Iterative learning control for near-field scanning optical microscope applications

Near-field scanning optical microscopes (NSOMs) construct images from optical properties recorded by a scanning probe as a sample is passed under a solid immersion lens (SIL) and a laser source. An air gap between the SIL and the sample must be maintained at a distance given in nanometers that is less than the wavelength of the laser source. Regulation of the air gap demands an accurate and fast controller. While lead-lag compensation has been successfully implemented as the feedback controller, the addition of feedforward or iterative learning control (ILC) can offer improved transient performance with significantly reduced overshoot. A serial ILC design is implemented upon a closed-loop system and the resulting performance is evaluated.