The gripper of the robot is designed for the picking and placing of blocks and the gripper is on the simple linkage category. There are two joints for the gripper and each joint is prismatic. Here is the joint definition of one gripper joint:

   <joint name="finger_joint1" type="prismatic"> 
    <parent link="gripper_roll_link"/> 
    <child link="gripper_finger_link1"/> 
    <origin xyz="0.0 0 0" /> 
    <axis xyz="0 1 0" /> 
      <limit effort="100" lower="0" upper="0.03" velocity="1.0"/> 
      <safety_controller k_position="20" 
                         k_velocity="20" 
                         soft_lower_limit="${-0.15 }" 
                         soft_upper_limit="${ 0.0 }"/> 
    <dynamics damping="50" friction="1"/> 
  </joint> 

Here, the first gripper joint is formed by gripper_roll_link and gripper_finger_link1, and the second joint is formed by gripper_roll_link and gripper_finger_link2.

The following graph shows how the gripper joints are connected in
gripper_roll_link: