Sensorless Implementation of a PPF Controller for Active Control of an AFM Microcantilever

Reducing the cantilever quality (Q) factor in the atomic force microscope (AFM), when operating in tapping mode, allows for an increase in imaging speed. Passive piezoelectric shunt control has several advantages over alternative methods of cantilever Q factor reduction. However, this technique uses a passive electrical impedance to modify the mechanical dynamics of the cantilever, which limits the amount of Q factor reduction achievable. This paper demonstrates that further reductions in the cantilever Q factor may be obtained with the use of an active impedance in the piezoelectric shunt control framework. The active impedance parameters are designed in such a way that the piezoelectric shunt controller emulates a positive position feedback controller in a displacement feedback control loop. A significant reduction in cantilever Q factor is obtained using an active impedance compared with that achieved with a passive impedance. The improvement in scan speed using this control technique is demonstrated with AFM images of a test sample.