Centroid Community CNC Support Forum

I'm experiencing an anomaly with rapid moves on a paired axis servo motor router gantry (4th axis paired to 1st axis).
Every so often throughout the day, (about 4 to 5 times over 8 hours), one of the paired servo motors suddenly tries to stop part way through a long rapid move (5m move for example).

With one servo motor rapidly stopping, and the other racing along at 40m/min, things don't end well.
The servo drive that's trying to stop a motor trips out with a "Regenerative discharge resistance overload" error.
Fortunately the Acorn sees the drive fault and shuts everything down.
From there it takes about 10 minutes to manually re-square the gantry.
This went on for a couple of days (anything to keep production going)
Over those couple of days I tried swapping cables, drives and motors in an attempt to identify what could be causing the anomaly.
But nothing obvious to point the finger at.

This morning I decided to focus on the possibility that the Acorn was responsible for the anomaly.
To test this out I connected both servo drives to the same step, direction, and enable outputs (axis 1 - screw terminals).
And, the router ran all day without a single occurrence of the anomaly.

I know there could confounding variables, but it seems there may be an issue with the Acorn hardware or software.

There's another anomaly occurring that may be related...
It seems that movements towards a home switch (negative moves) move a greater distance than moves that move away from the home switch (positive moves).
This is occurring on all 3 axes, XYZ.
It's only a small difference, but over the course of a day it adds up. (5.5mm out on the Y axis today)
Can't set up the ATC until I resolve this one.

This is a new Acorn install using CNC12 v4.82

Troy

"Regenerative discharge resistance overload"
Your servos are slowing down faster than they should. Add a bigger brake resistor or set them up to a slower deceleration.

Axis Label Motor Encoder Lash Comp. Limit Home Dir Screw
mm/rev counts/rev. (mm) - + - + Rev Comp
1 X 20.1062 2000 0.0000 4 4 1 0 N N
2 Y 20.1062 2000 0.0000 4 4 2 0 N N
3 Z 5.0000 2000 0.0000 4 4 0 3 N N
4 N 20.1062 2000 0.0000 4 4 0 0 N N
5 N 5.0800 200 0.0000 0 0 0 0 N N
6 N 5.0800 200 0.0000 0 0 0 0 N N
7 N 5.0800 200 0.0000 0 0 0 0 N N
8 N 5.0800 200 0.0000 0 0 0 0 N N

Axis are not set up in the recommended range, related reading:
https://centroidcncforum.com/viewtopic.php?f=63&t=1801

What kind of home switches you are using?

Uwe

Thanks Uwe

To be clear, the issue isn't with how well the servo decelerates.
It's the fact that it is trying to slow down, or stop, mid way through a rapid move.
This may not be the cause, but it's like it stops receiving pulses at some point during a rapid move.
It's random when it happens, but it always seems to be on the 4th axis, (1st and 4th paired).

I'm unsure what you mean by the axes are not set up in the recommended range.
I've read the post and nothing is jumping out at me.
Could you be more specific please?

Home switches are inductive proximity sensor - ???

I've rolled back CNC12 to v4.62 and this has resolved the negative axis creep issue.
It's possible that it's also resolved the other issue, but can't afford the downtime to test at the moment.

Troy

i'm running v4.82 on multiple machines using software paired axes and they run 40 hours a week with no issues since v4.82 has been released.
https://photos.app.goo.gl/VP1HT62UqC23P6kX7


what make model size motors and drives are you using?
photos please.

v4.64 has know bugs in it which may or may not effect you, i'd be using v4.82 for sure.
fyi.. v5.0 rev0 beta is about to go public, hopefully tomorrow.

Thanks Keith

I just remembered, I also swapped the Acorn controller out with another - as well as rolling back to v4.62
Needed to remedy the issue as quickly as possible.

If you're confident it's not v4.82 causing the mid rapid move issue, or the axis creep issue, then perhaps there's a hardware issue with that particular Acorn board.

Troy

Router and Plasma Machine Happy Zone
- Happy Zone Overall Turns Ratio for routers is 1.5 to 2.5 turns of the AXIS MOTOR yields 1" of table/gantry movement.
- Steps per Revolution of at least 1600

20mm/rev = 1.27 turns per 1"

"With one servo motor rapidly stopping, and the other racing along at 40m/min, things don't end well."

Your reports says 30m/min max.

Uwe

Happy zone... that's an unusual concept.
I wonder what the theory is behind this?

The router is actually designed to run at 60m/min, which is common for industrial CNC routers.
The only way to achieve that with 3000rpm max servo's, is to have 20mm travel per motor rev.

I initially set the rapid speed to 40m/min, but after the reoccurring rapid issue I slowed it down to 30m/min.
Once I have confidence in the system I'll bump it up to 60m/min.

Troy

The theory is to have a usable resolution in mm/step
Your setup will have only 0.01mm/step, not good for precision work.
I would at least set the step/rev to 8000 and Acorn to 400000 kHz if your servo driver can take this.

On my machines, Acorn runs absolutely reliable at high rates.

Uwe

and the machine design should have the motor operating in its useful, practical rpm/torque range , and the axis motor sized to meet the weight and inertia loads.