Controllers

Controllers are interfaces between policies and robots. The policy outputs actions to the controller, and the controller converts actions to control signals to the robot. For example, the arm_pd_ee_delta_pose controller takes the relative movement of the end-effector as input, and uses inverse kinematics to convert input actions to target positions of robot joints. The robot uses a PD controller to drive motors to achieve target joint positions.

The controller defines the action space of an environment. The robot can have separate controllers for its arm, gripper, and other components. The action space is a concatenation of the action spaces of all controllers.

Terminology

  • fixed joint: a joint that can not be controlled. The degree of freedom (DoF) is 0.

  • qpos ( \(q\) ): controllable joint positions

  • qvel ( \(\dot{q}\) ): controllable joint velocities

  • target joint position ( \(\bar{q}\) ): target position of the motor which drives the joint

  • target joint velocity ( \(\bar{\dot{q}}\) ): target velocity of the motor which drives the joint

  • PD controller: control loop based on \(F(t) = K_p (\bar{q}(t) - q(t)) + K_d (\bar{\dot{q}}(t) - \dot{q}(t))\). \(K_p\) (stiffness) and \(K_d\) (damping) are hyperparameters. \(F(t)\) is the force of the motor.

  • Augmented PD controller: Passive forces (like gravity) are compensated for the PD controller.

  • action ( \(a\) ): the input of the controller, also the output of the policy

Supported Controllers (Franka Emika)

The robot is Franka Emika, a.k.a Panda. The DoF of the arm is 7. We use the tool center point (TCP), which is the center between two fingers, as the end-effector.

All “pd” controllers translate input actions to target joint positions \(\bar{q}\) (and velocities) for the internal built-in PD controller. All the controllers have a normalized action space ([-1, 1]), except arm_pd_joint_pos and arm_pd_joint_pos_vel.

For simplicity, we use the name of the arm controller to represent each combination of the arm and gripper controllers, since there is only one gripper controller currently. For example, pd_joint_delta_pos is short for arm_pd_joint_delta_pos + gripper_pd_joint_pos.

Arm controllers

  • arm_pd_joint_pos (7-dim): The input is \(\bar{q}\). It can be used for motion planning, but note that the target velocity is always 0.

  • arm_pd_joint_delta_pos (7-dim): \(\bar{q}(t)=q(t) + a\)

  • arm_pd_joint_target_delta_pos (7-dim): \(\bar{q}(t)=\bar{q}(t-1) + a\)

  • arm_pd_ee_delta_pos (3-dim): only control position ( \(p\) ) without rotation ( \(R\) ).

    \(\bar{p}(t)=p(t)+a\), \(\bar{R}(t)=R(t)\), \(\bar{q}(t)=IK(\bar{p}(t), \bar{R}(t))\)

  • arm_pd_ee_delta_pose (6-dim): both position ( \(p\) ) and rotation ( \(R\) ) are controlled. Rotation is represented as axis-angle in the end-effector frame.

    \(\bar{p}(t)=p(t)+a_{p}\), \(\bar{R}(t)=R(t) \cdot e^{[a_{R}]_{\times}}\), \(\bar{q}(t)=IK(\bar{p}(t), \bar{R}(t))\)

  • arm_pd_ee_target_delta_pos (3-dim): \(\bar{p}(t)=\bar{p}(t-1)+a\), \(\bar{R}(t)=R(t)\)

  • arm_pd_ee_target_delta_pose (6-dim): \(\bar{T}(t)= T_{a} \cdot \bar{T}(t-1)\). \(T\) is the transformation of the end-effector. \(T_a\) is the delta pose induced by the action.

  • arm_pd_joint_vel (7-dim): only control target joint velocities. Note that the stiffness \(K_p\) is set to 0.

  • arm_pd_joint_pos_vel (14-dim): the extension of arm_pd_joint_pos to support target velocities

  • arm_pd_joint_delta_pos_vel (14-dim): the extension of arm_pd_joint_delta_pos to support target velocities

Gripper controllers (optional)

  • gripper_pd_joint_pos (1-dim): Note that we force two gripper fingers to have the same target position. Thus, it is like a “mimic” joint.

Mobile Manipulator

The mobile manipulator is a combination of sciurus17 connector and one or two Panda arms. The controller is named base_{}_arm_{}. Except for the base controller, the arm and gripper controllers are the same as in the stationary manipulator.

Base controllers

  • base_pd_joint_vel (4-dim): only control target velocities. The first 2 dimensions stand for egocentric xy-plane linear velocity and the 3rd dimension stands for egocentric z-axis angular velocity. The 4th dimension stands for velocity to adjust torso.