Electronics 120: Multi-Axis Coordination & Motion Planning
A practical engineering guide to coordinating multiple motion axes: how machines move in straight lines and curves, why acceleration matters more than speed, and how planners turn simple commands into smooth, safe motion for CNC machines, robots, plotters, and automation systems.
1) What is multi-axis coordination?
Multi-axis coordination ensures that multiple motors move together in a precisely timed way so the toolpath follows the intended geometry. If one axis arrives early or late, the path distorts.
2) Cartesian space vs motor space
Users think in Cartesian space (X, Y, Z). Controllers drive motor space (steps, currents, pulses).
- Cartesian planning defines the path
- Kinematics map path ? motor commands
- Timing synchronizes all axes
3) Linear interpolation (straight-line motion)
Linear interpolation breaks a straight move into many small, synchronized steps.
- Path defined in Cartesian space
- Step ratios computed per axis
- All axes complete together
4) Arc & curve interpolation
Curves are approximated by small linear segments or by true arc interpolation.
- G2/G3 arcs in CNC
- Parametric curves in robotics
- Trade-off: smoothness vs computation
5) Feedrate vs axis speed
Feedrate is the speed of the tool along the path — not the speed of any single axis.
6) Acceleration, deceleration & jerk
Instant speed changes are physically impossible. Motion must ramp smoothly.
- Acceleration: Rate of speed change
- Jerk: Rate of acceleration change
- High jerk excites vibration
7) Motion profiles
| Profile | Description | Use Case |
|---|---|---|
| Trapezoidal | Constant accel, constant speed | Simple systems |
| S-curve | Smooth accel & jerk limiting | High precision, quiet machines |
8) The motion planning pipeline
- Parse motion command
- Convert to Cartesian path
- Apply limits (speed, accel)
- Generate time-based profile
- Output synchronized axis steps
9) Axis synchronization & timing
Synchronization is achieved by scheduling steps against a shared timer.
- Master clock or timer interrupt
- Deterministic step scheduling
- No blocking operations
10) Lookahead & cornering
Lookahead allows planners to anticipate upcoming moves and adjust speed smoothly through corners.
- Prevents full stops at each segment
- Maintains toolpath continuity
- Reduces cycle time
11) Precision, rounding & numerical issues
Small numerical errors accumulate over long moves.
- Use sufficient precision (fixed-point or double)
- Avoid cumulative rounding
- Periodically re-anchor position
12) Design rules & best practices
- Separate planning from execution
- Use a single global time base
- Limit acceleration before speed
- Test with worst-case geometry
- Prefer smoothness over raw speed
