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Offline cncnc

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Jerky dual axis movement
« on: April 08, 2018, 04:28:55 PM »
Hi guys,

I have a problem with a jerky axis movement on my home built machine (with PoKeys 57CNC controller) and don't quite know where it comes from.
The only affected axis is the Y. It is a dual axis (two ball screws, two 8Nm closed loop steppers). Both bearings on the Y axis are quite preloaded but this shouldn't be a problem for the stepper motors.
During the milling cycle the Y axis starts to bang when it accelerates. At first I expected the worst, that both axis motors didn't move synchronously and the gantry is put under heavy stress. But turned out not to be the case, I measured the flatness of my X axis and it was just fine. If both Y motors would have moved different from one another, the gantry would bent. This shouldn't even happen though, because both stepper motors do have encoders and the same input signal. If they wouldn't move in sync, an alarm would be triggered.
So I watched both motors during a job and noticed that both seem to jerk simultaneously.
I don't think this comes from overload of the motors, they have plenty of power and in general they aren't equally strained during a job so a simultaneous jerking movement is quite unlikely.
Additionally these jerky movements don't appear when I jog the Y axis. This leads me to the assumption that there must be something wrong with Mach4 or the way I set it up.

I already thought this may come from too many steps per unit, but according to my calculations it should be ok and Mach4 (PoKeys plugin) didn't complain.
But I don't know how these step frequency calculations apply to a dual axis or when more than one axis is moving.

In order to give you an impression of what I mean I uploaded a short mp3 file where I face off my slot table. From 2,5 - 4 seconds you can clearly hear the banging of both motors in the background. AudioYaxismotors.m4a



I hope you have some ideas and can help me out!

Many thanks and best regards,
cncnc
« Last Edit: April 08, 2018, 04:30:34 PM by cncnc »
Re: Jerky dual axis movement
« Reply #1 on: April 08, 2018, 09:09:26 PM »
Hi,
what steppers and drivers are you using?
What microstepping are you using?
What are your native units?
What is your current 'steps per unit'?
What pitch are your ballscrews?
Do you have any gear reduction/belt reduction between the steppers and the ballscrews?

I suspect that you will need to use microstepping, it results in smooth movement which is what you want.
If you don't like calculating steps per unit....bad luck...its part of successful CNC.
Post the relevant data and we'll do the calculation.

Craig
My wife left with my best friend...
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Offline cncnc

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Re: Jerky dual axis movement
« Reply #2 on: April 09, 2018, 10:18:33 AM »
Hi Craig,

thanks for your help.

I'm using Leadshine Easy Servo ES-M23480 with the appropriate drivers ES-D1008.
They are configured to a 12800 steps per revolution. In combination with 10 mm pitch ball screws that's 1280 counts/steps per 1mm.
There is no gear reduction or similar involved, both ball screws are directly driven from the motors.

I don't think the problem is to less micro stepping. I initially thought it may come from too many step commands that Mach4 or the controller can't handle.
But I did all the calculations on it. And according to that there shouldn't be a problem. The PoKeys maximum step frequency is 125kHz, with 1280 steps per mm that leads to a maximum velocity of 5859 mm/s. I don't exceed this limit.

Did you listen to the audio file? What does is sound like to you? Does is it sound rather like a software or a hardware problem?
I already thought about loose couplings, but I tightened them as hard as I could and both couplings are quite oversized, they shouldn't have a problem to transmit the torque. And if they would slip, the axes wouldn't be square anymore, but they are.

In the end, I have no idea what's causing these problems and I'm glad for every help.

Best regards,
Yannik
Re: Jerky dual axis movement
« Reply #3 on: April 09, 2018, 03:02:58 PM »
Hi,
yes I listened to the audio and clearly the stepper is not happy. As to why sound alone doesn't tell me.

It looks to me that your 'steps per unit' is correct and given that you understand the signaling limit of the PoKeys board that should be OK.

How have you hooked your controller to the drivers? I thought the PoKeys boards are 3.3V output types and that would be insufficient for your drivers.

Craig
My wife left with my best friend...
     and I miss him!

