Piezo-electric transducer test apparatus

Abstract: The invention concerns an acoustic loading rod for use in testing transducers which are intended for use in a liquid medium. The rod comprises an elongated body (10) capable of simulating the normal acoustic impedance presented to a transducer to be tested by the medium in which the transducer is to operate, the rod (10) having a bore (30) in its main body portion containing a core of material having a close match of acoustic impedance with the material of the rod but a higher acoustic loss factor. A piezo-electric transducer (22) is mounted between the ends of the rod.

Description: The present invention concerns the testing of piezo-electric transducers. One application involving the use of piezo-electric transducers is in generating or detecting acoustic signals under water. In order to test such transducers it is necessary to simulate the normal acoustic impedance presented to them in the medium in which they are intended to operate. This presents problems when the operating medium is a fluid such as water. The present invention is concerned with providing apparatus for testing piezo-electric transducers intended for operation in a liquid medium without the need for immersing the transducers in the liquid medium. According to the present invention there is provided a loading rod for use in testing transducers and comprising an elongated body capable of simulating the normal acoustic impedance presented to a transducer to be tested by the medium in which the transducer is to operate, the rod having a bore in its main body portion containing a core of material having a close match of acoustic impedance with the material of the rod, characterised in that the core of material has a higher acoustic loss factor than the material of the rod. US-A-4475376 discloses a loading rod for use in testing transducers and comprising an elongated body capable of simulating the normal acoustic impedance presented to a transducer to be tested by the medium in which the transducer is to operate, the rod having a bore in its main body portion containing a core of material having a close match of acoustic impedance with the material of the rod. EP-A-0252764 discloses a loading rod for use in testing transducers and comprising an elongated body capable of simulating the normal acoustic impedance presented to a transducer to be tested by the medium in which the transducer is to operate. In order that the present invention may be more readily understood various embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which Figure 1 is a side view of one embodiment of a transducer test apparatus incorporating an acoustic wave guide and according to the present invention, Figure 2 is a view of one end of a second embodiment of transducer test apparatus, Figure 3 shows how a compression wave can be monitored in an acoustic wave guide forming part of apparatus according to the present invention,, Figure 4 shows a further modification of the waveguide shown in Figure 1, and Figure 5 shows an acoustic waveguide according to the present invention and incorporating all the features shown in Figures 1, 3 and 4. Referring now to Figure 1 of the accompanying drawings this shows an acoustic waveguide in the form of a rod-like member 10 made from a rigid, low-loss material such as an acrylic polymer. One suitable material is Perspex (RTM). The purpose of the rod 10 is to present a mechanical impedance to a piezo-electric transducer which corresponds to the complex load, normally referred to as Z L , which the transducer sees when immersed in water. The load represented by the rod is equivalent to a mass in series with a radiation resistance both of which vary with frequency. In use the test transducer end 11 of the rod 10 is placed in contact with the emitting head of the transducer being tested. The contact can be improved by the use of a couplant in the form of a suitably viscous fluid to exclude air bubbles. As can be seen from Figure 1, the transducer end 11 of the waveguide rod 10 has a waisted portion indicated at 12. This waisted portion is made of a pair of opposed frusto-conical sections. Figure 2 of the drawings shows the transducer end 11 of a second embodiment of acoustic waveguide according to the present invention. The two shapes shown in Figures 1 and 2 are to increase the frequency range over which a water load can be simulated by a single acoustic waveguide. The shapes are obtained by using an iterative procedure using a computer program. The program calculates the response of given shapes and compares them graphically to a set of theoretical water load impedances. Using a combination of transmission line theory and pragmatic experience a skilled operator can modify the shapes to obtain better matches. It will be appreciated that different frequency ranges and transducer diameters will provide different requirements on the final profiles. Referring now to Figure 3 of the drawings shows the central part of an acoustic waveguide basically similar to that shown in Figure 1 or 2. However in this embodiment the main body of the rod 10 has been separated into two portions 20, 21 and a transducer 22 placed between the two portions. In its operational state the two portions of the rod would be bonded to either side of transducer 22. The transducer 22 is capable of monitoring the amplitude and phase of the signals output by a transducer under test. The transducer 22 can also be used bi-directionally to inject a signal into the rod to simulate an external source and thus test the receiving abilities of any transducer under test. The transducer 22 is made from a piezo-electric polymer. One suitable material is marketed under the name "KAYNAR" (RTM) by the PENWALT CORPORATION. Other comparable materials such as PVDF or PVF2 are equally suitable (Poly Vinyleiden Fluoride). Referring now to Figure 4 of the drawings shows a section through the main body portion of an accoustic waveguide according to the present invention. As can be seen in this figure the main body 10 of the rod-like acoustic waveguide is provided with an internal tapered bore 30 with the taper extending towards the transducer end 11 of the rod. The bore 30 is filled with a core of material which has a close match of accoustic impedance to the material of rod 10 but has a very high accoustic loss factor. In the embodiment being described the core material is a metal loaded epoxy that is only partially polymerised. A suitable material is LOCTITE METAL SET S2 (RTM) mixed in the ratio of 100% hardener to 40% adhesive. This ratio may change slightly in accordance with requirements. The manufacturers recommended mix for full polymerisation is 100% hardner to 100% adhesive + or - 10%. Alternative materials may include soft plastics mixed with some high density loading material. The body 10 provides the main acoustic load and the provision of the tapered bore 30 and its core is intended to optimise the energy absorbing performance of the acoustic load. The exact nature of the taper in the bore is not critical but at some point within rod 10 the core material 31 must appear to occupy the cross-sectional area of the body. Figure 5 of the drawings shows a single acoustic waveguide incorporating all of the features previously described. Thus the acoustic waveguide 10 has a transducer end 11 which is waisted so as to produce the required frequency response characteristics. The rod also includes a transducer 22 of the kind described with regard to Figure 3, and includes a tapered bore housing a core of very high acoustic loss factor material as in the embodiment described with regard to Figure 4. The rod can be mounted in a test rig with similar rods so that an array of transducers can be tested simultaneously. Whilst the preceding description has been directed to piezo-electric transducers, it will be appreciated that the invention can be applied to any type of transducer in which an acoustic wave is generated for transmission through a medium of interest. An example of another type of transducer is the magnetostrictive transducer.

Claims: A loading rod for use in testing transducers and comprising an elongated body (10) capable of simulating the normal acoustic impedance presented to a transducer to be tested by the medium in which the transducer is to operate, the rod (10) having a bore (30) in its main body portion containing a core of material having a close match of acoustic impedance with the material of the rod, characterised in that the core of material has a higher acoustic loss factor than the material of the rod. A loading rod as claimed in Claim 1 and further characterised in that the bore (30) is of tapered construction. The loading road as claimed in either Claim 1 or Claim 2 and further characterised in that the material of the core is a metal loaded epoxy that is only partially polymerised. A loading rod as claimed in any one of the preceding claims and further characterised in that a piezo-electric transducer (22) is mounted between the ends of the rod. A loading rod as claimed in Claim 4 and further characterised in that the transducer (22) is made from a piezo-electric polymer and the main body of the rod (10) is separated into two portions (20, 21) bonded to either side of transducer (22). A loading rod as claimed in any one of the preceding claims and further characterised in that the rod (10) has a waisted portion (12) formed by a pair of opposed frusto-conical sections.

Inventors: GENTLES ROBERT, GENTLES, ROBERT
Applicants: GEC FERRANTI DEFENCE SYST, GEC FERRANTI DEFENCE SYSTEMS LIMITED
Application year: 1989
Main classification: G01H3
IPC codes: G01H3