Fengtian Han

Performance of an active electric bearing for rotary micromotors

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.

Experimental study of a variable-capacitance micromotor with electrostatic suspension

A variable-capacitance micromotor where the rotor is supported electrostatically in five degrees of freedom was designed, fabricated and tested in order to study the behavior of this electrostatic motor. The micromachined device is based on a glass/silicon/glass stack bonding structure, fabricated by bulk micromachining and initially operated in atmospheric environment. The analytical torque model is obtained by calculating the capacitances between different stator electrodes and the rotor. Capacitance values in the order of 10−13 pF and torque values in the order of 10−10 N m have been calculated from the motor geometry and attainable drive voltage. A dynamic model of the motor is proposed by further estimating the air-film damping effect in an effort to explain the experimental rotation measurements. Experimental results of starting voltage, continuous operation, switching response and electric bearing of the micromotor are presented and discussed. Preliminary measurements indicate that a rotor rotating speed of 73.3 r min−1 can be achieved at a drive voltage of 28.3 V, equivalent to a theoretical motive torque of 517 pN m. Starting voltage results obtained from experimental measurement are in agreement with the developed dynamic model.

Micromachined electrostatically suspended gyroscope with a spinning ring-shaped rotor

A micromachined electrostatically suspended gyroscope is described in this paper, in which a spinning ring-shaped rotor is suspended by an electric bearing in five degrees of freedom and driven by a three-phase variable-capacitance motor. The electric bearing provides contactless suspension of the spinning rotor, allowing the rotor through a torque-rebalance loop to precess about two input axes that are orthogonal to the spin axis. In this way, the micromachined spinning-rotor gyroscope can be used as a two-degree-of-freedom angular rate sensor by detecting the precession-induced torque. Design and simulation of the dual-axis torque-rebalance loop, by considering actual negative spring effect in rotor dynamics, are presented to investigate the loop stability and explain the experimental measurement. The prototype device has been fabricated by bulk micromachining technique and tested successfully with a suspended rotor spinning at a speed of 10?085?rpm. Initial measurements of the rate gyroscope shows that an input range of ?100??s?1, a noise floor of 0.015??s?1 Hz?1/2, and a bias stability of 50.95? h?1 have been achieved. The detailed results of the prototype device, electric bearing and motor spin-up are also described.

Capacitive Sensor Interface for an Electrostatically Levitated Micromotor

This paper presents the design and performance of a capacitive sensor interface dedicated to a microelectromechanical systems (MEMS) micromotor electrically levitated in five DOFs. The position and orientation of the rotor are detected by measuring differential rotor-electrode capacitances with a set of capacitance-to-voltage converters (CVCs). The sensor contains multiplexed electrodes for both capacitive sensing and force feedback, and a set of common electrodes for carrier exciting with an aim to eliminate ohmic connection with the levitated rotor. The proposed interface circuit is based on a symmetrical structure containing two half ac bridges, more robust against parasitic capacitances, capable of detecting capacitance changes with frequency higher than 10 kHz, and able to decouple multiaxis position signals of a levitated rotor. An electronic equivalent model of the sensing circuit has been developed and used to analyze the sensor performance. The major nonidealities and their effects on the accuracy of the position sensing are discussed. The performance of the sensing circuit was experimentally investigated on a prototype interface circuit. The experimental results confirm the principles of operation and the performance of the interface for the multiaxis levitated devices using capacitive position sensors.

Performance of a Sensitive Micromachined Accelerometer With an Electrostatically Suspended Proof Mass

A three-axis micromachined accelerometer with a proof mass suspended electrostatically in six degrees of freedom was designed and tested to evaluate its performance of this sensitive sensor for potential microgravity space applications. The device is based on a glass/silicon/glass bonding structure, fabricated by bulk micromachining process, and operated with force-balance technology. The motion of the proof mass with respect to each side is fully servo-controlled by capacitive position sensing and electrostatic force feedback. The design and simulation of multiaxis suspension control loops are presented based on the stiffness requirements for different full-scale ranges. The ground test of this sensitive accelerometer is facilitated by setting the vertical axis at a relatively high measurement range to counteract the one-g gravity, whereas the range in two lateral axes can be set as low as possible to achieve high sensitivity. Detailed experimental results of electrostatic suspension, threeaxis accelerometer, and its cross-axis sensitivity are presented with the device operated initially in an atmospheric environment. The preliminary test results of a prototype accelerometer show that a sensitivity up to 688.8 V/g and a noise density down to 3 μg/Hz1/2 can be achieved by setting an extremely low full-scale range of ±2.90 mg. The results also show that much different stiffness levels in the design of three-axis suspension is a major source of cross coupling effects in the prototype accelerometer.

Self-Locking Avoidance and Stiffness Compensation of a Three-Axis Micromachined Electrostatically Suspended Accelerometer.

