This paper is concerned with modeling of water hydraulic McKibben muscle. In particular, linear system identification technique is applied to the muscle in order to obtain a muscle dynamic model. However, the muscle has nonlinearity such as hysteresis and saturation characteristics. It is difficult to express the nonlinearity using linear system identification because the linear model is assumed. Then we propose the introduction of Bouc-Wen model which is widely used for expression of hysteresis characteristics for the identified muscle model. The application of Bouc-Wen model is easy because of the structure of the identified model and the proposed muscle model has good agreement with experimental results. In addition, the model for gait-training taking into consideration of its operation range can be obtained. Applying to limited operation range, the performance of the identified muscle model is improved and then the model which can express hysteresis and saturation characteristics can be obtained.

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Poppet valve is one of the most popular components in hydraulics, and it is also known as a trouble maker because it induces some unpredictable vibration in hydraulic system. In this research, thanks to the rapid development of visualization technology and digital simulation technology, we make a re-study on the well-known problem of unpredictable vibration phenomenon. Results show that; 1) stability of a poppet valve essentially depends on the components and parameters of the system, but the stability characteristics are influenced by the cavitation state at the downstream of the valve, 2) by changes in the compressibility of the hydraulic fluid with cavitation, frequency characteristics of the poppet valve are changed to unstable ones. Since the cavitation has complicated relations with various factors, the prediction of vibration of the poppet valve affected by cavitation is generally difficult. Therefore, the vibration may come out suddenly beyond prediction. The results of this paper can give a guide for the cavitation vibration of a poppet valve.

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