Distance coded glasscales possible?

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07 Jul 2014 17:55 #48485 by uli12us
On my VMC from DMG I have glass scales with distance coded reference marks, with these parts the control knows the axis position after a little more than 20mm running. Can liuxcnc work with these kind of scales. There is a table in the control which says the distance 19.99 is on this point, 20 on the other and 20.01 else and so on.

With the machine I have often trouble, that something don't runs correctly, so I want to try to built in a new control, because 1 day of a serviceman will cost me 2500$. I think all the hardware I need will not be more than 2 servicedays.

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08 Jul 2014 03:12 #48509 by cmorley
Are these heidenhain or acurite scales?

Linuxcnc currently cannot use the distance coded indexs, I don't know anyone playing around with adding it.
This would be a very nice addition but I bet there are few with the technical know how to do it.

But the scales will also produce quadrature signals that linuxcnc can use.
Do the motors have feedback as well?

Chris M

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08 Jul 2014 19:15 #48536 by uli12us
That are HH scales, they produce a 1,1Vpp sinusoidal signal, but that can easily transformed to quadrature with HH exes.

The Motors have feedback but the same 1,1V with an additional position sensor, because they are synchronous servos.

If I will change it, I think the best idea is, to change motors and drives completely and sell the used parts in Ebay or similiar. The problem with these parts is, the driver controller have cheap capacitors in it
I changed these parts, but with power source, computer unit, one big and 5 small inverter units and the matching controllers, you have many things, that can make a fault.

Actually I worked the complete last week, to find out the actual fault.
The z-Axis fell down a bit, because the brake opens a short time before the motor will be engaged.
I changed parts, but after each change, i must wait 1 1/2h until the new part have reached the working temperature, while the machine can't start.

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09 Jul 2014 05:34 #48549 by andypugh

Linuxcnc currently cannot use the distance coded indexs, I don't know anyone playing around with adding it.


it doesn't sound all that difficult in theory.
You need to set index-enable, move one way until it resets, then latch the counts at the next mark.
The Mesa Hostmot2 has some (completely unexplained an undocumented) pins that look like they are meant to do that:
git.linuxcnc.org/gitweb?p=linuxcnc.git;a...852e08a;hb=HEAD#l380
They probably behave like the same pins in the software encoder (also largely unexplained..)
www.linuxcnc.org/docs/devel/html/man/man9/encoder.9.html

I can imagine a per-joint hal component that twiddled the index-enable and latch-pins as the joint was jogged in a sensible direction, but I am not sure how to pass a new definition of the current position to the motion module.
The magic is likely to be here:
git.linuxcnc.org/gitweb?p=linuxcnc.git;a...010ad5907ad4732#l543
followed by twiddling a homing pin in halui (It is probably worth looking at how halui does its thing)

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10 Jul 2014 08:08 #48601 by cmorley
If you interested in trying this Andy, I can look up the math equation for that Heidenhain used to decode the marks.
But I think the problem is really linuxcnc assuming that index is always zero and in this case it will be an arbitrary number.
This would be nice to have for my lathe, as it has duel turrets so having a home switch on either extreme is not ideal.

Chris M

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