GB2147104A - Measuring output of ultrasonic transducer - Google Patents

Abstract

The invention provides a device for measuring energy adsorption, particularly ultrasound energy, in a body expansible in response to an input of energy; said device comprising a rigid body 1 provided with a dielectric 2. A transducer 10 is immersed in a coupling liquid 5 which is separated from the liquid 2 by means of a window 4. The dielectric 2 expands in response to energy imparted thereto by the transducer 10 and accordingly the dielectric 2 is forced between the plates 7 so that an electrical signal, which may then be analyzed in the signal analyzer is provided by sensing changes in the capacitance of the plates 7. <IMAGE>

Description

SPECIFICATION Dosemeter The use of ultrasound is now widespread, and extends to many fields (e.g. medicine, nondestructive-testing etc.). For almost all applications, the ultrasound is produced in the form of a propagating beam of longitudinal pressure waves emanating from an acoustic transducer.

Determination of the ultrasonic power output delivered by such a transducer is, however, not straightforward, particularly for non-plane (e.g. focussed) transducers. Due to the unwieldly nature of the equipment required and difficulties inherent to the procedures utilised, it is usually not feasible to perform the measurement at the actual point of use. Instead, the transducer must be despatched to a suitable acoustics laboratory. This not only precludes routine monitoring, but also discourages periodic checks of this important parameter.

Unlike previously described devices which utilised the expansion of an oil (e.g. Mikhailov IG, Ultrasonics, July/September 1964, pp 129-133), the present invention does not require visual observation of the extent of the expansion followed by manual correlation of this measurement to determine the ultrasonic power output from the transducer. Instead, the volume of the absorbing medium is monitored by means of a suitable electrical system, and following initial calibration of the system the transducer power may be derived by automatic analysis of these signals.

These present invention seeks to overcome these problems by providing a compact and portable (yet rugged) means of rapidly measuring the output acoustic power of ultrasound transducers by utilising the thermal expansion of a fluid as it absorbs ultrasound energy. A major advantage over previous instruments is that the new dosemeter to be described below is not restricted to plane transducers, but may also be used for other types, e.g. focussed.

According, therefore, to one aspect of the present invention, there is provided a method of measuring energy adsorption of a body which expands in response to an input of energy, which comprises causing said body to displace a liquid and measuring the liquid displacement so caused, converting said measurement to an electric signal, and analysing said signal.

In a second aspect of the invention, there is provided an energy adsorption measuring device comprising a body expansible in response to an input of energy, a liquid displaceably responsive to said expansion, and means for providing an output signal in response to movements of said liquid.

The energy is preferably ultra sound energy, and the liquid may be, at least in part, a dielectric. The body may be a liquid which is a bubble free dielectric contained in a rigid container.

A measurement of the liquid displacement may be effected by a pair of dielectric plates having their ends disposed in the dielectric liquid. The dielectric liquid, may constitute all or only part of the body. Where dielectric liquid is separated from the body. This must be done by means of a flexible diaphragm The rigid container may comprise one or more deflector plates.

The invention will now be described, by way of illustration only, with reference to the accompanying drawings, wherein; Figure 1 shows a vertical cross-section through a dosemeter of the invention, and Figure 2 shows a vertical cross-section through a second embodiment.

With particular reference to Fig. 1, the equipment consists of a rigid container 1 completely filled (to the exclusion of any air bubbles) with the absorbing material 2. To facilitate filling and emptying of the container and the removal of air bubbles, a hole 3 is located in one wall. This hole is tapped so that it may be blocked by a screw during normal use of the dosemeter.

The ultrasound is transmitted from the transducer 10 to this aborbing material 2 by means of an input window 4 and an acoustic coupling medium 5 (degassed water, for example) within which the sound emitting surface of the transducer is completely immersed. If desired, the propagation direction of the ultrasound beam within the absorber may be altered by an obliquely angled portion of the container wall or some other reflector located along the beam's path, such as that shown at 6 in Fig. 1.

