WO2007113503A2

WO2007113503A2 - Ultrasonic transducer/receiver

Info

Publication number: WO2007113503A2
Application number: PCT/GB2007/001161
Authority: WO
Inventors: Gordon Hayward; Walter Galbraith; Ewan Fraser Campbell
Original assignee: University Of Strathclyde
Priority date: 2006-03-31
Filing date: 2007-03-30
Publication date: 2007-10-11
Also published as: WO2007113503A3; GB0606506D0

Abstract

An ultrasonic transducer having a conduit (15) and means for causing vibrations (5) within the conduit.

Description

Ultrasonic Transducer/Receiver

Field of the Invention

The present invention relates to an improved ultrasonic transducer. In particular, the present invention relates to an ultrasonic transducer utilising resonant and non- resonant conduits.

Background to the Invention

Ultrasonic transducers are devices that generate and/or detect ultrasonic signals. A commonly used transducer/receiver is based on piezoelectric technology. Piezoelectric materials undergo a dimensional change when subjected to an electrical stimulus. In order to use the piezoelectric material as an ultrasound transmitter, a pulsed electrical stimulus of appropriate frequency is applied to the piezocrystal. This causes it to change dimensions with the frequency of the applied signal, which in turn can induce ultrasonic waves in an abutting medium. Often transducers/receivers will be tailored to specific applications using additional mechanical components such as backing and matching to extend the bandwidth. The relative mechanical stiffness of these devices makes them suitable for use with liquid and solid mediums.

Transducers/receivers may be grouped together into a phased array. In this configuration, the phases of the signal from each of the transducers in the array are matched such that the signal is reinforced in selected directions and suppressed in other directions. The overall shape of the array may also be used to control the spread of emitted radiation. However, there are some limitations in applying phased receivers to transducer/receiver systems. These include the generation of crosstalk, where the operation of one array element affects the performance of other array elements.

Summary of Invention

According to a first aspect of the present invention, there is provided a transducer/receiver having a conduit and means for causing vibrations within the conduit. The transmitter/receiver may be adapted for operation in the ultrasonic range of frequencies, preferably above 2OkHz.

The means for causing vibrations may include a membrane. Alternatively or additionally, the means for causing vibrations may include a piezoelectric device and/or a magnetic device and/or an optical device or any device capable of creating mechanical vibrations.

Optionally, the or each conduit opens into a cavity at one end.

A plurality of conduits may be provided. The means for causing vibrations may be operable to individually address each conduit.

Description of the Drawings

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:

Figure 1 is a schematic diagram of a first ultrasonic transducer; Figure 2 is a schematic diagram of a second ultrasonic transducer;

Figure 3 is a schematic diagram of a modified version of the ultrasonic transducer of Figure 1, and

Figure 4 is a schematic diagram of a third ultrasonic transducer.

Specific Description

Figure 1 shows an ultrasonic transducer/receiver that has a conducting substrate 10. Any suitable material may be used for the substrate 10, for example a semiconductor material. Defined through the substrate 10 is a conduit 15. At one end of the conduit 15 is a load medium 20 and at the other end is a thin membrane 5 that has. a conducting upper face positioned on top of the substrate 10. The membrane 5 may be shaped or patterned, for example by etching, cutting or coating in order to vary its vibrational characteristics. When in use as a transmitter, application of a DC bias voltage between the membrane 5 and the substrate 10 attracts the membrane 5 to the substrate 10. Application of an AC voltage modulates the bias voltage, causing the membrane 5 to vibrate with the frequency of the applied voltage. Vibration of the membrane 5 results in ultrasonic

5 agitation at the end of the conduit 15 giving rise to a vibrating air column within it. If correctly configured, the conduit 15 acts as a mechanical amplifier. The vibrating column of air within the conduit 15 acts on the membrane, improving the membrane displacement. The membrane continues to drive the vibrating column, resulting in an improvement in transducer performance of more than an order of magnitude greater

0 than that obtained from the membrane 5 alone.

When the apparatus is being used to receive signals, vibration in the load medium 20 causes a corresponding vibration within the conduit 15, which in turn causes, the membrane 5 to vibrate. The resulting change in capacitance may be detected in a 5 manner known in the art.

The geometry of the conduit(s) 15 may be selected in order to vary characteristics such as its harmonic frequencies and also the displacement, frequency and amplification of the sound wave produced. Li particular, conduits having a length to

O diameter ratio of at least 6:1 have been found to produce the most beneficial performance, with the increase in amplitude of the sound wave increasing with length to diameter ratio. The conduit 15 may be cylindrical or tapered and the cross section may be any shape, such as circular. In order for the conduit 15 to possess natural harmonic frequencies in the ultrasound range, it will be small, typically having a

'.5 length of less than 5mm and preferably less than 4.3mm. The vibrational characteristics of the membrane 5 may be tailored to match those of the conduit 15, and thereby enhance the sensitivity of the system.