Offline cncnc

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Re: Jerky dual axis movement
« Reply #4 on: April 18, 2018, 04:17:56 PM »
The PoKeys57CNC runs with 5V logic level, I don‘t see a problem there.
I think that this behavior is caused by the toolpath itself, not any communication errors or something like that since those jumpy movements exclusively occur at 2D adaptive toolpaths of relatively similar geometries.

Thanks for your time and help!
Re: Jerky dual axis movement
« Reply #5 on: April 21, 2018, 08:25:15 AM »
Hi,
reading the manual for your stepper/driver and its fitted with a 1000 line encoder which results in 4000 counts per rev. I'm intrigued as to how you get to a microstep regeime of 12800?

Craig
My wife left with my best friend...
     and I miss him!
Re: Jerky dual axis movement
« Reply #6 on: April 21, 2018, 08:48:28 AM »
Hi,
found the page with the DIP switch settings and 12800 is certainly on the list.

Given that your encoder is 4000 counts per revolution is there any thing to be gained by even greater number of micro steps? Looking at the list there is an option for 3200 steps per rev.
Each step would be achieved because the encoder has slightly finer resolution whereas your existing 12800 setting could only be guaranteed at those locations where the encoder positions are,
ie 4000 positions in a revolution.

Even with the microstepping dropped back to 3200 per rev the resolution would be 3.125um or just a little over 1/10 th of a thousandth of an inch, plenty good!

It might be worth a try, after all you only have to flick a couple of switches and redo the 'steps per unit'

Craig
My wife left with my best friend...
     and I miss him!

Offline cncnc

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Re: Jerky dual axis movement
« Reply #7 on: April 21, 2018, 12:36:23 PM »
Hi Craig,

you‘re absolutely right. In terms of increasing the accuracy that high number of steps/rev is total nonsense, since you loose quite a lot of step torque and probably will end up being less accurate. But this isn‘t my goal. I chose this specific motor because it has a lot of holding torque and I can increase the steps/rev in order to smooth the movements and reduce the induced vibrations but without sacrificing performance (by not falling below the required application torque with the step torque).
Nevertheless I‘ll give it a try and see if that‘s the problem‘s solution (I‘ve already cut it down by half). If so, I might dampen the vibrations of each motor with a rubber pad between motor and machine.

As a side note: I programmed all drivers with the dedicated software. There are some options that aren‘t accessible from the DIP switches. For example you‘re able to set even higher steps/rev than with switches. In fact I could set steps/rev exactly to 4000 if I wanted to.

BR
« Last Edit: April 21, 2018, 12:40:52 PM by cncnc »
Re: Jerky dual axis movement
« Reply #8 on: April 21, 2018, 04:19:51 PM »
Hi,
as you know most of us who use microstepping with regular steppers do it for smooth running. Its nice to think that we get increased resolution but due to the reducing
differential torque we get resolution of 400 steps per rev irrespective of the number of microsteps.

Getting smooth running without undue number of steps per rev is maximized with microstepping of 8 or maybe 16 microsteps per full step or about 1600 to 3200 microsteps per rev.

You have set your microsteps to 12800 steps per rev. Are you of the opinion that you are actually getting smoother motion as a result?

The reason I ask is because I wonder if your machine is attempting to make a huge number of very small steps at certain times, you mentioned 2D adaptive toolpath, not sure what that is, but
maybe it introduces a whole bunch of steps. I wonder that while your PoKeys controller can produce pulse streams of 100kHz it may be that at the top rate the pulses are getting attenuated by
the cabling and the drives are not getting a clean signal?

Craig
My wife left with my best friend...
     and I miss him!

Offline cncnc

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Re: Jerky dual axis movement
« Reply #9 on: April 21, 2018, 05:29:00 PM »
Hi,

at the moment microstepping is at 6400 per revolution. I‘ll reduce it to 3200 tomorrow for test purposes.
Initially I planned with 3200 microsteps because I‘m aware of step torque loss. But I found that there‘s still a little noise/vibration improvement with 6400 steps over 3200 steps.

The Pokeys should easily be able to handle the number of steps at the driven speed, it‘s not even in the top rate area. But I will reduce the microstepping furthermore, then we‘ll see, maybe there‘s indeed another transfer problem on the line