Electric Field Proximity Switch Failure - What Gives?

[HotPotato]

Hello All,

I have recently experienced the failure of a "linear magnetic effect detector". This detector was used to confirm rotation of a spinning bar that had magnets on both ends. This proximity switch worked well for nearly two years but just recently had been failing to detect the rotation at random times during a batch run, causing a trip.

We replaced the proximity switch and the issue was resolved. I would appreciate any information on what can cause failures in these non-mechanical proximity switches. What goes bad on the circuit board that it becomes unreliable? Any links, experiences, or words of wisdom is greatly appreciated.

A side question: Can the magnets that are installed at the end of the bars lose their magnetic fields? These magnets are exposed to large amounts of water, base and acid solutions (they are protected by a plastic cover), and temperatures up to 60C.

Thanks
Mike

 

[IRstuff]

You don't say what the proximity switch consists of, circuit-wise, but it's certainly not impossible to get circuit failures; that's way there's an entire industry built around MIL-HDBK-217 and the whole concept of MTBF. The whole gist of of MTBF is that the bulk of failures are RANDOM, but that they can be accelerated by temperature, vibration, and voltage stress.

TTFN (ta ta for now)
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[BrianE22]

Magnets or switch mounting loose?

 

[hacksaw]

Mike,

Sounds like you have a faulty reed switch that is supposed to close when the magnet approaches, and spring open once it passes.

Just replace the reed switch. You can try to demagnetize it using an ac demagnetizer.

Common switch set up in ornamental displays,,,

 

[itsmoked]

If indeed reed switches are involved they are mechanical and definitely have a fixed lifetime.

Magnets also definitely lose their magnetism over time based on shock, temperature, and associated magnetic fields that counter the magnet's.

I believe Hall effect switches also have a lifetime.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Hypothetically, a Hall-effect switch should not have any magnetic degradations, as it ought not have any magnets. At its simplest, it's measuring the resistance of a Hall-effect sensitive material, and the resistance changes as a function of the external applied magnetic field.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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