On Thursday 17 December 2015 13:55:53 Brian wrote: > On Thursday, December 17, 2015, Gene Heskett <[email protected]> wrote: > > On Thursday 17 December 2015 10:40:41 Viesturs Lācis wrote: > > > AFAIK it has been few years since Araisrobo and Yishin Li have > > > done it; take a look here: > > > https://github.com/araisrobo/linuxcnc/blob/master/src/emc/kinemati > > >cs/t p.c > > > > > > I found some threads from 2012 on developers mailing list, where > > > Yishin discussed their work on that. This one seems even older, > > > hopefully it would get you started on finding some more useful > > > information: > > > http://sourceforge.net/p/emc/mailman/message/27376772/ > > > IIRC it has not been implemented in LinuxCNC, because it would not > > > work for spindle-synchronised moves, but I might be totally wrong > > > on that :)) > > > > > > > > > Viesturs > > > > Early on, in playing with G76, I found that the actual lockup phase > > relationship was quite spindle-speed dependent, wrecking a few > > threads because I thought I could crank the speed up once the motion > > was verified. > > > > And was advised that this was the Z accel lag, and that as long as > > the spindle speed was maintained, it would not be a problem. Doing > > that, it hasn't been but I now restrict the spindle rpms to <300 > > also. I assume the same caveat applies to G33.1, rigid tapping, so > > I don't crank the revs up there either. > > > > But I have wondered why, in the grand scheme of things, that delay, > > which can be obtained in degrees without a huge hassle, is not > > turned into a pre-start, that based on the rpms, puts the actual > > start z point that fraction of a turn ahead of the index, which > > would result in the lock being obtained such that the index to > > spindle phase was within an encoder count (or closer) of zero. > > Doing this on every Z restart would allow us to run it one or two > > cycles at 50 rpms to check clearances etc, then crank the spindle up > > to 500 revs and get the job done, without wrecking the threads cut > > because the phase lag is a fixed time at that rpm. Obviously it > > would need stretching because of the higer speed Z needs to > > accelerate to at a higher spindle rpm. > > > > I haven't tested recently, perhaps this is something that Robert > > Ellenburg has addressed? > > > > Call me "Curious". > > > > Thanks. > > > > Cheers, Gene Heskett > > -- > > > > Theoretically, Z acceleration and jerk limits shouldn't be a major > > concern > > in spindle sync moves, because the Z motion is being derived from a > physical moving body (the spindle). The natural occurring physics of > the spindle motion should result in compatable motion for the Z axis. > > This breaks down if Z motion is to synchronize with an already moving > spindle. In this case, Z axis jerk and accel limits need to be > observed. This shouldn't be an issue though because it is expected > during the beginning of such a move that the Z axis will need a small > amount of motion to lock phase. > > Another edge case is a spindle that can out accelerate the Z axis. > IMO, The likelihood that the spindle is able to out accelerate the Z > axis is so unlikely that I wouldn't make handling it a priority. > > Regarding Gene's statement regarding the phase relationship changing > with spindle speed, that shouldn't be. This is what the I term in a > PID controller is there to do. This may be a bug or a significant > shortcoming of the code that should be looked at. > > Brian
I don't know as I would label it a bug. Its a known effect, 100% alleviated by maintaining a constant spindle rpms as the kick-in point is reached. For this reason, particularly for rigid tapping, where the tapping load CAN slow the spindle if the tap is too big, it isn't a problem because Z remains locked to the spindle right thru the reversal at the end of the tap, and stays in sync until it reaches the synch point going back away. At That point z stops and returns to the start point, while the spindle is doing its rev/fwd turnaround. What I suggest would seem to be a quite desirable thing for LinuxCNC to do. 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) Some mill pix are at: Genes Web page <http://geneslinuxbox.net:6309/gene/GO704-pix> ------------------------------------------------------------------------------ _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
