Development of a Closed-loop MEMS Capacitive Force Sensor
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Date
2009-08-04
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Abstract
This thesis describes a closed-loop microelectromechanical system (MEMS) based
on lumped-parameter modeling. Analytical models are derived for electrostatic comb drive
actuator (CDA) under force-controlled actuation, electrothermal actuator (ETA) under
displacement-controlled actuation, capacitive position sensor, including parallel plate capacitive
sensor (PPCS) and torsional plate capacitive sensor (TPCS), mechanical equation
of motion of a suspended shuttle, viscous air damping, folded
exure. These models are implemented
and simulated in finite element analysis softwares (ANSYS and FEMM). System
level simulation, implementing PID difierential feedback loop, is simulated in a numerical
simulation program (MATLAB).
The MEMS die is fabricated by following the standard PolyMUMPs process by
MEMSCAP. A series of MEMS packaging process and storage are done in the lab. All
peripheral circuitries are self-made.
A commercial capacitive readout IC (MS3110) is first used for open-loop capacitive
sensing, which achieves the resolution of 0.05fF, equivalent to 1nm in displacement. Due to
the disadvantage of MS3110 in closed-loop, AC bridge capacitance measurement method is
then implemented for closed-loop integration. The resolution of AC bridge sensor reaches
0.02fF, equivalent to 0.4nm in displacement. An additional function of AC bridge sensing
is accomplished which is simultaneously sensing and actuation of CDA. In the feedback
loop, the traditional analog PID controller is designed to transfer the voltage signal of
capacitance measurement to the voltage-force transducer which converts feedback voltages
to differential feedback force. Since the differential feedback force is limited by clamped
voltage, a force-balanced mode is observed under 5V actuation of CDA.
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Keywords
force-balanced, feedback, capacitive sensor, MEMS
Citation
Degree
MS
Discipline
Mechanical Engineering
