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

 

J. A. Márquez¹, R. Cortés², H. R. Siller*¹, V. Coello², D. Escamilla¹

 

¹ Centro de innovación en Diseño y Tecnología Tecnológico de Monterrey Monterrey, Nuevo León, México. *hector.siller@itesm.mx.

² Centro de Investigación Científica y de Educación Superior de Ensenada CICESE, Monterrey Km 9.5 Nueva carretera al Aeropuerto. PIIT. Apodaca, Nuevo León, México.

 

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 a constant distance between the tip and the surface. In this system, nanometric adjustments should be made in order to sense topographies at the same scale with an appropriate resolution. These kinds of devices varies its properties through short periods of time, and it is required a control algorithm capable of handle these changes. In this work a fuzzy logic control scheme is proposed in order to manage the changes the device might have through the time, and to counter the effects of the non-linearity as well. Two inputs are used to program the rules inside the fuzzy logic controller, the difference between the reference signal and the sample signal (error), and the speed in which it decreases or increases. A lock-in amplifier is used as data acquisition hardware to sample the high frequency signals used to produce the tuning fork oscillations. Once these variables are read the control algorithm calculate a voltage output to move the piezoelectric device, approaching or removing the tip-probe from the sample analyzed.

Keywords: Low-cost SNOM, fuzzy logic, nanometric control, shear-force.

 

RESUMEN

El control de la distancia entre la superficie y la punta de sensado de un microscopio de escaneo de campo cercano (SNOM por sus siglas en inglés) es esencial para un mapeo superficial confiable. El algoritmo de control tiene que ser capaz de mantener al sistema a una distancia constante entre la punta y la superficie de interés. Se requieren ajustes nanométricos para poder recuperar una topografía con una resolución apropiada, debido a que los cambios en la superficie son en escala nanométrica. Este tipo de dispositivos cambian sus propiedades a lo largo de un periodo de tiempo muy corto, para resolver este problema se necesita un algoritmo de control que sea capaz de manejar estos cambios. En este trabajo se propone un esquema de lógica difusa para de esta manera poder compensar los cambios que el dispositivo pueda presentar a través del tiempo, y para contrarrestar los efectos producidos por la no linealidad que presenta el sistema. Dos entradas fueron usadas para programar las reglas utilizadas en el controlador de lógica difusa, la diferencia entre la señal de referencia y la señal retroalimentada (error), y la velocidad en la cual disminuye o aumenta. Como dispositivo de adquisición de datos se utilizó un amplificador de amarre de fase para leer las señales de alta frecuencia usadas para producir las oscilaciones del diapasón de cuarzo. Una vez que se adquieren estas variables podemos manipularlas por medio del algoritmo de control para calcular una salida de voltaje la cual mueve el dispositivo piezoeléctrico, retrayendo o extendiendo la punta hacia la muestra analizada.

 

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Acknowledgements

The authors acknowledge financial support from CONACyT project 127589. The authors of this paper want to thank Dr. Juan Merlo for his support in this work and technical assistance. Additional support was provided by The Research Chair of Intelligent Machines at Tecnológico de Monterrey.

 

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