On 7/26/2011 6:11 PM, Steve Blackmore wrote: > On Tue, 26 Jul 2011 03:02:35 +0100, you wrote: > > >> See http://wiki.linuxcnc.org/emcinfo.pl?TrajectoryControl for explanation >> As discussed on the page you can use the exact path mode >> so it goes to the full extents. >> >> but it probably means your acceleration times need looking at. >> > Hi Dave, I came across a similar conundrum the other day, have some > changes been made to trajectory control in 2.4 ? > > I ran a file that consists of a series of curves (Fender Telecaster > outline) on the router. The performance was awful, speed up, slow down > almost stop ad infinitum. I last ran the same file a few years ago under > 2.3 and it seemed OK, I'm sure I'd have commented had it not been<G>. > > It seemed to be almost in exact stop mode, even with the addition of a > G64. The only way to get it smoother was completely throw away any > accuracy with large P values> 0.5mm ! > > As an experiment I ran the same file in Mach3, same acceleration, same > velocities and it ran much, much smoother. In fact, it finished 20 > seconds quicker than in EMC per pass. Now on a file that takes a little > under two minutes per pass that's an enormous difference :( > > Played about with accelerations but all that did was make movements very > jerky. They were conservative at 600mm/sec/sec. with a max velocity of > 5200 mm/min. > > I do know that Mach's trajectory planner uses look ahead, does EMC use > any look ahead, if so can it be set anywhere? > > BTW - Look ahead in Mach was set to 20 lines, tolerance was 0.1mm. > > (Art Fenerty's description is appended here) > > Mach3 is a bang-bang accelerative trapezoidal velocity planner, and a > very good one for almost all code types. Its CV is created by the simple > rule of beginning the acceleration of the next motion when the > acceleration of the current move goes negative. ( deceleration phase). > This means you are literally adding one move to the next , allowing for > the speed to increase. I'm sure many have read this before and can > picture it in the context of two motions where the second is added to > the first , thus making the deceleration phase virtually invisible ( > thus constant velocity ), but what I , or Brian, rarely mention is that > it operates in full look ahead, this means that in tiny segment code, > the code of a line may be passed over by the time sequence > of the planner as it progresses, but that motions speed must still be > added in to the total of the motions distance. This means its > an iterative process, where up to 100 lines or more may be actively > added together in terms of their deceleration phases being added to the > next motions acceleration phase. This can be hard to wrap the head > around. Each Gcode line has to be considered fully in the iterative > calculations involved. Imagine tiny segment code where the angle of the > next motion is constantly shifting ( typical 3d work ), and the > processor is trying to keep track of the real speed that's possible. > Since the consistency of the speed is a function of the additive > properties of each motions deceleration phase, it means the max speed an > individual motion can do is important in the functions total speed > accumulation. > > Steve Blackmore > -- > > > Steve,
Is this a stepper system? I'm running a waterjet with some short arc segments blended to create some compound curves and it is running up to 700 inches per minute and the movement is nice and smooth. This waterjet system uses parallel port generated steps run into some 1 KW servo drives, so it might be substantially different. The system is also running on 2.4. Dave ------------------------------------------------------------------------------ Got Input? Slashdot Needs You. Take our quick survey online. Come on, we don't ask for help often. Plus, you'll get a chance to win $100 to spend on ThinkGeek. http://p.sf.net/sfu/slashdot-survey _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
