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Tuning Jerk & Acceleration

Standard Grbl uses Trapezoidal motion planning: the machine ramps up to speed linearly, holds speed, and ramps down. This is efficient but can cause vibrations ("jerk") at the start and end of movements because the rate of change of acceleration is instant.

grblHAL introduces S-Curve Acceleration (Jerk Limited acceleration), which smooths out these transitions.

Why use Jerk Limiting?​

  • Smoother Motion: Reduces machine vibration and resonance.
  • Less Wear: Lower mechanical stress on belts and screws.
  • Higher Quality: Reduces "ringing" or "ghosting" in prints/cuts.
  • Trade-off: S-Curve motion is computationally more intensive and complicates "junction deviation" (cornering) calculations.

Configuring Jerk​

In the current version of grblHAL, Jerk is primarily configured via Compile-Time options in config.h.

1. Enable S-Curve Acceleration​

You must compile grblHAL with S-Curve enabled. In the Web Builder or config.h:

  • Ensure the Jerk/S-Curve option is checked or defined.

2. Tuning ACCELERATION_TICKS_PER_SECOND​

This setting in config.h controls the resolution of the acceleration curve.

  • Default: 100 (10ms segments). Good for 8-bit or lower-end 32-bit boards.
  • Higher Quality: 500 - 1000. Provides much smoother motion but requires a faster processor (Teensy 4.1, STM32F4/H7).
  • Warning: Higher values consume more CPU power.

3. Tuning SEGMENT_BUFFER​

If you increase ACCELERATION_TICKS_PER_SECOND, you must increase the segment buffer size to prevent starvation.

  • Recommendation: Buffer should hold at least 40-50ms of motion.
  • If Ticks = 1000 (1ms), you need a buffer of at least 50-100 segments.

Tuning Process​

Once firmware is flashed:

  1. Set Max Acceleration ($120-$122):

    • Start conservative.
    • Find the point where motors stall, then reduce by 20%.

  2. Set Jerk (Compile Time or Plugin):

    • Rule of Thumb: Start with Jerk = 10x Acceleration.
    • Example: If Accel = 500 mm/s², Jerk ≈ 5000 mm/s³.
    • Higher Jerk = More aggressive (closer to Trapezoidal).
    • Lower Jerk = Smoother (rounder corners, slower starts).

Known Issues​

  • Jogging Delays: There is a known issue where long jogging moves may take time to process or stop due to the complexity of the S-curve planner.