Trajectory Planner with Jerk limits

[TFrenken]

Hi Tom,
I currently use the KLP factor to reduce jerk during coordinated motion. But as this introduces a geometric error in some circumstances I am hoping for the implementation of a jerk limiter to the trajectory planner. Of course this is only needed for dynamic machines but as I am using KMotionCNC on a DMG DMU50 with 8 m/s2 and 22m/min it becomes quite important when using milling toolpaths with high feed rates and some rapids as feeds in between.
I do understand that this will add a lot of complexity to the trajectory planner but maybe it could be implemented in the future to get the KMotionCNC even closer to high grade industrial controls.

Regards,
Thomas

[TomKerekes]

Hi Thomas,

Yes we are always looking into this but it is very complex. For example following a U shaped "smooth" path will still require infinite jerk unless the path is reshaped. Doing this with potentially hundreds of 3D points/segments through multi dimensional paths is extremely difficult. Many systems that claim to do jerk limiting do not do it properly or optimally.

If you have suggestions or examples on how this could be achieved we'd be open for discussion.

Are you aware that Rapid moves are normally performed as 3rd order Jerk limited moves? See here. One caveat is that with non-linear systems (using non-linear Kinematics or geo Correction) a point to point move in actuator space will not be a straight line in CAD space. In systems that are nearly linear the non-linearity might not matter for the rapid move.

Another option might be to change the KLP factor with a custom MCode for when accuracy is or isn't as critical.

[TFrenken]

Hi Tom,

thanks for the reply. As a mechanical engineer I am only as much into software and programing as needed. I would try to get in touch with people from heidenhain or siemens.
Rapid moves work fine with jerk limitation but with trochoidal milling or high feed milling the paths with high feedrate (sometimes the same speed as rapids) have to use G02 or G03. But I will look into my CAM Software if some paths could be converted into linear G00 segments.

As for M Codes to change the KLP and maybe acceleration settings this might be a good idea to get the needed accuracy. Is it possible to change the acceleration setting of the trajectory planner with a custom C program?

Regards,
Thomas

[TomKerekes]

Hi Thomas,

Rapid moves work fine with jerk limitation but with trochoidal milling or high feed milling the paths with high feedrate (sometimes the same speed as rapids) have to use G02 or G03. But I will look into my CAM Software if some paths could be converted into linear G00 segments.

Well that wouldn't really help or work well. G0 Rapids are always performed point to point starting from a stop and then ending in a stop in one linear motion. There is no blending/smoothing between blocks. That is why it is relatively easy to perform 3rd order jerk limited motion for those cases.

As for M Codes to change the KLP and maybe acceleration settings this might be a good idea to get the needed accuracy. Is it possible to change the acceleration setting of the trajectory planner with a custom C program?

Not currently (other than the rapid parameters that the trajectory planner extracts from KFLOP/Kogna). We could add that if you would like to test.

You might also post some GCode fragment (with all your parameters) that you think is not executing smoothly or optimally for us to look at.