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del Corro, P. G., M. Imboden, D. J. Bishop, and H. Pastoriza. "Comb Drive Designs With Minimized Levitation." Journal of Microelectromechanical Systems 25, no. 6 (2016): 1025–1032.
Abstract: This paper presents two capacitive comb drive
designs for electrostatic actuation of MEMS with the aim to
eliminate the levitation effect often observed in such systems.
By placing a shield over the comb drive fingers, it is possible
to balance the electric field and suppress vertical forces while
maintaining the desired lateral motion. By optimizing the comb
geometry, we demonstrate that our approach is able to reduce the
levitation by an order of magnitude and unwanted coupling of
motion from out-of-plane to in-plane by a factor of 7 compared
with standard comb architectures fabricated using PolyMUMPs
technology, without the need of alternating comb finger polarities
or additional control electrodes. Levitation was reduced to
160 nm, for 3.6-Âµm lateral displacement at a driving voltage
of 80 V.
del Corro, P. G., M. Imboden, D. J. Pérez, D. J. Bishop, and H. Pastoriza. "Single ended capacitive self-sensing system for comb drives driven XY nanopositioners." Sensors and Actuators A: Physical 271 (2018): 409–417.
Abstract: This paper presents the implementation of a system to capacitively self-sense the position of a comb drive based MEMS XY nanopositioner from a single common node. The nanopositioner was fabricated using the multi-users PolyMUMPs process, on which comb capacitors fringe fields are large and out of plane forces cause considerable deflection. An extensive analysis of the comb-drive capacitance including the levitation effects and its correlation to the measurements is presented. Each axis is independently measured using frequency division multiplexing (FDM) techniques. Taking advantage of the symmetry of the nanopositioner itself, the sensitivity is doubled while eliminating the intrinsic capacitance of the device. The electrical measured noise is 2.5aF/Hz, for a sensing voltage Vsen=3Vrms and fsen=150kHz, which is equivalent to 1.1nm/Hz lateral displacement noise. This scheme can also be extended to N-degree of freedom nanopositioners.