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motion_primitive_controllers

Package to control robots using motion primitives like LINEAR_JOINT (PTP/ MOVEJ), LINEAR_CARTESIAN (LIN/ MOVEL) and CIRCULAR_CARTESIAN (CIRC/ MOVEC)

Description

This project provides an interface for sending motion primitives to an industrial robot controller using the ExecuteMotionPrimitiveSequence.action action from control_msgs. The controller receives the primitives via the action interface and forwards them through command interfaces to the robot-specific hardware interface. Currently, hardware interfaces for Universal Robots and KUKA Robots are implemented.

• Supported motion primitives:

  • LINEAR_JOINT

  • LINEAR_CARTESIAN

  • CIRCULAR_CARTESIAN

If multiple motion primitives are passed to the controller via the action, the controller forwards them to the hardware interface as a sequence. To do this, it first sends MOTION_SEQUENCE_START, followed by each individual primitive, and finally MOTION_SEQUENCE_END. All primitives between these two markers will be executed as a single, continuous sequence. This allows seamless transitions (blending) between primitives.

The action interface also allows stopping the current execution of motion primitives. When a stop request is received, the controller sends STOP_MOTION to the hardware interface, which then halts the robot’s movement. Once the controller receives confirmation that the robot has stopped, it sends RESET_STOP to the hardware interface. After that, new commands can be sent.

This can be done, for example, via a Python script as demonstrated in the example python script in the Universal_Robots_ROS2_Driver package.

Command and State Interfaces

To transmit the motion primitives, the following command and state interfaces are required. All interfaces use the naming scheme tf_prefix_ + "motion_primitive/<interface name>" where the tf_prefix is provided to the controller as a parameter.

Command Interfaces

These interfaces are used to send motion primitive data to the hardware interface:

• motion_type: Type of motion primitive (LINEAR_JOINT, LINEAR_CARTESIAN, CIRCULAR_CARTESIAN)

• q1 – q6: Target joint positions for joint-based motion

• pos_x, pos_y, pos_z: Target Cartesian position

• pos_qx, pos_qy, pos_qz, pos_qw: Orientation quaternion of the target pose

• pos_via_x, pos_via_y, pos_via_z: Intermediate via-point position for circular motion

• pos_via_qx, pos_via_qy, pos_via_qz, pos_via_qw: Orientation quaternion of via-point

• blend_radius: Blending radius for smooth transitions

• velocity: Desired motion velocity

• acceleration: Desired motion acceleration

• move_time: Optional duration for time-based execution (For LINEAR_JOINT and LINEAR_CARTESIAN. If move_time > 0, velocity and acceleration are ignored)

State Interfaces

These interfaces are used to communicate the internal status of the hardware interface back to the motion_primitives_forward_controller.

• execution_status: Indicates the current execution state of the primitive. Possible values are:

  • IDLE: No motion in progress

  • EXECUTING: Currently executing a primitive

  • SUCCESS: Last command finished successfully

  • ERROR: An error occurred during execution

  • STOPPING: The hardware interface has received the STOP_MOTION command, but the robot has not yet come to a stop.

  • STOPPED: The robot was stopped using the STOP_MOTION command and must be reset with the RESET_STOP command before executing new commands.

• ready_for_new_primitive: Boolean flag indicating whether the interface is ready to receive a new motion primitive

Architecture Overview

Architecture for a UR robot with Universal_Robots_ROS2_Driver in motion primitives mode.

Architecture for a KUKA robot with kuka_eki_motion_primitives_hw_interface.

Demo-Video with UR10e

Demo-Video with KR3

Parameters

This controller uses the generate_parameter_library to handle its parameters. The parameter definition file located in the src folder contains descriptions for all the parameters used by the controller.

tf_prefix (string)

All interfaces use the naming scheme tf_prefix_ + motion_primitive/<interface name>

Read only: True

Default: “”

An example parameter file for this controller can be found in the test directory:

test_motion_primitives_forward_controller:
  ros__parameters:
    tf_prefix: ""