Abstract
A novel constant force feedback mechanism based on fuzzy logic for tapping mode Atomic Force Microscopes (AFM) is proposed in this paper. A mathematical model for characterizing the cantilever-sample interaction subsystem which is nonlinear and contains large uncertainty is first developed. Then, a PID-like fuzzy controller, combing a PD-like fuzzy controller and a PI controller, is designed to regulate the controller efforts and schedule the applied voltage of the Z-axis of the piezoelectric tube scanner to maintain a constant tip-sample interaction force during sample-scanning. Using the PID-like fuzzy controller allows the cantilever tip to track sample surface rapidly and accurately even though the topography of the surface is arbitrary and not given in advance. This rapid tracking response facilitates us to observe samples with high aspect ratio micro structures accurately and quickly. Besides, the overshoot which will result in tip crash in commercial AFMs with a traditional PID controller could be avoided. Additionally, the controller efforts can be intelligently scheduled by using the fuzzy logic. Thus, continuous manual gain-tuning by trial and error such as those in commercial AFMs is alleviated. In final, computer simulations and experimental verifications are provided to demonstrate the effectiveness and confirm the validity of the proposed controller.
Highlights
In 1986, Binnig, Quate, and Gerber invented the Atomic Force Microscopes (AFM) to investigate nano-scale surfaces of insulators in a noninvasive manner [1]
A novel constant force feedback mechanism based on fuzzy logic for tapping mode Atomic Force Microscopes (AFM) is proposed in this paper
The main purpose of this study is to propose a novel, alternative method to schedule the feedback dynamics of tapping mode AFMs, exploiting the PID-like TakagiSugeno fuzzy logic controller technique (TSFLC) [13,14,15,16,17,18,19] to improve tracking response both in speed and accuracy
Summary
In 1986, Binnig, Quate, and Gerber invented the AFM to investigate nano-scale surfaces of insulators in a noninvasive manner [1]. There are three operation modes in AFMs, including the contact mode, noncontact mode, and tapping mode. Tapping mode AFMs have received more attention and is focus of this study. In this operation mode, a microcantilever with a sharp tip is vibrated near its 1mechanical resonant frequency by an external sinusoidal input and the tip contacts with the sample periodically in each cycle of the. The system consists of a microcantilever driving subsystem, a microcantilever deflection detection subsystem, a signal conversion subsystem, a sample-positioning subsystem, and an intelligent feedback controller subsystem. The microcantilever driving subsystem consists of a microcantilever with a sharp tip driven mechanically by a piezoelectric bimorph. The driving signal of the piezoelectric bimorph is generated by a high frequency resolution numerically-controlled oscillator. The microcantilever deflection detection subsystem, consisting of a position-sensitive photo detector (PSPD), a current-to-voltage converter, a preamplifier, and a low-pass filter, is designed to opti-
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