actuators Review Piezoelectric Motors, an Overview Karl Spanner and Burhanettin Koc * Physik Instrumente GmbH & Co. KG, Auf der Roemerstrasse, 1, Karlsruhe 76228, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-721-4846-2416; Fax: +49-721-4846-2399 Academic Editor: Kenji Uchino Received: 1 December 2015; Accepted: 17 February 2016; Published: 26 February 2016 Abstract: Piezoelectric motors are used in many industrial and commercial applications. Various piezoelectric motors are available in the market. All of the piezoelectric motors use the inverse piezoelectric effect, where microscopically small oscillatory motions are converted into continuous or stepping rotary or linear motions. Methods of obtaining long moving distance have various drive and functional principles that make these motors categorized into three groups: resonance-drive (piezoelectric ultrasonic motors), inertia-drive, and piezo-walk-drive. In this review, a comprehensive summary of piezoelectric motors, with their classification from initial idea to recent progress, is presented. This review also includes some of the industrial and commercial applications of piezoelectric motors that are presently available in the market as actuators. Keywords: piezoelectric; inverse piezoelectric effect; friction coupling; motor 1. Introduction Generation of electrical charge on certain materials in response to applied mechanical stress is known as the piezoelectric effect. The same materials have the ability to convert electrical energy into mechanical motion directly and that is known as the inverse piezoelectric effect. The range of mechanical movement generated on a piezoelectric element is very small, from 1 µm to 100 µm. This small movement is the key for obtaining extremely high precision positioning in the order of 1.0 nm. Piezoelectric motors are the devices that generate unlimited rotary or linear movements by harvesting this very small motion. By the different drive and functional principles for generating unlimited rotary or linear movement, piezoelectric motors can be classified into three categories: (1) resonance drive (Ultrasonic motor), where vibration at ultrasonic frequency range on an oscillating element is transferred to a moving element through frictional coupling; (2) inertia-drives, where a movement is generated by speed dependent friction coefficient; and (3) piezo-walk-drives, where various piezo actuators are used for alternatively clamping and shifting a moving element. The actuators in these motors operate in a quasi-static mode. 2. Early Structures of Piezoelectric Motors Since the discovery of piezoelectricity, there have been several attempts to obtain longer mechanical motion using the inverse piezoelectric effect [1–4]. In a US patent titled “Converting electrical oscillations into mechanical movement” filed by Meissner in 1927 [1]. The structure of the motor described in this patent consists of a piezoelectric plate with a center-attached shaft and asymmetrically-attached two lever arms. When a driving signal is applied through two conductive plates to top and bottom surfaces of the piezoelectric plate, torsional vibration modes are excited Actuators 2016, 5, 6; doi:10.3390/act5010006 www.mdpi.com/journal/actuators Actuators 2016, 5, 6 2 of 18 the piezoelectricActuators 2016, 5plate, 6 at a suitable torsional mode causes the piezoelectric plate and the center2 of 18 shaft to rotate (Figure 1). Actuators 2016, 5, 6 2 of 18 due to asymmetrically-attached lever arms. It seems that the conductive plates are not only used for applyingthe piezoelectric a driving plate oscillatory at a suitable signal, torsional but also mode to create causes a frictional the piezoelectric coupling. plate Generated and the vibration center shaft on theto rotate piezoelectric (Figure plate1). at a suitable torsional mode causes the piezoelectric plate and the center shaft to rotate (Figure1). Figure 1. Piezoelectric oscillating element to obtain mechanical movement as described in the US Patent invented by Meissner [1]. Figure 1.1. 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In the same patent, a structure to convert stressingmotion force to ato rotary increase motion frictional by frictional coupling coupling was was applied also proposed. through Themag necessarynetic attraction pre-stressing force force (Figure gyratory motion to a rotary motion by frictional coupling was also proposed. The necessary pre- to increase frictional coupling was applied through magnetic attraction force (Figure2). 2). stressing force to increase frictional coupling was applied through magnetic attraction force (Figure 2). Figure 2. Piezoelectric motor proposed by Williams et al. in 1942 [2]. FigureFigure 2. Piezoelectric 2. Piezoelectric motor motor proposed proposed by by WilliamsWilliams et et al. al. in in 1942 1942 [2] [2]. 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Actuators 2016, 5, 6 3 of 18 Actuators 2016, 5, 6 3 of 18 were used to convert radial vibratory motion into tangential motion via angularly-attached lamina Actuators 2016(Figure, 5, 64 )[5]. 3 of 18 Figure 3. Piezoelectric motor introduced by V. Lavrinenko in 1965 [3]. Further investigations of piezoelectric motors led to a recognition of fundamental design principles that following structures could be operated bidirectional [6–8]. As a result of these studies by various people, a book on piezoelectric motor was written in 1981 by Bansevicius