Abstract: Effective bulk modulus of hydraulic oil is an important factor of dynamic characters in a hydraulic control system and it’s regarded as one of the essential parameter for monitoring the condition of the sets at real operation. Based on transient flow theory and gas-liquid two-phase flow theory, a novel soft-sensing model is suggested for the online measurement of the effective bulk modulus and numerical simulations of the model are carried out. The model represents the relationship among natural frequency, pressure, bubble volume fraction and effective bulk modulus of oil in a straight pipe. The key problem of using this model in practice is how to measure the natural frequency online. The feasible solution is installing piezoelectricity type pressure sensors at two oil pressure fluctuation monitoring points selected in the hydraulic power system, and through online excitation, the oil pressure frequency response function is dynamically measured, thus the natural frequency of oil is identified. Moreover, the effective bulk modulus may be estimated by the above model. This model is tested on the experiment platform of multi-source information acquisition of the hydraulic system. The results indicate that the model can be used successfully in the online measurement of the effective bulk modulus of hydraulic oil.

Key words: Effective bulk modulus, Hydraulic system, Online measurement, Soft-sensing model