Advanced ODrive Tutorials: Closed-Loop Control, Trajectory Planning, and TuningIntroduction
ODrive is an open-source motor controller designed to deliver high-performance, cost-effective control of brushless DC (BLDC) motors for robotics, CNC machines, gimbals, and other motion systems. While basic ODrive usage—spinning a motor open-loop or running simple velocity commands—is straightforward, building reliable, accurate, and high-performance motion systems requires understanding closed-loop control, trajectory planning, and careful tuning. This article dives deep into those advanced topics, providing practical explanations, configuration examples, and troubleshooting tips to help you get the most from your ODrive-based projects.
Table of Contents
- System overview and prerequisites
- Closed-loop control fundamentals
- ODrive control modes and state machine
- Trajectory planning and motion profiles
- Tuning strategies and parameter selection
- Practical examples and step-by-step walkthroughs
- Common issues and troubleshooting
- Resources and next steps
1. System overview and prerequisites
Before tackling advanced topics, ensure you have the following:
- An ODrive board (v3.x recommended) with appropriate firmware.
- One or more BLDC motors with hall sensors or an encoder (incremental or absolute).
- A compatible power supply and proper wiring (motor phases, encoder/Hall, power, and logic).
- A PC with odrivetool installed (Python-based CLI) and a USB connection, or an equivalent control interface (CAN, UART).
- Basic familiarity with BLDC motor operation, PID control concepts, and safety precautions for power electronics.
Important safety note: high currents and voltages are involved. Always disconnect power before changing wiring, and use appropriate fuses, current limiting, and protection.
2. Closed-loop control fundamentals
Closed-loop control uses feedback (usually from an encoder or Hall sensors) to correct errors between a desired setpoint and the actual motor state. The three main regulated variables in ODrive are:
- Position (θ) — for precise angular control.
- Velocity (ω) — for steady speed with disturbance rejection.
- Current (I) — for torque control or limiting.
Why closed-loop? It provides accuracy, stability under load, and predictable dynamic response. The typical control chain in ODrive is hierarchical:
- Position controller (outer loop) — computes a velocity or setpoint to drive position error toward zero.
- Velocity controller (middle loop) — computes a current command from velocity error.
- Current controller / FOC (inner loop) — performs field-oriented control (FOC) to regulate motor phase currents and produce torque.
Understanding loop bandwidths and separation is key: inner loops run faster and must be stable before outer-loop gains are increased.
3. ODrive control modes and state machine
ODrive supports multiple control modes; relevant ones for advanced users:
- IDLE — motors are not powered.
- MOTOR_CALIBRATION / ENCODER_INDEX_SEARCH — used during startup to identify motor/encoder alignment.
- CLOSED_LOOP_CONTROL — full closed-loop operation (position/velocity/current setpoints valid).
- HOLD, VELOCITY_CONTROL, POSITION_CONTROL — higher-level conveniences that map to closed-loop behavior.
Control modes (set via axis.controller.config.control_mode or axis.requested_state):
- CONTROL_MODE_VELOCITY_CONTROL — the controller interprets input as target velocity (rad/s).
- CONTROL_MODE_POSITION_CONTROL — interprets input as absolute position (turns or encoder counts).
- CONTROL_MODE_CURRENT_CONTROL — direct current/torque control.
State machine: move from IDLE -> MOTOR_CALIBRATION -> ENCODER_INDEX_SEARCH (if needed) -> CLOSED_LOOP_CONTROL. Proper calibration ensures accurate phase-encoder offset for FOC.
4. Trajectory planning and motion profiles
Directly commanding position setpoints can lead to aggressive, jerky motion. Trajectory planning smooths commands, limits jerk/acceleration, and ensures feasibility.
Common profiles:
- Trapezoidal velocity profile — constant acceleration up to max velocity, cruise, then constant deceleration. Simple and efficient for point-to-point moves.
- S-curve profile — shapes jerk for smoother transitions, reducing mechanical stress and vibrations.
- Multi-segment trajectories — piecewise polynomial (e.g., cubic, quintic) for synchronized multi-axis moves or precise path following.
ODrive features for trajectories:
- axis.controller.pos_setpoint and axis.controller.vel_setpoint can be updated in real time to follow a computed profile.
- The built-in position control implements a basic trajectory generator (configurable with vel_limit, accel_limit, and input_mode settings). Use axis.controller.config.vel_limit and axis.controller.config.accel_limit to constrain moves.
- For advanced trajectories (S-curves, coordinated multi-axis), compute the setpoints externally (in host code) and stream them at a sufficient rate to the ODrive (via USB, UART, or CAN). Use real-time update rates and consider CAN for lower latency/multi-axis systems.
Practical notes:
- Choose sampling/update rates consistent with your controller bandwidth; for high-performance systems, stream at 1 kHz or as high as communication link reliably allows.
- For multi-axis coordination, generate trajectories offline (e.g., quintic polynomials) and feed time-synchronized setpoints to each axis.
5. Tuning strategies and parameter selection
Tuning ODrive involves setting gains for current, velocity, and position loops and configuring motor and encoder parameters. A systematic approach:
A. Motor and encoder setup
- Set motor config: pole pairs, torque constant (if known), motor type. For custom motors, measure or calculate Kv/Kt and set appropriately.
