On Tuesday 24 January 2017 10:41:51 sam sokolik wrote: > As Ed just mentioned in another thread.. Why oh why use steppers ;) > (although we have a few stepper machines and haven't had any real > issues with them) > > anyhoo... > > we have a couple compact 5 cnc (integrated cnc control) that are dead. > The original steppers are non-hybrid 72 steps per rev. They worked ok > but quite under powered, Hot and slow - 30ipm max. We purchased some > small pittman servos a few years ago cheap. iirc they where like > $9.99 with a heds 500 line encoder on the end. We also have some amc > pwm/dir input drives. Peter was nice enough to make me a bit file for > a 5i25 that had pwm gens on the external plug and encoder counters on > the internal plug. The emcos have 2.5mm (approx 10tpi) pitch ball > screws with a 2.5 belt reduction. > > we made the pullys fit the servos and new mounting plates > > http://electronicsam.com/images/emco/EmcoServo.JPG > > http://electronicsam.com/images/emco/EmcoCompare.JPG > > and after a bit of tuning we got the following error down to .0002 > with a top speed of 150ipm 30in/s^2. (testing with a lowly 1a 25v > archer power supply) > > https://www.youtube.com/watch?v=lTNzBLNBNiE > > I think we could tune them a bit better. Peter suggested adding more > I until it is unstable - then backing it off. I am really surprised > how well it works. We thought the servos might be a bit undersized - > but now playing with it - it seems to be a good match. I don't think > the servo will turn the ball screws inside out. > > This is basically the setup > Linuxcnc -> mesa 5i25 pwm -> Cheap Bob -> AMC drive -> Servo -> > Encoder -> mesa 5i25 encoder counter -> Linuxcnc > > Not that I am trying to cheap out by not buying a mesa analog daughter > board.. We just didn't need that much i/o for this lathe. So - 2 > printer ports worth of i/o is more than enough for this machine. > (good match for the 5i25 or 7i92...) > > sam > That does indeed look like a good match. But I can't see one of those moving a 25x5x1450mm long screw on the saddle of an 11x36 Sheldon w/o enough gear reduction to make the top speed drop below 20 ipm. Then too, Sam, you obviously have enough servo experience to "just know" if it would work. I don't, and the motor prices I see on ebay are need a laundromat scary for something in the size range needed. Top that off with the fact that NONE of the stuff for sale on ebay is described as anything but a servo, no measurements or data on what sort of an encoder it has, voltage or amps it needs.
They're like Sgt Schultz from Hogans Hero's, they know nuttink. I know servo's as applied to professional vtr's well enough that I can take a now 40+ year old Ampex VR-1200 2" machine, normally expected to produce airable video phase locked to house synch in 10 seconds from a dead headwheel start, but I do make it do it in 5 seconds. FWIW, 10 years later they were doing it in 400 milliseconds from a dead headwheel, or around 40 milliseconds from a hot, spinning headwheel. It spins 14,400 rpms... They just kept adding pairs of 6550 vacuum tubes to the final stage of the servo amp till they had enough power to manhandle a tiny little 3 phase, 400 hz motor that usually ran in air bearings, that had a 2" diameter wheel with 4 heads on its rim on one end of the motor shaft and a 4 pole rotary transformer just beyond that. :) Thats my experience with servo's. The servo's feedback signal was the output color video, and the time accuracy was about 3 nanoseconds. And properly setup, it Just Worked. If it didn't, my phone or pager went off with a paniced voice on the other end. But none of the tuning controls were labeled as P, I, or D. So that experience does not well translate, even to steppers. I'd be willing to learn, if the price tag today wasn't so #%^& high. 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
