Strange failure of magnetic sensors

[xyzz]

I have a very strange issue with magnetic sensors in my new
project.
Four Numatics 940-300-032 Hall effect sensors are attached to two 50mm bore SMC pneumatic cylinders and connected to Allen Bradley Micrologix 1500 inputs.

The sensors repeatedly go out of order, one or several at a time.
Sometimes they become continuously on, sometimes do not switch on anymore.
I tryed inputs on another module of the PLC.
I introduced 2.4k current limiting resistors into lines.
No effect.
Another similar sensors on the machine work normally.

This is the first such case in my 20 years practice.
Any ideas?

 

[automatic2]

Best be checking for shorts. Increased loading on the devices will alter thier operating characteristics, as will compromised supply feeds.

 

[automatic2]

And, check that the retaining straps are non magnetic and that metallic objects are not in close proximity. Stainless is typically used to reduce disruption of the pistons magnetic field.

 

[xyzz]

Shorts are not the case, since I tried to limit
the sensors output current with no success.

Concerning your second post: we are not talking about false signals or missing signals about magnetic piston position.
The sensors are permanently damaged. Some of them even melt locally near the chip inside (the body is plastic).
Do you mean that magnetic field disruption or too strong field may bring a hall effect sensor out of order?

 

[automatic2]

field disruption by adjoining objects (metallic). Excessively high loading on the output can however alter the switching characteristics of the device. If you are having a melt, you likely do have a short. where did you try to current limit?. Check your power supply for an AC component, and maybe insert an opto isolator on the output, local to the device.