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General Mach Discussion / Re: new computer can't jog faster than 50 ipm?
« on: November 27, 2015, 02:41:54 AM »
Hi all,
my take on the timing variation is
When a stepper is running at max speed, lets say 1200 rpm at 500 step per rev (I use Vexta 5-phase steppers) corresponds to 10,000 pulses per second.
If the pulse generator (ie the PC) is stable the period between rising edges is 0.1ms. If the PC isn't doing as well then the period between pulses will vary.
If we assume just 10% variation the periods will vary from 0.09ms to 0.11ms which corresponds to a variation in speed of 1333rpm and 1090rpm.
Further these speed variations can occur in rapid succession and constitute VERY high accelerations/ deaccelerations and is no wonder the steppers can't
keep up.
My 'big' scope, while old, is quite good and has a histogram function which allows me to measure % variation on a nominally constant pulse stream. I have found
that more than 2-3% severely limits high speed running. At much lower cutting speeds variations of 10 and more % can be tolerated.
I don't believe that the PP, breakout board or stepper controller have that much influence provided they work of course!
As comments in this thread and others make clear an external motion controller is really the only way to eliminate pulse speed variations. I use a single board
Atom based PC for my controller and it works fine so I haven't bothered with an external controller.
I also use an embedded OS (Windows 7) which allows me the choice of what modules run in the machine. I don't run any Ethernet or Ineternet capable modules
as they have security checking processes which 'clogs' things up when trying to run Mach3.
Craig
my take on the timing variation is
When a stepper is running at max speed, lets say 1200 rpm at 500 step per rev (I use Vexta 5-phase steppers) corresponds to 10,000 pulses per second.
If the pulse generator (ie the PC) is stable the period between rising edges is 0.1ms. If the PC isn't doing as well then the period between pulses will vary.
If we assume just 10% variation the periods will vary from 0.09ms to 0.11ms which corresponds to a variation in speed of 1333rpm and 1090rpm.
Further these speed variations can occur in rapid succession and constitute VERY high accelerations/ deaccelerations and is no wonder the steppers can't
keep up.
My 'big' scope, while old, is quite good and has a histogram function which allows me to measure % variation on a nominally constant pulse stream. I have found
that more than 2-3% severely limits high speed running. At much lower cutting speeds variations of 10 and more % can be tolerated.
I don't believe that the PP, breakout board or stepper controller have that much influence provided they work of course!
As comments in this thread and others make clear an external motion controller is really the only way to eliminate pulse speed variations. I use a single board
Atom based PC for my controller and it works fine so I haven't bothered with an external controller.
I also use an embedded OS (Windows 7) which allows me the choice of what modules run in the machine. I don't run any Ethernet or Ineternet capable modules
as they have security checking processes which 'clogs' things up when trying to run Mach3.
Craig