Limit Sensor Options / Questions - Onefinity Build

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Ziggy
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Limit Sensor Options / Questions - Onefinity Build

Post by Ziggy »

Hi I am trying to convert my Onefinity CNC over to Acorn. I have made great strides with the help from everyone here.

At this point I am trying to put together some parts for my Home / Limit sensors. I was hoping someone could give me some insight before I spend money where I shouldn’t. So I have a few questions?

Does a larger sensor work better? Does it produce a larger/stronger field?

The Centroid website recommends a 2CM sensor from Amazon:
https://www.amazon.com/Twidec-Inductive ... ss_tl&th=1

I was hoping to use a smaller sensor such as a 12mm with a longer sensing distance. I was looking at this one:
https://www.automationdirect.com/adc/sh ... pnm6-cn-4h


Is a longer sensing distance better?

I am hoping to sense my aluminum gantry. From what I understand, you multiply the sensing distance by .4. So a 4mm sensing distance would only give me a 1.6 sensing distance. (I have also read that a larger object will be sensed less than a thinner one due to the dissipation of eddy currents.)

Here is a picture. I was planning on mounting a sensor on top of the Y end-stop and sense the gantry.
IMG_004260.jpg

Should I use a shielded cable?
These cables will be running in a drag chain with my control and spindle cable. All cables – Including AC – in my setup are shielded. Do these need to be shielded?

If so, I have been using IGUS wire. Does this look OK for limit sensors?
https://www.igus.com/product/442?artNr=CF881-05-03

Also, how much current do these things pull?
The Amazon description says a 300mA output on that sensor. Output???? If each sensor pulls 300mA I am going to have to come up with a different power supply because I think the 24V power supply that came with my Acorn is only 1 amp.

Does anyone know of any good sources for these sensors?

I see the Chinese sensors all over Amazon. Are they any good? I know that some sensors have multiple oscillators and even compensation for swarf buildup… along with a hefty price. I just want something that is reliable and reasonably accurate.

I spent way too long at Mouser, Newark, Digikey, Grainger, McMaster Carr, Amazon, Ebay and such. Most places are sold out or only have 1 or 2 in stock. The Automation Direct sensors linked above require a M12 plug which will evidently add another $10 or $15 to the cost of each…. AD is sold out of the wired version. It is all very frustrating.

Finally, do you think if I mounted a sensor perpendicular to my Z axis that the sensor would pick up the bearing?

Here is another picture:
IMG_004261.jpg
So if anyone has any suggestions or advice it will be most welcome and appreciated. As always, Thanks.
ShawnM
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by ShawnM »

Any of the sensors you linked to will work. I've used the expensive Balluff sensors and the Chinese ones and they all worked. These inductive proximity sensors sense ferrous metals A LOT better than other metals and this will effect the sensing distance. They will sense aluminum but not as good and it may not be as repeatable. They make sensors just for aluminum if you need them. If you use proximity sensors I'd recommend steel flags for the target. Also, it's not recommended to have the target move AT the sensor but have it move BY the sensor. This minimizes damage to the sensor if things go awry. If I had a dollar for every post I read where the builder stated they smashed up their proximity sensors because they mounted it in harms way, well, you know, I'd have a lot more money today. :D

Shielded wire is a great idea for these if you have it but not a must. Be sure to buy NPN NC sensors.

It is best practice to use a separate power supply for all ancillary devices and sensors, the Acorn power supply is ONLY for the Acorn.
Ziggy
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by Ziggy »

Go "by" and not "at." !!!

Great advice! That sorely complicates things on my end... but is something I needed to hear!

I have a 36V meanwell PS that I use for my Gecko drivers. I have some regulators that I used for 24V fans. I could probably just connect there. I may do a little current measuring when I get the sensors.

Thanks again!
Ziggy
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by Ziggy »

Hello again. I am resurrecting my own old thread. :)

It has been about 4 months since I started looking to add sensors to my build.

During that time I had to put a hold on buying parts due to just trying to be financially responsible. However, I kept doing research and decided on optical sensors. ( Shawn's advice about going THROUGH rather than AT made a lot of sense to me.) During my self-imposed financial hiatus, I did a lot of CAD work and a lot of 3D printing getting things ready.

I finally saved enough to buy the parts. (This upgrade also included upgrading my drag chain setup.)

Now that I have the parts, I was bench testing them and they didn't behave the way I was expecting. Maybe my design will not work? I sure hope not.

I decided to use optical sensor rather than proximity sensors for several reasons. The optical sensors use less current and would work out better for mounting in my case. I bought the Panasonic PM25 NPN NC sensors. (Datasheet is attached Page 10) I have one switch for the X axis and two for the Y axis. I also decided to use a regular spdt three wire microswitch for the z axis. I didn't see why that wouldn't work but I now get the feeling this is about to come back and bite me. Here is a link to the switch. https://www.newark.com/honeywell/v7-2b1 ... dp/50F5137

I thought that the sensors would behave like the switch. I thought that the output would switched from power to ground when "triggered."

My benchtesting of the optical sensors gave me unexpected results.

My sensors use the same Brown (VCC) Blue (GND) and Black (Output) wires as shown on page 14 of the paired axes acorn user guide. The schematic seems to indicate that in the NC (Non triggered) state that IN1 and IN2 should be effectively at a COM or ground state,

However, when testing my sensors with a battery, I did not get the results I expected. I connected the positive post of a 9v battery to the brown wire and the negative post to the blue wire. The LED indicator lit up and the sensor is working. Blocking the beam turns the LED off as expected.

Here is where I didn't get the results I expected. I connected my volt meter between the black output wire and ground and got a few millivolts so I blocked the beam and nothing really changed. My sensor has a white wire for NO and it behaved the same way. I tried connecting the ohm meter between the output and ground and it would cycle from about 3.5 Meg ohms to infinite resistance.

Finally I connected my volt meter between the black output wire and the positive side of the battery and I finally saw a result. With my volt meter red lead on the positive post and the black lead on the black output wire, in the NC (non triggered) state I read 9v. When I trigger the sensor it goes to 0v.

Is this the way it is supposed to work? Are the IN1 and IN2 looking for power rather than ground in the NC (non triggered) state?

Sigh... I know this is long. Have I screwed up and spent way too many hours to count designing and building mounts for sensors that aren't right? If I have just mis-understood the Acorn schematic, is it OK to use a mechanical switch for Z?

I have NOT connected these to my Acorn board yet. I wanted to do a bench test first. Hence my mild panic.

Also, I have fully shielded flex wires from IGUS for this setup. (Somewhat paranoid about EMI)

As always, any help is greatly appreciated. This includes berating me for not completly RTFM. I need to get this documentation printed but I don't have a printer and the print shops want a kidney per page. My old eyes can only stare at a screen for so long so I may have missed where the expected inputs to the ACORN are explained.

Thanks!!!
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paired_axes_acorn_user_guide.pdf
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pm-254565_e_cata.pdf
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cncsnw
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by cncsnw »

Finally I connected my volt meter between the black output wire and the positive side of the battery and I finally saw a result. With my volt meter red lead on the positive post and the black lead on the black output wire, in the NC (non triggered) state I read 9v. When I trigger the sensor it goes to 0v.
Yes, that is how NPN transistor outputs work. The "load" that they are controlling needs to be between +V and the transistor output (black wire). The NPN transistor provides a path to 0V when active. It never provides a path to +V, even when inactive. See the circuit diagrams on the top half of page 10 of the Panasonic data sheet that you posted.

As such, they should work just fine with an Acorn PLC input. Acorn PLC inputs are a load, with +24V on one side (internal to the Acorn), waiting for you to connect the other side to 0V.

Whether you can combine two or three of these sensors in series to a single "HomeAll" input is something you will have to experiment with. I would try hooking just one sensor up to an Acorn input first, to prove that they work as expected.

I have no doubt you could connect one optical sensor in series with your electro-mechanical limit switch: just include the normally-closed switch contact between the output of the optical sensor (black wire) and the Acorn input.
Ziggy
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by Ziggy »

Great!! Thanks!!! Much relief on my mind!

And thanks for the Heads Up on only including only the normally closed switch contact on the mechanical switch.

I thought I read somewhere in one of the manuals that you shouldn't use a 2 wire switch. But now that I have gone back to the install manual I see that it was talking aobut a 2 wire sensor rather than a mechanical switch.
ShawnM
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by ShawnM »

Good luck with the build and the optical sensors but I'm not sure they were the best choice for a router. You do know that routers make chips and dust that can easily trip an optical sensor, yes? :shock:

As for the current draw from an optical versus a proximity, it's not an issue as you'll use a dedicated power supply for the sensors and other ancillary devices. You will only ever trip one sensor at a time anyway.
Ziggy
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by Ziggy »

Thanks Shawn. That is a good point. This time though I did think of that. That was the biggest downside.

My CNC was never designed for sensors. It was designed around stall homing.

So I designed a mount to where the sensor is nearly completly enclosed except for the opening which is recessed 5mm.

I also have the sensors mounted on the outside / behind the rails so they are somewhat out of the line of fire.

Whether they work in reality is yet to be seen.


Here is a picture with the sensor inside the enclosure.
IMG_3888.jpg
Here is one with the cap.
IMG_3889.jpg
And this is the opposite side showing the "probe" attached to the X/Z carriage. (Not sure what to call it.)
IMG_3890.jpg
ShawnM
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by ShawnM »

Sounds good, time will tell I guess. I call it a "flag" and not a probe. A probe is really a measuring device.
Ziggy
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Re: Limit Sensor Options / Questions - Onefinity Build

Post by Ziggy »

Good news! I did a bench test using jumper wires with all three optical devices and the mechanical switch and it worked.

I used the <alt> <i> to look at IN1 and IN2. They both showed "lit." When I tripped Z, Y Master or X, IN1 would go "out" When I tripped Y Slave, IN2 went out!

The next step is to actually put them in place and wire them properly.
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