CMU MEMS Laboratory Publication Abstract


in Ph.D. Thesis, May 2002, Carnegie Mellon University, Pittsburgh, PA.
Parallel-Plate Micro Servo for Probe-Based Data Storage
M. S. Lu
This thesis describes the use of closed-loop voltage control to extend the travel range of a parallel-plate electrostatic microactuator beyond the open-loop pull-in limit of one third of the gap. A general controller design procedure is presented to deal with the nonlinearities and unstable characteristic of the parallel-plate actuator. The resulting linear controller design is implemented easily for the desired application of a probe-based mass data storage device. In the envisioned data storage system, an array of parallel-plate tip actuators are employed to position the read/write probe tips about 10 nm away from the magnetic media for data access. Fabrication of tip actuators and servo and data channel circuits are conveniently integrated by use of the CMOS-MEMS micromachining process developed at Carnegie Mellon.

Controller design ensures system stability in the presence of a unstable pole beyond the pull-in limit. Desired transient response is achieved by a pre-filter added in front of the feedback loop to shape the step input command. The fabricated microactuator is characterized by static and dynamic measurements, with a spring constant of 0.17 N/m, mechanical resonant frequency of 12.4 kHz, and effective damping ratio from 0.55 to 0.35 for gaps between 2.3 to 2.65 µm. The minimum input-referred noise capacitance change is 0.5 aF/√Hz measured at a gap of 5.7 µm, corresponding to a minimum input-referred noise displacement of 0.33 nm/√Hz. Measured closed-loop step response illustrates a maximum travel distance up to 60% of the initial gap with a rise time less than 5 ms.
© 2002 Carnegie Mellon University, Department of Electrical and Computer Engineering.
Full paper (PDF) (opens in new window).

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