Arm

The robotic arm is controlled by two servo motors and features a parallel mechanism (see arm.urdf.xacro) that maintain the orientation of the end effector and make the gripper to always stay parallel to the chassis.

This module wraps class rm:robomaster.robotic_arm.RoboticArm through which we can control the arm servo motors and get update about the end effector position.

The arm needs to be calibrated to work properly. Perform the short calibration procedure in the Robomaster App that consists of fully retracting the arm: when calibrated the servos will both report a value of 180 degrees to the SDK and 0 degrees in the App, which corresponds to the default values of arm.left_motor.zero and arm.right_motor.zero.

State estimation

Parameter chassis.rate control the rate at which the arm state is gathered and republished to ROS.

• End effector position (estimated onboard) from rm:robomaster.robotic_arm.RoboticArm.sub_position() is republished to arm_position. The accuracy of this estimation depends on the arm calibration procedure performed in the Robomaster App but not from the calibration parameters passed to ROS (see next point).

• Joint states from rm:robomaster.servo.Servo.sub_servo_info() is republished to joint_states_p with the state of three joints using

  • rod_joint (left motor, using arm.left_motor.zero, arm.left_motor.angle, arm.left_motor.direction)

  • arm_1_joint (right motor, using arm.right_motor.zero, arm.right_motor.angle, arm.right_motor.direction)

  • rod_3_joint (one of the passive joint, whose angle is computed from the arm parallel kinematics as the difference between right and left motor angles

  The other joints in arm.urdf.xacro are dependable of these three and automatically updated by robot_state_publisher.

Control

The only SKD native way to directly control the arm is through an action move_arm to reach a target goal position for the end effector. Unfortunately, there is no way to configure the target speed during the action and the robot does not offer end effector velocity control. The only way to move the end effector slowly, is to use a sequence of interpolated position towards the target (a functionality not exposed by the ROS driver).

Important

The arm can be indirectly controlled providing target angles or target angular speeds for the two servos. In fact, there are two mutually exclusive ways to control the arm: - arm related SDK commands from rm:robomaster.robotic_arm.RoboticArm - servo related SDK commands from rm:robomaster.servo.Servo. Arm commands are enabled if the arm has been configured in the Robomaster App, else servo commands are enabled.

This fact is reflected in ROS: the ROS control commands described here are only effective if the arm has been configured. .. As this information is not available to SDK clients, users are responsible to pick the correct interfaces.

We also expose an stateless interface to send a relative target point ignoring action feedback and states using subscription cmd_arm.

Parameters

parameter arm.enabled bool [Default: false]

Enables all the ROS interface described here, which by default is disabled, like all other modules.

parameter arm.rate int [Default: 10]

The rate in Hz at which arm_position and joint_states_p (with arm joints) are published. One of 0, 1, 5, 10, 20, 50 (else it will approximate to the nearest value). A value of 0 will effectively disable updates.

parameter arm.right_motor.zero int [Default: 1024]

The raw right servo value when the arm is in the zero configuration. 1024 corresponds to 180 degrees.

parameter arm.left_motor.zero int [Default: 1024]

The raw left servo value when the arm is in the zero configuration. 1024 corresponds to 180 degrees.

parameter arm.right_motor.angle float [Default: -0.274016]

The right servo joint (arm_1_joint) position when the arm is in the zero configuration. When zero configuration has the arm fully retracted, it corresponds to the lower limit of arm_1_joint.

parameter arm.left_motor.angle float [Default: 0.073304]

The left servo joint (rod_joint) position when the arm is in the zero configuration.

parameter arm.right_motor.direction int [Default: -1]

The orientation of the right motor with respect to the joint (arm_1_joint).

parameter arm.left_motor.direction int [Default: 1]

The orientation of the left motor with respect to the joint (rod_joint).

Subscriptions

subscription cmd_arm geometry_msgs/msg/Vector3

Listen for relative target position commands for end_point_link in arm.urdf.xacro), which are passed to the robot without using SDK actions. This provides a similar interface to move_arm but able to update the target position without having to cancel the action. You can you this interface to continuously control the end effector cartesian position or velocity.

Warning

This interface is experimental. To be safe, use move_arm.

Publishers

publisher arm_position geometry_msgs/msg/PointStamped

Publishes the position, estimated onboard, of the arm end effector (end_point_link in arm.urdf.xacro) in the arm_base_link frame. If accuracy is low, redo the arm calibration procedure in Robomaster App.

Action Servers

action server move_arm robomaster_msgs/action/MoveArm

Move the end effector towards a target position, which can be absolute (i.e., with respect to arm base) or relative (i.e., with respect to the current arm position). If speed is too high, try interpolating the target position.

The control is performed on-board, most probably using inverse kinematics (i.e., no motion planning seems involved). Target position may not be reachable by the controller even when feasible. To overcome this limitation, you may plan a path in cartesian space and then pass waypoints to this action.

Warning

New goals are not accepted if a current goal is active. To change goal, you first need to cancel the current goal.