Passive Control of a Hydraulic Human Power Amplifier Using a Hydraulic Transformer

The hydraulic human power amplifier is a tool that uses hydraulic actuation to amplify the force that the human exerts on it. Our control objective and framework are to make the system behave like a passive mechanical tool when interacting with the human and with the work environment with a specified power scaling factor. A virtual velocity coordination control approach casts the human power amplifier problem into one of velocity coordination by generating a fictitious reference mechanical system. Force amplification becomes a natural consequence of velocity coordination. This control has been previously demonstrated using servo valves which is a major contributor to energy loss in hydraulic system. In this paper, a hydraulic transformer, which does not rely on throttling to accomplish its control function is used instead of a servo valve to achieve human power amplification. In addition, a passivity based control approach that makes use of the natural energy storage of the hydraulic actuator is used to define the flow requirement. This approach fully accounts for the non-linearity due to the pressure dynamics. The controller was experimentally validated with good force amplification and velocity coordination performance on a single degree of freedom hydraulic human power amplifier.