Robots must have similar mechanical impedance characteristics to humans in order to make safe and efficient contact. This impedance requirement applies not only to the surface but also to the actuation mechanisms. The objective of this research is to develop inherently flexible actuator by realizing backdrivability. A class of hydraulic actuation called electro-hydrostatic actuator was applied to knee joint in humanoid robots to satisfy flexibility and large torque output simultaneously. This paper explains the methodology of performance evaluation of actuators and design concept of joint mechanism. Mathematical model of electro-hydrostatic transmission is also presented. Evaluation of backdrivability, inertia modification control, and compliance control of developed mechanism are performed.

This paper proposes a new torque sensing method, Cr-N alloy strain sensitive thin-film based torque sensors and distributed joint torque servo systems that enable human support robots to have capabilities to make physical interaction in adaptive and safe operation tasks. Stiffer torque sensing with stable and high-resolution sensing to meet practical level of the developed torque sensors have been achieved. We have developed the joint torque control based 4-DOF arm models in order to verify practical effectiveness of the proposed joint torque servo systems. We also demonstrated the joint torque control based bilateral master slave system exploring future applications.

When robots cooperate with humans it is necessary for robots to move safely on sudden impact. Joint torque sensing is vital for robots to realize safe behavior and enhance physical performance. Firstly, this paper describes a new torque sensor with linear encoders which demonstrates electro magnetic noise immunity and is unaffected temperature changes. Secondly, we propose a friction compensation method using a disturbance observer to improve the positioning accuracy. In addition, we describe a torque feedback control method which scales down the motor inertia and enhances the joint flexibility. Experimental results of the proposed controller are presented.

Although backdrivability is recognized as an important property of actuators, this term is often used without clear definition. In order to design mechanisms with advanced controllability, it is important to understand the fundamental mechanism of backdrivability. In this paper, we introduced idea of series elasticity and series dissipation of actuators. Based on this idea, total/output backdrivability and their fundamental properties are stated. EHA was shown to be series dissipative and it was confirmed from the model of the actuator. Utilizing the backdrivability of EHA, position based impedance control was implemented and evaluated. Application of this EHA in a robot hand is also reported.