Incomplete gravity compensator for a 4-DOF manipulator

This paper proposes gravity compensators for a 4-degree-of-freedom (4-DOF) humanlike manipulator. Eighteen springs (or 1-DOF gravity compensators) are required to achieve complete static balancing of a 4-DOF manipulator. Because locating 18 springs is impractical, incomplete gravity compensators are designed for practical implementation in this paper. Springs are selected using an objective function of the gravity compensation and design cost. The design cost indicates the complexity of the mechanisms. As a result, four- and two-spring designs are obtained. Optimizations of spring constants of the four- and two-spring designs are conducted for the objective function of gravity compensation. The torque ratios for the four-spring design are computed as [18.64%, 11.92%, 77.68%, 81.14%]. The torque ratios for the two-spring design are computed as [16.03%, 20.22%, 100.00%, 100.00%] and indicate that gravity compensation is made only at proximal joints to the base. Dynamic simulations are conducted, and simulation results show that the ratios of gravity compensation are achievable.