Mixed stepping/scanning mode control of stick-slip SEM-integrated nano-robotic systems

The ability to do dexterous automated and semi-automated tasks at the micro- and nano-meter scales inside a Scanning Electron Microscope (SEM) is a critical issue for nanotechnologies. SEM-integrated nano-robotic systems with several Degrees Of Freedom (DOF) and one or several end-effectors have therefore widely emerged in research laboratories and industry. The Piezoelectric Stick-Slip (PSS) is one of the best actuation principle for SEM-integrated nano-robotic systems as it has two operating modes, namely a coarse positioning mode with long travel range, and a fine positioning mode with a resolution of the order of the nanometer. The main contribution of this paper is the design of a switch control strategy to deal efficiently and in a transparent way from the user's point of view, with the transition between the coarse and the fine operating modes of PSS actuators. The aim is to be able to perform positioning tasks with a millimeter displacement range and a nanometer resolution without worrying about the mode of operation of the actuator. The coarse mode and the fine mode are respectively controlled with a frequency/voltage proportional control and a H-infinity control. The switch control is based on an internal model of the actuator. Experimental results show the effectiveness of the new mixed coarse/fine mode control strategy to satisfy closed-loop stability and bumpless specifications at the switching time. For the best knowledge of the authors, this result is the first demonstration of such a control capability for PSS actuators. (C) 2018 Elsevier B.V. All rights reserved.