This paper proposes a new version of a variable gravity compensation mechanism (CVGC-II) for mobile manipulation robots such as mobile manipulators and exoskeletons. CVGC-II, which incorporated an energy-free variable mechanism in a smaller design space, exhibited improved performance with a wider range of variable torque. These enhancements were achieved by adopting an optimal curved lever and hybrid spring system. First, the mechanism concept and specifications of the CVGC mechanism are described. Next, the contribution of the curved lever and hybrid spring system to the performance enhancement of CVGC-II is explained. Subsequently, the design methodologies for the main mechanical elements, including the lever, hybrid spring, and cam, are introduced. In the lever design, Hermite spline and B-spline curve model based optimization is performed to generate optimal curved shape of the lever. The hybrid spring system, consisting of compression coil springs and layered leaf springs, helps achieve a wider variable range by maximizing the storable energy in a limited design space. Finally, the enhanced performance of CVGC-II is evaluated experimentally. The results showed that CVGC-II exhibited a 6.8 times wider variable range and energy-free variable mechanism under a 1.5 and 1.3 times smaller size and weight, respectively.

Funding Information
* The Samsung Research Funding & Incubation Center of Samsung Electronics under Project Number SRFC-IT1903-02
* The Chung-Ang University Graduate Research Scholarship in 2020