Development of Acoustic Wave Filters by Using Magnetostrictive Nickel Gratings

Acoustic wave devices have gained enormous interest for sensor applications. In the current practice, the surface acoustic waves are usually generated and measured by piezoelectric transducers. In this work, however, we propose to generate the acoustic waves by using a magnetostrictive transducer using magneticstrictive nickel strip gratings and a sensing coil. The coil encircling the gratings converts the magnetic flux change induced by the strain of the strips into voltage change, and vise versa. The main motivation using magnetostrictive transducers, not piezoelectric transducers, is that the wave generation and measurement can be achieved without direct wiring to the magnetostrictive nickel gratings. In addition, the transducer can be costeffective. It is shown that the grating distance can be so tuned to the center frequency that narrow-band wave signals can be effectively excited and measured. Introduction: Most of currently-available acoustic wave filters use piezoelectric transducers. In this work, however, we will consider the possibility of using alternative magnetostrictive transducers to develop an acoustic wave filter. The magnetostrictive effect refers to the coupling phenomena between the mechanical field and the magnetic field. When a mechanical load is applied to a ferromagnetic material, its magnetic field distribution changes, and vice versa. When piezoelectric materials are used for acoustic wave filters, they are arranged in a inter-digital type. However, we arrange the magnetostrictive nickel strips in a grating type because of the different mechanics involved. Obviously, the grating distance affects the center frequency of the developed acoustic filter. To investigate the frequency characteristics of the proposed magnetostrictive grating-type transducer, several experiments were conducted. The main motivation to use the magnetostrictive materials is due to the possibility of generating and measuring waves without direct electric wiring to the materials. In this work, the possibility to tune the desired frequency by the magnetostrictive grating distance will be investigated. The actual experiments were conducted on aluminum pipes where torsional waves were generated by the proposed approach. The suggested acoustic filter consists of several nickel strips, a coil surrounding the strips and some measurement equipment. Nickel is used as a magnetostrictive material in the present application. Since nickel strips are quite flexible and possess a reasonable degree of magnetostriction, they can be bonded on the curved pipe surface. In the present work, experiments were conducted on an aluminum pipe where the torsional wave was generated and measured by the proposed transducer. Since torsional waves are non-dispersive, it is easy to process the measured wave signals. Although only the torsional waves in a pipe were investigated, the idea using the nickel grating can be extended to the design of surface acoustic wave filters. Results: 1. Sensing mechanism by the magnetostrictive transducer Figure 1 shows nickel strips attached to a pipe. Two magnets are installed in order to produce the magnetic flux along the nickel strip direction. The motivation to use the magnets will be explained later. Figure 2 shows the schematic diagram of the experimental configuration. As indicated in Fig. 1, the magnetostrictive transducer consists of nickel strips, magnets and a solenoid coil. If stress is developed in the region of a pipe where the solenoid coil encircles, the stress causes the change in the magnetic field of the nickel strip. Then the change will be picked up by the solenoid coil as the voltage change. The solenoid coil serves both as the signal transmitter and receiver. To generate torsional waves, an electric pulse is sent through the solenoid coil. ADDRESS CORRESPONDENCE TO: Abstract Number: 487 Name: Ik Kyu Kim Phone: 82-2-880-1688 Fax: 82-2-872-5431 E-mail: [email protected] (a) (b) Fig. 1. Nickel strips attached to a pipe: (a) Nickel gratings (b) Nickel gratings under a bias magnetic field Power Amplifier for launching pulses PreAmplifier SR560 for measured signals Function Generator Agilent 33120A Oscilloscope Lecroy LT354M RAM 5000 PC L= 2m Aluminum Pipe A B Magnetostrictive transducer Nickel strips inside the coil Fig. 2. The schematic diagram of the experimental setup