Feature extraction is of great importance for running states monitoring and performance evaluation of mechanical electro-hydraulic systems (MEHS). However, due to the complexity of the multi-domain energy conversion property of MEHS, especially during the varying operation conditions, it is quite difficult to extract the desired features effectively. In addition, the conventional signals are difficult to be collected and analyzed, as different kinds of coupled information are mixed together. Therefore, based on a power distribution analysis of MEHS, it is found that the change rate of the kinetic energy (CRKE) can be considered as a suitable index for evaluating the performance, such as energy saving and output stationarity of the considered MEHS. In order to characterize the magnitude of CRKE, a cooperation analysis method is proposed by using internal and external features. In the proposed method, the kinetic energy stiffness (KES) is selected as the internal feature, while the instantaneous speed fluctuation (ISF) is chosen as the external feature. According to a Lissajous figure-based information fusion approach and the order tracking technology, a systematic method is developed to obtain the magnitudes of KES and ISF. Furthermore, based on the complementary advantages and mutual relationship of KES and ISF, the performance of MEHS is analyzed under varying operation conditions. The proposed method is verified through experiments with a real rig. The results show that the changes of KES and ISF can effectively reflect the change in the operation condition, and lower KES loss can improve the efficiency of the system and also restrain the ISF.

Instantaneous speed fluctuation of hydraulic motors is a significant physical parameter, which contains abundant information reflecting the working condition of the whole hydraulic system. Monitoring instantaneous speed fluctuation change under different operational conditions helps to reveal the performance of the system, such as flow fluctuation, oil compressibility, and internal leakage. Therefore, a new evaluation method for hydraulic systems using the instantaneous speed fluctuation of the hydraulic motor is proposed in this paper. The proposed method includes measurement of the instantaneous speed and instantaneous speed fluctuation extraction. By means of the software counting algorithm, the instantaneous speed of the hydraulic motor can be obtained based on data acquisition card and industrial personal computer. The process of the instantaneous speed fluctuation extraction adopts order analysis, which converts the instantaneous speed signal from the time domain to the angle domain. Moreover, by tracking the order spectrum, the instantaneous speed fluctuation change under varying operational conditions can be identified. The effectiveness of the proposed method was validated by experiments under different working conditions. The experimental results show that the instantaneous speed fluctuation of the hydraulic motor can analyze the performance change of the hydraulic system with different motor speeds, loads, and moments of inertia, which provides a new method to evaluate the operational states of hydraulic systems.

Oil is a key medium for transmitting power and coupling information in the hydraulic transmission system. Accurate calculation and measurement of the dynamic compressibility of the oil have profound significance to the system performance analysis. The present study primarily focuses on the effect of steady pressure on the bulk modulus of static oil. However, changing pressure and flowing oil are the general working conditions in most of hydraulic apparatus. There are few researchers paying attention to the influence of pressure on the compressibility of flowing oil. Considering the log-normal distribution of bubble size in oil, an improved static oil model (Model B) is developed to calculate the bulk modulus of the motionless oil under the dynamic pressure. Then, by deriving Model B, this paper proposes an original flowing oil model (Model C) to determine the effective bulk modulus of flowing oil. Finally, based on the inherent pressure pulsation of the axial piston pump, an innovative online method for measurement of the bulk modulus of flowing oil is presented as it has the advantage of avoiding interference with flow stability. It has been proved that the changes in the flow velocity corresponds to the crucial effect on the effective bulk modulus of flowing oil, especially under the low-flow and low-pressure operation conditions. Those results and analysis provide promoting support for identifying and determining the effective bulk modulus of oil, analyzing the system stiffness, improving the control accuracy, as well as optimizing the mathematical models. INDEX TERMS Bulk modulus, bubble size distribution, flowing oil, online measurement.

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