Resonant Circuits for Low Voltage Electrostatic Drive and Position Sensing
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Monday, March 16, 2009
CIS 101X 3:00-4:00pm
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Baris Cagdaser
Senior Integrated Circuits Designer
InvenSense, Sunnyvale, CA
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Abstract
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This talk presents a single MEMS electrostatic interface circuit that
does not require high voltage electronics, but does provide high
voltage actuation, position sensing, and extends the range of
parallel-plate actuators.
Electrostatic actuation finds widespread use in micromechanical
systems. Despite being simple in implementation, achieving a
sufficiently large force is often challenging. Increasing the
actuator voltage results in increased force, but requires special
interfaces with high voltage electronics. Furthermore, position
feedback is commonly used to control the dynamic response of the
actuator. In principle, the position dependent actuation capacitance
can be used for sensing. But, in practice, measuring this capacitance
is challenging due to small levels of signal and large feedthrough
from the drive system. So, most actuators need separate sense
electrodes as well as circuit techniques such as time division
multiplexing.
The core of the proposed circuit is an electrical RLC tank formed by
the actuation capacitor and an inductor. When driven at its
electrical resonance, the tank amplifies the voltage across the
actuator capacitor. So, the actuation voltage becomes much higher
than the drive signal provided by electronics. The prototype
implementation uses only a 4Vpeak drive signal to actuate a MEMS
mirror that requires 45VDC under conventional voltage control. Since
the actuator capacitor is position dependent, achieving the maximum
amplification requires an oscillator circuit that automatically
follows the tank's electrical resonance frequency as the actuator
moves. So, the actuator motion translates the oscillator frequency,
which is now used to measure the MEMS displacement in the same
interface circuit. Finally, the position dependent nature of the tank
also increases the range of parallel-plate actuators by providing
inherent position feedback and changing the pull-in behavior.
Biography
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Baris Cagdaser received his BS degree from the Middle East Technical
University in Ankara, Turkey, where he worked with Prof. Tayfun Akin
on MEMS capacitive ice detectors. During his PhD studies at the
University of California at Berkeley, he worked with Prof. Bernhard E.
Boser focusing on MEMS interface electronics. Specifically, he
developed a low voltage electrostatic interface for electrostatic
actuation and position sensing. This circuit was used for the
positioning of MEMS micro-mirrors. Since earning his PhD degree in
2005, Baris Cagdaser has been with InvenSense, a Silicon Valley MEMS
start-up company, as a Senior IC Designer working on integrated
interface electronics for MEMS inertial sensors.
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