%0 Conference Paper %B Proceedings of IMECE 2006, 2006 ASME International Mechanical Engineering Congress and Exposition, Nov. 5-10, Chicago, USA %D 2006 %T On-off control for full-gap positioning of parallel-plate electrostatic MEMS %A Mol, Lukas %A Rocha, Luis %A Cretu, Edmond %A Wolffenbuttel, Reinoud %C Chicago, Illinois, USA %I ASME %U http://www.asmeconferences.org/Congress06/TechnicalProgramOverview.cfm#27 %X IMECE2006-14503 Technical Publication On-Off Control for Full-Gap Positioning of Parallel-Plate Electrostatic MEMS Authors Lukas Mol, Delft University of Technology Luis A. Rocha, University of Minho Edmond Cretu, University of British Columbia Reinoud F. Wolffenbuttel, Delft University of Technology Abstract Electrostatic parallel-plate actuation is classically limited to displacements up to 1/3 of the gap due to the pull-in effect [1,2]. This limits the use of electrostatic parallel-plate actuation in many applications. In order to overcome this limitation several techniques have been proposed: geometry leverage [3], series feedback capacitor [4], current drive methods [5,6] and closed-loop voltage control [7]. Stable displacement over the full available range has not been achieved with these approaches, except for the geometry leverage technique, which is limited by the higher voltage levels required and the larger dimensions. The method presented in [8] is recently introduced to overcome these difficulties. It is based on the comparison of the measured momentary actuator displacement with the desired displacement. The voltage applied to the actuator is changed between two values (unlike traditional feedback): between a high level, if the measured displacement is lower than the reference, and a low level, if the actuator displacement is higher than the reference value. The method relies strongly on the nonlinear dynamic behavior of the MEMS devices. By changing the applied voltage a shift occurs between stable and unstable trajectories. This implies that the device must be overdamped or critically damped in order not to be in the oscillatory operating regime. At very small gaps (i.e. large displacements), the damping forces are huge due to the rarefaction effects. These damping forces slow even further the structure displacements, improving the dynamic device response when operated with the on-off method. Due to the feedback topology, there is a finite time delay that results in a small ripple of the electrode displacement. Two important parameters are available for reducing the remaining ripple. Although the voltage levels are not critical for proper operation, adjusting the high and low level is affecting the device response and can be used to improve the performance. Furthermore the feedback loop delay must be reduced as much as possible, for example by integrating the displacement measurement circuits with the actuator. Careful selection of device geometry, gas type, pressure and actuation voltage levels, can yield greatly reduced position ripple, while maintaining bandwidth and quick response time. This could effectively reduce an existing device size with a factor three for the same operating displacement range. References [1] H.A.C. Tilmans, and R. Legtenberg, Sensors and Actuators, A 45 (1994) 67-84. [2] L.A. Rocha, E. Cretu and R.F. Wolffenbuttel, J. Microelectromech. Syst. 13 (2004) 342-354. [3] E.S. Hung and S.D. Senturia, J. Microelectromech. Syst. 8 (1999) 497-505. [4] E.K. Chan and R.W. Dutton, J. Microelectromech. Syst. 9 (2000) 321-328. [5] R. Nadal-Guardia, A. Deh{\'e}, R. Aigner and L.M. Casta{\~n}er, J. Microelectromech. Syst. 11 (2002) 255-263. [6] J.I. Seeger and B.E. Boser, J. Microelectromech. Syst. 12 (2003) 656-671. [7] Y. Sun, D. Piyabongkarn, A. Sezen, B.J. Nelson and R. Rajamani, Sensors and Actuators, A 102 (2002) 49-60. [8] L.A. Rocha, E. Cretu and R.F. Wolffenbuttel, J. Micro-electromech. Syst., 15 (2006), pp. 69-83. Session: MEMS-10 Micro and Nano Sensors and Actuators %8 November 5-10 %9 inproceedings