A loading probe for use in testing transducers

Abstract: A loading probe (1) 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 main body of the loading probe (1) comprising a tube (4) made from a rigid material, a rod (5) having an elongated convergent portion (6) extending into the interior of the tube (4) so as to define a cavity which has a varying cross section, the cavity housing a core (7) of material having a close match of acoustic impedance with the material of the tube (4) and rod (5) but a higher acoustic loss factor, and means (8) for sealing the cavity. The active end (2) of the loading probe (1) carries a piezoelectric element (10) having a planar form, and a protective end cap (13) is bonded to the active end (2) of the loading probe (1) to enclose the piezoelectric element (10).

Description: The present invention concerns the testing of transducers such as piezo electric transducers. 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. One application involving the use of transducers is in generating or detecting acoustic signals under water. In British Patent Specification No.2217952B (EPA-339 823) there is described a loading probe or rod for use in testing transducers and which comprises 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 but a higher acoustic loss factor. In this prior specification a preferred embodiment of the rod is manufactured from an acrylic polymer and has a tapered bore in its main body portion. However, this arrangement has proved difficult and expensive to manufacture and the present invention is concerned with meeting this problem. British Patent Specification No. 2192514 (EPA-252764) discloses a measurement probe for carrying out the same type of measurement operation and in which a piezo electric transducer is mounted in the main body of the probe after the impedance matching network. The present application is also concerned with providing a measurement probe or loading rod which can give more accurate results than that disclosed in the aforesaid prior specification. According to the present invention there is provided a loading probe 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, characterised in that the main body of the loading probe comprises a tube made from a rigid material, a rod having an elongated convergent portion extending into the interior of the tube so as to define a cavity which has a varying cross section, the cavity housing a core of material having a close match of acoustic impedance with the material of the tube and rod but a higher acoustic loss factor, and means for sealing the cavity. In order that the present invention may be more readily understood an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a side elevation of an acoustic loading probe, Figure 2 is a section through a main body portion of the probe of Figure 1, Figure 3 is a similar view showing the assembled probe, Figure 4 is a partially exploded view showing how a transducer can be mounted at the active end of the probe, and Figure 5 shows the assembled transducer end. Referring to the drawings, Figure 1 is an outer view of a probe 1 which carries a piezo electric transducer at its active end or foot 2. The purpose of the probe 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 1 is equivalent to a mass in series with a radiation resistance both of which vary with frequency. In use the test transducer or active end 2 of the rod 1 is placed in contact with the emitting head of a transducer being tested. The contact can be improved by the use of a couplant in the form of a suitably viscose fluid to exclude air bubbles. As can be seen from Figure 1, the transducer end 2 of the probe 1 has a waisted portion indicated at 3. This waisted portion is made of a pair of opposed frusto conical sections. The actual shape of the probe is obtained by using an iterative procedure using a computer program which 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 experience a skilled operator can modify the shapes to obtain appropriate matches. Figure 2 of the drawings shows the main part of the probe of Figure 1 prior to final assembly. Thus this main body part comprises a hollow tube 4 made from a rigid, low-loss material such as an acrylic polymer. One suitable material is perspex. Into this tube 4 is placed a plastic rod 5 of a similar material which has been machined to produce a spiked shape 6 at one end which fits neatly into the tube 4. The spiked rod 5 is bonded to the tube 4 and the cavity inside the tube 4 then filled with an acoustically absorbent material 7. This material can be a partially polymerised metal loaded epoxy resin. The material has a close match of acoustic impedance to the material of tube 4 and rod 5 but has a very high acoustic loss factor. A suitable material is LOCTITE METALSET S2 mixed in the ratio of 100% hardner to 40% adhesive. This ratio may change slightly in accordance with requirements. The manufacturers recommended mix for full polymerisation is 100% hardener to 100% adhesive plus or minus 10%. Alternative materials may include soft plastics mixed with some high density loading material. The unit is completed by sealing the end of the probe 1 opposite its active end 2 with an end cap 8. The final unit is shown in Figure 3. In the probe described in prior Patent No.2192514 a piezo electric transducer is mounted in the main body of the probe after the impedance matching network. However in the present embodiment more accurate results are obtained by mounting a piezo electric element 10 on the face of the probe foot 2. The wires 11 connecting the piezo electric element 10 are routed through a hole 12 in the probe foot 2 and a protective end cap 13 is then bonded to the probe foot 2 enclosing the piezo electric element 10. Figures 4 and 5 show the steps in this procedure.

Claims: A loading probe (1) 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, characterised in that the main body of the loading probe (1) comprises a tube (4) made from a rigid material, a rod (5) having an elongated convergent portion (6) extending into the interior of the tube (4) so as to define a cavity which has a varying cross section, the cavity housing a core (7) of material having a close match of acoustic impedance with material of the tube (4) and rod (5) but a higher acoustic loss factor, and means (8) for sealing the cavity. A probe (1) according to Claim 1 wherein the elongated portion (6) of the rod (5) extending within the tube (4) is in the form of a pointed spike (6). A probe (1) according to Claim 1 or Claim 2 wherein the sealing means (8) comprises a cap (8) having a boss which is a fit within the internal cavity of the tube (4). A probe (1) according to Claim 1 or Claim 2 or Claim 3 wherein the active end (2) of the probe (1) carries a piezo electric element (10) having a planar form, and a protective end cap (13) is bonded to the active end (2) of the loading probe (1) to enclose the piezo electric element (10). A probe (1) according to Claim 4 wherein the active end (2) of the probe (1) has a bore (12) through which connecting wires (11) for the piezo electric element (10) pass.

Inventors: GENTLES ROBERT, GENTLES, ROBERT
Applicants: GEC MARCONI AVIONICS HOLDINGS, GEC-MARCONI AVIONICS (HOLDINGS) LIMITED
Application year: 1992
Main classification: G01H1
IPC codes: G01H3