Confused about LinuxCNC and Fanuc Redcaps on a running machine

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31 Oct 2018 20:54 - 31 Oct 2018 22:27 #119727 by toolmaker69
Hi all,

I'm at the point where I would like to change my business from production to piecework. I have a 1988 Pratt Tape Mate that is fully functional and I love it.

However, it is only 3 axis and has little memory. I have some rotary tables that I use as indexers, but I would like to use them as full axes so I can do some four and five axis work.

The Fanuc motors are redcaps, and I would like to use the drivers, powers supply etc., and just send signals and read the encoders with LinuxCNC. The large Cushman rotary table has a stepper motor on it, and the small rotary has a servo motor that I guess I would need to replace, since it uses a tachogenerator.

My questions:

Would the Pico kit work with this motor?
Do I need anything beyond the Pico kit for the redcaps to make this work?
Do I need to figure out where the PWM signals go on the Fanuc, or is there information somewhere in an archive on how to hook them up?

I may have done wrong, but I have Mesa 5i25/7i77 cards.

Attached is a pic of the Z axis motor plate.

Thanks for any help.

Edit: I forgot to mention, the control is a Fanuc 0MB (some call it OMB).
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Last edit: 31 Oct 2018 22:27 by toolmaker69.
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01 Nov 2018 15:11 #119762 by andypugh
Which Pico kit do you mean? It seems that you already have a set of Mesa cards and the original amps.

Also, the tachogenerator servo will be perfectly OK with LinuxCNC as long as the drive has a tacho input. Alternatively you can probably just ignore the tacho and control on encoder-only
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02 Nov 2018 02:35 #119800 by toolmaker69
Thank you for the answer!

Again, I may be all mixed up. I've had 4 tabs open with various LinuxCNC threads for over a month, trying to figure out what I need.

1. I thought I needed some kind of kit to translate the Fanuc encoder output to something the Mesa card would understand. Is that not correct? Or is that just when using non-Fanuc amps?

2. I thought one should never run tacho-generator servos without tacho feedback. I have lots of tacho-generator servo motors around. Could I put them on the rotary tables, or even use them for a 3 axis mill without using the tachogenerators?

3. If I was ignorantly reading threads that were based on just using the motors and not the Fanuc drives, then is it pretty easy to just find the PWM inputs on the drives and run the machine from there? (I know the Robodrill type toolchanger will be a challenge to program, but I think I can handle that.)

Thanks again Andy.
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02 Nov 2018 12:57 #119812 by andypugh

toolmaker69 wrote: 1. I thought I needed some kind of kit to translate the Fanuc encoder output to something the Mesa card would understand. Is that not correct? Or is that just when using non-Fanuc amps?

It depends on what type of encoder it is. The 4-channel commutation + quadrature type can be used with a normal encoder counter for position and the bldc HAL component can use the 4-channel commutation signals for motor commutation.
If you are using Fanuc drives then it might be that the drives themselves interpret the commutation signals.
If the motors have the Aa64 serial encoders (or similar) then there is a module for the Mesa cards that interprets the data and retains the absolute position information in to HAL. I _think_ that the Pico converter for serial encoders only converts to quadrature and index, so you lose the absolute data. I might be wrong.


2. I thought one should never run tacho-generator servos without tacho feedback. I have lots of tacho-generator servo motors around. Could I put them on the rotary tables, or even use them for a 3 axis mill without using the tachogenerators?

You should certainly not run them with a velocity-mode drive that expects tacho feedback, but if you had that then you wouldn't be asking the question, I assume. If the motor speed / position loop is closed some other way (typically an encoder) then you can ignore the tacho.


3. If I was ignorantly reading threads that were based on just using the motors and not the Fanuc drives, then is it pretty easy to just find the PWM inputs on the drives and run the machine from there?

That slightly depends on which drives, and how many PWM channels you need. I am terrible with names and this is a new thread, are you the chap who already knows that he needs 6 PWM channels?
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02 Nov 2018 15:23 #119828 by toolmaker69
Thank you again.

andypugh wrote:
"... then is it pretty easy to just find the PWM inputs on the drives and run the machine from there?"

That slightly depends on which drives, and how many PWM channels you need. I am terrible with names and this is a new thread, are you the chap who already knows that he needs 6 PWM channels?


No, not me. I think I saw that on one of the threads I was studying. Maybe the one with the opto-isolators, so I assumed it did not apply to me.

I'll open the cabinet and also study the Maintenance and AC Servo motor manuals before I continue posting.

I appreciate your help.
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05 Nov 2018 16:33 #120007 by toolmaker69
Reply from Mesa:

"We do not have a interface for the Fanuc PWM drives, it would require a custom breakout since the Fanuc interface needs differential PWM outputs that can be direction reversed to read drive status and 2 analog channels per axis to read phase currents "

I guess new drives to run the fanuc motors would be prohibitively expensive.

Oh well, I guess that's that.

Attached is a copy of the Fanuc control-to-drive cable pinout.
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05 Nov 2018 17:05 #120011 by PCW
I'm not saying its impossible but its not trivial to do this.

Also there are many variants of Fanuc PWM drives, newer ones using high density I/O connectors (that include the encoder signal) and some with differential PWM signaling

This subject has come up often enough that I have considered making an interface but it is a fair sized project
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05 Nov 2018 17:32 #120012 by toolmaker69
Thanks.

It's certainly way over my head.

Meanwhile I'll put the machine up for sale, and use it until sold. While it's for sale, I'll look for a machine that is easier to move to 5-axis.

If an interface becomes available meanwhile, I'll take the machine off the market.
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