Payload

The gripper plugins that open and close a two-finger gripper in Gazebo or a servo on a real drone.

The gripper behavior opens and closes a gripper, and its plugin knows how to drive it.

Plugin	Drives
`two_fingers`	The two-finger gripper of the Gazebo simulation
`dc_servo`	A gripper moved by a servo, through a PWM driver you provide

The behavior serves the GripperHandlerBehavior action, of type as2_msgs/action/GripperHandler. Its goal is one flag, request_gripper. True closes the gripper and false opens it. The feedback's state_gripper is true while it's closed.

The gripper moves as soon as the goal arrives, and the behavior keeps running while it holds. It never finishes on its own, so don't wait for a result. Send a new goal to move the gripper again, and stop the behavior when you're done:

ros2 action send_goal /drone0/GripperHandlerBehavior as2_msgs/action/GripperHandler \
  "{request_gripper: true}"

ros2 service call /drone0/GripperHandlerBehavior/_behavior/stop std_srvs/srv/Trigger

There's no Python API module for the gripper yet. From a mission, use an rclpy action client.

Running it

Each plugin has its own launch file:

ros2 launch as2_behaviors_payload two_fingers.launch.py namespace:=drone0
ros2 launch as2_behaviors_payload dc_servo_gripper.launch.py namespace:=drone0

Every parameter below is also a launch argument, such as angle_to_open:=10.0. A file given as plugin_config_file is merged over the defaults.

Two fingers

two_fingers sends a position to each finger joint of the Gazebo two_fingers_gripper model. Add the gripper to the payload of an f330 drone in the world file:

config/world.yaml

drones:
  - model_type: "f330"
    model_name: "drone0"
    payload:
      - model_type: "two_fingers_gripper"
        model_name: "gripper"

The other drone models also carry the gripper, but its fingers only move when the drone is named drone0.

Parameter	Default	What it does
`l_finger_open`	1.0	The left finger's angle when open, in radians
`r_finger_open`	-1.0	The right finger's angle when open, in radians
`l_finger_close`	0.0	The left finger's angle when closed, in radians
`r_finger_close`	0.0	The right finger's angle when closed, in radians
`topic_l_finger`	`/gz/<namespace>/joint/r_gripper_l_finger_joint/cmd_pos`	The left finger's command topic
`topic_r_finger`	`/gz/<namespace>/joint/r_gripper_r_finger_joint/cmd_pos`	The right finger's command topic

The launch file builds the finger topics from the namespace and ignores them on the launch line. To change them, use plugin_config_file.

When the behavior ends for any reason, the gripper opens. Keep the behavior running for as long as the drone carries something.

DC servo

dc_servo turns an angle into a PWM duty cycle and publishes it as a std_msgs/msg/Float64 on gripper_pwm, in the drone's namespace. A servo driver on the drone has to apply it, and Aerostack2 doesn't ship one. The duty cycle grows linearly from duty_min at 0 degrees to duty_max at max_angle, so with the defaults the gripper opens at 4.5% and closes at 8.0%.

Parameter	Default	What it does
`angle_to_open`	6.43	The servo angle that opens the gripper, in degrees
`angle_to_close`	51.4	The servo angle that closes it, in degrees
`max_angle`	90.0	The servo's full range, in degrees
`duty_min`	4.0	The duty cycle at 0 degrees, in percent
`duty_max`	11.0	The duty cycle at `max_angle`, in percent
`topic_pwm`	`gripper_pwm`	The topic the duty cycle is published on

When the behavior ends, it publishes -1 instead of opening the gripper, for the driver to release the servo.

Point gimbal

The payload package also has the point gimbal behavior, which aims a gimbal at a point. It has no plugins, and the Python API runs it with the point_gimbal module.

Running it

Two fingers

DC servo

Point gimbal

On this page