Acceloration, cv settings and cut speed

[matttargett4]

Hi,

Ive recently built my second plasma table using servos, my first was using steppers. I have been having trouble reaching the recomended cut speeds in the book for the plasma but have found that when i decrease the acceloration value i can achieve much higher speeds, using the servo configuartor sofware i have i have tuned the servo drives so that the following error is pretty small so i think that is ok but if i reduce the acceloration enough to get the correct cut speeds my accuracy is hopeless, i never encountered this problem with the steppers, is it likely a cv problem or a servo problem, im sure the first response will be asking for more detail about the machine but i thought i should ask a general question and hope for some advice.

Thanks alot

Matt
 

[HimyKabibble]

That says your servos do not have enough torque to do the job.  Either your motors are too small, you're not driving them properly, or the PID settings are far from ideal.

Regards,
Ray L.

[matttargett4]

Thanks for the reply,

So maybe i need to learn more about servo tuning, is there anywhere good to start? or do you have any good pointers and ways to test if im getting good results? I have whale3 drives from cnc drives.com and used their servo config software which showed me i had a following error of only a few steps which is fractions of a mm.

Thanks again

Matt

[matttargett4]

I tried a few tests this morning,

I attached a pen tot he torch and drew some shapes with the machine, a 100mm square with a 50mm circle inside at 1200mm/min and 2000mm/min, the shapes were closer than i could measure to the right size so im having trouble understanding how it could be a problem with my servo tuning, is there any better way to show the problems? My only guess is to increase the feed rate to try to force some backlash or something although 1200 to 2000mm/min is the sort of speed i would typically aim to cut at.

Thanks

Matt

[BR549]

Part of the problem may be in your Gcode. A lot of plasma files are made up of microsegmented code it has thousands of short segments of straight lines. IF your motor are a bit weak in the torque department then they will not be able to rocket up to speed before they get to the end of the segmnent and have to start over. Like trying to run 100 mph in rush hour traffic. VERY hard to do with all the stop go motion.

SO the average speed usually falls short of your fast projected speeds.

SO stronger motors may help and better Gcdoe may help

Just a thought, (;-) TP

[matttargett4]

An interesting thought, i will take a look, surely there must be some cv settings that can help with that?

On the subject of cv settings what should i be using for plasma?

Matt

[BR549]

NONE

(;-) TP

[TOTALLYRC]

Hi Matt,
to add to TP's point,
I have setup several servo machines and the pid tuning isn't that difficult, just more trial and error than anything else. If you are cutting at 2000mm/min, what are your rapid speeds?? If you can't rapid a lot faster and not fault out then you choice of servos and drive mechanics may not be up to the task.

On my mill I can reliably rapid at 200 ipm but wouldn't cut much faster than 100 ipm without a lot of testing

[BR549]

The default internal settings for the CV seem to work best.  plasma is very sensitive to feed speeds and quality of cut.