Performance of an active electric bearing for rotary micromotors

Abstract

An electric bearing used to support a micromachined rotor of variable-capacitance motors was designed and tested in order to study the characteristics of this frictionless bearing. Electrostatic suspension of a ring-shaped rotor in five degrees of freedom is required to eliminate the mechanical bearing and thus the friction and wear between the rotor and the substrate. Bulk microfabrication-based glass/silicon/glass bonding is chosen for this device, allowing the fabrication of large area sense capacitors and rotor, which make the device potentially suitable for the development of an electrostatically suspended micromachined gyroscope. The device and its basic operating principle are described, as well as the dynamics of the rotor and basic design considerations of the electric bearing system. A theoretical relationship to relate the characteristics of a classical lag–lead compensator to the stiffness properties of the electric bearing is developed to explain the experimental bearing measurements. The experimental results of closed-loop frequency response, suspension stiffness and drive voltage effects are presented and discussed for the bearing operated initially in the atmospheric environment. The performance of a tri-axial electrostatic accelerometer has also been experimentally investigated on the prototype of the electric bearing system.

Decoupling Control of Micromachined Spinning-Rotor Gyroscope with Electrostatic Suspension

“…Then the compensated results can be observed and evaluated via measuring the closed-loop frequency response and suspension stiffness. To test the closed-loop frequency response, ${\phi }_{\mathrm{ex}}\left(s\right)$ and ${\theta }_{\mathrm{ex}}\left(s\right)$ are set as the input and output while ${\phi }_{\mathrm{ex}}\left(s\right)$ and $V_{\mathrm{dx}}\left(s\right)$ are set as the input and output in the following suspension stiffness frequency-sweeping test [7]. Three curves C1, C2, and C3 in Figure 10 indicate the under-, proper- and over-compensation in the closed-loop frequency responses, respectively.…”

“…The traditional spinning-rotor gyroscopes [3], as the most highly-accurate gyroscopes in inertial navigation and pointing applications, function as a two-degree-of-freedom angular rate or position sensor with various suspension techniques [3,4,5]. In order to minimize the mechanical friction of the spinning rotor and achieve high accuracy, different miniaturized gyroscope suspension mechanisms, such as contactless electrostatic and electromagnetic bearings [6,7,8] and liquid-suspended and gas-lubricated bearings [9,10,11], have been studied and developed. Among them, electrostatic suspension is comparatively compatible with existing microfabrication techniques and ideally suited for micromachined spinning-rotor gyroscopes.…”

“…Based on our prior work, such as the fabrication process by bulk micromachining [20], designing and testing an active electrostatic bearing [7], and performance testing of the MESG in vacuum [21], the micromachined device can be used as a dual axis angular rate gyroscope by spinning-up the rotor rate over 10 4 r/min [19]. Since the MESG is a kind of free spinning rotor gyroscope, an inherent coupling error resulting from a rigid body rotation controlled by a torque-rebalance loop exists in this dual-axis angular rate sensor.…”

Levitating Micro-Actuators: A Review

“…Employing an active approach for stable levitation, the gap between the levitated disc and surrounding electrodes was 2 µm. The small distance made it possible to generate a drive torque of 10 −11 N m by applying a nominal drive actuator potential of 5 V. This concept of actuation and its application in micro-motors [27][28][29], gyroscopes [30][31][32][33], multi-inertial sensors [34,35] and accelerometers [36][37][38][39], was further improved by other research groups in terms of micro-machining fabrication and signal conditioning. As a result, in 2005, an electrostatic micro-motor which was able to rotate a 1.5 mm diameter ring-shaped rotor having a speed up to 74,000 r.p.m., was demonstrated by Nakamura [35].…”

Squeeze-Film Air Damping of a Five-Axis Electrostatic Bearing for Rotary Micromotors

“…To maintain stable suspension, the rotor is actively suspended in five DOFs: translations along the x -, y -, and z -directions, and rotations around two in-plane axes. If the cross-coupling effects among the different axes are ignored, the dynamics of the rotor can be modeled by five uncoupled 1-DOF systems [10]. $\{\begin{array}{l}\left(1\mathrm{normala}\right)& m{\ddot{e}}_i+b_i{\dot{e}}_i=F_i\left(1\mathrm{normalb}\right)& J{\ddot{\alpha }}_j+b_j{\dot{\alpha }}_j=M_j\end{array}$where m and J are the mass and moment of inertia of the rotor, e and α are the linear and angular displacements of the rotor away from its equilibrium position, b is the air-film damping coefficient, F and M are the electrostatic feedback force and torque, the subscript i = x , y , z denotes the axis along which the force is produced, and j = φ x , φ y denotes the axis around which the torque is produced, respectively.…”