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 ∞ 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.

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International audienceThis paper deals with robust closed-loop control of a nano-robotic system dedicated to fast scanning probe microscopy. The nano-robotic system is actuated by piezoelectric stick-slip actuators able to produce a millimeter range displacement with a nanometer resolution. In order to meet the requirements of fast scanning in terms of closed-loop bandwidth and vibration damping, robust control strategies are studied. We first show that a commonly used one degree of freedom (1-DOF) H∞ controller is limited to satisfy robust performances required for fast and accurate positioning of the actuators. As such, the control strategy is defined considering two closed-loops. Results show that the 2-DOF H∞ control scheme allows robust performances for the positioning of nanorobotic systems and lead to new perspectives for fast scanning probe microscopy using stick-slip actuators