Proximity Sensor Wiring Issue? Help needed.


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    Default Proximity Sensor Wiring Issue? Help needed.

    Hi guys.
    I have 3 proximity sensors on my cnc gantry that are used as homing switches. They are normal closed sensors and I believe they are NPN. When they get power the led on them is on. When they sense a metal object the led goes of and the state of the input changes to give the signal. I have been using it for a year now but I have started to think that there is an issue with the way they are wired and I would like some advice.

    Here is a photo of my control box.

    The ribon cable from the break out board connects to an adapter like this one.

    One day after working on my control box I found out that the home switches stopped working. After a lot of trial and error I found out that the reason they stopped working was a strange issue with the adapter. This adapter is glued on the side of the control box where there is also an aluminum sheet. When the metal surrounding portion of the adapter is in contact with the aluminum sheet, the switches work fine. If I push a little bit the adapter and the surrounding metal portion is not in contact with the aluminum sheet the home switches power down and seem tripped on mach3.
    I measured continuity between that aluminum sheet and my -12v connection on the 12v supply.

    Here is how my break out board, 5v supply, 12v supply, and proximity sensor wiring is done at the moment.

    And an illustration of it on my control box too.


    So from what I get the return path to -12v is done through the aluminum sheet which has continuity with the -12v and when the metal portion of the adapter loses contact with the that aluminum sheet the switches power down.

    What I have temporarily done to get it more reliable at the moment is to run a cable screwed on the -12v and the little bolt on the parallel adapter.
    Here is a wiring diagram of this and a photo of the actual cable. It is the grey cable.


    The home switches work fine this way. But I feel that this wiring is wrong and could cause some other issues. I had some issues with a touch plate that I am using to set zero which was triggering randomly before touching the touchplate some times. I installed a 0.1uF capacitor on the input of the touch plate to fix this.
    From what I understand there is already a connection between -12v and ground. When the metal portion of the adapter touches the aluminum sheet another connection with the ground occurs which could create a ground loop??
    After some research I think I found that the correct way to wire my proximity sensors is to not run the ground wire from the sensor to the break out board, but to the -12v terminal? And have -5v and -12v connected together? Like this diagram?


    Any help would be great guys!

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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    It depends on your electronics whether you should be using NPN or PNP prox sensors. I bought a heap of NPN ones and then found out I should have bought PNP ones. The solution was to add a pullup resistor. This video I found very useful.



    Rod Webster
    www.vehiclemods.net.au


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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    My bob has pull up resistors internally.



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    to investigate why I had unexpected results while testing the same BOB
    I reverse engineered it to produce this circuit diagram

    Proximity Sensor Wiring Issue? Help needed.-chinese-breakout-board-jpg

    the 74HC14 used to buffer the inputs connected to the printer port pins 10 to 13 & 15 does not have the +5V supply connected to the IC's supply pin 14 !!!
    so its only powered by any input or output pin that's at a logic 1
    (about +4V depending on the volt drop across the 10K pullup resistors)

    I added a wire link to pin 14 on the next 74HC14 IC

    Proximity Sensor Wiring Issue? Help needed.-chinese-bob-jpg

    the input 10K pull up resistors are a little high and an external 1K pull up input resistor may improve things

    I would not use a screw to connect the 12V negative to the 25pin D-Type connector shell

    Proximity Sensor Wiring Issue? Help needed.-proximity-sensor-wiring-jpg


    John

    Last edited by john-100; 02-11-2018 at 11:12 AM. Reason: add proximity switch wiring diagram


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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    Thanks for he reply. i have all my input pins occupied. Isn't what you show (connect -12v to ground on bob) the same as connecting -12v to -5v directly?



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    yes , but the reason I would connect the 12V power supplies negative connection to the BOB's input gnd terminal instead of the 5V power supplies negative terminal
    is to avoid any electrical noise from the BOB's step & direction output signals being mixed with the BOB's inputs from the Proximity Sensors & Estop

    to use all 5 inputs
    you could connect the negative supply wires from two of the Proximity Sensors to one of the BOBs ground terminals
    ( I am assuming the input ground wires are small enough for 2 wire to go into one screw terminal )

    John



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    Hmmm
    Why would any noise be mixed if the connection is made between -12v and -5v?
    From what I understand ground terminal on the bob is the same as -5v terminal on the bob. Right? Or are there any components between those two terminals? I maybe thinking something wrong here. So can you explain please? Thanks in advance!



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    the BOB does not have an electrolytic capacitor (or two) and several ceramic capacitors decoupling the 5V supply

    as the 74HC224 buffer outputs switch between 0V & 5V a current of about 12mA will flow into the stepper driver opto-isolator

    with 2 or 3 axis moving at once you can have several 12mA square wave currents being taken from the BOB's 5V supply

    at high frequencies the supply wires acts as an inductors
    and the impedance of the wire increases as the frequency increases
    and can be very large compared to the resistance you will measure with your multimeter

    depending on how long the negative 5V supply wire is ,your going to have a complex voltage dropped across the wire
    that can be a problem, if the proximity sensors negative supply wiring is connected to the 5V supplies negative terminal instead of the BOB's gnd terminal

    John



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    Cool I will connect it the way you suggested then and check how it works.



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    is this more like your intended wiring setup ?

    Proximity Sensor Wiring Issue? Help needed.-proximity-sensor-wiring-v3-jpg

    connecting the BOB's ground to the machines spindle can result random triggering of the probe


    John

    PS
    this is a view of the added link to power the input 74HC14
    I made reference to in post 4

    Proximity Sensor Wiring Issue? Help needed.-5v-link-jpg



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    The spindle ground is connected to my bob, so I have only the touch plate connected on my input pin. So there is a free ground terminal after all.
    Also what is the link of the power to input 74hc14 you mentioned for?



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    Default Re: Proximity Sensor Wiring Issue? Help needed.

    Also for the issue with the probe triggering on before touching some times, I added a 0.1uf capacitor between its input pin and ground which fixed it. I plan to add my 4 closed loop stepper drives fault outputs to my e stop chain. If I have any random estop trigger can I add another capacitor to the e stop input? Or will there be any problem with the one already installed?



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Proximity Sensor Wiring Issue? Help needed.

Proximity Sensor Wiring Issue? Help needed.