Development of a gravity compensation device for rotary joint using magnetic energy

A gravity compensation (GC) device compensates for the torque originating from a constant mass or payload, which occupies a large part of the capacity and energy consumption of an actuator on the joint. Adapting a GC device can reduce energy consumption and capacity of the actuator. A GC device comprises an energy-storage component and a motion-converting mechanism. The energy storage component stores and releases energy according to the change in gravitational energy as the mass of the joint rotates. The motion-converting mechanism matches the energy from the energy storage component to the gravitational energy of the rotating mass. The majority of GC devices use springs as the energy storage component, and they are connected to the body by motion-converting mechanisms, such as gears and slide cranks. A GC device that uses magnetic energy as an energy storage component was proposed in this study. It uses noncontact permanent magnets (PMs) as energy storage components. It is designed to have a simple structure and compact size, and can be easily connected to the actuator module, similar to commercial gear reducers. It comprises two identical structures, consisting of one yoke and two PMs. The two structures are assembled as the PMs face each other and generate attractive and repulsive forces depending on the relative angle between the two facing PMs. The shapes of the PMs were determined to generate a sinusoidal torque profile to compensate for the gravitational torque by a mass. The designed mechanism is verified through simulations and experiments.