The active controlled hydrostatic bearing is becoming more and more popular because of its accuracy, safety, as well as low vibration and noise. In this paper, we present a design approach for a hydrostatic thrust bearing system, where the analytical nonlinear state space equation of the system is established first, and then three kinds of control inputs are investigated and compared to each other. It is found that, by selecting the supply pressure as the control input, we could obtain an affine nonlinear system, which could be linearized by the feedback linearization method, and its robustness could be enhanced by the sliding mode control method. The tracking control law could be easily obtained with the linearized system. The simulation verifies the effectiveness of the nonlinear control law. The proposed nonlinear control model might have a positive effect on the improvement of the machining accuracy, safety, and vibration absorption.
The motion errors of hydrostatic lead screws can be smaller than the pitch errors due to the averaging effect of oil films, which is the so-called averaging effect on the pitch errors (AEPE). This paper investigates the factors influencing AEPE under low speeds and a constant external load. An equivalent model of hydrostatic lead screws with continuous helical recesses is established, and the axial motion errors of the nut are obtained by combining the dichotomy and finite difference methods. The results show that (a) the motion errors caused by the pitch errors of the lead screw have additivity, and the pitch errors of the nut have little influence on the motion errors, (b) when the lead screw only has cumulative pitch errors, the motion errors are equal to the average value of the cumulative pitch errors located on the two flank surfaces of the lead screw, and (c) when the lead screw only has periodic pitch errors, the number of nut threads with helical recesses n and the period of the pitch errors of the lead screw T are the main influencing factors on AEPE.
Tamarix nabkha is one of the most widespread nabkhas, distributing in the arid region of China. Based on the observations outdoors and the simulation experiments in laboratories, analysis in this paper refers to the biological geomorphologic features and growth process of Tamarix nabkhas in the middle and lower reaches of the Hotan River, Xinjiang. And the results indicate that the ecological type of Tamarix in the study area is a kind of Tugaic soil habitat based on the deep soil of the Populus Diversifolia forests and shrubs. This type of habitat can be divided into three kinds of sub-habitats which demonstrate the features of ecological environment of Tamarix nabkhas during the differential developed phases. Meanwhile, the Tamarix nabkha can exert intensified disturbance current on wind-sand flow on the ground, and its root and stems not only have strong potential of sprouting but are characteristic of wind erosion-tolerance, resistance to be buried by sand and respectively tough rigid of the lignified branches, for it has a rather longer life-time. Thus, the wind speed profile influenced by the Tamarix nabkha is different from the Phragmites nabkha and Alhagi nabkha. And the structure of the wind flow is beneficial to aeolian sand accumulating in/around Tamarix shrub,which can create unique Tamarix nabkhas with higher average gradient and longer periodicity of life. Tamarix nabkha evolution in the area experienced three stages: growth stage, mature and steady stage and withering stage. In each stage, morphological features and geomorphic process of Tamarix nabkha are different due to the discrepant interaction between the nabkha and aeolian sand flow.
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