Dynamic proportional-integral-differential controller for high-speed atomic force microscopy

In tapping mode atomic force microscopy, the cantilever tip intermittently taps the sample as the tip scans over the surface. This mode is suitable for imaging fragile samples such as biological macromolecules, because vertical oscillation of the cantilever reduces lateral forces between the tip and sample. However, the tapping force (vertical force) is not necessarily weak enough for delicate samples, particularly for biomolecular systems containing weak inter- or intramolecular interactions. Light tapping requires an amplitude set point (i.e., a constant cantilever amplitude to be maintained during scanning) to be set very close to its free oscillation amplitude. However, this requirement does not reconcile with fast scans, because, with such a set point, the tip may easily be removed from the surface completely. This article presents two devices to overcome this difficulty; a new feedback controller (named as “dynamic proportional-integral-differential controller”) and a compensator for drift in the cantilever-excitation efficiency. Together with other devices optimized for fast scan, these devices enable high-speed imaging of fragile samples.