A high-frequency, high-stiffness piezoelectric actuator for microhydraulic applications
Introduction
In order to realize high specific power transducers, a novel approach integrating microhydraulics, piezoelectric materials and micromachining technology is being developed [1], [2], [3]. These microhydraulic transducers (MHTs) rely on a high-frequency fluid pumping mechanism and equally fast actively controlled valves to achieve high system flow rates (∼1 ml/s) against large differential pressures (∼1–2 MPa). The pumping and valving functions in MHT systems require a positive-displacement actuator structure that can produce fluid volume oscillations at high frequency (10–20 kHz) and exhibit high structural stiffness so as to minimize compliance against the large differential system pressures. The actuator is required to be compact, low mass, and amenable to integration with other micromachined components of the hydraulic system. Motivated by the need for such an actuator for use in MHT technology, and potentially, other similar microhydraulics applications, a novel piezoelectrically driven actuator has been developed and tested. The paper highlights the development of this actuator.
Conceptually, the proposed actuator incorporates a circular piston structure supported from beneath by one or more small bulk piezoelectric cylinders and suspended circumferentially from a surrounding support structure by a thin annular micromachined tether. This compact “piston-type” design enables high-frequency actuation against large external pressurizations due to the high stiffness of the piston structure and integration of miniature bulk piezoelectric elements beneath the piston using a thin-film bond layer. Previous work in the area of piezoelectric micropumping devices has focused on either piezoelectric “bender” or “bimorph” actuators that are limited in their frequency of operation and pressurization capabilities [4], [5], [6], or piezoelectric stack actuators that are relatively bulky and incorporate thick bonding agents such as epoxy [7]. In contrast to these previous works, the piezoelectric actuator introduced in this paper is designed to be compact and achieve high-frequency, high-stiffness actuation against large differential pressures. Furthermore, the integration of high-performance (high strain, low hysteresis) single-crystal PZN-PT piezoelectric material over standard polycrystalline PZT-5H piezoelectric material allows for enhanced actuation capabilities [8].
Development of the actuator reported in this paper involved overcoming several technical challenges including (1) reliable bonding of bulk piezoelectric material to silicon using a thin-film AuSn eutectic bond, (2) tight control tolerances of piezoelectric cylinder lengths and surface roughness, and (3) controlled Si etching of the thin tether structures. The following sections present the details of design, microfabrication, and assembly of the actuator, and experimental validation of the actuator performance.
Section snippets
Geometry
A 3D cut-away schematic of the piezoelectric actuator structure is illustrated in Fig. 1. The device incorporates one or more 1 mm thick, 1 mm diameter bulk piezoelectric elements beneath a silicon micromachined piston. This piston is supported along its circumference by a 10 μm thick silicon tether. The lateral dimensions of the tether are designed to make the tether compliant enough to allow for rigid piston motion up and down, yet stiff enough to resist bowing under pressurization caused by the
Fabrication
As shown in Fig. 2, the actuator structure consists of five layers, two of which are silicon and three of which are glass, in addition to the piezoelectric material cylinders. The following subsections detail the fabrication and processing of all components.
Assembly
As shown in Fig. 5, a series of four die-level anodic bonding steps are carried out to produce a completed device. All anodic bonding procedures are carried out at 300 °C with an applied voltage of 1000 V. The first and second steps involve the bonding of the top glass wafer to the top silicon wafer and the bonding of the bottom glass wafer to bottom silicon wafer, respectively. The third step creates a bond between the middle glass layer and the bottom silicon layer. The fourth step in the
Experimental
Three distinct actuator devices have been fabricated and tested for their performance characteristics. All testing occurred sufficiently long after device poling (several days) to ensure relaxation of the piezoelectric material. The piezoelectric cylinder placement within each device is outlined in Fig. 7(a). Device 1 incorporates a single PZT-5H cylinder centrally located beneath the tethered piston and Device 2 incorporates a single PZN-PT cylinder centrally located beneath the piston, as
Piezoelectric material characterization
Fig. 9 plots the displacement behavior versus applied voltage for a representative PZT-5H and PZN-PT material cylinder under a drive voltage of 500±500 V at frequency f=100 Hz. This low frequency was chosen to ensure quasi-static operation. As shown by the data, PZN-PT exhibits larger displacement capability and reduced hysteresis in comparison to PZT-5H. Finite-element models of the piezoelectric microactuator devices indicate that the stiffness of the tether structure is insignificant compared
Conclusions
A piezoelectric actuator device for use in high-frequency fluidic micropumping applications has been developed. The actuator features small bulk piezoelectric cylinders integrated within a micromachined tethered piston structure using a AuSn eutectic bond. This unique design enables high-stiffness actuation within a compact volume (). A series of microactuator devices was fabricated and experimentally characterized to frequencies in excess of 100 kHz. Experimental modal and
Acknowledgements
This research was sponsored by DARPA under Grant #DAAG55-98-0361 and by ONR under Grant #N00014-97-1-0880.
