A theoretical study of weight-balanced mechanisms for design of spring assistive mobile arm support (MAS)

This paper studies the underlying theory of weight-balanced mechanism for the design of a class of spatial mobile arm support (MAS), a spring assistive MAS. Conventional designs of spring assistive MAS and their associated spring balancing techniques are analyzed based on the stiffness matrix analysis in order to highlight the structural novelty of the proposed MAS design concept. This MAS comprises two ideal zero-free-length springs directly installed to the arm mechanism without using any auxiliary link. Through the passive assistance provided by the springs, the MAS can facilitate the arm movement in space by the complete weight compensation of the upper limb at any possible posture. The design is believed to have benefited from its simple structure and the easiness of adjustment compared to other conventional designs. The conceptual design of the MAS is proposed and followed with a simulation model. The gravity balancing is verified with an example of a quasi-static motion. The results show that the MAS is capable of fully balancing the weights of user's upper limb and the device during the full range of motion.

► Underlying theory of the existing weight-balanced mechanisms is investigated.
► A new spring assistive mobile arm support (MAS) without auxiliary links is proposed.
► The spatial MAS is uniquely derived from a planar balancing mechanism.
► The MAS design can be adjusted to adapt to different levels of gravity balancing.
► Practicality of the design is suggested.