Pilz Industrial Motion Planner

pilz_industrial_motion_planner provides a trajectory generator to plan standard robot motions like PTP, LIN, CIRC with the interface of a MoveIt PlannerManager plugin.

User Interface MoveGroup

This package implements the planning_interface::PlannerManager interface of MoveIt. By loading the corresponding planning pipeline (pilz_industrial_motion_planner_planning_pipeline.launch.xml in your *_moveit_config package), the trajectory generation functionalities can be accessed through the user interface (c++, python or rviz) provided by the move_group node, e.g. /plan_kinematics_path service and /move_group action. For detailed usage tutorials please refer to Move Group C++ Interface.

Joint Limits

For all commands the planner uses maximum velocities and accelerations from the parameter server. Using the MoveIt setup assistant the file joint_limits.yaml is auto-generated with proper defaults and loaded during startup.

The limits on the parameter server override the limits from the URDF robot description. Note that while setting position limits and velocity limits is possible in both the URDF and the parameter server setting acceleration limits is only possible via the parameter server. In extension to the common has_acceleration and max_acceleration parameter we added the ability to also set has_deceleration and max_deceleration(<0.0).

The limits are merged under the premise that the limits from the parameter server must be stricter or at least equal to the parameters set in the URDF.

Currently the calculated trajectory will respect the limits by using the strictest combination of all limits as a common limit for all joints.

Cartesian Limits

For cartesian trajectory generation (LIN/CIRC) the planner needs an information about the maximum speed in 3D cartesian space. Namely translational/rotational velocity/acceleration/deceleration need to be set on the parameter server like this:

cartesian_limits:
  max_trans_vel: 1
  max_trans_acc: 2.25
  max_trans_dec: -5
  max_rot_vel: 1.57

The planners assume the same acceleration ratio for translational and rotational trapezoidal shapes. So the rotational acceleration is calculated as max\_trans\_acc / max\_trans\_vel \* max\_rot\_vel (and for deceleration accordingly).

Planning Interface

This package uses moveit_msgs::MotionPlanRequest and moveit_msgs::MotionPlanResponse as input and output for motion planning. The parameters needed for each planning algorithm are explained below.

For a general introduction on how to fill a MotionPlanRequest see Planning to a Pose goal.

You can specify “PTP”, “LIN” or “CIRC” as the planner\_id``of the ``MotionPlanRequest.

The PTP motion command

This planner generates fully synchronized point-to-point trajectories with trapezoidal joint velocity profiles. All joints are assumed to have the same maximal joint velocity/acceleration/deceleration limits. If not, the strictest limits are adopted. The axis with the longest time to reach the goal is selected as the lead axis. Other axes are decelerated so that they share the same acceleration/constant velocity/deceleration phases as the lead axis.

PTP velocity profile with trapezoidal ramps - the axis with the longest duration determines the maximum velocity

Input parameters in moveit_msgs::MotionPlanRequest

  • planner_id: PTP

  • group_name: name of the planning group

  • max_velocity_scaling_factor: scaling factor of maximal joint velocity

  • max_acceleration_scaling_factor: scaling factor of maximal joint acceleration/deceleration

  • start_state/joint_state/(name, position and velocity: joint name/position/velocity(optional) of the start state.

  • goal_constraints (goal can be given in joint space or Cartesian space)

  • for a goal in joint space
    • goal_constraints/joint_constraints/joint_name: goal joint name
    • goal_constraints/joint_constraints/position: goal joint position
  • for a goal in Cartesian space
    • goal_constraints/position_constraints/header/frame_id: frame this data is associated with
    • goal_constraints/position_constraints/link_name: target link name
    • goal_constraints/position_constraints/constraint_region: bounding volume of the target point
    • goal_constraints/position_constraints/target_point_offset: offset (in the link frame) for the target point on the target link (optional)

Planning results in moveit_msg::MotionPlanResponse

  • trajectory_start: first robot state of the planned trajectory
  • trajectory/joint_trajectory/joint_names: a list of the joint names of the generated joint trajectory
  • trajectory/joint_trajectory/points/(positions,velocities,accelerations,time_from_start): a list of generated way points. Each point has positions/velocities/accelerations of all joints (same order as the joint names) and time from start. The last point will have zero velocity and acceleration.
  • group_name: name of the planning group
  • error_code/val: error code of the motion planning

The LIN motion command

This planner generates linear Cartesian trajectory between goal and start poses. The planner uses the Cartesian limits to generate a trapezoidal velocity profile in Cartesian space. The translational motion is a linear interpolation between start and goal position vector. The rotational motion is quaternion slerp between start and goal orientation. The translational and rotational motion is synchronized in time. This planner only accepts start state with zero velocity. Planning result is a joint trajectory. The user needs to adapt the Cartesian velocity/acceleration scaling factor if the motion plan fails due to violation of joint space limits.

Input parameters in moveit_msgs::MotionPlanRequest

  • planner_id: LIN
  • group_name: name of the planning group
  • max_velocity_scaling_factor: scaling factor of maximal Cartesian translational/rotational velocity
  • max_acceleration_scaling_factor: scaling factor of maximal Cartesian translational/rotational acceleration/deceleration
  • start_state/joint_state/(name, position and velocity: joint name/position of the start state.
  • goal_constraints (goal can be given in joint space or Cartesian space)
    • for a goal in joint space
      • goal_constraints/joint_constraints/joint_name: goal joint name
      • goal_constraints/joint_constraints/position: goal joint position
    • for a goal in Cartesian space
      • goal_constraints/position_constraints/header/frame_id: frame this data is associated with
      • goal_constraints/position_constraints/link_name: target link name
      • goal_constraints/position_constraints/constraint_region: bounding volume of the target point
      • goal_constraints/position_constraints/target_point_offset: offset (in the link frame) for the target point on the target link (optional)

Planning results in moveit_msg::MotionPlanResponse

  • trajectory_start: first robot state of the planned trajectory
  • trajectory/joint_trajectory/joint_names: a list of the joint names of the generated joint trajectory
  • trajectory/joint_trajectory/points/(positions,velocities,accelerations,time_from_start): a list of generated way points. Each point has positions/velocities/accelerations of all joints (same order as the joint names) and time from start. The last point will have zero velocity and acceleration.
  • group_name: name of the planning group
  • error_code/val: error code of the motion planning

The CIRC motion command

This planner generates a circular arc trajectory in Cartesian space between goal and start poses. There are two options for giving a path constraint:

  • the center point of the circle: The planner always generates the shorter arc between start and goal and cannot generate a half circle,
  • an interim point on the arc: The generated trajectory always goes through the interim point. The planner cannot generate a full circle.

The Cartesian limits, namely translational/rotational velocity/acceleration/deceleration need to be set and the planner uses these limits to generate a trapezoidal velocity profile in Cartesian space. The rotational motion is quaternion slerp between start and goal orientation. The translational and rotational motion is synchronized in time. This planner only accepts start state with zero velocity. Planning result is a joint trajectory. The user needs to adapt the Cartesian velocity/acceleration scaling factor if motion plan fails due to violation of joint limits.

Input parameters in moveit_msgs::MotionPlanRequest

  • planner_id: CIRC
  • group_name: name of the planning group
  • max_velocity_scaling_factor: scaling factor of maximal Cartesian translational/rotational velocity
  • max_acceleration_scaling_factor: scaling factor of maximal Cartesian translational/rotational acceleration/deceleration
  • start_state/joint_state/(name, position and velocity: joint name/position of the start state.
  • goal_constraints (goal can be given in joint space or Cartesian space)
    • for a goal in joint space
      • goal_constraints/joint_constraints/joint_name: goal joint name
      • goal_constraints/joint_constraints/position: goal joint position
    • for a goal in Cartesian space
      • goal_constraints/position_constraints/header/frame_id: frame this data is associated with
      • goal_constraints/position_constraints/link_name: target link name
      • goal_constraints/position_constraints/constraint_region: bounding volume of the target point
      • goal_constraints/position_constraints/target_point_offset: offset (in the link frame) for the target point on the target link (optional)
  • path_constraints (position of the interim/center point)
    • path_constraints/name: interim or center
    • path_constraints/position_constraints/constraint_region/primitive_poses/point: position of the point

