Add jerk-limited motion planning

[MitchBradley]

No description provided.

[Copilot]

Pull request overview

This PR adds per-axis jerk configuration and starts integrating jerk-limited ramp shaping into FluidNC’s planner/stepper motion pipeline.

Changes:

• Adds _jerk axis configuration and YAML handler support.
• Adds jerk limiting calculations alongside acceleration/rate axis limits.
• Extends planner blocks and stepper segment prep to use jerk-aware acceleration state.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
FluidNC/src/Stepper.cpp	Adds current acceleration state and jerk-shaped accel/decel segment logic.
FluidNC/src/Planner.h	Extends planner blocks with max acceleration and jerk fields.
FluidNC/src/Planner.cpp	Computes jerk limits and effective acceleration for lookahead.
FluidNC/src/NutsBolts.h	Declares jerk axis-limit helper.
FluidNC/src/NutsBolts.cpp	Implements per-axis jerk limiting and unit conversion.
FluidNC/src/Machine/Axis.h	Adds default per-axis jerk setting.
FluidNC/src/Machine/Axis.cpp	Exposes jerk as jerk_mm_per_sec3 config item.

Comments suppressed due to low confidence (2)

FluidNC/src/Stepper.cpp:647

• The deceleration path has the same integration error as acceleration: it updates current_acceleration before applying it over the whole timestep, so jerk-up/down segments use a constant acceleration instead of the linear jerk integral. That over-decelerates segments and can force an abrupt correction to exit_speed near the end of the block.

                        if (prep.current_speed > jerk_rampdown) {
                            prep.current_acceleration = MIN(prep.current_acceleration + accel_var, pl_block->max_acceleration);
                        } else {
                            prep.current_acceleration = MAX(prep.current_acceleration - accel_var, accel_var);
                        }
                        speed_var = prep.current_acceleration * time_var;

FluidNC/src/Stepper.cpp:645

• This clamp likewise prevents deceleration from tapering below one segment's accel_var, so the deceleration ramp cannot smoothly return acceleration to zero at the target exit speed. That undermines the jerk limit and can leave the final segment relying on the hard current_speed = exit_speed correction.

                            prep.current_acceleration = MAX(prep.current_acceleration - accel_var, accel_var);

[MitchBradley]

The documentation, suitable for a FluidNC Wiki page, is at https://gist.github.com/MitchBradley/efc5772e52ce9e65a72f7250087fcd53