High performance feedback for fast scanning atomic force microscopes

We identify the dynamics of an atomic force microscope (AFM) in order to de sign a feedback controller that enables faster image acquisition at reduced imaging error compared to the now generally employed proportional integral differential (PID) controllers. First, a force model for the tip-sample in teraction in an AFM is used to show that the dynamic behavior of the cantil ever working in contact mode can be neglected for control purposes due to t he relatively small oscillation amplitude of the cantilever in response to a defined topography step. Consequently, the dynamic behavior of the AFM sy stem can be reduced to the behavior of the piezoelectric scanner making the design of a model based controller for the AFM possible. Second, a black b ox identification of the scanner of a commercial AFM (Nanoscope IIIa, Digit al Instruments) is performed using subspace methods. Identification yields a mathematical model of the scanner which allows us to design a new control ler utilizing H-infinity theory. Finally, this controller is implemented on an existing AFM and operated in contact mode. We demonstrate that such an H-infinity-controlled AFM system, while scanning at rates five times faster than conventional PID-controlled systems, operates with reduced measuremen t error and allows scanning at lower forces. (C) 2001 American Institute of Physics.