Control issues of MEMS nanopositioning devices

In this chapter, the control issues of microelectromechanical system (MEMS) nanopositioning devices are introduced and discussed. The real-time feedback control of a novel micro-machined 1-degree-of-freedom (1-DoF) thermal nanopositioner with on-chip electrothermal position sensors is presented. The actuation works based on the thermal expansion of V-shaped silicon beams. The sensing mechanism works based on measuring the resistance difference between two electrically biased identical silicon beams. The resistance difference varies with displacement. The heat conductance of the sensor beams varies oppositely with the position of the movable stage, resulting in different beam temperatures and resistances. A pair of position sensors are operated in differential mode to reduce low-frequency drift. The micro-machined nanopositioner has a nonlinear static input-output characteristic. The electrothermal actuator has a dynamic range of 14.4 µm and the electrothermal sensor has a low drift of 8.9 nm over 2000 s. An open-loop controller is first designed and implemented. It is experimentally shown that uncertainties result in unacceptable positioning performance. Hence, feedback control is required for accurate positioning. The on-chip displacement sensor is able to provide high-resolution displacement control. Therefore, a real-time closed-loop feedback control system is designed using a proportional-integral (PI) controller together with the nonlinear compensator used for the open-loop control system. The closed-loop system provides acceptable and robust tracking resolution for a wide range of set point values. The step response results show a positioning resolution of 7.9 nm and a time constant of 1.6 ms in a 10 µm stroke. For triangular reference tracking, which is required in raster-scanned scanning probe microscopy (SPM), the steady-state tracking error has a standard deviation of 20 nm within a wide range of 10 µm.