Development of 6-DOF manipulator driven by ceramics reinforced hydrostatic actuators and its experimental evaluations on force control and payload abilities

The purpose of the Hydracer project is to discuss the system performance of the EHAs (Electro-Hydrostatic Actuators) when integrated into robotic systems. We have developed EHAs focusing on compactness, light-weight, force controllability, and back-drivability. The requirements such as compactness and light-weight lead us to the adoption of high-pressure, which unfortunately contradicts with the other requirements, namely, force controllability and back-drivability. The contradiction indicates the necessity of study to find the balanced specification between them, which was the question we raised when we started the development of Hydracer, a 6-DOF manipulator driven by EHAs. The system design started following the specifications of an industrial standard of small-size manipulators. In the previous paper, we reported on the design and evaluation of actuators, namely, the linear EHA modules for the first three joints from the base and the ceramic-reinforced vane motors for the last three. We could also discuss the preliminary results of force control of the last three (wrist) joints. In this paper, we report on the performance of force-controllability and backdrivability of the first three joints of the manipulator after integrating force controllers and gravity compensation. It provides useful information about the total performance of manipulators driven by EHAs. The RMSEs (root mean square errors) of torques of the first three joints under the gravity compensation were 0.53, 1,7, and 1.2 Nm respectively, while the manipulator could carry 10 kg payload at the horizontally stretched configuration for 5 s.