Study On The Dynamic Characteristics Of Micro Bearing-Rotor System

The first electrostatic micromotor was developed in University of California at Berkeley in 1987; thereby it indicates the advent of the epoch of MEMS (Micro-Electro-Mechanical Systems). The great success of MEMS technologies has promoted a microminiaturization revolution in micro rotating machinery and the technologies have been used to fabricate the structures and systems of micro/nano rotating machinery. All of these have accelerated the lasting development of micro-rotor systems, which become the research hot spot in MEMS. As the power source of the MEMS, the development of Micro motor is on the focus. In order to increase the power density, the rotor is needed to run at a very high speed. The lubrication between the rotor and stator became very important. The micro gas bearing came to the first choice in MEMS condition for the advantages, for example, the gas bearing can support very high-speed rotor, there are very low drag and almost no running wear, what's more, and the gas would not stain the engine. On the fabrication aspect, the Gas Turbine Laboratory of MIT has developed a series of high-speed rotating devices supported by gas bearing.At present, many researches on the micro rotating machinery mainly focus on the fields of fabrication technologies, static characteristic analyses and measurements. However, the dynamic characteristics of the micro bearing-rotor systems, particularly in the vibration characteristics, friction and wear, dynamical lubrication, dynamic measurements, control technologies and reliability, have been investigated rarely or have not been involved yet. The advent and development of MEMS puts forward a new challenge for bearing-rotor dynamics. Theories and experiment researches on the dynamics of micro bearing-rotor systems have turned into an exigent task. Therefore, this investigation has important science and application values to promote the development of the micro bearing-rotor dynamics, accelerate the step of the applications and industrialization of micro rotating machinery and improve the work efficiency and reliability of applications.The content of this thesis involves the introduction on the development of the MEMS, the research of the friction and wear in the micro rotating machinery, the effect of the velocity slip on the mechanical performance of a steady operating micro gas bearing and the studies of stability and the dynamical behaviors of a micro gas bearing. Finally, experiment researches have been carried on the vibration characteristics of the thin film electromagnetic micromotors.First, the domestic and oversea research actualities of the micro rotating machinery have been summarized, the traits of various power driven modes have been analyzed and the micro-scaling effects on them have been discussed. The rotor dynamics and the nonlinearity of the micro rotating machinery in MEMS condition are introduced, and the issue on friction, wear and the lubrication are presented. The applications of the micro gas bearing in MEMS are introduced, including the fabrication and the dynamics of the micro gas bearing are analyzed and the micro scale effect on it is discussed.Micro-scaling effects make a great impact on the friction and wear characteristics. With the decrease of the scaling, the friction coefficient increases in an approach exponential form when elastic contact and it makes exponential increase or decrease when plastic contact. At the same time, the wear coefficient is reducing when the scaling decreases. According to Archard law, the friction and wear characteristics of three wear models, are analyzed and the effects of the wear and torque under different working conditions on the micro-rotor systems are investigated in detail. The wear rates depending on the service parameters and the shape of the pivot. Consider a typical variable-capacitance micromotor, a linear sliding wear model with ratcheting effects is proposed to describe the wearing process and a simplified mathematical method is presented to simulate the wear of the rotor bushing sliding on the ground plane. It is indicated that the distance of the bushing away from the center of the rotor and the radius of the rotor bushing have great impact on the wear rates. Friction and wear behaviors of the micro-rotor system can be changed by various geometric structures of micro-rotor bushings and they can be reduced by decreasing the distance of the bushing away from the center of the rotor or increasing the radius of the rotor bushing, which will increase the rates of dimensional and volumetric wear.Due to the limitation of micro-machining technology, the minimum clearance of the micro gas bearing is approximating 1μm or even smaller. Thus, the characteristic length of the micro flow is comparable to the mean free path of the gas. In such condition, the continuum regime is instead of the slip regime, in which the flow velocity slips at the solid surface. What's more, the common mean free path is derived from hard sphere (HS) model, which is the simplest model and is not realistic. In order to provide a good prediction, the variable hard sphere (VHS) model and the variable soft sphere (VSS) model are used for the analysis of micro flow. Combined with the VHS and VSS model, some slip models are adopted to acquire modified Reynold's equations, including first order slip model, second order slip model and 1.5-order slip model. The flow rates of a micro flow are acquired with analytical method. With the decrease of the inverse Kundsen number, the continuum flow model leads to a mistake result. For the different inverse Knudsen number range, the slip models respectively provide accurate predictions.The steady mechanical performances of two micro gas bearings, including the micro gas journal bearing and the micro gas thrust bearing, are pressented and the slip effects are discussed. Solving the modified equation with finite difference method (FDM), the gas pressure distribution, load-carrying capacity and the attitude angle are acquired. According the above results, the second slip model and the 1.5 order slip model are respectively adopted for the analysis of the micro journal bearing and thrust bearing, and the results are compared with that of the continuum flow model and the first slip model. It is found that the velocity slip at the solid boundary reduce the gas pressure in the bearing, hence the load-carrying capacity. In the analysis of the micro gas thrust bearing, the expansion course at the bushing end is discussed, a low pressure zone is found and it will cause the decrease of the load carrying capacity. Besides, the velocity variations at the bushing surface result in a dissymmetry pressure distribution.For a micro motor, a stable operation and power output are needed, thus it requires the assurance that the rotor will spin stably, and can sustain a radial loading force with slight shock. For a micro rotor, which is supported by a micro gas journal bearing, a large disturbance will cause the crash, the small disturbance problem is on the focus of research. The stability of the micro bearing-rotor system depends on the eigenvalue of the system motion equation. In the analysis, the linear bearing force increments are adopted to get the linear threshold speed for stable operation, and the nonlinear bearing forces, which are acquired by FDM, are adopted to get a more accurate simulation of the dynamical response. It is found that the stability of the system are depend on the rotating speed of the rotor. Given a slight disturbance, the rotor will crash into the bearing, whirl around the static equilibrium point with small amplitude or converge to its static equilibrium point. Only when the rotating speeds over the linear threshold speed, the micro gas bearing-rotor system will keep stable operation. For a realistically micro rotor, the rotor's density is not homogeneous exactly. As a result, the rotor is mass eccentricity more or lest. The stability of an unbalance rotor depends on the unbalance ratio. With the increase of the unbalance ratio, the velocity range for stable operation decreased, and the amplitude of whirl increased. The unbalance rotor can not keep stable when the unbalance ratio is too big.The dynamic behaviors of the micro-rotors, which is supported by dry friction bearing, have been studied experimentally. Whether the diameter of the thin film electromagnetic micromotor is 2mm or 6mm, the micro-rotor system appears serious rub phenomena and the pocketed heat issue during the rotation process and can not be run for a long time. With the increase of the rotating speed of the rotor, the friction force increases, the time with no rub-impact and the contact time with rub-impact shortens, the rub-impact becomes frequent and serious, then the system gives birth to the heat issue and finally the rotating speed increases difficultly. Since the dynamic behavior of micro-rotor system is a challenging problem, some aspects to be further studied and discussed in depth is put forwards.The results of this thesis may be worthily used to understand and develop the properties of friction and wear, the mechanical performance, the stability, the dynamic response of a micro rotor-bearing system, and vibration testing in the micro-rotor systems.