Design, Modeling, and Control of a Micromachined Nanopositioner With Integrated Electrothermal Actuation and Sensing

In this paper, a real-time feedback control of a novel micromachined one-degree-of-freedom thermal nanopositioner with on-chip electrothermal position sensors is presented. The actuation works based on thermal expansion of silicon beams. The sensing mechanism works based on measuring the difference between the electrical resistances of two electrically biased identical silicon beams. The difference increases with displacement, as the heat conductance of the sensor beams varies oppositely with position, resulting in different beam temperatures and resistances. The sensor pair is operated in differential mode to reduce low-frequency drift. The nanopositioner has a nonlinear static input-output characteristic. An open-loop controller is first designed and implemented. It is experimentally shown that uncertainties and sensor drift result in an unacceptable nanopositioner performance. Hence, feedback control methods are necessary for accurate nanopositioning. A closed-loop feedback control system is designed using a proportional-integral controller together with the nonlinear compensator used for the open-loop control system. The closed-loop system provides an acceptable and robust tracking performance for a wide range of set point values. For triangular reference tracking, which is needed in raster-scanned scanning probe microscopy, the tracking performance of the closed-loop system is further improved by incorporating a feedforward controller.