In reinforcement learning training, the left-right symmetric opening and closing of the Franka Panda gripper are ensured by PDJointPosMimicController (imitation controller). The working principle of this special controller is as follows:

1. Single control parameter: Although Panda has two gripper joints panda_finger_joint1 and panda_finger_joint2, the mimic controller allows both joints to share the same control signal. A single value is sufficient to control the opening and closing of the entire gripper.

2. Code implementation:

   pythonclass PDJointPosMimicController(PDJointPosController):
       def _get_joint_limits(self):
           joint_limits = super()._get_joint_limits()
           diff = joint_limits[0:-1] - joint_limits[1:]
           assert np.allclose(diff, 0), "Mimic joints should have the same limit"
           return joint_limits[0:1]
   

This code indicates that the imitation controller only needs to know the constraints of the first joint, and then the same control signal will be applied to all joints.

3. Controller configuration:

   pythongripper_pd_joint_pos = PDJointPosMimicControllerConfig(
       self.gripper_joint_names,  # 包含两个夹爪关节
       lower=-0.01,  # 最小位置（稍微闭合）
       upper=0.04,   # 最大位置（完全打开）
       ...
   )
   

4. Simplified action space:

   • Although two joints are physically controlled, only one dimension is required in the action space.
   • For example, when input is 0.04, both joints will move to the 0.04 position (fully open).
   • When input is 0.0, both joints will move to the 0.0 position (fully closed).

Advantages of this design:

• Simplified learning: Reinforcement learning algorithms only need to learn to control one parameter, rather than controlling two fingers separately
• Ensuring symmetry: No matter what value is input, the gripper always opens and closes symmetrically on both sides
• Accommodating physical reality: The real Franka Panda gripper is designed to move synchronously and symmetrically in terms of its physical structure

In the GEN72-EG2 robot, the same control method is used. Although there are 4 gripper joints, they can maintain synchronized symmetric motion through PDJointPosMimicController.