On Saturday 26 August 2017 17:16:40 Chris Albertson wrote: > I just went to Mouser.com and did a parts search. It is really easy > to find low cost optical isolators that runs at 20MHz. 20Mhz seems > to the common rate for modern parts but if you look for faster parts > you can find 50Mhz without having to spend over $4 each. > > If you really need very high speed you'd use a tiny one inch long > fiber optic cable for isolation but that costs more. These run at the > GHz range. and required specialist engineering skills to use. > > But in any case normal hobbyists can do 20MHz without need for > "exotic" parts. The $2 parts from Mouser or Digikey would work. > > But can your stepper motor run that fast? I wonder what the upper > limit is. It is like a function of the motor inductance and how many > volts you can spell put into the motor. > I have run a bare, non-loaded 473oz nema 23 motor at a hair over 300 kilohertz. Set for a/8 in the driver, that was a hair north of 3000 rpms.
Very very fussy about the drive and offset/duty cycle from a 2MHz function generator. And the stalls are violent. Looking at the current flow into the drivers opto's, I was surprised I could go that fast as it was never truly off. This was using about 55 volts and a DM860 driver set to put 4.5 amps into an 8 wire motor, wired parallel. Based on that, I came to the conclusion that the real world max revs for a decent low inductance stepper, weren't being limited by the rest of the driver, but by the speed of its own opto-isolated inputs. Reliable at 750 to maybe 1500 revs. But watch for stall inducing resonances at fractions of that speed. I had made some fender washers separated by rubber sheets whose central hole was quite tight on the drum, 9 to 15 washers deep, and stuck them on the back shaft of the motors on my toy mill. Ditto the back shaft of the z motor on TLM, which trippled its speed. By absorbing the magnetic spring resonance, I trippled the mills speeds. I am tempted to see about putting something similar on the rear shaft of the X motor on the Sheldon as its stalls very easily at 40 ipm. But its crowded back there, maybe 12mm's to play with before it starts hitting the bed. In the work I've done with it so far, the 36" I have it limited to seems more than enough to get the job done, so if its not broke, I don't fix it. Ditto the Z, which can move better than 100", but 60 seems like a good limit. Noisy tho. Something in my shop made motor and thrust bearing mount seems to have a natural resonance excited by the steppers steps, which on a 1600 oz/in motor, are violent. I need to drop it to a 960. Need$ $$, works ok now. > On Sat, Aug 26, 2017 at 5:00 AM, Gene Heskett <[email protected]> wrote: > > On Saturday 26 August 2017 07:38:28 Mark wrote: > > > > At one point in the video they talk about very high frequency > > drivers, but you can't get much over 250 kilohertz through an > > opto-isolator reliably. BTDT with my own testing, So unless they are > > using the capacitive couplers even in their motor driver hardware, > > an expensive item, they are full of it. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
