This paper presents a design method for compensating the gravity effect of a three-degrees-of-freedom (3-DoF) hybrid robotic manipulator with variable payloads. The manipulator is constructed by a pair of 2-DoF parallelogram-based five-bar parallel mechanisms combined with a 1-DoF serial mechanism. The balancing design of the manipulator is realized by attaching two balancing units to the five-bar parallel mechanism, where each unit is made by a geared seven-bar mechanism with a linear spring. The parameters of the balancing units are analytically derived from the static equilibrium of the mechanism. The design method is also effective when the payload is varied and the spring stiffnesses are prescribed. Examples are then given to illustrate the effectiveness of the proposed method. The method was also validated by simulation software. The simulation results showed that, by using the balancing design, the actuation torques of the manipulator could be reduced by more than 98% with different applied payloads.

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