In Kelvin probe force microscopy, a conducting cantilever is scanned over a surface at a constant height while an AC+DC potential is applied.   The AC signal is a sinusoid whose frequency matches the mechanical resonance of the cantilever.  The cantilever is driven into oscillation by electrostatic forces where the DC potential difference between the surface and the cantilever is non-zero.  Using a four-quadrant detector and an A/D to detect cantilever motion, the feedback circuit drives the DC signal to the surface potential, minimizing cantilever motion and resulting in a map of the work function of the surface.
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