GB2330724A

GB2330724A - Method of manufacturing an electro-acoustic transducer

Info

Publication number: GB2330724A
Application number: GB9722466A
Authority: GB
Inventors: Ronald Gregory Hare; John William Thomas
Original assignee: Federal Industries Industrial Group Inc; Milltronics Ltd
Current assignee: Siemens Canada Ltd; Federal Industries Industrial Group Inc
Priority date: 1997-10-24
Filing date: 1997-10-24
Publication date: 1999-04-28
Anticipated expiration: 2017-10-24
Also published as: GB2330724B; GB9722466D0

Abstract

An electro-acoustic transducer is manufactured by taking an transducer assembly comprising at least one piezoelectric element 2 secured by means of a screw 8 between a steel driver block 4 and a steel loading block 6 wherein electrodes 16, 18 provide electrical connection to the assembly. Successive superposed layers of material having different acoustic properties are applied around the transducer to provide a moulded transducer with at least one layer of rubber 34 covering the radiating surface of the transducer assembly to form an acoustic matching layer. A casing 38 surrounds the remainder of the transducer with at least one layer of acoustic isolating material 36 being disposed between the casing and transducer assembly.

Description

Method of Manufacturing an Electro-Acoustic Transducer This invention relates to electro-acoustic transducers used for example in acoustic pulse-echo ranging system.

Such transducers are typically piezoelectric in operation, with one or more piezoelectric elements and associated contact electrodes clamped between loading blocks to provide a relatively high-Q assembly that will oscillate at a predetermined frequency when excited by an alternating electric potential at that frequency so as to transmit acoustic energy. The same transducer is commonly used to receive reflected acoustic energy at that frequency and convert it back to electrical energy.

Since the potential required to excite such transducers are typically quite high, and they are high impedance devices, and may need to be located some distance from a transceiver which generates the excitation signal and processes received signals, they are usually associated with an impedance matching transformer and possibly also with temperature sensing components and preamplifying or preprocessing circuits for received signals. Furthermore, it is usually necessary to provide acoustic matching between the transducer assembly and d a surrounding medium, usually gaseous, to provide the transducer assembly with suitable directional properties, to protect the transducer assembly from the surrounding medium, and to isolate the transducer assembly as far as possible from the structure on which it is mounted.

The above requirements must be accommodated by the enclosures applied to house such transducers. Typically, the transducer assembly is wrapped on its non-radiating surfaces with a material such as cork, and paced in a moulded or fabricated metallic or moulded plastic shell which may be selectively lined with material such as cork and is then filled with a potting compound so as to embed the transducer assembly. The shell may be open on a side corresponding to the radiating surface of the transducer, in which case a layer or layers of acoustic matching material may be cast into the shell so as to cover the radiating surface of the assembly, or it may be closed by a thin diaphragm at that surface, in which case matching material must be installed within the diaphragm prior to inserting and potting the transducer. In either case, the assembly process is slow, laborious, and labour intensive, must be carefully controlled so that components are properly located within the shell. The installation of transformers and other electronic components further complicates the process.

Typical transducers are described in U.S. Patent No.

3, 674, 945 (Hands) and 5,339,292 (Brown et al).

It is an object of the present invention to provide an improved technique for packaging acoustic transducers which simplifies and improves control over the assembly process.

According to the invention, a method of manufacturing an electro-acoustic transducer comprises forming a transducer assembly comprising at least one piezoelectric element, electrodes contacting the element, and acoustic loading blocks, and moulding successive layers of material of differing acoustic properties around said transducer assembly to provide a moulded transducer, while maintaining electrical coupling between the transducer assembly and the exterior of the transducer, the moulded layers providing at least one acoustic matching layer covering a radiating surface of the transducer assembly, a casing surrounding the remainder of the transducer assembly, and at least one layer of acoustic isolating material between the casing and the transducer assembly.

The invention is described further with reference to the accompanying drawing, in which Figure 1 is a cross-section through an exemplary embodiment of transducer manufactured in accordance with the invention.

The manufacture of a transducer in accordance with the invention will be described with reference to Figure 1.

A first stage in manufacture is the assembly of a transducer assembly comprising a ceramic piezo-electric transducer element 2 secured between a steel driver block 4 and steel loading block 6 by a machine screw 8 engaging a threaded socket 10 in the block 4 and bearing on the block 6 through an electrically insulating washer assembly 12. The screw is further isolated from the block 6 by an insulating sleeve 14 surrounding the stem of the screw where it passes through the block 6. The element 2 and the masses of the blocks 4 and 6 are selected so that the assembly will have a desired resonant frequency typically in the low ultrasonic range.

In order to provide electrical connection to the assembly, rigid wires 16 and 18 are spot welded respectively to the block 4 and the head of the screw 8 so as to project upwardly from the top (as shown in the drawing) of the transducer assembly.

