LinuxCNC S-Curve Accelerations

Out of curiosity, what is the purpose of these external axis offsets? When would someone use them?

Why not add the offset to the position command sent to the TP rather than using a separate TP and combining their output (which requires that they "split" the total v_max and a_max budget between them through the OFFSET_AV_RATIO setting)?

rodw: The external axis offsets seem to work on cartesian position rather than joint position. How are they functionally equivalent? Just trying to understand what they are used for.

External offsets allow applying an offset to an Axis position that is external to the motion controller.
Its used extensively for Torch Height Control in the Plasmac config (where the axis position is adjusted using arc voltage as a process control variable. There is a strong linear relationship between torch height and arc voltage.

There are a number of cool sims demonstrating how else External offsets can be used in v's 2.8 & 2.9 you might like to look at. Machining a cam lobe might be a practical example. Andy has a video demonstrating how this might work. Plasmac is also part of 2.8 & 2.9

In my experiments I tried to jerk-limit not inidividual axis but the tangential movement. Even that should improve things quite a bit.

Dealing with jerk-limits per axis and going back and forth through kinematics will be very interesting.

I've had to take a break from looking at this unfortunately, but yeah, my idea is also to jerk-limit the tool's movement rather than the individual axes (although I've been looking at both tangential and radial jerk limiting). It won't guarantee a certain jerk limit for joints in non-cartesian machines, but it could hardly make things worse either?

(Cute cat BTW. )

just for reference: forum.linuxcnc.org/38-general-linuxcnc-q...oth-velocity-profile

I agree code could be organized better. Unfortunately, my approach leads to acceleration constraint violations (that should not be there and which I don't fully understand) and leads to some really strange behaviour.

You said it well in that thread:

rmu wrote: Explicit constructions of an S-Curve with constant derivative of jerk followed by constant jerk, constant acceleration, constant velocity phases and at the end the other way back seems to be very messy and have a ton of cases and corners, especially if you want to continue a given velocity/acceleration/jerk.

It's an interesting challenge though. If there was just more energy available each day.