Development of Hydraulic Tough Motors with High Power Density and their Application to a 7-axis Robotic Arm

A combination of electric motors and speed reducers are widely used as robot actuators. However, owing to problems such as low impact resistance and poor backdrivability, it is hard to use robots in actual disaster environments. Hydraulic actuators used in construction machines are not only applicable to such environments, but they also have good force density and power density. However, most conventional actuators are large, heavy, and have poor controllability and it is difficult to apply these to robots. In this research, we developed two actuators (a semi-rotary motor and an axial piston motor) with the aim of improving miniaturization and reduce weight for use in robots and evaluated their characteristics. The developed semi-rotary motor utilizes pure titanium and a hollow shaft form. They contributed to weight reduction and facilitated hydraulic robot composing method. The developed axial piston motor is a combination of a remodeled off-the-shelf motor and a special tooth-type planetary gear reducer that is expected to have high backdrivability. Both the actuators can bear high input pressures (35 MPa) and equip servo valve, absolute encoder, and pressure sensors directly to enhance their applicability to robots. The semi-rotary motor (high output, medium output) and axial piston motor (medium output, small output) achieved T/M (torque / mass) ratios of approximately 88.5 [Nm/kg] and 105.2 [Nm/kg], respectively, which are 5.6 times and 8.2 times greater than that of conventional oil hydraulic motors. In the evaluation test, we confirmed the possibility of estimating the torque by using the pressure sensors and found that it is possible with an accuracy of approximately ±13% of the rated torque for the high-output semi-rotary motor. In addition, as a prototype of a hydraulic legged robot, we developed a robot arm with seven degrees of freedom that is highly compatible with the four-limb electrically-actuated robot, WAREC-1, for use in disasters. WAREC-1 was mainly developed at Waseda University. The length of the developed robot arm is 1.2 m, which is similar to that of WAREC-1. The mass of the robot is 56 kg, which is heavier than WAREC - 1; however, the joint torques are equal or greater than those of WAREC-1. With respect to the movable range, it was able to be equal to or more than 214 degrees with WAREC-1 by using hollow shaft actuators and swivel joints. The power density and the backdrive torque ratio (backdrive torque / rated torque) of the developed actuators are 5.9 times higher and 1/26 smaller than those of the actuators of WAREC-1. In the demonstration, the arm succeeded in breaking a stuck of three concrete-boards, each 30mm-thick.