On Friday 02 November 2012 15:13:07 Peter C. Wallace did opine: > On Fri, 2 Nov 2012, Gene Heskett wrote: > > Date: Fri, 2 Nov 2012 12:07:45 -0400 > > From: Gene Heskett <[email protected]> > > Reply-To: EMC developers <[email protected]> > > To: [email protected] > > Subject: Re: [Emc-developers] ferror calculation incorrect in motion > > > > On Friday 02 November 2012 11:44:04 Jon Elson did opine: > >> On 11/01/2012 12:25 PM, Stuart Stevenson wrote: > >>> Gentlemen, > >>> > >>> What is ferror used for in motion or any calculation? I thought > >>> it was > >>> > >>> only for human feedback during the tuning phase. > >> > >> It is used to stop motion when the error is excessive. On my > >> machines, it causes an E-stop. > >> > >>> PID error and ferror are not two different things? > >> > >> Well, I think that's the crux of Peter's complaint, is that they > >> should not be different, but his claim is that the ARE different. > >> If you want to demand tight tolerance for following of the commanded > >> path, then you really need the PID to be striving to minimize the > >> same thing that is used to stop the system when it doesn't. > >> > >> I have not explored this very deeply, but I think Peter may be on > >> to something, and I believe I have seen the same problem but I didn't > >> find where it was coming from. > >> > >> Jon > > > > I'm not sure, but in my attempts to tune the lathes spindle speed > > servo, I may have some instability created by that, which leads to > > over correction and which is then prevented from slowing down by the > > rc time constants in the pwm to electronic speed potentiometer > > interface. Due to the filtration on Arturo's card, the opamp can > > speed it up faster than it can slow it down. The smoothing capacitor, > > a 10uf unit, is IMO about 40x too big. Before I blew up the first > > card, and my temp controlled iron because I'd forgotten to unplug > > everything but the grounded tip iron, it was running well on a .2 uf > > cap in that pair of holes. In retrospect, I should have bought > > another PMDX-106. That would have Just Worked(TM) with some ice cube > > relays to boost it. But what isn't in the flying swarf on the mill, > > is drowning in it on the front of that lathe. > > Much as I would like to blame all the worlds problems on LinuxCNCs > ferror calculation pipelining issue, I doubt your spindle speed control > troubles are related. I think that a classical PID control has problems > controlling low cost spindle drives that do not have 4 quadrant drive > cabability but rather assymetrical drive (so you can accelerate much > faster that you can slow down) > > Pehaps a little googling might find a better control algorithm that pure > PID for a "plant" with this behaviour
I have it working well enough to cut good threads, which was the general idea. It only gets obviously jerked around when below about 2 rps, and I generally cut the size threads I'm doing at 5 to 10 rps. But, I just had a thought that could maybe improve the spindle speed vs thread position drift the present G76 method gives with changes in spindle speed. How about, when waiting to start the forward cutting trace, an extra rev is spent measuring the spindle period, then calculating when in the next revolution that z, given its accel limits, would have to be started in order to arrive at the same z position at the start of the work regardless of the rpms, and do an estop right there if it cannot be done due to the z speed and accel limits? On the face of it, it seems doable even with my relatively coarse 39 slot encoder. 1/39th, actually 156 signal edges, one signal edge isn't going to screw up the average thread even at 48 tpi (which I haven't tried yet) is it? It seems a shame to have to waste a part finding it can be sped up, specially when that operation is near the end of that parts cut sequence. Something to consider for 3.0 after a new kernel is working? Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! "Just think, with VLSI we can have 100 ENIACS on a chip!" -- Alan Perlis ------------------------------------------------------------------------------ LogMeIn Central: Instant, anywhere, Remote PC access and management. Stay in control, update software, and manage PCs from one command center Diagnose problems and improve visibility into emerging IT issues Automate, monitor and manage. Do more in less time with Central http://p.sf.net/sfu/logmein12331_d2d _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
