Frequencies, max speed and random questions

[fbmstar]

I have a couple questions regarding turbocnc.

1.) What is the max frequency i can use?
On startup, Turbocnc reports min time 4us, max 268us with the best computer/parallel port configuration i can come up with. I figure that means i can set my max frequencies to 3700hz (1/0.000268) without fear of losing steps. Is this correct?

I have 400step/rev. motors and 10u step drivers, and my max speed comes out to be around 4-1/2" per second, which is rather unimpressive.

2.) 10% jitter frequency? This number is around 10,000 hz. I figure this number is machine/controller/motor dependent. If anyone can shed the light on this subject and when i would need to worry about it, i'd appreciate it.

3.) Accelerations and start velocities. I really have no idea what to use for these values. I've played around with them a bit, but am still clueless on how to determine the best settings.

4.) What frequencies are some of you using, with what parallel port times, and what kind of accuracies?

thanks,

Brian

[ger21]

Originally Posted by fbmstar I have a couple questions regarding turbocnc. 1.) What is the max frequency i can use? On startup, Turbocnc reports min time 4us, max 268us with the best computer/parallel port configuration i can come up with. I figure that means i can set my max frequencies to 3700hz (1/0.000268) without fear of losing steps. Is this correct?

No. You realy need to figure out your max velocity through a bit of trial and error. You didn't say exactly how fast the PC is, but even a 486 should get you 8-10Khz.

Losing steps would usuually be more a function of motor torque (or lack of) than TurboCNC's frequency limit, which would just limit your speed, unless the pulse train gets rough near the limit.

Originally Posted by fbmstar 3.) Accelerations and start velocities. I really have no idea what to use for these values. I've played around with them a bit, but am still clueless on how to determine the best settings.

First thing you do, is set your accel and start relatively low, and setup the Max velocity. This is your rapid speed. Do some rapid moves on your machine, while increasing the Max frequency until you notice your machine starts to lose steps. Once you find a max that seems to work well, back it off by about 20% for a safety margin.
Setting Start and Accel are done similarly, slowly increasing one at a time until you have problems, then again, back off Accel about 20%.

One thing to note, is that Start and Accel are somewhat dependant on each other. With a very low start speed, you should be able to use a much higher Accel. The downside to that is the machine will come to a complete stop between each move, Increasing the start speed can make the machine run a little smoother, at the expense of acceleration.

Originally Posted by fbmstar 4.) What frequencies are some of you using, with what parallel port times, and what kind of accuracies? thanks, Brian

This is really machine dependant, and even identical machines might need different settings.

[dkowalcz]

OK fbmstar, here are my replies:

1 - The max pulse rate to have absolutely perfect pulses is 3700 Hz, as you have ascertained. Almost everybody goes much faster than this, as the 268 uS dwell probably only occurred on the first pulse.

2 - I've determined from user feedback and just my own experience building machines that when the pulse to pulse error approaches 10% of the time between them, things start to get "iffy" with driving stepper motors.

They're pretty tolerant of erratic pulse spacing, I know for a fact that two of my competitors use an algorithm that produces erratic pulse streams and both are well respected softwares. One manufacturer of drives that I know of has a circuit that deliberately changes the regularity of the pulse stream to eliminate motor resonance.

So the 10% jitter frequency is a pretty good guess of the "speed limit range" for your computer.

This doesn't consider whether you'll have enough torque at that speed to still move your machine, this is a measure of when the pulse stream starts to roughen up by a defined amount.

I'm running 3 kHz on the Z axis and 40 kHz on the X axis - computer is a 550Mhz AMD. The disparity comes from using ACME and a 1996 vintage stepper drive on Z with tight gibs, ballscrews, linear rails, and a gecko on X.

In terms of real world speed, that's 12 ipm and 200 ipm respectively. Yeah, I know... I bought a ballscrew for Z, but haven't had a chance to take things apart to make the mods yet.

As Ger21 mentioned, experimentation is the only way to find out where the true max is. I've run up to 12-14 kHz on machines that told me 10% jitter is 6000, and conversely you can see from my Z axis that mechanical issues can cause a lot of speed loss.

Hope this helps...

[dkowalcz]

Whoops - missed your question on start and accel:

Start vel - I'd not go below 100Hz on this one. This is the pulse rate that you can instantly hit your motors with at a dead stop and have them move. For a 10 microstep drive, I'd think 1000-2000 Hz would be reasonable to start from.

Accel - This is gain or loss in Hz per second. Needs to be trial and error, but getting to the max and back down in about 1-2 seconds seems to work for most. I'd try 5000 at first, and then play around.

You'll want to change these a bit, take a test rapid, then modify them again and go back and forth. Be sure to try the whole area of the machine, ways and screws are typically are tighter at the ends and looser in the middle, etc..

You can cheat a bit - the setup screen says changes aren't valid until the ports are reset, that's only true if you change the pin settings, the accel and max speed are immediately valid.

I want to set up a motor tuner utility to take the hassle out of doing this, but for now expect it to take a good 20 min or so to tune each axis.