Variable structure control of a piezoelectric actuator for a scanning tunneling microscope

Scanning probe microscopes are now widely used in the field of material science and engineering for surface imaging at atomic scale. Their principle is based on the surface probing by a sharp tip approached at a nanometric distance of the surface. The probe is fixed to piezoelectric actuators allowing its displacement above the surface. An electronic command of a scanning tunneling microscope (STM) has been designed and tested. The regulation feedback loop of the tunnel current includes an integral controller, as is the case in commercial equipment. An extra control by variable-structure system has been implemented on this electronic command. Its principle is based on the commutation of the feedback sign. The effect on the system performance of the variable structure control is presented and discussed. An STM head has been modeled and all the model parameters have been determined. The model has been validated by comparison of the experimental and simulated responses of the system under excitation.