The geometry of the locating is sho

The geometry of the locating is shown in Fig. 11b. We concentrate the masses of the PGCU and link 2 at point D as mD. Since link 2 is always vertical, mD can be moved to point B based on the observation that the gravitational moments at point A generated by the mass at point D and generated by the mass at point B are equal. Note that there are two masses, namely, mD and m1, mounted on the LBB, similar to the geometry and mass locations of the PRR-based gravity compensator (see Fig. 8b). The spring stiffness of the PGCU that achieves complete gravity balance can be calculated similar to Eq. (13).