Demystifying Spindle Sensors

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08 Mar 2016 18:49 #71245 by Trispectiv
Hello,
I am trying to convert a Denford Orac to LinuxCNC. I had it running ok on Mach3 until I tried to thread. Hence I am now trying to move it to LinuxCNC. The problem is that I cannot find a straight plain and simple tutorial on how to setup spindle sensors.

I will be using some of these:
i.ebayimg.com/images/g/2VgAAMXQLoZR24W-/s-l300.jpg
(building 3D printers, I have tons of these on stock)
which I hope they can pulse fast enough: 2000RPM x 100 slots.

I routed a simple disk with 100 holes that will attach to the back of the spindle bore. The disc has one longer hole that can be used as 1 pulse per revolution.

As I understand, I will need to mount 2 sensors in the 100 holes per rev area. I will also need to mount a 3rd sensor in the 1 hole per rev area. Wouldn't counting full 100 impulses on the other sensor be the same information as 1 pulse on this sensor (which would both equate to one revolution)?

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11 Mar 2016 11:06 #71386 by cncbasher
100 hole is fine for an orac ,
i used 2 sensors on my orac , as index and the other as channel a , you dont need channel b , but if you have fitted one thats fine .
setup is straight forward , the complication comes in checking your rpm and scaling this to actual rpm etc .
it took me some time in adjusting the readout to be actual speed , with not too much runout when changing between forward and reverse when threading , so you dont end up hitting the shoulder etc .
i just used the origional orac sensors , but adjusted the pullup resistors to get a clean signal as i dropped the input down to 5v ( just made life easier ) .
what breakout board are you using ? ,

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11 Mar 2016 11:38 #71387 by Trispectiv
Well.. I see I spelled it wrong: I made a 50 holes disc, I was counting divisions of 360 degrees needed to get 50 holes.
File Attachment:


Anyway, threading works fine. I only tried the sample included with LinuxCNC on a random piece of aluminium tubing:

and I got this:
File Attachment:
File Attachment:

I guess I needed a little bit more RPM to get a higher surface speed.

Do you still have the original (Swedish ?) motor? Does it lack power while threading as it is only 1/2HP? Does it get hot at low RPM as the fan at its back doesn't cool it properly?

I tried using the original Orac sensors, but i killed the board that was powering them while I was testing things out. I couldn't find any schematics for it, so I had to put them aside.

One thing I don't understand: you have 50 holes on phase A. Hence, you already know when you have a full revolution (50 holes have passed). Why would you need another index pulse to count revolutions?
100 holes on phase A means 2 revolutions, etc.

I used this breakout board:
img.alibaba.com/img/pb/382/733/750/750733382_553.jpg

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11 Mar 2016 14:11 #71400 by andypugh

One thing I don't understand: you have 50 holes on phase A. Hence, you already know when you have a full revolution (50 holes have passed). Why would you need another index pulse to count revolutions?


The index is used to synchronise the start of each threading pass.
In theory you could synthesis it from counting the A-phase pulses, as you suggest.
Using an index guarantees that the threads always start at the same point even through a machine reset. I doubt that matters to many people, though. With a parallel port you will find that the encoder loses counts at max spindle speed. The index means that when you slow back down to threading speed this does not matter.

LinuxCNC can thread with only the index pulse, but only slightly better than Mach3 does (LinuxCNC will at least update the traverse speed once per rev)

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11 Mar 2016 14:37 #71403 by Trispectiv
Yes, I was looking through HAL scope and at the 2000RPM that the Orac can handle, I only have about 20 pulses out of the 50 holes. Is it my sensor, or the parallel port? Also, the wave becomes wider on the active pulse (high), while the width of the low part is very tiny. This even if the hole represent 3.6 degrees opening.

What I am stuck at, is the spindle RPM display. I added that panel that is supposed to show the RPM, but it is about 1.33 ratio lower then the expected RPM. Any particular part of the HAL file I should look at?

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11 Mar 2016 14:43 #71406 by andypugh

Yes, I was looking through HAL scope and at the 2000RPM that the Orac can handle, I only have about 20 pulses out of the 50 holes. Is it my sensor, or the parallel port?


Probably the parallel port. To count 50 holes quadrature you need to sample more than 200 times per revolution. So a sample rate of at least 10kHz. What is your base thread rate?

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11 Mar 2016 15:01 #71408 by Trispectiv
20000
(at the moment I am not at quadrature, just A and Z)

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11 Mar 2016 15:35 #71412 by andypugh
In that case you might well have a problem with the speed of the opto-sensor.

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11 Mar 2016 15:37 #71413 by Trispectiv
Regarding the spindle speed display - isn't it supposed to read it off the sensors? The displayed value is about 1.33 ratio smaller then the actual speed. That is even for low speeds.

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11 Mar 2016 15:40 #71415 by andypugh

Regarding the spindle speed display - isn't it supposed to read it off the sensors? The displayed value is about 1.33 ratio smaller then the actual speed. That is even for low speeds.


How do you know that your "actual speed" measurement isn't 1.33 times too high? What are you measuring it with?

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