Industrial proximity sensors. Magnetic or inductive? - Page 1
Forum section: electronics / repair

author: oliver602
author_rank: Regular Contributor
timestamp: June 09, 2021, 01:00:41 am
content: HiI 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?

author: Renate
author_rank: Super Contributor
timestamp: June 10, 2021, 11:40:10 am
content: 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?

author: oliver602
author_rank: Regular Contributor
timestamp: June 10, 2021, 12:02:53 pm
content: 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.

author: TheMG
author_rank: Frequent Contributor
timestamp: June 10, 2021, 01:21:17 pm
content: 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.

author: Renate
author_rank: Super Contributor
timestamp: June 10, 2021, 02:55:10 pm
content: Quote from: oliver602 on June 10, 2021, 12:02:53 pmHmm, 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?

author: oliver602
author_rank: Regular Contributor
timestamp: June 10, 2021, 04:52:41 pm
content: 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.

author: fordem
author_rank: Regular Contributor
timestamp: June 10, 2021, 06:44:02 pm
content: 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.

author: mikerj
author_rank: Super Contributor
timestamp: June 10, 2021, 09:20:58 pm
content: Quote from: fordem on June 10, 2021, 06:44:02 pmThis isn't true, the inductive proximity sensors will detect static positions. They are not variable reluctance sensors that only respond to moving targets.

author: Renate
author_rank: Super Contributor
timestamp: June 10, 2021, 09:49:33 pm
content: Well, it's definitely a Hall effect sensor.Quote from: oliver602 on June 10, 2021, 04:52:41 pmAre the two magnets on the machine showing the same polarity?There is a strange beast of alatchingHall 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/552In a pinch you could even use a reed switch if it will get your machine back online.