But this is a BLDC servo and free wheels quite easily when powered down. I put the .2" lead screws back on today and re-tuned with a couple of new tricks and got much better results than I've had in the past.
The drives use torque commands, and had been difficult for me to tune well and get stiff and quiet. They need a lot of I, but using a lot of I caused them to be unstable. The new trick I've tried was to use the pid.N.maxerrorI. I turned up the I-term till it started to get unstable (large overshoots not oscillating), then I set an error limit on I, and made it smaller and smaller until the I induced overshooting was minimized. ----- Original Message ----- From: "Gene Heskett" <[email protected]> To: [email protected] Sent: Thursday, August 18, 2016 5:04:09 PM Subject: Re: [Emc-users] Leadscrew Efficiency? On Thursday 18 August 2016 11:16:02 Todd Zuercher wrote: > They are both almost identical screws and nuts except for their leads. > They are both Kirk Motion VHD series LEAD screws. > http://www.haydonkerk.com/LinearActuatorProducts/LeadScrewsAndNuts/Nut >s/VHDSeriesNuts/tabid/148/Default.aspx > > So you are saying that all things being equal the turning force should > be about the same for the 2 screws. Which must leave me with the > conclusion that the older lower lead screw I pulled off the shelf is > probably mostly worn out. I'll go disassemble it and inspect the > threads in the nut. > > I'm considering swapping to ball screws, but I'm a little concerned > about it back-driving when off, and I don't think I can find a 0.1" > lead ball screw (screws are currently direct coupled to the motor). > So far, I have not found that to be a problem as the steppers I have used have a sufficient cogging action when powered off that the only time I have had one unwind and drop the head was when I took the 1600oz/in off the G0704 to replace it with the 940oz/in. It unwound and stopped on some relatively 'cushionable' wood stuffs laying on the mills table, no damage to anything other than my surprise when I realized and understood why it was happening. I won't say its not happened, like the knee on a bigger bridgeport, which weighs about 15x what the head on a G0704 weighs, but I've not seen it happen, just read about it. So I'd go a decent way out on a persimmon limb and say thats a servo problem 99% of the time. If anyone here has had a problem like that while using steppers, please speak up, we would like to hear about it. > ----- Original Message ----- > From: "John Kasunich" <[email protected]> > To: [email protected] > Sent: Thursday, August 18, 2016 10:50:59 AM > Subject: Re: [Emc-users] Leadscrew Efficiency? > > Are they LEADscrews or BALLscrews? > > With ballscrews, the finer pitch screw should indeed provide a much > better mechanical advantage. > > With leadscrews, the mechanical advantage almost doesn't matter, > because friction is by far the dominant force. Somewhere between 60% > and 95% of the torque required to turn a leadscrew under load is due > to the friction between the screw and nut rather than the force > actually needed to raise the load. > > I'm not sure why the 10tpi screw would actually be worse - I would > expect it to be roughly the same. However, many factors come into > play: > > 1) materials: are both screws steel? both nuts bronze? (pr plastic, > or whatever) The materials and lubrication can make a huge difference > in the amount of friction. > > 2) surface quality: a rough screw or nut will have more friction than > a polished one > > 3) thread form: 60 degree threads have a lot more friction than Acme > due to the wedging action of the 60 degree flanks. Square threads are > best because the flanks are perpendicular to the load, but Acme is > almost as good. > > 4) diameter: if all of the above are equal, the friction FORCE will > be the same. But the TORQUE required depends on the radius of the > screw, so a larger diameter screw will require more torque. > > On Thu, Aug 18, 2016, at 10:14 AM, Todd Zuercher wrote: > > I have a machine that I converted from step-motors to servos, and > > I'm having a little trouble with the Z axis. It has an anti backlash > > lead screw with a 5tpi screw. This sort of worked, but the servo was > > working hard to move the head, and I wanted a little higher encoder > > resolution for better tuning. So I swapped in a nearly identical > > 10tpi lead screw set I happened to have on hand (removed from > > another stepper machine to get better speed and perfomance). I > > thought that the 10tpi screw should be easier for the servo to turn, > > but I'm finding that the opposite is true and the servo can't raise > > the motor without counter balance assistance when it could with the > > 5tpi. Does that make sense, or is the problem more likely that the > > 10tpi screw and nut are worn out and binding? > > > > -- > > > > ======================================== > > > > Todd Zuercher > > mailto:[email protected] > > > > ======================================== > > -------------------------------------------------------------------- > >---------- _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users 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> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
