Rough movement in cnc

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26 Jun 2017 20:55 #95007 by suspension
suspension created the topic: Rough movement in cnc
I am using linuxcnc to control my Chinese 6040CNC. I am quite happy with the machine and the software except that jogging at high speeds is not smooth. Max jitter reported is around 24000ns. Since correct timing values for my stepper driver is not available from the documentation, I am using some approximate values. If I make these values too high, then the max jog velocity reduces considerably, but it runs smooth. If I put lower values, although I get a fairly high jogging velocity, movement is not smooth.
However occasionally, jogging runs smoothly for few seconds. I also noticed that the way I log in to machine also has some effect. Currently I am using a remote XServer to connect to the machine from a windows desktop or an android based remote desktop client that runs on a tablet. Normally, desktop connection gives smoother jogging. Could this be related to the video card? The card is a pci -express nvidia card that use the default open source driver. Any other pointers to make it smoother?
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27 Jun 2017 03:59 #95017 by Todd Zuercher
Todd Zuercher replied the topic: Rough movement in cnc
Assuming you are asking for realistic step rates from software stepping system. Jogging via a remote interface will be fraught with problems, especially on a software stepping system. As high step-rate settings will consume a lot of system resources, the remote user interface will get preempted by the real time resources of the machine. If you want absolutely smooth movement you may need to switch to real-time inputs for your jog commands, and/or reduce the demands on the system by switching to hardware stepping.
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27 Jun 2017 06:24 #95020 by suspension
suspension replied the topic: Rough movement in cnc
I have ordered a mpg pendent which I plan to use for jogging. Maybe this will improve things a bit.
However even with remote connection I don't see any issues in the user interface. It just does not jog smooth. Also what do I need to do to switch to hardware stepping?
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27 Jun 2017 12:06 #95032 by Todd Zuercher
Todd Zuercher replied the topic: Rough movement in cnc
Install a hardware step generating interface, such as a Mesa card or Pico Systems USC. (those are just a couple examples.)

Are you getting real time alarms? If configured right, Linuxcnc can control "fast" jogging smothly even when using software stepping.
But first some sanity tests.
How fast are you considering fast?
What is your step scale?

I have software stepping machines that can jog nicely at speeds approaching 1000IPM.
But they are not using fine micro-stepping, or really high resolution. (step scales are near 1000 steps/inch.) and the PC has max jitter less than 10000.
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27 Jun 2017 12:50 #95043 by suspension
suspension replied the topic: Rough movement in cnc
I will take a look at those cards.
Yes I get these latency warning occasionally.

I like to have at least around 4000 mm/M. I am not sure about my step scale, I need to look in the config. Will post it once I am home. 1000IPM is around 25400 mm/M, which seems far higher than what mine can do at the moment.
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27 Jun 2017 13:53 #95051 by suspension
suspension replied the topic: Rough movement in cnc
After looking at the card, I am not sure if I am using software or hardware step generation. My CNC is this: www.ebay.com/p/USB-Four-4-Axis-6040-800w...708?iid=201933154777
It certainly got final stepper driver stages, but not sure if it does step generation as well.
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27 Jun 2017 14:09 #95052 by Todd Zuercher
Todd Zuercher replied the topic: Rough movement in cnc
It is almost certainly software stepping. (If you used the Stepconfig wizzard to build your Linuxcnc config it is software stepping.)

I'd hardly consider 4000mm/min fast. (I'm working with wood carving machines where high feed rates are important.)

If you are having realtime errors, they can easily cause your motion to not be smooth.
Run the real time test for at least a few hours with a couple of GLXgears windows open, and performing some other tasks, like updates, and surfing the net.

Reduce micro stepping settings on the drive. High micro stepping does not increase the resolvable accuracy of a stepper motor much (if at all) beyond about 4x micro-stepping.
for 4000mm/min, and a 25000ns jitter, I would suggest not having a step scale higher than about 100steps/mm (more like 50 steps/mm might be better.)
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28 Jun 2017 01:09 #95085 by tommylight
tommylight replied the topic: Rough movement in cnc
Chinese machine so most probably it uses chinese BOB with slow opto couplers, so set the latency to 100.000, then yoi can play with step timings a bit to get max speed.
Do this AFTER you check what Todd has sugested above.
Almost all cheap BOB's will cause all kinds of wierdness with anything under 100.000 for latency, had over 15 versions of them.
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28 Jun 2017 08:15 #95095 by suspension
suspension replied the topic: Rough movement in cnc
Sure I will do these today. Not sure how to change micro-stepping settings, but will google and see.
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28 Jun 2017 08:19 #95096 by suspension
suspension replied the topic: Rough movement in cnc
When you say set the latency to 100,000 are you saying I should set the max jitter to 100,000?

Re - BOB: Are you referring to these:
users.skynet.be/ldt/CNC%20electronics/BOB%27s/BOB4_descr.html

This looks like the one I have inside. I need to re-check. In that case, knowing the schematics will be really helpful.
Do you know any better boards that I can use to replace above without much changes? I like to have to support for a PWM and coolant control too. Although this board has a relay which switches on/off with coolant on/off button in EMC, relay does not seem to be connected to any power line so it useless.
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