0885–3010/$25.00 © 2009 IEEE 1716 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 56, . 8, AUGUST 2009 Abstract—Actuators remain a limiting factor in robotics, especially in microrobotics where the power density of actua- tors is a problem. A 3 × 3 × 8.7 mm 3-axis piezoelectric ultra- sonic micromotor system is described here in an effort to help solve this problem. Formed from 4 bulk lead zirconate titanate (PZT) thickness-polarized elements placed around the periph- ery of a rectangular rod, the stator is designed to combine axial and flexural vibrations in a way that permits rotation of a hardened steel ball as a rotor about an arbitrary axis. A simple prototype of the micromotor was found to produce at least a rotation speed of 10.4 rad/s with 4 μN-m torque about all 3 orthogonal directions at an excitation frequency of about 221 kHz, demonstrating the feasibility of a 3 degree-of-freedom millimeter-scale piezoelectric motor. I. I T  paper describes the design, analysis, construction, and testing of a 3 × 3 × 8.7 mm triple-axis piezoelec- tric ultrasonic micromotor system producing triple degree- of-freedom motion by flexural-axial coupled vibrations of 4 thickness-poled piezoelectric elements. The study em- ployed finite element analysis and accurate measurement methods to demonstrate the successful performance of a torsional micromotor capable of producing motion with 3 degrees of freedom. A common theme in microrobotics research in all its potential applications is the concern with the lack of ef- fective actuators at small scales [1], [2]. Although applica- tions and proposed concepts of actuation in nano-scale technology are being explored, it should be noted that these do not often apply to the tasks in the micro- and millimeter-scale window. Actuation in this range is gen- erally best served through mechanical means. Many im- portant applications lie within this range, for example, critical surgical procedures on various parts of the human body. Eye, brain, middle ear, and gastrointestinal surgical procedures are some of the applications that require very precise motion, dealing with objects in the millimeter to micrometer range, where compact and light micromotors are essential in delivering precise motion, sufficient torque, and robust performance. Although there are a variety of methods for performing actuation and certainly innovative research into new ma- terials and methods, a technique that remains promising due to its flexibility, power density, and inherent brak- ing among other features is the use of ultrasonic resonant excitation of flexible structures with piezoelectric mate- rials—so-called ultrasonic motors. Piezoelectric materials have been used for some time to produce actuators [3]–[6]. Further, in contrast to many other technologies, the scal- ing of both the piezoelectric materials and the actuators that use them to smaller scales is entirely feasible [7]–[11]. Electrostatics, in particular, has long been considered for actuation through the use of silicon microfabrication techniques [12], producing electrostatic micromotors of a few tens of micrometers [13]. However, as motors, these generate very low torque at high speed (on the order of 10 pN-m and 10 000 rpm) [14], requiring reduction gears for most practical applications accompanied by the long- known problem of friction and wear [15], especially rel- evant at these scales. Beyond the straightforward application of piezoelectric materials to form motors regardless of the size, research- ers have used the flexibility inherent in ultrasonic actua- tion to provide devices capable of rotating objects about 3 orthogonal axes [16]—even for representing the human neck [17]. Although small actuators have been formed to provide this kind of motion [7], the planar configuration is inconvenient for many microbotic applications, as is the high voltage (200 V) required to obtain motion. In this paper, a piezoelectric micromotor is designed and developed to produce a triple-axis or multi-degree- of-freedom (DOF) micromotor. The paper is organized as follows. The fundamental configuration of the motor is analyzed in the next section, followed by impedance and vibration analysis to identify the location of the resonance modes, finishing with measurement of the performance of a motor prototype. It is finally shown that the resulting micromotor is capable of generating rotation about an ar- bitrary axis. II. C  A In prior work, a single-DOF motor was created by us- ing the in-plane shearing deformation of rectangular lead Triple Degree-of-Freedom Piezoelectric Ultrasonic Micromotor via Flexural-Axial Coupled Vibration Ter Fong Khoo, Dinh Huy Dang, James Friend, Member, IEEE, Denny Oetomo, Member, IEEE, and Leslie Yeo Manuscript received April 25, 2008; accepted March 29, 2009. This work was supported in part by an Australian Research Council Discov- ery Grant DP0773221 and the Monash University Small Grant and New Faculty Grant schemes. The authors are with the MicroNanophysics Research Laboratory, De- partment of Mechanical Engineering, Monash University, Clayton, Vic- toria, Australia (e-mail: james.friend@eng.monash.edu.au). D. Oetomo is with the Department of Mechanical Engineering, Uni- versity of Melbourne, Victoria, Australia. Digital Object Identifier 10.1109/TUFFC.2009.1236 Authorized licensed use limited to: Monash University. Downloaded on August 14, 2009 at 02:49 from IEEE Xplore. Restrictions apply.