Closed loop stepper driver configuration help

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29 Jan 2024 18:48 #291917 by spacestate1
I'm trying to get my PnCConf configurations dialed in for a closed-loop stepper driver that doesn't have much documentation. I've attached the manual for the drivers.I have a Nema 34 Servo Motor closed-loop 86EBP181ALC and a ZDM-2HA865 stepper driver. I know my ball screws have a lead of 5mm, but that's about all the info I can get for them. The Nema 34 has a 1.8° step angle, and the drivers say they are set to '16' on P001. I'm not sure what '16' means; I think it's 16 microsteps, but I'm not sure how many revolutions that is. The manual says it can be set to these options: 2, 4, 5, 8, 10, 16, 20, 25, 32, 40, 50, 64, 100, 128, 200, 256, with the maximum 256 subdivision being 51200 pulses per revolution.So far, here are my settings. They get me into the ballpark for movement accuracy, but I'm pretty sure they could use some fine-tuning/corrections.
  • PnCConf
  • Step on time: 5000
  • Step space: 5000
  • Direction hold: 10000
  • Direction Setup: 10000
  • Microstep Multiplication Factor: 16
  • Leadscrew Metric Pitch: 5.000
  • Motor Steps per revolution: 196
  • Motor steps per unit: 627.2
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29 Jan 2024 19:08 #291921 by tommylight
Motor steps per revolution should be 200, not 196.
200x16=3200/5=steps per unit, or steps per mm in this case.
Make sure the screws are actually 5mm, as there are imperial ones at 5.08mm per turn.

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29 Jan 2024 19:33 #291924 by spacestate1
They are definitely metric 5mm (from China). But I'm thinking the 16 might represent something different because setting Motor Steps per revolution to 200 makes it move too far, (or not enough, I can't remember).

I'll try setting it to 200 again and see if it makes a difference this time around.   

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30 Jan 2024 16:26 #291991 by spacestate1
I set the motor steps per revolution to 200 and, yes I think it's the correct setting. The issue now is can the accuracy be increased via stepper settings or other configs ?

When I do a test cut, I get about +- 0.10 mm and I'm pretty sure it can do better. I've read about ball screw mapping, is there a guide or info on how to do this in LinuxCNC?

The machine's specs say the ball screws are this type ? There's not much info out there on this.
TBI C7 grade 2005-4

thanks again

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30 Jan 2024 21:16 #292017 by spumco
Couple suggestions before you try to chase this down inside LCNC:
  • Check the drive parameter P111
    • See if this is set to "1" as this sets the drive's internal positioning accuracy.  If it's not a "1", the drive will consider 'close enough' as acceptable.
  • Check your ballscrew fixed bearings for end-play.  If you have C7 ballscrews it's possible (likley) the bearings have some slop or compliance.
    • Disconnect the motor and put an indicator on the free end of the ball screw.
    • Push the ballscrew back and forth and see if you've got some play.
  • Same thing for ball-nut.  There may be some internal backlash in the ballnut.
  • Same thing for ballscrew coupler.  See if it's rotationally stiff or if there's some spring/slop
A bit of play in the bearings, plus a bit in the nut, plus a bit in the coupler, plus some compliance in the whole machine, plus maybe a bit in the drive...

could all add up to 0.1mm of repeatability error.

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31 Jan 2024 17:08 #292079 by spacestate1
Thanks, I'll give this a shot. if I do find that things are loose, what is the corrective action ? just tighten the screws or is there anything else that can be done ?

The driver's P111 setting is set to 1.

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31 Jan 2024 18:06 #292085 by spumco
  • (free) Alignment
    • Check axis alignment and free movement.  You haven't said what this machine is, or whether this is a dovetail, box, or rolling element way machine.
    • Misalignment in the axis way components or improperly adjusted gibs can cause positioning inaccuracy - either in certain locations or across the whole length of travel.
    • Check ballscrew alignment.  Is the torque required to move the axis the same down the whole length of travel?  Does it bind up anywhere or at the ends?
Assuming you can't fix it by just tightening mounting screws or similar free things, you can throw money at it and replace any component or assembly that has backlash or is contributing to the inaccuracy.
  • ($) Bearings
    • Buy some angular contact bearings and replace the (almost certainly) deep-groove ball bearings in the fixed end ballscrew support.  That won't be a ton of money - you don't need to buy top-of-the-line AC bearings... anything that isn't crunchy will be better than DGBB's.
  • ($-$$) Ballnut
    • Re-ball the ball-nut with oversized balls (not expensive, but tedious and not for the faint of heart)
    • Replace the ballnut and hope the new one has a bit of preload
    • Replace the ballnut with a double-nut that has some preload (assuming you don't mind possibly losing a bit of travel)
  • ($$$) Ballscrew assembly
    • Buy a new or used ballscrew assembly of higher quality
  • ($) Stretch Goal
    • Buy a better quality coupler that has less rotational compliance or backlash

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31 Jan 2024 18:56 - 31 Jan 2024 20:50 #292093 by spacestate1
Not that it's possible to determine much from these but here's what they look like, on the Y axis. 

It's a LyCNC cast Iron 4020 mill, I'm piecing together information as I find it but it's not been easy.

X=380MM,Y=220MM,Z=280MM

thanks again
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Last edit: 31 Jan 2024 20:50 by spacestate1.

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31 Jan 2024 20:57 #292105 by spumco
Those are significantly nicer components than I was imagining.

The linear rails appear to be properly side-clamped against ground registration surfaces with those screws & dowels, and I believe the rails are name-brand Hiwins.  I'd be surprised if there's any serious misalignment-induced binding.

You may actually have AC bearings in the combination ballscrew/motor mount.  And your coupler is a zero-backlash type.

I'd remove the motor and proceed with checking screw fixed bearing slop.  If there is any you can try snugging up the nut.  The machine looks clean but used, so it's possible the bearings are a little worn out.  On heavy or fast machines ballscrew fixed bearings take a beating.

After that, the nut, nut adapter, and motor mount screws are now (in my mind) the most likely culprits if there's a mechanical problem.  Likely the nut & balls, TBH.

Aliexpress has a few vendors of Taiwanese "C3" double-nut ball-screws around $400-$800 depending on length & dia. I don't know what the machine is, but it looks nice enough under the hood to be worth a set of ground ballscrews if you can scrape up the money. 

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03 Feb 2024 03:23 #292274 by spacestate1
I conducted a few more test cuts on aluminum, and it's not ±0.10 mm; it's consistently +0.20 mm. I'm not sure if this indicates that an adjustment can be made on the software side or not?Apart from always being +0.20 mm, the variance is about ±0.04 mm.

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