am thinking that mounting them surrounded by a metal plate with a hole in it that is earthed may help to remove the static charge on the items being detected.

Les.
 

MaxHeadRoom

Joined Jul 18, 2013
19,006
Where are you located?
In many large cities there is a AB office, I have contacted them before for questions and queries on products, it may be worth contacting one and run it by them to see if they can suggest a solution.
They are now owned by Rockwell Automation and a fairly large Co.
Max.
 

LesJones

Joined Jan 8, 2017
2,317
Sensacell,
That is a good point. It could be the nature of the load causing the output device (Relay contact, transistor, mosfet etc.) to fail short circuit. I did not find the data sheet. I was just looking for what method it used to detect proximity.

Les.
 

MaxHeadRoom

Joined Jul 18, 2013
19,006
According to the spec sheet it is a 2 wire AC type, 24-240vac.
2 wire types typically cannot switch very high loads, for this a 3 wire type is generally used with open collector device etc.
If it had been a 3 wire DC type, these are sometimes used to switch a relay direct, and if the BEMF diode is not fitted the device can fail prematurely.
In this case it would help to know what the AC out is operating into.
Max.
 

MisterBill2

Joined Jan 23, 2018
3,860
A capacitive proximity switch has a very high impedance input that may not have adequate protection in this particular application. That is why I have avoided them. My suggestion is to change to a different type of switch, such as an inductive prox switch, if there can be a metal thing for it to sense. Or consider a reflective photo switch as a possibility. Some parts of some food processes do generate thousands of volts constantly. In that case a change of type is the best choice.
 

MaxHeadRoom

Joined Jul 18, 2013
19,006
A capacitive proximity switch has a very high impedance input that may not have adequate protection in this particular application. That is why I have avoided them. .
In many cases an inductive proximity cannot be used due to ferrous metal occurring in the target field and can be ignored with a capacitive sensing.
Max.
 

ebp

Joined Feb 8, 2018
2,332
Tough problem - so many things that might work would probably interfere with the sensor (e.g. a ground grid of very fine stainless steel wires in front)

I would be inclined to try well-grounded tinsel surrounding the sensing end as closely as possible, with the notion of providing a whole bunch of little lightning rods. This may well be completely incompatible with the physical arrangement or wash-down requirements. I have no idea where one might procure corrosion-resistant tinsel. A little crown of sharply pointed stainless steel wires might help, but that is a worker hazard along with the other problems.

If there is a conveyor belt, which is likely the culprit, another similar possibility might be to use conductive bristle brushes, similar to those found on the output slot of things like laser printers and scanners, to discharge static from the belt near the edge where the sensor is mounted. Same problems as before.

Static neutralizer bars or blowers are used on lots of industrial equipment, but once again there is the compatibility issue and possibly a worker safety issue with the bar type. They aren't particularly cheap, but might pay off reasonably quickly vs. cost of sensors & downtime.
 

MisterBill2

Joined Jan 23, 2018
3,860
In many cases an inductive proximity cannot be used due to ferrous metal occurring in the target field and can be ignored with a capacitive sensing.
Max.
There are a whole lot of different types and styles of inductive proximity switches, most don't require a ferrous target, but some do. AND there are some that have very small detection zones for conditions just as Max described..
 

LesJones

Joined Jan 8, 2017
2,317
I have now found a link to the datasheet on these sensors. The sensor is a screened AC two wire sensor with NO contact type. (Page 4-17 of the data sheet.) It looks like it derives it's power from the leakage current on the open state and from a small voltage drop in the closed state. It would be interesting to know if the failed devices had a full short across their terminals. A build up of a film of dirt on the face of the sensor could cause it to falsely indicate proximity to the target.

Les.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
3,860
I have now found a link to the datasheet on these sensors. The sensor is a screened AC two wire sensor with NO contact type. (Page 4-17 of the data sheet.) It looks like it derives it's power from the leakage current on the open state and from a small voltage drop in the closed state. It would be interesting to know if the failed devices had a full short across their terminals. A build up of a film of dirt on the face of the sensor could cause it to falsely indicate proximity to the target.

Les.
I have used a lot of the two wire sensors over the years and in some instances they require what they call a load resistor if they are feeding something like a PLC input. The load resistor is typically a 4R7K ohm (4700 ohm) device in a 120 VAC system. So they require a bit more than typical leakage current to operate. And the ones that I have used maintain about 10 volts across the device in the "on" state, and the full line voltage in the "off" state. In a very high static-electricity application my guess is that eventually the static protection diodes fail and the rest of the switch follows shortly after that. And if a good brand fails then probably the cheap ones will not last as long.
So I stick with my suggestion of trying to find a different technology for the application. A remote mounted reflective sensing switch is one suggestion. Also, possibly, the AB customer assistance folks may be able to help.
 

MaxHeadRoom

Joined Jul 18, 2013
19,006
The sensor is a screened AC two wire sensor with NO contact type. (Page 4-17 of the data sheet.) It looks like it derives it's power from the leakage current on the open state and from a small voltage drop in the closed state. .
This is how just about all two-wire proximities work.
They cannot switch loads that the likes of open collector 3 wire, types, they are mainly used for just a high impedance indicator, not a actual operator.
Max.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
3,860
This is how just about all two-wire proximities work.
They cannot switch loads that the likes of open collector 3 wire, types, they are mainly used for just a high impedance indicator, not a actual operator.
Max.
Certainly correct, Max, but there are a lot of instances where the only PLC input module had to be one that accepted 120VAC, and in addition there was no room or budget for a DC supply. In those cases a 2-wire AC switch is the only choice. This case is like some of those that allowed me to create novel work-around solutions for problems that left a lot of folks totally stumped. They were often a lot of fun, coming up with "ways to do what could not be done."
 

MisterBill2

Joined Jan 23, 2018
3,860
Here is a poem about a challenge:

They said it was an impossible task
And to look at the job, who wouldn't.
Then we tackled the task that couldn't be done
And you know what? It couldn't!

I hope that the formatting of that little piece holds when I post it.
 

panic mode

Joined Oct 10, 2011
1,782
wow, 15 messages so far and still no basic info like what is being sensed, from what distance, what is the sensor connected to etc. :confused:
 
Top