prox switches in series

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ShawnM
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Re: prox switches in series

Post by ShawnM »

gttool wrote: Tue Oct 26, 2021 5:03 am Hey Shawn i just realised in the video how the first is used as the trigger , dont think i was wiring it that way
spoke to my Omron rep , he said it wasn't possible , haha
Finished all but the limit switches , tt and e stop wiring
Next one Gary will have the ETH1616 - router with tool changer and dual tt
Thanks for the advice
Did you wire it just like the diagram I posted or the video? It works, don't overthink it. I've done this with NC Omron switches as well as the cheap Chinese ones. Not sure where the issue is with what you are doing. Wire it just like the diagram I posted and see if it works. Do your switches have the same color wires, brown, blue and black? Is brown + VDC, blue is ground or 0 VDC and the black is the output?
gttool
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Re: prox switches in series

Post by gttool »

Machine is all up and running
Had another go today with sensors,wired as the diagram ( wires are different colors ) tested all first and they would all trigger individually
Wired only two together
24v in , would see 23.5v out when triggered doesnt make sense
only one would induce voltage drop
Have some others NO coming for the limits
ShawnM
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Re: prox switches in series

Post by ShawnM »

gttool wrote: Tue Oct 26, 2021 11:38 pm Machine is all up and running
Had another go today with sensors,wired as the diagram ( wires are different colors ) tested all first and they would all trigger individually
Wired only two together
24v in , would see 23.5v out when triggered doesnt make sense
only one would induce voltage drop
Have some others NO coming for the limits
What color are the wires? Can you share a pic or wiring diagram? Voltage out when triggered? Are you sure you are doing this correctly? I'm so confused. Good luck with those NO limit switches.

I also did a Google search for your sensor part number and everything I find tells me that it is a NPN NO switch. Can you verify that you have the correct number or that it is a NC switch? When you power it up is the light on the sensor on or off when there is no metal in front of it?
gttool
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Re: prox switches in series

Post by gttool »

I did google it also and it came up as a NO but the output is 24v light off - 0v light on , these were all wire to a mach3 board in the machine ,
not to worry
looking at some absolute servos , wont need any switches with them , another topic
drdennis
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Re: prox switches in series

Post by drdennis »

(Assuming you are correct about your switches being NC/NPN)

As @ShawnM said, the problem is that your wiring was incorrect. Your sensors are NC which means that in the inactive state the output NPN transistor is on (closed) which makes the output sit at 0V, not 24V as you assumed. So to properly daisy chain the sensors the output of the previous sensor must go to the ground terminal of the next one. The power (24V) terminals must be connected together.

This is electrically a perfectly fine way to daisy chain the sensors and should make your setup work without any additional hardware.
ShawnM
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Re: prox switches in series

Post by ShawnM »

drdennis wrote: Thu Oct 28, 2021 11:33 am (Assuming you are correct about your switches being NC/NPN)

As @ShawnM said, the problem is that your wiring was incorrect. Your sensors are NC which means that in the inactive state the output NPN transistor is on (closed) which makes the output sit at 0V, not 24V as you assumed. So to properly daisy chain the sensors the output of the previous sensor must go to the ground terminal of the next one. The power (24V) terminals must be connected together.

This is electrically a perfectly fine way to daisy chain the sensors and should make your setup work without any additional hardware.
Thank you drdennis, I've been banging my head against my keyboard for days with this topic. It'll work if he'd just wire it as I posted. Now he's moving on to absolute servo motors, that should prove to be interesting.
gttool
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Re: prox switches in series

Post by gttool »

Thanks for your votes of confidence ,
i will revisit them for yours and my sanity
the sensors were marked in the machine 24v 0v S
no absolute servos for the acorn
drdennis
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Re: prox switches in series

Post by drdennis »

I'd like to add a few words of caution to what I said earlier. I was mainly reflecting on the wiring mistake described in the original post -- the goal being correcting the logic in order to make it work as desired. But here I want to make a few more subtle, but equally important points.

TLDR: Always verify compatibility in standard configurations, and doubly so in "creative" non-standard ones.

Longer version:
People may want to treat the proximity sensors as plug-and-play devices, but note that there is no standard for the sensors to comply with. So compatibility is an issue. Unlike with Ethernet or USB where you know that [extremely likely] a device will work (because it must comply with a well-defined spec) these sensors while often having similar specs do not have a standard to follow. Worse yet, the designers of the controller input circuit also have considerable latitude for implementing input characteristics. These sensors are not switches -- they are analog devices that, with some care, can be treated in a binary (on/off) fashion -- as long as each device matches the input design of where it is being plugged. One could argue this is analogous to digital circuits which we comfortably treat as digital even though technically they are analog, but the difference is that with digital logic the signal levels (both input and output) are well specified and well controlled.

In the world of proximity sensors, we solve the compatibility by introducing huge margins -- both the sensor and the controller inputs are carefully designed to work with a wide range of parameters. The important point to note is that when you start creating non-standard configurations, you may be changing (cutting) the margins. It is definitely possible since you start with a lot of headroom, but one does need to be careful.
Unlike an NC switch which is truly a short when closed, an NC NPN device has a BJT transistor on its output which will (in the "closed" state) sink current provided by the input of the controller. The current flowing from the controller into the sensor "output" will produce proportional output voltage (on the sensor) due to the nature of the BJT transistor being the output element. That voltage (typically <2V @ 200mA) will be ignored by the controller input as long as it does not exceed certain, likely undocumented threshold. Chaining these devices puts the outputs effectively in series thus adding these undesirable output voltages -- something to watch for, especially in applications where the controller outputs high current on it "inputs". Chaining as described will also cause the sensor oscillator/detector circuit currents to be added to the controller current thus increasing the total sink current on the "lower" devices in the chain -- another parameter to check.

The bottom line -- check with your friendly electrical engineer if you are putting together anything other than vendor recommended devices and setups. Things may very well work way outside of what the vendor was willing to guarantee, someone just has to do the homework.
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