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Thread: Tachometer

  1. #16
    Ueee's Avatar
    Ueee is offline Blacksmith, Cabinetmaker, Machinist, Messmaker
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    Quote Originally Posted by BobL View Post
    Do these proximity sensors generally work with the same displays as the magnetic ones use or do they require a special display?
    I see they usually have 4 choices (Dry contact、PNP、NPN、Pulse) presumably the display needs to be wired to suit?
    I used an NPN (?) type sensor on the Mars. It just needed a pull up (or was it down....i can't remember if it goes high or low when an object is sensed) resister to hold the non sensing time "steady" so no ripples would effect the readout. An oscilloscope really helps to see what is going on. It will all be in that Mars lathe DC conversion thread..

    Ew
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  3. #17
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    > Do these proximity sensors generally work with the same displays as the magnetic ones use or do they require a special display?
    > I see they usually have 4 choices (Dry contact、PNP、NPN、Pulse) presumably the display needs to be wired to suit?

    The best is IMHO to decide on the display first and read its manual, then buy a sensor to suit. Of course it is always possible to convert the signal between sensor and display, but this needs extra components in between. It is always simpler, quicker, cleaner and more reliable if the sensor and display are a match to begin with. For me, the only reason to do a signal conversion is when I buy nice sensors or displays as NOS eBay bargains, just because I think they may one day come in handy. Then when I need an rpm display I have to use the parts I have collected

    There are three common proximity sensor types that can be used. Hall (simplest of all), Inductive (mid-price), capacitive (most expensive) and photo-electric (from a cheap light barrier to pricy intelligent detectors). Each of these four types comes in a wide variety of shapes and packages, in a variety of sensing distances, in normally open or closed output, in PNP or NPN output, or in PNP or NPN open collector output. Hundreds of permutations to choose from.

    - A Hall sensor simply senses a magnetic field. It is good at things like sensing a moving magnet attached to the piston inside an Alu air cylinder to detect end of stroke.
    - An inductive sensor uses an oscillator and a coil to create its own magnetic field. It detects changes in oscillator frequency as an object or objects interferes with the field. The sensing distance is fixed.
    - A capacitive sensor uses an oscillator to create an electric field. It detects changes in oscillator frequency as an object or objects interferes with the field. It often has an adjusting screw that lets the user adjust the sensing distance.
    - Photoelectric sensors (light barriers using usually invisible infrared light) have the greatest sensing distances and can not only detect objects, but sometimes also shape and motion.

    Chris

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