1) Locating part The locating part is constructed by the kinematic chain from the class II Assur group. This part solves the problem of misalignments to achieve the kinematic compliance, and provides place to mount the timing belt mechanism (TBM). 2) Anti-gravity torque generating part The PGCU is used to generate anti-gravity torque which balances the LBB to achieve the static compliance. 3) Torque transmission part The TBM mounted on the locating part is used to transmit the angle and torque between the LBB and the PGCU. This makes it possible to locate the heavy PGCU near the base to reduce the distal inertia, and thus meets the dynamic compliance.