Getting Started

Installation

We recommend use mplib for motion planning with SAPIEN. mplib is a lightweight python package that includes common functionalities for motion planning. You can use mplib to plan a collision-free trajectory for a robot, calculate inverse kinematics, and take point cloud observation as an environment model. Unlike MoveIt, mplib is decoupled from ROS, and it’s easy to set up and use with simple python APIs.

Please use pip to install mplib:

pip install mplib

Supported Python versions: 3.6, 3.7, 3.8, 3.9

Supported operating system: Ubuntu 18.04+

Note

mplib is a standalone package and can be used independently without SAPIEN.

Planner Configuration

In Getting Started with Robot, we learned how to load a robot in SAPIEN:

        loader: sapien.URDFLoader = self.scene.create_urdf_loader()
        loader.fix_root_link = True
        self.robot: sapien.Articulation = loader.load("../assets/robot/panda/panda.urdf")
        self.robot.set_root_pose(sapien.Pose([0, 0, 0], [1, 0, 0, 0]))

To use mplib, we need to first set up a planner for the robot:

    def setup_planner(self):
        link_names = [link.get_name() for link in self.robot.get_links()]
        joint_names = [joint.get_name() for joint in self.robot.get_active_joints()]
        self.planner = mplib.Planner(
            urdf="../assets/robot/panda/panda.urdf",
            srdf="../assets/robot/panda/panda.srdf",
            user_link_names=link_names,
            user_joint_names=joint_names,
            move_group="panda_hand",
            joint_vel_limits=np.ones(7),
            joint_acc_limits=np.ones(7))

• The URDF file describes the robot, while the SRDF file complements the URDF and specifies additional information for motion planning. For example, mplib loads the disable_collisions pairs in SRDF to ignore collisions between certain pairs of links. Currently, SRDF files are generated by MoveIt Setup Assistant. In the future, we may provide other tools for generating SRDF files.

• To align the link order and joint order between SAPIEN and mplib, we need to provide user_link_names and user_joint_names. After that, mplib can tell which joints the qpos vector (from SAPIEN) is referring to. Please note that we only care about the active joints (i.e., revolute and prismatic joints) and ignore the fixed joints.

• move_group specifies the target link 1 for which we may specify target poses to reach. The end-effector of an agent is typically specified as the move_group. After specifying the move_group, mplib only focuses on those active joints along the path from the root link to the move_group, since other joints doesn’t affect the pose of the move_group. For example, for our panda robot arm (7 DoF), the end-effector is panda_hand. Only the first seven active joints affect the pose of panda_hand, while the last two finger joints don’t.

• For safety, the robot cannot move arbitrarily fast. joint_vel_limits and joint_acc_limits specify the maximum joint velocity and maximum joint acceleration constraints for the active joints along the path from the root link to the move_group. mplib takes the constraints into account when solving the time-optimal path parameterization.

After setting up the planner, we can use it to solve many motion planning tasks.

1

mplib currently only supports a single link as move_group.