In the next stage, the assembly is placed in a mould and a damping layer of relatively soft polymer is moulded around the entire assembly except for the bottom surface the block 4 which is the radiating surface of the transducer assembly.

The layer 20 has an external configuration which is generally cylindrical except for a cylindrical axial boss 22 at its top end through which the wires 16 an 18 protrude. The layer 20 may be formed for example of flame retardant nit rile rubber of 25 durometer hardness.

Following this stage, a toroidal matching transformer 24 is located coaxially on the boss 22, and a small printed circuit board 26 is secured above the transformer by soldering the wires 16 and 18 to it. Connections from the primary and secondary of the transformer are also soldered to the board, to which the conductors of a lead-in cable 28 are also secured. The board also carries such components as a tuning capacitor 30, a temperature sensing thermistor and other small components associated with matching the transducer to the line through the transformer, as well as establishing connections between the various components. The configuration of the transformer and associated components may be in accordance with known practice, for example as disclosed in U.S. Patent No. 5,347,495 (Cherek et al), and forms no part of the present invention beyond the necessity to accommodate components thereof in the transducer. This stage completes electrical assembly of the transducer.

In a following stage, further layers 32, 34 of harder rubber are moulded on to surround the entire assembly, except for the distal portion of cable 28. In the example shown, these layers are of 70 durometer hardness flame retardant nitrile rubber. The layer 34 covers the bottom face of the block 4 to form an acoustic matching layer as described further in the Hands and Brown et al patents referenced above. If the properties required in the matching layer 34 are different from those required in the layer 32, then they may be moulded separately from different materials. Such separately moulded layers are in any case preferred to facilitate positioning of the assembly in the mould. If the transducer is to be utilized in a particularly aggressive atmosphere, it is possible to cast in a very thin diaphragm (not shown) of stainless steel or other resistant material beneath the layer 34 to protect the radiating face of the latter.

Next, a further damping layer 36, typically of the same material as the layer 20, is applied around the layer 32.

According to the design of the transducer, the layer 36 may not be required in all cases.

Finally an external casing 38 in form of an outer generally cylindrical shell of rigid synthetic resin such as polypropylene or polyvinylidene fluoride is moulded over the entire assembly and around a strain relief portion 40 of the layer 32 around the cable 28. The casing 38 may be moulded with a mounting thread 42. Instead of the layer 38 being moulded in situ over the layer 36, it may be preformed in a separate operation, and the assembly with layer 36 applied pressed into it.

Particularly if the layers 32 and 34 are moulded separately, none of the layers of material moulded onto the transducer assembly completely envelopes the assembly produced by the preceding stage, thus facilitating rigid positioning of the assembly within each mould used defining a mould cavity into which the material of a layer is injected.

It will be noted that successive superposed layers of moulded material between the cylindrical wall of the casing 38 have in general alternating degrees of hardness, thus contributing to isolating the active components of transducer acoustically from its outer casing.

Claims

CLAIMS: 1. A method of manufacturing an electro-acoustic transducer

comprising forming a transducer assemblylat least one piezoelectric element, electrodes contacting the element, and acoustic loading blocks, and applying successive superposed layers of material of differing acoustic properties around said transducer assembly to provide a moulded transducer, while maintaining electrical coupling between the transducer assembly and the exterior of the transducer, the moulded layers providing at least one acoustic matching layer covering a radiating surface of the transducer assembly, a casing surrounding the remainder of the transducer assembly, and at least one layer of acoustic isolating material between the casing and the transducer assembly.
2. A method according to claim 1, wherein successive layers of material are moulded onto the assembly such that they do not completely envelope the assembly thus allowing for accurate support of the assembly in moulds used to form those layers.
3. A method according to claim 2, wherein superposed layers of material moulded around the transducer assembly within a cylindrical wall of the casing have alternating degrees of hardness.
4. A method according to claim 2 or 3, wherein the layer forming the casing is separately moulded, and pressed onto the assembly after the moulding of the successive layers of material thereon.
5. A method of manufacturing an electro-acoustic transducer, substantially as hereinbefore described with reference to the accompanying drawing.
6. An electro-acoustic transducer manufactured by the method of any one of the preceding claims.
7. An electro-acoustic transducer comprising a transducer assembly comprising at least one piezoelectric element, electrodes contacting the element, and acoustic loading blocks, and successive superposed layers of material of differing acoustic properties moulded around said transducer assembly to provide a moulded transducer, electrical coupling being maintained between the transducer assembly and the exterior of the transducer, the moulded layers providing at least one acoustic matching layer covering a radiating surface of the transducer assembly, a casing surrounding the remainder of the transducer assembly, and at least one layer of acoustic isolating material between the casing and the transducer assembly.