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Actuators of a robot that physically interacts with objects or human require backdrivability to obtain force controllability, impact resistance and compactness. An electro-hydrostatic actuator (EHA) is a servo pump type hydraulic system that has a potential to satisfy backdrivability. However, the previous mechanical design of a revolute EHA did not achieve compact design compared to the other actuators. The hydraulic pump and vane motor were separately located, which made the size of EHA system larger. To solve this problem, we propose a design methodology of a revolute EHA using a frameless motor. Commercially available frameless motor is suitable for actuating the hydraulic pump. Moreover, the pump can be built in the hollow space of the frameless motor, and this design makes the EHA downsized. We present a prototype design based on the formulation of the output power. Through the experimental validations, we show that developed EHA has higher backdrivability than the comparable harmonic drive, and obtain the knowledges on design factors that are crucial to the EHA with high power-to-weight ratio. [ABSTRACT FROM AUTHOR]