Fuzzy logic scheme for tip-sample distance control for a low cost near field optical microscope

Abstract

The control of the distance between the surface and the tip-sample of a Scanning Near Field Optical Microscope(SNOM) is essential for a reliable surface mapping. The control algorithm should be able to maintain the system in aconstant distance between the tip and the surface. In this system, nanometric adjustments should be made in order tosense topographies at the same scale with an appropriate resolution. These kinds of devices varies its propertiesthrough short periods of time, and it is required a control algorithm capable of handle these changes. In this work afuzzy logic control scheme is proposed in order to manage the changes the device might have through the time, andto counter the effects of the non-linearity as well. Two inputs are used to program the rules inside the fuzzy logiccontroller, the difference between the reference signal and the sample signal (error), and the speed in which itdecreases or increases. A lock-in amplifier is used as data acquisition hardware to sample the high frequency signalsused to produce the tuning fork oscillations. Once these variables are read the control algorithm calculate a voltageoutput to move the piezoelectric device, approaching or removing the tip-probe from the sample analyzed.