A controller that manages a Feetech servo bus.

This controller wraps the Feetech GenServer and provides a shared ETS table for actuators to write commands to. The controller runs a fixed-rate control loop that batches all pending commands into efficient bulk writes (sync_write) and reads positions via bulk reads (sync_read).

On each loop tick, the controller:

1. Reads all pending commands from the ETS table
2. Batches them into sync_write operations for the serial bus
3. Clears the command fields
4. Reads all servo positions via sync_read
5. Updates the ETS table with current positions
6. Publishes JointState messages for positions that exceed deadband

Configuration

The controller is typically defined in the robot DSL:

controller :feetech, {BB.Servo.Feetech.Controller,
  port: "/dev/ttyUSB0",
  baud_rate: 1_000_000,
  control_table: Feetech.ControlTable.STS3215,
  loop_interval_ms: 10
}

Options

• :port - (required) The serial port path, e.g., "/dev/ttyUSB0"
• :baud_rate - Baud rate in bps (default: 1_000_000)
• :control_table - The servo control table to use (default: Feetech.ControlTable.STS3215)
• :loop_interval_ms - Control loop interval in ms (default: 10, i.e. 100Hz)
• :status_poll_interval_ms - Status polling interval in ms (default: 1000, set to 0 to disable)
• :disarm_action - Action to take when robot is disarmed (default: :disable_torque)
  • :disable_torque - Disable torque on all servos (safe default)
  • :hold - Keep torque enabled (servos hold position)

ETS Table Structure

Each registered servo has a row in the ETS table:

{servo_id, actuator_path, position_deadband,
 last_position_raw, present_position, present_temperature,
 present_voltage, present_load, hardware_error,
 goal_position, goal_speed}

Actuators write goal_position (raw motor units) and goal_speed (motor rad/s) via :ets.update_element/3. The controller reads and clears them each tick and uses BB.Actuator.to_joint_space/3 to translate encoder readings to joint-space before publishing JointState.

Safety

This controller implements the BB.Safety behaviour. When the robot is disarmed or crashes, torque is disabled on all known servo IDs using sync_write for speed.