Planning results in moveit_msg::MotionPlanResponse

  • trajectory_start: first robot state of the planned trajectory
  • trajectory/joint_trajectory/joint_names: a list of the joint names of the generated joint trajectory
  • trajectory/joint_trajectory/points/(positions,velocities,accelerations,time_from_start): a list of generated way points. Each point has positions/velocities/accelerations of all joints (same order as the joint names) and time from start. The last point will have zero velocity and acceleration.
  • group_name: name of the planning group
  • error_code/val: error code of the motion planning

Example

By running

roslaunch prbt_moveit_config demo.launch

the user can interact with the planner through rviz.

rviz figure

Using the planner

The pilz_industrial_motion_planner::CommandPlanner is provided as a MoveIt Motion Planning Pipeline and, therefore, can be used with all other manipulators using MoveIt. Loading the plugin requires the param /move_group/planning_plugin to be set to pilz_industrial_motion_planner::CommandPlanner before the move_group node is started.

To use the command planner cartesian limits have to be defined. The limits are expected to be under the namespace <robot_description>_planning. Where <robot_description> refers to the parameter under which the URDF is loaded. E.g. if the URDF was loaded into /robot_description the cartesian limits have to be defined at /robot_description_planning.

An example showing the cartesian limits which have to be defined can be found in prbt_moveit_config.

Sequence of multiple segments

To concatenate multiple trajectories and plan the trajectory at once, you can use the sequence capability. This reduces the planning overhead and allows to follow a pre-desribed path without stopping at intermediate points.

Please note: In case the planning of a command in a sequence fails, non of the commands in the sequence are executed.

Please note: Sequences commands are allowed to contain commands for multiple groups (e.g. “Manipulator”, “Gripper”)

User interface sequence capability

A specialized MoveIt capability takes a moveit_msgs::MotionSequenceRequest as input. The request contains a list of subsequent goals as described above and an additional blend_radius parameter. If the given blend_radius in meter is greater than zero, the corresponding trajectory is merged together with the following goal such that the robot does not stop at the current goal. When the TCP comes closer to the goal than the given blend_radius, it is allowed to travel towards the next goal already. When leaving a sphere around the current goal, the robot returns onto the trajectory it would have taken without blending.

blend figure

Implementation details are available as pdf.

Restrictions for MotionSequenceRequest

  • Only the first goal may have a start state. Following trajectories start at the previous goal.
  • Two subsequent blend_radius spheres must not overlap. blend_radius(i) + blend_radius(i+1) has to be smaller than the distance between the goals.

Action interface

In analogy to the MoveGroup action interface the user can plan and execute a moveit_msgs::MotionSequenceRequest through the action server at /sequence_move_group.

In one point the MoveGroupSequenceAction differs from the standard MoveGroup capability: If the robot is already at the goal position, the path is still executed. The underlying PlannerManager can check, if the constraints of an individual moveit_msgs::MotionPlanRequest are already satisfied but the MoveGroupSequenceAction capability doesn’t implement such a check to allow moving on a circular or comparable path.

See the pilz_robot_programming package for an example python script that shows how to use the capability.

Service interface

The service plan_sequence_path allows the user to generate a joint trajectory for a moveit_msgs::MotionSequenceRequest. The trajectory is returned and not executed.

Open Source Feedback

See something that needs improvement? Please open a pull request on this GitHub page