Just Another G0704 CNC

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11 Oct 2017 18:24 #100222 by Kyle
Kyle created the topic: Just Another G0704 CNC
Hello,

I think this is my first post, I have been lurking for quite some time. Over a year ago I bought a G0704 with a ball screw and mount kit as well as some glass linear encoders (Grizzly / GS10 type from DRO PRO) all as a pile of parts. I've got it mostly put together, back injury - took a long break from the shop - long story, but I'm better now and have to make some decisions to get this finished.

A few months ago I purchased the 5i25 and 7i77 with the plan of going with servos. I also have some nice Advanced Motion Controls servo drives I got on EBAY for a song while every other hobby CNC person in the world was sleeping :)

I have not bought motors yet.

My issue is the more reading and research I do the more my instincts are telling me to go with with steppers, one other thing I should admit is out of pure laziness I can just bolt the steppers directly to the NEMA 23 mounts on my mill.

I want closed loop position control based off the linear encoders so my question to you is do I move forward with servos and figure out the gear reduction needed to make the available ~50 oz-in/amp NEMA 23 servos work (KL23-130-60) or do I move to steppers?

If I move to steppers can I set the 7i77, and LinuxCNC up to give a velocity frequency command (say to some G210x or other step/direction drive) and use the linear quadrature encoders to close the position loop in LinuxCNC? The would be no angular velocity feedback from the motor but the axis velocity could be derived from the linear position sensors. I have just not researched enough to know if this is possible in LinuxCNC but I suspect it is. One other thought is I could, maybe, whip up a little circuit to convert the voltage outputs on the 7i77 to frequency.

I would like to avoid if possible buying the hybrid servo/stepper type drive and close the loop in LinuxCNC.

I am open to any and all comments the hive brain has to offer, thanks for looking.

Kyle
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11 Oct 2017 18:49 #100227 by PCW
PCW replied the topic: Just Another G0704 CNC
I would use the DC servos as they are a better match to linear scales
Note that it's difficult to tune systems with only linear feedback because of backlash/stiction

Most systems with linear scales and ball screws have dual feedback (on the motor and from the linear scales)
The following user(s) said Thank You: Kyle
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11 Oct 2017 19:49 #100232 by Kyle
Kyle replied the topic: Just Another G0704 CNC
PCW, You gave me the piece I was not understanding / finding. A little more Googling with the term 'dual feedback' came up with this link and answered my question of 'can I run the stepper with a velocity command as an open loop,' I just did not ask it that elegantly... :) Dual-Loop Feedback is discussed towards the bottom.

granitedevices.com/wiki/Dual-loop_feedback_position_control

That should get me going, thanks again!
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12 Oct 2017 23:44 #100311 by andypugh
andypugh replied the topic: Just Another G0704 CNC
I would still always suggest servos over steppers, especially if you already have them.
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