A micromachined electrostatically-suspended accelerometer (MESA) is a kind of three-axis inertial sensor based on fully-contactless electrostatic suspension of the proof mass (PM). It has the potential to offer broad bandwidth, high sensitivity, wide dynamic range and, thus, would be perfectly suited for land seismic acquisition. Previous experiments showed that it is hard to lift up the PM successfully during initial levitation as the mass needs to be levitated simultaneously in all six degrees of freedom (DoFs). By analyzing the coupling electrostatic forces and torques between three lateral axes, it is found there exists a self-locking zone due to the cross-axis coupling effect. To minimize the cross-axis coupling and solve the initial levitation problem, this paper proposes an effective control scheme by delaying the operation of one lateral actuator. The experimental result demonstrates that the PM can be levitated up with six-DoF suspension operation at any initial position. We also propose a feed-forward compensation approach to minimize the negative stiffness effect inherent in electrostatic suspension. The experiment results demonstrate that a more broadband linear amplitude-frequency response and higher suspension stiffness can be achieved, which is crucial to maintain high vector fidelity for potential use as a three-component MEMS geophone. The preliminary performance tests of the three-axis linear accelerometer were conducted under normal atmospheric pressure and room temperature. The main results and noise analysis are presented. It is shown that vacuum packaging of the MEMS sensor is essential to extend the bandwidth and lower the noise floor, especially for low-noise seismic data acquisition.

Performance of a high-voltage DC amplifier for electrostatic levitation applications

High-voltage amplifiers as a means of amplifying the low-output voltage signals of the feedback controllers to the suspension voltages typically in the kilovolts range are often required for electrostatic force generation in electrostatic levitation. This paper proposes a high-voltage do amplifier including an amplitude modulator, a power amplifier, a step-up transformer, a pair of peak detectors, and a voltage feedback channel to stabilize the amplifier outputs in an effort to provide high suspension voltage and fast dynamic response. Since the various carrier frequencies have virtually no effect on the power consumption of the do amplifier by filtering out the high-frequency carrier components with peak detectors while keeping the input signal unaffected, satisfactory dynamic performance can be achieved by choosing a sufficiently high carrier frequency. The operating principle of the dc amplifier is analyzed, followed by an experimental performance evaluation and discussion for electrostatic levitation applications. The experimental results demonstrate the superiority of the high-voltage do amplifier over classical ac amplifiers in terms of dynamic response, force-voltage coefficient, voltage ripple, power consumption, and long-time stability using a carrier frequency of 30 kHz and the closed-loop control scheme.

A sensitive three-axis micromachined accelerometer based on an electrostatically suspended proof mass

A three-axis micromachined accelerometer where a free proof mass is suspended electrostatically in six degrees of freedom is proposed and tested in order to evaluate this sensitive electrostatic accelerometer for potential microgravity space applications. The micromachined device is based on a novel glass/silicon/glass bonding structure, fabricated by bulk micromachining technique and operated with closed-loop forcefeedback technology. The motion of the proof mass with respect to each side is fully servo-controlled by capacitive position sensing and electrostatic force feedback. To facilitate ground test of this low-g accelerometer, the full input range in the vertical z axis is set at a relatively high value of 3.68g in order to counteract the gravity in one g condition, while the range in the lateral x axis is set as low as 2.90mg to achieve high sensitivity. Initial test of this MEMS accelerometer shows that a sensitivity of 688.8V/g is achieved by setting a low bias voltage of 1V.

Rotation Control and Characterization of High-Speed Variable-Capacitance Micromotor Supported on Electrostatic Bearing

This paper presents the rotation control design and experimental performance of a microelectromechanical systems (MEMS) variable-capacitance motor where a free-spinning rotor is suspended and centered in an evacuated vacuum cavity by a contactless electrostatic bearing. The micromachined device is based on a glass/silicon/glass bonding structure, fabricated by bulk micromachining, driven by a three-phase electrostatic motor, and used as an angular rate gyroscope by spinning-up the rotor rate over 104 r/min. A closed-loop phase commutation scheme is proposed in our rotation design where three-phase drives are switched depending on one channel of the rotors angular position. The design of the micromotor spin-up and constant-speed operation is described based on the proposed electronic commutation. Experimental results of the motor spin-up process under different vacuum settings, static, and dynamic characteristics of the constant-speed control loop together with scale factor of the spinning-rotor gyroscope are described for the device operated in vacuum. It is indicated that the rotor can be spun up to 2.5 × 104 r/min within 400 s, and up to 2.96 × 104 r/min in steady state under a drive voltage of 11.8 V. Measurement data in constant-speed control mode show that the standard deviation of the spin rate error is 0.07 r/min at a rated speed of 1.5 × 104 r/min.

Research on gas film damping of an electrostatically levitated micromachined accelerometer

Squeeze film damping and slide film damping for an electrostatically levitated ring-shaped proof-mass are calculated and measured. This paper has derived three-dimensional linearized Reynolds equations for the electrostatically levitated accelerometer based on slip flow condition and Couette fluid model of slide film damping respectively. The gas film damping is determined by using analytic solution with the motion of the proof-mass in five degrees of freedom. Both motion of the proof-mass and temperature effect have been taken into account in gas film damping calculation. The simulated results show that squeeze film damping has dominant effect on dynamics of levitated systems. Electrometric method is utilized to test gas film damping of a levitated accelerometer in axial direction. Experimental results are compared with theoretical analysis.