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agribot_control configures the ROS control layer for AGRIBOT’s four-wheel skid-steer drivetrain. It uses ros_control effort controllers with PID tuning for each wheel joint, and a joint state controller for publishing current joint positions and velocities. The package contains a single YAML configuration file and one launch file — all the logic lives in the ros_control framework itself, which the configuration wires together.

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Controller Configuration

The full agribot_control.yaml is shown below. It defines five controllers under the /agribot namespace: one joint state controller and four per-wheel effort controllers, one for each corner of the skid-steer chassis. 
agribot:
  joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 50

  leftwheelR_effort_controller:
    type: effort_controllers/JointEffortController
    joint: base_link_to_left_wheel_R_joint
    pid: {p: 100.0, i: 0.1, d: 10.0}

  leftwheelF_effort_controller:
    type: effort_controllers/JointEffortController
    joint: base_link_to_left_wheel_F_joint
    pid: {p: 100.0, i: 0.1, d: 10.0}

  rightwheelR_effort_controller:
    type: effort_controllers/JointEffortController
    joint: base_link_to_right_wheel_R_joint
    pid: {p: 100.0, i: 0.1, d: 10.0}

  rightwheelF_effort_controller:
    type: effort_controllers/JointEffortController
    joint: base_link_to_right_wheel_F_joint
    pid: {p: 100.0, i: 0.1, d: 10.0}
All four wheel controllers share identical PID gains, reflecting that the chassis is symmetric and the wheels have equivalent inertia and friction properties.

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PID Parameter Reference

ParameterValueDescription
p (Proportional)100.0Main torque output per unit error — high value enables fast response to velocity commands
i (Integral)0.1Eliminates steady-state error; kept low to avoid windup on a wheel that is momentarily slipping
d (Derivative)10.0Dampens oscillation from the high proportional gain
publish_rate50 HzRate at which joint_state_controller publishes to /agribot/joint_states

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Launching the Controllers

roslaunch agribot_control agribot_control.launch
The launch file performs two actions in order:
  1. Loads parameters — rosparam uploads agribot_control.yaml to the ROS parameter server under the /agribot namespace.
  2. Spawns controllers — controller_manager/spawner starts all five controllers inside the /agribot namespace:
    • joint_state_controller
    • rightwheelR_effort_controller
    • leftwheelR_effort_controller
    • rightwheelF_effort_controller
    • leftwheelF_effort_controller
The launch file also re-processes the Xacro description and loads it as robot_description, ensuring the hardware interface and URDF are consistent when starting controllers standalone (outside of my_world.launch).

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Joint Names

The four joints controlled by the effort controllers map directly to the joints defined in agribot_body.urdf.xacro. The joint_names_agribot.yaml config file records these names for tooling that needs to enumerate controllable joints:
Joint NamePosition
base_link_to_left_wheel_F_jointFront-left wheel
base_link_to_right_wheel_F_jointFront-right wheel
base_link_to_left_wheel_R_jointRear-left wheel
base_link_to_right_wheel_R_jointRear-right wheel
Each joint is of type continuous (unbounded rotation), which is required for driven wheels. The effort_controllers/JointEffortController accepts a torque command in Newton-metres on the topic /agribot/<controller_name>/command.
The skid-steer drive plugin (libgazebo_ros_skid_steer_drive.so) directly subscribes to /agribot/cmd_vel and handles differential drive kinematics internally — it bypasses the effort controllers entirely during normal autonomous navigation. The effort controllers are used for direct joint-level control, hardware-in-the-loop testing, and scenarios where precise torque commands are needed per wheel.

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