EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: oliver602 on June 09, 2021, 01:00:41 am
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Hi
I need to replace a M12 threaded cylindrical proximity sensor on some farm machinery.
There are two magnets spaced apart to trigger the sensor at different positions as a hydraulic cylinder is retracted or extended.
The output is normally open npn. And there is a fair amount of slop in the machinery so a reliable sensing distance of at least 5mm.
I thought this was a hall effect proximity sensor, but when I searched the big distributors those types of sensors were for speed sensing.
I think a magnet sensor would be more selective considering the dirty environment. Is an inductive sensor ok for this application?
What kind of sensor should I search for?
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It sounds like this is a Hall effect sensor.
Whether you use this for a limit switch or a speed sensor is just your application.
Does this have a 3 wire connection including (probably) a +5V source?
Why do you need to replace them?
Multiple ones going bad sounds fishy.
Are you sure that you didn't lose your supply voltage?
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It's a 3 wire sensor and the supply is 12V. It's a fairly old machine and the sensor is exposed to the weather so I'm assuming water ingress. But I don't know the history. The output was shorted to + supply.
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The difference between the two types is that the inductive prox sensors will trigger on anything metallic, while the magnetic sensors will only trigger in the presence of a magnet.
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The output was shorted to + supply.
Hmm, that's a bit strange.
If it's a normal open collector output there's possibly none or minimal components between the collector and the +V supply.
Are you sure what it's feeding is not shorted? Or the wires?
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Yep I checked the wires and electronics on the machine. The sensor output is connected to a 40106 schmitt trigger inverter through a diode. I also had the sensor off the machine hooked up to a power supply. I had blue and brown wires connected to the power supply and tried triggering the sensor with one of the magnets I took off the machine from both north and south pole and the voltage on the black wire was always 12V. Then I measured the resistance between all the wires and found black shorted to brown.
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An inductive sensor senses disruption of it's magnetic field, whereas a magnetic sensor senses the presence of a magnetic field, so the first can only sense when the hydraulic cylinder moves into a given position, but not if it's already in that position.
If it's limit switch application, then a magnetic sensor is what's required, and what you're describing sounds like a hall effect sensor - you could also use something like a reed switch which would close when the magnet is nearby, and open when it's not, but that would be a two wire device, not three.
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An inductive sensor senses disruption of it's magnetic field, whereas a magnetic sensor senses the presence of a magnetic field, so the first can only sense when the hydraulic cylinder moves into a given position, but not if it's already in that position.
This isn't true, the inductive proximity sensors will detect static positions. They are not variable reluctance sensors that only respond to moving targets.
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Well, it's definitely a Hall effect sensor.
... one of the magnets I took off the machine from both north and south pole ...
Are the two magnets on the machine showing the same polarity?
There is a strange beast of a latching Hall effect sensor that gets turned on by a south pole and turned off by a north pole.
If this hydraulic cylinder was supposed to only extend and retract fully this could be the (weird) case.
It's probably just a regular Hall effect sensor.
I presume there is a pullup on the input of that Schmitt trigger?
There's a lot of M12 sensors here: https://www.digikey.com/en/products/filter/magnetic-sensors-position-proximity-speed-modules/552 (https://www.digikey.com/en/products/filter/magnetic-sensors-position-proximity-speed-modules/552)
In a pinch you could even use a reed switch if it will get your machine back online.