I think the reason for a jerk limit is to limit changes in the force applied to the machine's structure. A high constant acceleration applies a constant force and hence constant deflection. "A constant deflection" is a fancy way to way "no motion". But any jerk means the force and deflection is non-constant, a fancy way to so the machine's frame is moving.
This matters on 3D printers and other lightly constructed machines a lot more than on a CNC mill. On Mon, Aug 23, 2021 at 1:45 PM Todd Zuercher <to...@pgrahamdunn.com> wrote: > But you need to look at why you have your jerk limit is set to what it is > in the first place. Is it not there so that the motor (joint) can > accelerate in a controlled fashion as quickly as possible within its > capabilities smoothly? Therefore the vector sum is also the vector sum of > the combined joints capabilities, so shouldn't that also be the same amount > higher? > > Todd Zuercher > P. Graham Dunn Inc. > 630 Henry Street > Dalton, Ohio 44618 > Phone: (330)828-2105ext. 2031 > > -----Original Message----- > From: Chris Albertson <albertson.ch...@gmail.com> > Sent: Monday, August 23, 2021 3:39 PM > To: Enhanced Machine Controller (EMC) <emc-users@lists.sourceforge.net> > Subject: Re: [Emc-users] jerk control > > [EXTERNAL EMAIL] Be sure links are safe. > > On Mon, Aug 23, 2021 at 7:16 AM Todd Zuercher <to...@pgrahamdunn.com> > wrote: > > > To my lay persons eyes I would think it would be enough to jerk limit > > in joint space. The limiting in Cartesian space would then take care > > of itself. > > > > Let's work a simple example to see. Assume jerk is limited to 1 meter per > second cubed. Assume the machine is just 2D, X and Y pen plotter. If > both axes are running at the same speed and the current X and Y jerk is at > the 1m/s^3 limit then the pen (Cartesian space) has jerk equal to 1.414 > (square root of two) which is over the limit. > > Controllering only joint limits can't work if the cartesian motion can be > expressed as the vector sum of joint motions because the sum of two vacters > can have greater magnitude then either of the two joint vectors > > Simply not bumping into a limit is not really hard but it becomes a hard > problem when you want to move as fast as possible, but no faster. You can't > do it without doing a full simulation and numeric optimization. > > > > > I think this is true on a 3-axis machine where all the axes are linear and > orthogonal. Basically wherever "trivial kinematics" applies. > > > > > > > > As to the jogging question, does it matter? Why would jogging have to > > have the same acceleration limits as planned motion? Set the jogging > > limits to something safe and conservative that won't matter if they > > are jerk limited. > > > > Todd Zuercher > > P. Graham Dunn Inc. > > 630 Henry Street > > Dalton, Ohio 44618 > > Phone: (330)828-2105ext. 2031 > > > > -----Original Message----- > > From: andy pugh <bodge...@gmail.com> > > Sent: Monday, August 23, 2021 4:50 AM > > To: Enhanced Machine Controller (EMC) > > <emc-users@lists.sourceforge.net> > > Subject: Re: [Emc-users] jerk control > > > > [EXTERNAL EMAIL] Be sure links are safe. > > > > On Mon, 23 Aug 2021 at 04:40, Chris Albertson > > <albertson.ch...@gmail.com> > > wrote: > > > > > Actually for a machine tool, why not run the simulation off-line and > > > use as much time and computer power as it takes. > > > > Feed-override? > > > > Do you allow infinite jerk on abort? You might think that is an easy > > question, except that continuous jog is implemented as a move to the > > limit that is aborted on key release. > > > > And, do we need to jerk-limit in joint space or cartesian space, or both? > > > > -- > > atp > > "A motorcycle is a bicycle with a pandemonium attachment and is > > designed for the especial use of mechanical geniuses, daredevils and > lunatics." > > - George Fitch, Atlanta Constitution Newspaper, 1912 > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flist > > s.sourceforge.net%2Flists%2Flistinfo%2Femc-users&data=04%7C01%7Cto > > ddz%40pgrahamdunn.com%7Cdd7544f442b14a481c5d08d9666dccd5%7C5758544c573 > > f47cebee96c3e0806fb43%7C0%7C0%7C637653444482037937%7CUnknown%7CTWFpbGZ > > sb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3 > > D%7C3000&sdata=2OcIVDtgOdrHp0UmetoWxlAJ3mdvi%2FoT5hRpOH3Oc4Q%3D&am > > p;reserved=0 > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flist > > s.sourceforge.net%2Flists%2Flistinfo%2Femc-users&data=04%7C01%7Cto > > ddz%40pgrahamdunn.com%7Cdd7544f442b14a481c5d08d9666dccd5%7C5758544c573 > > f47cebee96c3e0806fb43%7C0%7C0%7C637653444482037937%7CUnknown%7CTWFpbGZ > > sb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3 > > D%7C3000&sdata=2OcIVDtgOdrHp0UmetoWxlAJ3mdvi%2FoT5hRpOH3Oc4Q%3D&am > > p;reserved=0 > > > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.sourceforge.net%2Flists%2Flistinfo%2Femc-users&data=04%7C01%7Ctoddz%40pgrahamdunn.com%7Cdd7544f442b14a481c5d08d9666dccd5%7C5758544c573f47cebee96c3e0806fb43%7C0%7C0%7C637653444482037937%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=2OcIVDtgOdrHp0UmetoWxlAJ3mdvi%2FoT5hRpOH3Oc4Q%3D&reserved=0 > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
