Abstract
Air-film damping, which dominates over other losses, plays a significant role in the dynamic response of many micro-fabricated devices with a movable mass suspended by various bearing mechanisms. Modeling the damping characteristics accurately will be greatly helpful to the bearing design, control, and test in various micromotor devices. This paper presents the simulated and experimental squeeze-film air damping results of an electrostatic bearing for use in a rotary high-speed micromotor. It is shown that the boundary condition to solve the three-dimensional Reynolds equation, which governs the squeeze-film damping in the air gap between the rotor and its surrounding stator sealed in a three-layer evacuated cavity, behaves with strong cross-axis coupling characteristics. To accurately characterize the damping effect, a set of multiphysics finite-element simulations are performed by computing both the rotor velocity and the distribution of the viscous damping force acting on the rotor. The damping characteristics varying with several key structure parameters are simulated and discussed to optimize the device structure for desirable rotor dynamics. An electrical measurement method is also proposed and applied to validate the numerical results of the damping coefficients experimentally. Given that the frequency response of the electric bearing is critically dependent on the damping coefficients at atmospheric pressure, a solution to the air-film damping measurement problem is presented by taking approximate curve fitting of multi-axis experimental frequency responses. The measured squeeze-film damping coefficients for the five-axis electric bearing agrees well with the numerical solutions. This indicates that numerical multiphysics simulation is an effective method to accurately examine the air-film damping effect for complex device geometry and arbitrary boundary condition. The accurate damping coefficients obtained by FEM simulation will greatly simplify the design of the five-axis bearing control system and facilitate the initial suspension test of the rotor for various micromotor devices.
Highlights
Various types of microfabricated motors have been successfully designed, fabricated, and tested for microsensors and microactuators [1,2,3]
This paper presents a numerical solution of the squeeze-film damping coefficients for a five-axis electrostatic bearing system operated at atmospheric pressure
The finite-element method (FEM)-based numerical solution is utilized to provide an accurate prediction of air-film damping coefficients which is crucial in device design and modeling of the bearing dynamics
Summary
Various types of microfabricated motors have been successfully designed, fabricated, and tested for microsensors and microactuators [1,2,3]. The rotor of a rotary micromotor is typically supported on a mechanically attached bearing using silicon microfabrication technology, the contact friction and wear between the surfaces of a micro-scale rotor and its bearing are typically large during the rotation operation [4,5]. Inherent friction and wear caused by the mechanical micro-bearing greatly restricts the lifetime and efficiency of traditional micromotors. Active electrostatic bearings are ideally suited for rotary micromachined devices as they are comparatively compatible with existing microfabrication technology and capable of integrating bearing control electronics into the same chip to realize low-voltage and low-power operation [9,10,11]. Several electrostatic bearing systems were developed as an effective solution to the problem of friction and wear in traditional micro-electro-mechanical system (MEMS) micromotors
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