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Toward the application to the tapping mode AFM (Atomic Force Microscope), we propose a new control method to keep the state of a soft impact between an object surface and the tip of a cantilever whose supporting point is periodically and laterally excited. In the practical usage of AFM, the surface of the object is scanned by the excited cantilever. Depending on the profile of the surface, the distance between the surface and the center of the excitation is varied and then, the state of the contact is changed. To keep the state of contact, the center of the excitation is automatically shifted in AFM. As a result, we can know the profile of the object from the shift of the center while scanning. By the way, it is known that various bifurcations of the responses of the beam occur according to impact conditions and variation of the excitation frequency. In our previous study, we indicated theoretically and experimentally that when the excitation frequency N is set to 2.5 times the first natural frequency ω1 of the cantilever, i.e., N=2.5ω1, the contact produces the frequency component of the 1.25 time the first natural frequency in the cantilever response, i.e., 1.25ω1. In this paper, by using this phenomenon, we propose a control method to shift the center of the excitation according to the profile of the object surface, keeping soft contact. For this purpose, a real-time signal processing method is suggested to determine the magnitude of the targetted frequency component. Also, a servo system is designed to periodically excite and to control the center of the excitation precisely. Finally, validity of the proposed control method is experimentally confirmed by using a macro-beam.

Control of an Impacted Cantilever toward Application of an Atomic Force Microscope in Tapping Mode