Most existing mechanical gravity compensators have been developed for revolute joints that are found in majority of articulated robot arms. However, robots such as patient transport robots use prismatic joints, which need to handle a heavy payload. In this study, a high-capacity linear gravity compensator (LGC), which comprises pure mechanical components, such as coil springs, a rack-pinion gear, a cam, and a wire, is proposed to compensate for the payload applied to a prismatic joint. The LGC is designed to generate a constant compensation force regardless of the payload position. The device can be manufactured at a low cost and has a significantly long lifespan because it uses coil springs to serve as an elastic body. Experiments demonstrate that the robot with the LGC can handle a load of 100 kg more than the robot using the same motors without it.

The use of high-capacity 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 wall-mounted 3-DOF roll-pitch-pitch counterbalance mechanism (CBM). Unlike previous CBMs used for floor-mounted industrial robot arms, the proposed CBM can be applied to wall-mounted 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 belt-pulley system. A 6-DOF robot arm equipped with the proposed CBM was fabricated to verify the performance of gravity compensation. It is shown that the proposed 3-DOF CBM can effectively compensate for gravitational torques, thus drastically reducing the motor torques required to operate the robot.

Most collaborative robots use high-power motors for a good weight-to-payload ratio, thus leading to not only an increase in manufacturing cost but also possibility of injury at a collision between a human and a robot. To maintain high-performance with low-power driving units, a spring-based counterbalance mechanism (CBM) and a robot arm based on these CBMs were developed in our previous study. In this study, a 6-DOF collaborative robot equipped with a multi-DOF CBM is proposed. A double parallelogram linkage and a slider-crank mechanism are employed for a compact and durable design of a multi-DOF CBM. Unlike the previous prototypes in which some portions of CBMs were protruded out of the robot body due to their large volume, the proposed CBMs can be embedded inside the robot links. The performance of the developed CBM and collaborative robot were verified based on simulations using dynamic simulation software. Simulation results show that the proposed CBMs can effectively reduce the joint torques required to operate the robot. This reduction in the torque enables low-power motors to be used in a collaborative robot, thus significantly improving collision safety and energy efficiency.

In most 6 DOF robot arms, considerable amounts of gravitational torques due to the robot's own weight are applied to pitch joints of the robot, which causes most arms to use high capacity motors and speed reducers. A spring-based counterbalance mechanism can compensate for this gravitational torque, thus leading to a significant reduction in the effect of gravity. However, a simple installation of counterbalance mechanisms at each pitch joint does not work properly because the gravitational torque at each joint is dependent also on the other joints. To achieve multi-DOF counterbalancing, we propose a parallelogram linkage combined with dual counterbalance mechanisms, each being composed of a slider-crank mechanism and springs. Simulations and experimental results showed that the counterbalance robot arm based on the proposed counterbalance mechanisms effectively reduced the torques required to support the robot mass, thus allowing the prospective use of much smaller motors and speed reducers than traditional industrial robots.