Advances in Precise Positioning using the Electrostatic Glass Motor R. Moser 1 , L. Sache 1 , A. Cassat 2 , H. Bleuler 1 1 Laboratory of Robotics (LSRO) 2 Laboratory of Integrated Actuators (LAI) Ecole Polytechnique Federale de Lausanne EPFL Lausanne, Switzerland roland.moser@ieee.org T. Higuchi Department of Precision Machinery Engineering The University of Tokyo Tokyo, Japan Abstract—This paper reports the continuation of the authors work in the field of linear and rotary sub-micrometric positioning using electrostatic glass motors. Electrostatic glass motors, a recent development where a glass rotor, once polarized using a static electrostatic field, is synchronously following a moving electrostatic field, have been described previously. The potential application to precise positioning was proven feasible and promising. The present report describes a fully operational rotary prototype, featuring remarkable performances. It will be shown that sub arc-second positioning is possible in open loop operation, over an infinite operation range and over a very broad speed range. After a brief recapitulation of the propulsion principle, the realized system will be presented and characterized, its performance experimentally determined and discussed. Keywords: Electrostatic glass motors; linear nanometric positioning; rotary sub arc-second positioning; electrostatic induction. I. INTRODUCTION The principle of electrostatic glass motors has been observed in [1] and its application to precise positioning was suggested and verified in [2] . The basic of this novel kind of actuator is the possibility of non-contact charge induction into glassy substances. These induced charge arrangements have a rise and decay constant in the order of tens of seconds, so a static field is required for the build-up of the charges. These relatively stable charge arrangements can be used for synchronous propulsion of the glass rotor, similar to a electret motor. Figure 1 sketches the principle. A glass rotor is kept at constant gap g over an array of stripe shaped electrodes with width w and mutual gap d. If a three phased excitation is applied to the electrodes (b), due to the slow relaxation time of glassy dielectrics, stable charge arrangements are created in the in the glass regions that face the gap d. Negative and positive charge arrangements are indicated by Q- and Q+, respectively. If the electrostatic field moves (c), lateral restriction forces attributable to the induced charges force the rotor to follow synchronously the field. The distribution and amplitude of the charge densities is function of the electrodes geometrical parameters, the air-gap and the applied voltages. Since the movement is synchronous, a point on the rotor sees always the same applied potential, therefore the induced charges are not changing. When the rotor is stopped, the excitation must be ‘freezed’ at DC mode and then can be restarted at any time. Figure 1. Principle of electrostatic glass actuators a) Geometric parameters b) Charge induction due to DC excitation c) Lateral restriction force when the electrostatic field moves. An analytical and experimental description of the forces and torques that are created by such a device is provided in [3]. A very interesting property of these actuators is the smoothness of the rotor movement. This property is unmatched by other actuation principles and is described in detail in [4]. In short, given the fact that the charge distribution on the rotor tends to mirror perfectly the amplitude distribution of the stator field, a sinusoidal stator field yields to a sinusoidal rotor potential. This interaction results in a ripple free synchronous open loop movement, an interesting property for compact and cost-effective precise positioning (obsolescence of position sensors). The generous support by the Gebert Rüf foundation (Gebert Rüf Stiftung), Switzerland, has made this research possible. w d g V+ V- 0 V+ V- Q- Q+ V+ V- 0 V- 0 a) b) c)