David C. Roberts received his BS and MS degrees in mechanical engineering from the Massachusetts Institute of Technology in 1995 and 1998, respectively. He is currently completing his PhD research in the Active Materials and Structures Laboratory at the Massachusetts Institute of Technology, focusing on the development of high-performance piezoelectrically driven microvalve and microactuator devices for hydraulic applications. Research areas of interest include mechanical design, structural
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David C. Roberts received his BS and MS degrees in mechanical engineering from the Massachusetts Institute of Technology in 1995 and 1998, respectively. He is currently completing his PhD research in the Active Materials and Structures Laboratory at the Massachusetts Institute of Technology, focusing on the development of high-performance piezoelectrically driven microvalve and microactuator devices for hydraulic applications. Research areas of interest include mechanical design, structural dynamics, and MEMS.
Hanqing Li received his BS degree in physics from Peking University, Beijing, China in 1982 and his MS degree in material science at the General Research Institute for Non-Ferrous Metals, Beijing, China in 1985. He received his PhD degree from the University of Nebraska-Lincoln in 1998. He was a guest scientist at NIST-Boulder from 1995 to 1998, a postdoc at MIT between 1998 and 1999, and is currently a research scientist at MIT. His primary research interests are in MEMS fabrication and testing and superconducting devices.
J. Lodewyk Steyn received his BEng degree in mechanical engineering from the Department of Mechanical and Aeronautical Engineering, University of Pretoria in 1998. He is currently pursuing a doctoral degree in the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology. His research interests include the fabrication, packaging, instrumentation and testing of piezoelectrically driven microfluidic devices for actuation and power generation applications.
Kevin T. Turner received his BS in mechanical engineering from the Johns Hopkins University in 1999. In 2001, he received his MS in mechanical engineering from the Massachusetts Institute of Technology. His masters research focused on silicon fusion bonding, mechanical strength of silicon, and thin-film solder bonding. He is currently pursuing a PhD at the Massachusetts Institute of Technology with a focus on mechanics and materials issues for MEMS.
Richard Mlcak received a BEng degree in 1989 from the Materials Science Department at McMaster University in Ontario, Canada. In 1994 he received a science doctorate in electronic materials from the Department of Materials Science and Engineering at MIT, researching photoelectrochemical micromachining processes. Dr. Mlcak presently holds the positions of president and chief technical officer at Boston Microsystems Inc. His research interests focus on the development of bulk micromachined harsh environment-compatible MEMS sensors fabricated from SiC and other refractory semiconductors.
Laxminarayana Saggere received his BE degree in 1987 from Osmania University, India, an MS degree in mechanical engineering in 1993 from the University of Rhode Island, Kingston, RI, an MS degree in aerospace engineering and a PhD degree in mechanical engineering from the University of Michigan at Ann Arbor in 1998. He served as a scientist/engineer in the Indian Space Research Organization, Sriharikota, India, from 1987 to 1991, and as a research scientist in the Department of Aeronautics and Astronautics at MIT from 1999 to 2001. He is currently an assistant professor of mechanical engineering at the University of Illinois at Chicago. His current research interests are in the development of novel MEMS-based miniaturized transducers and mechanisms for engineering and biomedical applications.
S. Mark Spearing is the Esther and Harold E. Edgerton associate professor of aeronautics and astronautics at MIT, where he has been employed since 1994. He received his BA in engineering from Cambridge University in 1986 and the PhD in 1990 and then worked as a research engineer at the University of California, Santa Barbara and a research specialist at BP’s Research Center in Cleveland, OH. His technical interests are in the mechanics of composite materials and structures and materials, structures and packaging for MEMS devices.
Martin A. Schmidt received his BSEE in electrical and computer engineering from Rensselaer Polytechnic Institute in 1981. He received his MS (1983) and his PhD (1988) in electrical engineering and computer science from MIT. In addition to his professorship in the Department of Electrical Engineering, he is the director of the Microsystems Technology Laboratories at MIT. Professor Schmidt investigates microfabrication technologies for integrated circuits, sensors, and actuators; design of micromechanical sensor and actuator systems; mechanical properties of microelectronic materials, with emphasis on silicon wafer bonding technology; integrated microsensors; and micro fluidic devices. His current research projects involve novel applications of MEMS technologies to a variety of fields, including miniature gas turbines, miniature chemical reactors, microswitches, biological applications and sensors monolithically integrated with electronics.
Nesbitt W. Hagood is an associate professor of aeronautics and astronautics at MIT. He specializes in the analysis, design and development of controlled structures and materials. He got BS (1985), MS (1988), and PhD (1991) degrees from the Department of Aeronautics and Astronautics at MIT. His primary research interest is in the design and modeling of active structural systems. This includes research in the areas of modeling of structures incorporating active material actuators and sensors, development of new active material systems and supporting design tools. He is an ONR young investigator and has received the Presidential Early Career Award in Science and Technology. He is also a co-founder of Continuum Control Corporation.