Phase-locked loop-based proportional integral control for spiral scanning in an atomic force microscope

The design of a phase-locked loop (PLL)-based proportional integral (PI) controller for improving the spiral scanning of a piezoelectric tube scanner (PTS), used in an atomic force microscope (AFM), is demonstrated in this paper. Spiral motion of the PTS for scanning of material surfaces or biological samples using an AFM is achieved by applying two sinusoidal signals with a 90 degree phase-shift and of varying amplitudes to the X and Y-axes of the scanner. If there is an exact 90-degree phase shift between the sinusoidal displacements of the two scanner axes, a true circular image is generated. Otherwise, the image will distort into an elliptical shape, which can lead to missing data points or changes in the lattice structure of the sample surface to be scanned. The phase error between the X and Y-axes positions increases with increasing scanning speeds. In this proposed control scheme, the phase error between the displacements of the two lateral axes of the scanner is measured by a phase detector and is minimized using a PI controller. Experimental results for reference tracking and imaging performance confirm the efficiency of the proposed control scheme.