A high displacement ultrasonic actuator based on a flexural mechanical amplifier
Introduction
Commonly used high displacement ultrasonic actuators are composed of a power vibration generator (the Langevin transducer) and of a displacement amplifier. This structure is employed in a large variety of industrial applications [1], [2], [3], [4], as well as in the biomedical field as an ultrasonic bystoury [5]. Depending on the application, this actuator is able to produce displacements up to hundred microns in the ultrasonic frequency range up to hundred kHz.
The Langevin transducer is basically composed of a couple of piezoceramic disks sandwiched between two metal masses. It can be excited to resonate in length–extensional mode at low frequency, avoiding the need of high driving voltages. The structure is usually pre stressed in order to increase mechanical strength of piezoceramic elements and is suitable to absorb high electrical power. Whenever the longitudinal dimension is sufficiently higher than the radial dimension the Langevin transducer can be adequately described by means of 1D analytical models [6], [7].
In application where very high displacement are required, the output displacement of the Langevin transducer has to be amplified by means of a mechanical structure commonly called displacement amplifier.
Among possible displacement amplifiers, sectional ultrasonic concentrators, made from rods of variable and constant cross section, are those that have been mainly exploited in applications. Basically, sectional concentrators are designed to resonate in length–extensional mode at the same frequency of the Langevin transducer and the displacement amplification depends on the ratio between the back and the front sections.
Sectional concentrators have been widely analyzed by Merkulov and Kharitonov [8], [9] for several shapes (conical, exponential and catenoidal). These authors concluded that the maximum displacement amplification is achieved for a stepped horn when the two sections are both one-quarter wavelength long. In this case, the amplification factor is equal to the ratio between the areas of the two end sections.
In this work, a new type of ultrasonic actuator, which is able to provide higher displacements than classical ultrasonic actuators based on sectional concentrators, is proposed. It is composed of a symmetrical Langevin transducer working in a length–extensional mode and of a displacement amplifier that is designed to vibrate in flexural mode at the same working frequency of the Langevin transducer. The displacement amplifier is able to transform the almost flat axial displacement provided by the Langevin transducer at its back end into a flexural deformation that produces the maximum axial displacement at the center of its front end.
A FEM software has been used as analysis tool. Performances of the proposed actuator have been evaluated by using a classical ultrasonic actuator, based on a stepped horn concentrator, as a benchmark. Numerical comparisons between mechanical amplifications of the two amplifiers as well as between maximum displacements achieved when the two amplifiers are joined to the same Langevin transducers have been performed.
Results of numerical simulations have been validated by experimental measurements carried out on two manufactured prototypes.
Section snippets
Working principle
Fig. 1 a schematically shows the proposed actuator: it is composed of a symmetrical Langevin transducer joined to a displacement amplifier designed to work in a flexural mode. The Langevin transducer is composed of a couple of piezoceramic disks with radius a and thickness , poled along z direction but with opposite polarities, and of two cylinder shaped steel masses, which have radius identical to that of the disks and thickness t. The displacement amplifier is composed of a cylinder shaped
Finite element model
FE analysis was performed by using a commercial package (ANSYS).
The symmetrical Langevin transducer was modelled by imposing the continuity of the displacements both in radial and in axial directions at the interfaces between the piezoceramic disks (PZ26 by Ferroperm) and the steel masses (mass density kg/m3, Young modulus N/m2, Poisson ratio ), as well as at the interface between each couple of piezoceramic disks. Furthermore, for the piezoceramic elements, electrodes
Numerical analysis
A modal analysis of the Langevin transducer, used as a vibration driver for both the mechanical amplifiers, was first carried out. The radius of both masses and piezoceramic disks was mm, the thickness of the piezoceramic mm, and masses’ length mm. The fundamental length-extensional resonance frequency obtained by simulation was kHz.
In order to model a flexural amplifier resonating at the same frequency of the Langevin transducer, four variables are to be determined
Experimental results
In order to experimentally validate results obtained by simulations, two actuators were manufactured by following FE design. However, as it is well known, Langevin transducers need to be prestressed; therefore, two couples of piezoceramic rings (inner diameter 5 mm) were used instead of disks in order to allow a bolt to pass through them. Prestress operation was accomplished by means of a torque wrench, in order to ensure its repeatability. A torque of 4 N m was applied to both actuators. As in
Conclusions
In this work a high displacement ultrasonic actuator has been proposed. It is composed of a symmetrical Langevin transducer joined to displacement amplifier vibrating in a flexural mode. In analogy to classical actuators that use sectional concentrators, the Langevin transducer and the flexural amplifier have been separately designed to work at the same frequency. Design and analysis of the actuator have been performed by using a FEM software. Performances of the proposed actuator have been
Antonio Iula was born in Salerno, Italy, on February 10, 1966. He received the DrSc degree from the University of Salerno and the PhD degree from the University Roma Tre in electronic engineering in 1992 and 1999, respectively. From 1992 to 1996 he worked in the Department of Electronic Engineering at the University of Salerno, as a postgraduate researcher. In 1996 he joined the Department of Electronic at the University Roma Tre as a PhD student. Since 1999 he has been teaching “Electronic” as
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Antonio Iula was born in Salerno, Italy, on February 10, 1966. He received the DrSc degree from the University of Salerno and the PhD degree from the University Roma Tre in electronic engineering in 1992 and 1999, respectively. From 1992 to 1996 he worked in the Department of Electronic Engineering at the University of Salerno, as a postgraduate researcher. In 1996 he joined the Department of Electronic at the University Roma Tre as a PhD student. Since 1999 he has been teaching “Electronic” as an assistant professor, and he has been involved in several national and international research programs. Antonio Iula worked mainly on piezoelectric transducers modelling and characterization for ultrasound applications in biomedical and industrial fields and on piezoelectric motors. Antonio Iula is author of more than 60 works on these fields published on international journals and conferences proceedings.
Lorenzo Parenti was born in Roma (Italy) in June, 1978. He received the MS degree in electronic engineering with a doctoral dissertation on FE analysis and experimental characterization of ultrasonic transducers from the University Roma Tre of Rome in 2004. His main scientific interests are in analytical and FEM modelling of ultrasonic transducers.
Fabio Fabrizi was born in Roma, Italy, in 1975. He received the MS and PhD degrees in electronic engineering from the University Roma Tre of Roma, in 1999 and 2005, respectively. His main research areas are in the field of ultrasonic transducers, where he is experienced FEM modelling, design and characterization of piezoelectric actuators.
Massimo Pappalardo is full professor in electronic at the University of Roma Tre. He received the DrSc degree in electrical engineering from the University of Naples in 1967; he started his research activity with a scholarship at the Istituto di Acustica C.N.R., where, in 1968, he came in the staff as a researcher. In 1978, he became scientific manager of the Department of Ultrasound and Acoustic Technology and in 1981 he was member of the Scientific Committee. During the period 1968–1985, he made research activity at the University of Birmingham, he was responsible of several national and international research programs and he taught as a contract professor at the Universities of Calabria and Salerno. In 1985, he became full professor at the Department of Electronic of the University of Salerno where he was the director for many years. Since 1995, he is full professor at the Department of Electronic of the University of Roma Tre, where he is the director. Massimo Pappalardo worked mainly on the field of ultrasound applications and acoustical imaging for biomedical and underwater prospecting. He is currently engaged in research in transducer modeling, piezoelectric devices, capacitive transducers, and echographic imaging elaboration. Massimo Pappalardo is author of more than 200 papers on these fields published on international magazines and Conferences Proceedings. Presently, he is an associate editor of IEEE-UFFC and member of the TPC of the IEEE International Ultrasonic Symposium.