Real-Time Reconstruction of Multimode Tip Motion of Microcantilevers in Dynamic Atomic Force Microscopy

A modal projection filtering method is proposed for the dynamic atomic force microscopy (AFM) to accurately reconstruct the multimode tip motion of the AFM microcantilever from the single measurement signal of the optical lever method in real time. The optical lever method is inherently a slope detection method, wherein each individual dynamic mode pertinent to the slope detection principle of the measurement system has its own distinct measurement sensitivity because of the unique dynamic mode shape per a mode. The proposed modal projection filtering method uses the experimentally identified multimode dynamic model of the AFM microcantilever along with the calibrated measurement sensitivities of individual dynamic modes to design the modal projection filters to extract the responses of individual dynamic modes from the single measurement signal. The tip motion of the AFM microcantilever is then accurately reconstructed by using a linear superposition of the extracted responses of multiple dynamic modes. The designed modal projection filters were implemented in a field programmable gate array for real-time computation with an update rate of 1 MHz. Computer simulations and experimental investigations were performed to validate the principle of and to demonstrate the performance of the proposed method.