To optimise performance, the frequency of vibration of the membrane 5 is preferably i0 selected to be equal to a harmonic frequency of the conduit 15. However, the conduit 15 can still be excited by a membrane 5 operating at a vibrational frequency different to that of the conduit 15. In particular, a membrane 5 operating at a higher frequency than the resonant frequency of conduit 15 can be used to excite the conduit 15. As an example, in one experiment, a membrane 5 operating at 20OkHz, with no detectable output below 10OkHz was used to excite output from a conduit 15 having a resonant frequency of 4kHz. Under certain circumstances, it may also be possible for a membrane 5 operating at a fundamental frequency lower than the resonant frequency of the conduit 15 to excite the conduit 15 to produce a signal. This is because a good membrane 15 under burst excitation may produce signals up to double that of the fundamental frequency. As an example, a membrane 15 operating at a frequency greater than half the resonant frequency of a conduit 15 may be used.

Figure 2 shows a variation of the transducer of Figure 1. hi this case, a cavity 25 is formed between the conduit 15 and the acoustic source/receiver 5. The dimension and shape of the cavity 25 can be tailored to match or alter the frequency effects of the acoustic source/receiver 5 and conduit 15. For the device of Figure 2, the conduit length is typically less than 9mm, and in some cases less than 8.7mm, for operation in the ultrasound range. Because of the presence of the cavity 25 in addition to a conduit 15, the device supports a different (but related) range of frequencies to a system having a conduit 15 only. Such a cavity 25 and conduit 15 system would be sensitive to even and odd harmonics.

hi order to create a larger radiating aperture, an array of resonating conduits 15 may be provided, as shown in Figure 3. Patterned electrodes may be used to independently drive sections of the membrane 5 in order to create a transducer/receiver array. The membrane may be arranged to allow each conduit 15 to be individually addressed and/or addressed simultaneously. The greater rigidity of the substrate 10 compared to that of the material within the conduits 15 means that ultrasonic vibrations within one conduit are unlikely to affect the vibrations within neighbouring conduits, thereby significantly reducing crosstalk within the array.

By leaving the membrane 5 open to the load medium 20, the device can be configured to have two radiating faces. An amplified signal will be produced via the conduits 15 and a conventional signal will be produced directly by a differing face of the acoustic source 5. This is utilised in a further embodiment of the invention as shown in Figure 4. This transducer/receiver includes a housing 30 located above the membrane 5. The housing 30 has its own resonant frequency. Additionally or alternately, chambers 35 may be formed in the housing. The chambers 35 are open at one end. The open ends of the chambers 35 are in communication with the membrane 5. The form and dimensions of the chambers 35 can be selected such that they have a resonant frequency matching that of the membrane 5 and conduits 15. Thus, resonance in the chambers 35 serves to reinforce the signal emitted via the conduits 15.

The conduit(s) 15 of the devices in which the present invention is embodied may contain a material that maximises compatibility with the load medium 20. For example, for a device designed to create ultrasonic waves in gaseous media such as air, the conduits 15 may be hollow and left open to be filled by the gas of the load medium 20. As another example, the conduits 15 may be filled with a polymeric material having qualities that make it particularly suitable for ultrasonic propagation into water or other liquids. The conduits 15 and cavities 25 may be produced by any means known in the art, such as laser boring or masking and etching.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, whilst the conduits are generally presented as being cylindrical with a circular cross-section, they may be of any suitable shape, hi addition, although in the specific embodiments a membrane is used to cause vibrations in the conduits, in alternate embodiments, a piezoelectric or a magnetic transducer/receiver or any other ultrasonic transducer/receiver may be used. In embodiments having piezoelectric or magnetic acoustic sources 5, the substrate 10 need not be conducting. Accordingly the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1. An ultrasonic transducer/receiver having a conduit and means for causing vibrations within the conduit.

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2. An ultrasonic transducer/receiver as claimed in claim 1, wherein the means for causing vibrations include a membrane.

3. An ultrasonic transducer/receiver as claimed in claim 1 or claim 2, wherein D the means for causing vibrations include a piezoelectric device and/or a magnetic device and/or an optical device.

4. An ultrasonic transducer/receiver as claimed in any of the preceding claims, wherein one end of the conduit opens into at least one cavity.

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5. An ultrasonic transducer/receiver as claimed in any of the preceding claims, wherein a plurality of conduits is provided.

6. An ultrasonic transducer/receiver as claimed in any of the preceding 3 claims wherein the or each conduit is cylindrical.

7. An ultrasonic transducer/receiver as claimed in any of claims 1 to 5 wherein the or each conduit is tapered.

> 8. An ultrasonic transducer/receiver as claimed in any of the preceding claims, wherein the conduit is defined in a substrate, for example a semiconductor substrate.

9. An ultrasonic transducer/receiver as claimed in any of the preceding ) claims, wherein the conduit has a length to diameter ratio of at least 6:1.

10. An ultrasonic transducer/receiver as claimed in any of the preceding claims, wherein the length of the conduit is less than 9mm.

11. An ultrasonic transducer/receiver as claimed in any of the preceding claims, wherein the means for causing vibrations operate at a frequency substantially corresponding to a harmonic frequency of the or each conduit(s).