Latency problems on Dell GX620

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10 Sep 2010 15:00 #4100 by photomankc
Had a whole long post and the forum just ate it so I'm going to summarize... I'm not typing the whole thing out again.

System is a Dell GX620 small form factor:
2GB of RAM
3.0GHz processor


Trying to run EMC2 w/10.04
I get the following jitter numbers

Servo thread ~325,000
Base thread ~25,000


Step pulses are confirmed on an Oscilloscope but they seem to dance around a lot indicating that they are not real steady in rate. The video is on-board and there are no expansion slots to use any external card. No BIOS settings to disable much. The SMI instructions on the wiki don't seem to work as I can't even get the step 1 command to get the sources with apt-get to execute. Do I just have a dog here that I can't do much with? Wanted small to go in the limited space near the mill and I don't have the cash at present to just buy and try hardware. Is the servo thread jitter a huge problem? How much jitter in the pulse train can I reasonably get away with considering a mill that will likely run around 50-60IPM max and much lower when cutting?

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11 Sep 2010 11:11 #4118 by Rick G
Hello,

I have one machine with a Dell GX520 which has a big latency spike when started in the first couple minutes then seems to not reoccur. I let the computer warm up for a couple of minutes then test it and it seems fine.
I have run it this way for several hours without a problem. Not sure if your system will react the same.
SMI problems seem to generally re occur constantly every so many seconds (90?).
The amount of speed you can run will depend not only on latency but also on your micro steps setting, gear ratio etc.

Rick G

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12 Sep 2010 07:34 #4137 by photomankc
Looks like the GX620 is just not going to work out that well. I have not been able to drop that servo thread number and the jitter becomes pretty obvious much past about 65IPM. On 1/8 microstep anything north of 40IPM will cause the stepper to stall with no load. I think I can get it to run reasonably at 55IPM - 60IPM and until I retrofit the mill with ballscrews that is probably optimistic anyway. I'll have to see if I can catch one of the models in the known working systems list later on for better performance. Saw a dude with the same mill running mach and ballscrews getting 200IPM with a 70lb load on the table! Hopefully I can find a system that will allow me to the same with EMC.

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12 Sep 2010 10:38 #4138 by Rick G
Hello,

Did you use Stepconf to set up your mill using the results from the latency test?
Can you set your stepper drivers to full step or half step? If the limiting factor for speed is the step pulse stream at full step you could go 8 times faster than at 1/8 micro steps.
Keep in mind the software step generator uses the Base Period. There is great information on getting the best from software step generation in the Integrator manual.

Rick G

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12 Sep 2010 11:06 #4139 by Rick G
Also keep in mind that stepper motors have a very limited rpm range and their torque drops off rapidly after a certain point.
Make sure you are not trying to run them beyond their range. You can also try adjusting their acceleration to see if an improvement can be made.

Rick G

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13 Sep 2010 12:17 #4168 by andypugh
photomankc wrote:

> The SMI instructions on the wiki don't seem to work as I can't even get the step 1 command to get the sources with apt-get to execute.

This is no longer necessary. If you read carefully (it should perhaps be made more obvious) the SMI kernel module is now part of the standard installation.
You only need to edit the rtapi.conf file (step 7 here: wiki.linuxcnc.org/cgi-bin/emcinfo.pl?FixingSMIIssues )

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16 Sep 2010 18:40 #4217 by photomankc
Rick G wrote:

Hello,

Did you use Stepconf to set up your mill using the results from the latency test?
Can you set your stepper drivers to full step or half step? If the limiting factor for speed is the step pulse stream at full step you could go 8 times faster than at 1/8 micro steps.
Keep in mind the software step generator uses the Base Period. There is great information on getting the best from software step generation in the Integrator manual.

Rick G


Yeah, I was using stepconf.and using 30,000 from the base thread as I had never seen it jump above 27,000 in any test and figured a little headroom is good. I still am not sure what the servo latency would affect, but I'm assuming that's reading inputs and noticing control signals and such? I can do 1/2 or 1/8 (I think 1/4 would be a good compromise but alas I don't have it) and I am set right now to 1/2 because the 1/8 setting sounds bad after 30IPM (you can hear the RPM jumping around) and randomly stalls after about 40 IPM. This is all without any load, clamped to the bench. In 1/2 step I could get up to close to 70IPM before it started to sound bad and then after about 80 it starts stalling. That could just be the upper limits of the stepper's RPM though. This was a 285ozin motor.

I'm now using some bigger 425ozin steppers and they can't run as fast anyway. So it looks like it should work OK until I retrofit again to ballscrews and probably upgrade the steppers later on to something that's faster. I guess the HobbyCNC 425's are not known for being too speedy. The video I saw was running 570's that have 1/3 less impendence than I do.

I am really liking EMC2 though. Been running a few test programs from CAM trials I am using and it's easy to pick up on. Just need to get my Joypad pendent setup and I'll be ready to rock when I finally get the X/Y motors all mounted up.

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17 Sep 2010 11:11 #4231 by Rick G
Hello,

Sounds like you are on your way. The RPM will most likely need to be tuned again after installing the stepper motors and with some load. In your hal file you can see what is assigned to the servo thread, but yes inputs, outputs and components that do not need really fast times.

For your joypad pendant you can start here.wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Simple_Remote_Pendant

Rick G

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19 Sep 2010 10:06 #4249 by Rick G
Just thinking....

Have you double checked your Step Time and Step Space settings? Maybe experiment with longer Step Space and Step times. You could also try inverting the step signal.

Rick G

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