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, joint_name, center_angle, reverse?, position_deadband,
 last_position_raw, goal_position, goal_speed}

Actuators write goal_position (raw units) and goal_speed (rad/s) via :ets.update_element/3. The controller reads and clears them each tick.

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.