- Configure encoder: CPR (counts per revolution), mode (incremental, Hall, absolute), and whether to use index for homing.
B. Current loop (inner-most)
- Current loop uses FOC and typically has highest bandwidth. ODrive’s low-level firmware handles this, but you can adjust current control gains (iq_ramp, current_lim). If you have custom firmware or advanced boards, ensure current loop PI gains are appropriate. Verify stable current behavior under step current commands, watching phase currents.
C. Velocity loop
- Set velocity_gain and velocity_integrator_gain. Start conservatively: low proportional gain, zero integrator.
- Increase velocity P until you see acceptable response but not oscillation. Then add small I to eliminate steady-state error.
- Confirm stability by applying step changes in desired velocity and observing overshoot, settling time, and oscillations.
D. Position loop (outer-most)
- Position gains (pos_gain primarily) determine stiffness. Start low, slowly increase until you see crisp tracking without oscillation.
- If using derivative (vel_feed_forward / pos_gain with velocity feedforward), tune to reduce overshoot. ODrive supports velocity feedforward via velocity_setpoint when used with position control.
E. Limits and safety
- Configure torque/current limits conservatively at first.
- Use current_lim, brake_resistance, and thermal limits to prevent damage.
Tuning workflow:
- Calibrate motor/encoder and enter closed-loop.
- Validate current loop safety with simple current commands.
- Tune velocity loop with velocity steps under light load.
- Tune position loop with small position steps.
- Increase speed/loads and iterate.
6. Practical examples and step-by-step walkthroughs
Example A — Basic closed-loop velocity tuning (outline):
- Calibrate and enter CLOSED_LOOP_CONTROL.
- Set axis.controller.config.vel_limit to a safe value (e.g., 10 rad/s).
- Start with velocity_gain = 0.1, velocity_integrator_gain = 0.
- Command axis.controller.input_vel = 2 rad/s and watch response.
- Increase velocity_gain until rise time is acceptable; if oscillations appear, reduce gain.
- Add small integrator to remove steady-state error.
Example B — Position move with trapezoidal profile using ODrive internal limits:
- Configure vel_limit and accel_limit.
- Set axis.controller.input_pos to desired target. The ODrive’s internal controller will respect limits and generate a smooth profile.
- Monitor axis.encoder.pos_estimate and axis.controller.state to confirm motion.
Example C — External S-curve trajectory streaming (host-side):
- Compute time-indexed position and velocity setpoints in host code using an S-curve generator.
- Stream setpoints via CAN/USB at ~500–1000 Hz to axis.controller.pos_setpoint and axis.controller.vel_setpoint.
- Use axis.controller.config.input_mode = INPUT_MODE_PASSTHROUGH or a mode that accepts direct setpoints.
Code snippets (Python, using odrivetool/pyodrive):
import odrive from odrive.enums import * from fibre.protocol import ChannelBrokenException import time import math print("Finding ODrive...") odrv = odrive.find_any() # assume single axis ax = odrv.axis0 # ensure closed loop ax.requested_state = AXIS_STATE_CLOSED_LOOP_CONTROL # configure limits ax.controller.config.vel_limit = 10.0 # rad/s ax.controller.config.accel_limit = 20.0 # rad/s^2 # simple trapezoidal move: set position and let internal controller plan ax.controller.input_pos = 5.0 # radians # streaming example: feed position & velocity setpoints def stream_trajectory(points, rate_hz=500): dt = 1.0 / rate_hz for pos, vel in points: ax.controller.pos_setpoint = pos ax.controller.vel_setpoint = vel time.sleep(dt)
7. Common issues and troubleshooting
- Motor won’t enter CLOSED_LOOP_CONTROL: ensure motor/encoder calibration completed, check encoder counts, and confirm correct wiring and motor config (pole pairs). Look for errors in board state.
- Oscillation or instability: reduce outer-loop gains, verify current limits, and ensure the current loop is stable. Increase damping (reduce pos_gain or vel_gain).
- Torque ripple or cogging: improve encoder resolution or use FOC tuning parameters; ensure correct phase/encoder offset calibration.
- Position drift or incorrect zero: verify encoder CPR and indexing; use encoder index pulse to align absolute position if available.
- Communication latency: for high-performance streaming, prefer CAN or a dedicated real-time link over USB.
8. Resources and next steps
- Official ODrive documentation and firmware release notes for version-specific features.
- Example projects: CNC, 3D-printed robots, gimbal stabilizers.
- Community forums and GitHub issues for troubleshooting and firmware patches.
References (for deeper reading)
- ODrive project docs and GitHub repository (search the latest docs for your firmware version).
- Field-Oriented Control primers and BLDC motor control textbooks for theoretical background.
- Motion planning literature for trajectory generation strategies (trapezoidal, S-curve, polynomial splines).
If you want, I can convert one of the example walkthroughs into a full ready-to-run script for your exact ODrive and motor specs — tell me your ODrive model, motor Kv/Kt or torque constant (if known), encoder CPR, and your target max speed/torque.