3DOF Gravity Compensation Mechanism for Robot Waists with the Variations of Center of Mass* This paper presents a threedegreesoffreedom (DOF) gravity compensation mechanism for the waists of cooperative robots whose centers of mass change according to the arm poses. It comprises 2DOF and 1DOF gravity compensation mechanisms in series, and is capable of natural and agile motions such as bobbing and weaving. The 2DOF mechanism for the upper part of the waist acts as a universal joint. To compensate for the gravity of this 2DOF motion, a simple and robust link mechanism with one compressive spring is proposed. Moreover, for the complete gravity compensation regardless of the arm poses, a threedimensional (3D) parallel link structure that maintains the upper body parallel to the ground is introduced. The 1DOF gravity compensation mechanism, which provides the motions of the hip and thigh, also has a parallelogram structure to maintain the waist parallel to the ground. The implemented 3DOF waist mechanism can compensate for up to 23 kg and its own weight. It has an adjusting mechanism that can easily calibrate the amount of compensation in case the weight of the upper body changes. The large workspace enables the upper body to move in any direction in a 3D space without singularity. Therefore, the robot with this waist can touch the ground to pick up objects on the ground as well as reach an object at a height of 2145 mm.

WallMounted Robot Arm Equipped with 3DOF RollPitchPitch Counterbalance Mechanism The use of highcapacity motors and harmonic drives increases the manufacturing cost and the energy consumption of the robot. It is well known that most motor torques are used to support the robot mass and payload. To deal with this problem, we propose a wallmounted 3DOF rollpitchpitch counterbalance mechanism (CBM). Unlike previous CBMs used for floormounted industrial robot arms, the proposed CBM can be applied to wallmounted robots in which the first three joints are all coupled and affected by gravity. This mechanism is composed of pure mechanical components such as coil springs, differential gear structure, and beltpulley system. A 6DOF robot arm equipped with the proposed CBM was fabricated to verify the performance of gravity compensation. It is shown that the proposed 3DOF CBM can effectively compensate for gravitational torques, thus drastically reducing the motor torques required to operate the robot.