New Trajectory Planner - Testers/programs wanted
15 Jan 2014 02:07 #42779
by rellenberg
Thanks! It wouldn't have been possible to get here without all the help from the community and the many hours of testing from other developers and users like skunkworks.
Engraving-type toolpaths are definitely where these changes can help, especially on faster machines. I'm always looking for more test cases, so if you have code that works well (or conversely, shows any performance losses), it would be a great help.
Here is my original proposal for this method, which describes the main concepts and what tradeoffs there are. Circular arc blending is not a perfect solution, but there were a number of advantages to the method. It works by inserting tangent arc segments in between line pairs as they are added to the queue. An optimization similar to "lookahead" walks over the queue and calculates the maximum safe velocity for each segment (this runs only when a new segment is added). The new TP also handles tangent segments without slowing to a stop, which means that motion can be much faster than the speed limit imposed by having to stop at the end of each segment.
I'm interested in limited jerk planning as well, and have done a fair amount of reading / research into the subject. My conclusion initially was that limited jerk planning might not be worth the effort if we can't attain high movement speeds. I suspect that at least part of the perceived need for limited jerk comes from how machines are tuned for linuxcnc. With the stock trajectory planner, the only way to get high speeds with small segments is to turn the acceleration up. Unfortunately, this leads to bigger jerk spikes.
Even though the new TP is technically infinite jerk, it means that you don't need to crank accelerations up really high to reach your desired feeds in fine work. If your toolpaths are very smooth, you might be able to cut your axis accelerations in half or more without a major loss of performance (in terms of total program time).
All this aside, I'd love to be able to do limited jerk planning at some point. There are other development efforts such as araisrobo's linuxcnc mirror which have implemented limited jerk planning and NURBS. It's an exciting prospect, though I haven't kept up with their development recently.
Replied by rellenberg on topic New Trajectory Planner - Testers/programs wanted
Thanks! It wouldn't have been possible to get here without all the help from the community and the many hours of testing from other developers and users like skunkworks.The very conservative motion planner of LinuxCNC was one of my complaints about the system. 3D contouring toolpaths manage to slow down the current TP to a crawl, and even at tangent line-arc transitions I could hear the motors slow down and accelerate. I will definitely give this new code a try.
Engraving-type toolpaths are definitely where these changes can help, especially on faster machines. I'm always looking for more test cases, so if you have code that works well (or conversely, shows any performance losses), it would be a great help.
Is there more information about this new TP anywhere? How far can it look ahead the programmed path? How does it work? Any plans for jerk/jounce limited motion? Etc.
Here is my original proposal for this method, which describes the main concepts and what tradeoffs there are. Circular arc blending is not a perfect solution, but there were a number of advantages to the method. It works by inserting tangent arc segments in between line pairs as they are added to the queue. An optimization similar to "lookahead" walks over the queue and calculates the maximum safe velocity for each segment (this runs only when a new segment is added). The new TP also handles tangent segments without slowing to a stop, which means that motion can be much faster than the speed limit imposed by having to stop at the end of each segment.
I'm interested in limited jerk planning as well, and have done a fair amount of reading / research into the subject. My conclusion initially was that limited jerk planning might not be worth the effort if we can't attain high movement speeds. I suspect that at least part of the perceived need for limited jerk comes from how machines are tuned for linuxcnc. With the stock trajectory planner, the only way to get high speeds with small segments is to turn the acceleration up. Unfortunately, this leads to bigger jerk spikes.
Even though the new TP is technically infinite jerk, it means that you don't need to crank accelerations up really high to reach your desired feeds in fine work. If your toolpaths are very smooth, you might be able to cut your axis accelerations in half or more without a major loss of performance (in terms of total program time).
All this aside, I'd love to be able to do limited jerk planning at some point. There are other development efforts such as araisrobo's linuxcnc mirror which have implemented limited jerk planning and NURBS. It's an exciting prospect, though I haven't kept up with their development recently.
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