Fluid power systems consist of components like pumps, valves and actuators and of the lines and hoses interconnecting these systems. Simple interconnections without branch points can be modelled as hydraulic two-port networks. This paper demonstrates the identification of linear state space models describing the input-output behaviour of hydraulic two-port networks in terms of pressure and flow-rate. The linear modelling approach restricts the applicability to the case of laminar flow with negligible influence of convective terms. Special attention is paid to a priori knowledge of certain model properties: The numerical optimization procedure used in the proposed identifcation method guarantees the passivity of the models and allows for instantaneous coupling of collocated pressure and flow variables according to the Joukowsky relation. The method takes experimental frequency response data as an input and generates a series of state-space approximations with increasing system order starting at order one. A hydraulic hose is presented as an example.

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