Jingxin Dong

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.

A Novel Digital Closed Loop MEMS Accelerometer Utilizing a Charge Pump

This paper presents a novel digital closed loop microelectromechanical system (MEMS) accelerometer with the architecture and experimental evaluation. The complicated timing diagram or complex power supply in published articles are circumvented by using a charge pump system of adjustable output voltage fabricated in a 2P4M 0.35 µm complementary metal-oxide semiconductor (CMOS) process, therefore making it possible for interface circuits of MEMS accelerometers to be integrated on a single die on a large scale. The output bitstream of the sigma delta modulator is boosted by the charge pump system and then applied on the feedback comb fingers to form electrostatic forces so that the MEMS accelerometer can operate in a closed loop state. Test results agree with the theoretical formula nicely. The nonlinearity of the accelerometer within ±1 g is 0.222% and the long-term stability is about 774 µg.

A closed-loop MEMS accelerometer with capacitive sensing interface ASIC

A closed-loop MEMS accelerometer with capacitive sensing interface ASIC (application specific integrated circuit) is presented. The parasitic-insensitive switched-capacitor sample-charge architecture is used to implement the capacitive sensing, which is crucial to the case where sensor and interface ASIC are combined in a two-chip approach to implement the closed-loop MEMS accelerometer. Based on the 0.35 µm CMOS sensing interface ASIC, an accelerometer prototype has been implemented, in which force-rebalance with the lag-proportional–integral controller is applied to improve the system stability and frequency response performance, and the testing results indicate the sensitivity of the presented accelerometer is 650 mV/g, the full measurement range ±15 g, the non-linearity 0.098% and the noise floor 23.17 µg/rt-Hz.

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.

Elimination of nonlinearity in sigma delta MEMS accelerometer

This paper presents a method of eliminating nonlinearity in sigma delta MEMS accelerometer by keeping the proof mass fixed at the geometrically symmetrical position (the zero position). Published architectures on sigma delta MEMS accelerometer fail to maintain the proof mass at the zero position with nonzero static input. The output voltage is nonlinear to the input because of the displacement The proposed accelerometer system consists of sense element fabricated in SOG process and ASIC in 2P4M 0.35μm CHART CMOS process. Circuit implementations in ASIC generate excitation signal to sense the input acceleration and feedback signal to form electrostatic feedback force. Time division multiplexing is applied so that excitation and feedback signal can work simultaneously on the three plates sense element Both simulations with SIMULINK and test results show that the proof mass is maintained at the zero position with different static input within the range. A nonlinearity of 0.065%, 10μg noise floor and an input range of about ±3g are achieved.