Design of Spring Static-Balanced Serial Manipulators for Reduced Spring Attachment Adjustments

This paper presents a design of spring static-balanced manipulators for reduced spring attachment adjustments. Gravitational joint torque is balanced by spring torque to maintain static balance, but joint reaction force by gravity force and spring force is still an important issue for manipulators. Springs, with their stiffness and attachment parameters, cause torque on the same joint, and then there is a torque-sharing effect between them, and the parameters of one spring can be represented by other springs. The sharing ratio between coupled springs is defined as the ratio of the torque due to the spring attached to the succeeding link to the gravitational torque. For adjacent springs, the bounds of the sharing ratio are from 0 to 1; for non-adjacent springs at a succeeding link or preceding link, the upper bound of the sharing ratio is determined, or the sharing ratio is determined, respectively. The 3-DOF manipulators are an illustrative example: the relationship between joint reaction force and the joint torque-sharing ratio is investigated, and on the optimum joint reaction force, the best sharing ratio and spring attachment installations are found. It is shown that the joint reaction force is reduced in manipulators, and this method is used in spatial manipulators with a systematic spring static balance method.