This article presents the design and experimental testing of a unidirectional variable stiffness hydraulic actuator for loadcarrying knee exoskeleton. The proposed actuator is designed for mimicking the high-efficiency passive behavior of biological knee and providing actively assistance in locomotion. The adjustable passive compliance of exoskeletal knee is achieved through a variable ratio lever mechanism with linear elastic element. A compact customized electrohydraulic system is also designed to accommodate application demands. Preliminary experimental results show the prototype has good performances in terms of stiffness regulation and joint torque control. The actuator is also implemented in an exoskeleton knee joint, resulting in anticipant human-like passive compliance behavior.

In order to reduce the weight and improve the energy efficiency of the lower extremity exoskeleton, a novel pump-valve coordinated controlled (PVCC) hydraulic system is presented. This hydraulic system only uses one electro-hydrostatic unit (EHU) and two valves to drive two hydraulic cylinders at the hip and knee of the lower extremity exoskeleton. The PVCC hydraulic system has the advantage of high energy conversion efficiency of the electro-hydrostatic actuator (EHA), which consists of one EHU and one hydraulic cylinder. To meet the requirements of the moment and speed of each joint of the exoskeleton, the proportional valve and on-off valve are added to adjust the flow into two hydraulic cylinders. The performance of EHU is tested by some hydraulic experiments, and the performance of the PVCC hydraulic system is analyzed by AMESim. The results show that the novel hydraulic system can only use one EHU to drive two hydraulic cylinders simultaneously under the premise of meeting the functional requirements of the exoskeleton.

An indicator of a passive biped walker’s global stability is its domain of attraction, which is usually estimated by the simple cell mapping method. It needs to calculate a large number of cells’ Poincare mapping result in the estimating process. However, the Poincare mapping is usually computationally expensive and time-consuming due to the complex dynamical equation of the passive biped walker. How to estimate the domain of attraction efficiently and reliably is a problem to be solved. Based on the simple cell mapping method, an improved method is proposed to solve it. The proposed method uses the multiple iteration algorithm to calculate a stable domain of attraction and effectively decreases the total number of Poincare mappings. Through the simulation of the simplest passive biped walker, the improved method can obtain the same domain of attraction as that calculated using the simple cell mapping method and reduce calculation time significantly. Furthermore, this improved method not only proposes a way of rapid estimating the domain of attraction, but also provides a feasible tool for selecting the domain of interest and its discretization level.

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