The change in volume of the absorbing medium is automatically detected and measured by means of an electrical bridge circuit and associated components. Thus, in Fig. 1, expansion of the absorber causes the liquid level between metal plates 7 to rise, which results in a change in the electrical signal output from the bridge circuit 8. From this, the signal analyzer 9 (e.g.

microprocessor) is able to determine the ultrasonic power output of the transducer.

In Fig. 2, a rigid frusto conical container 11 is positioned generally vertically with its smaller diameter downwardly. The container 11 is completely filled with a bubble-free absorbing material 1 2. Ultrasound is transmitted from a transducer to the material 1 2 via an input window 1 4 and an acoustic coupling medium 15, in which the sound emitting surface of the transducer is immersed.

A measuring unit 21 is operatively connected to the rigid body 11, via a connector piece 22.

The unit 21 is a hollow body containing an element 22 which has a generally Y-shaped configuration. The upper arms of the Y shape terminate in a pair of concentric vertically aligned concentric cylinders in spaced relation, the space provided thereby enabling liquid 1 2 to move therebetween. The levels of the liquid 1 2 relative to the concentric cylinders 1 6 can be adjusted by rotating screw head 20 and thereby causing plunger 1 7 to reciprocate, thus changing the volume of the liquid 1 2.

The components of the dosemeters will now be discussed in greater detail.

As large a fraction as possible of the ultrasound produced by the transducer should pass into the absorber. The coupling medium should therefore have a low ultasound attenuation coefficient, and the path length of the ultrasonic beam within this medium should be short. Care should be taken to ensure that, following immersion of the transducer face in the coupling medium, no gas bubbles lie in the path of the ultrasound beam. The thickness of the input window and the acoustic impedances of the coupling and absorbing media should be such that maximal transmission of ultrasound is obtained. This implies that the window should be thin.

preferably with Pw < Aw/4 where p = thickness of the input window; A,,, = wavelength (in the window) of the highest frequency sound with which the dosemeter is to be used.

If this condition exists, then

where Ita = intensity of ultrasound transmitted into the absorbing medium; = = intensity of ultrasound incident on the window from the coupling medium; Za = acoustic impedance of the absorbing medium; Zc = acoustic impedance of the coupling medium, and the fraction of incident sound transmitted is independent of the acoustic impedance of the window material.Furthermore if Z = Z then Ita/l,c = 1 (i.e. complete transmission), so to further increase the transmitted fraction it is desirable that Za and Zc should be approximately equal, as is the case for example with water as coupling medium and castor oil as absorber: Z = Zwater = 1.5 x 10-6 kg m -2 - l; Za = Zcaste,o = 1.4 X 10-6 kg m-2s-l Despite the requirement that the input window be thin, it should nevertheless undergo negligible bending as the volume of the absorbing medium changes. If, as with the vertical metal plate arrangement of Fig. 1, volume changes of the absorber result in the rise or fall of a liquid level then the hydrostatic pressure on the input window will change.Because of this, the input window should satisfy either or both of the following conditions.

(i) be sufficiently rigid to withstand these pressure changes with negligible bending. (The considerations regarding window thickness and material outlined above need to be borne in mind, however. One possibility is a thin, tightly stretched foil of metal or other suitable material).

(ii) be situated at a height relative to the changing liquid level such that the percentage change in hydrostatic pressure at the window due to variation of absorber volume is negligible.

One such arragement is shown in Fig. 2; the vertical distance over which the liquid level relative to the metal plates may rise or fall, h, (ie the length is small compared to the relative height of the input window). If this is the case, the window need not be inherently rigid but may be, for example, a thin, flexible plastic membrane, since it will be held under tension by the hydrostatic pressure difference.

In order to achieve a large change in dielectric volume between the metal plates for only a small change in dielectric level, the plates may be in the form of vertical concentric cylinders, as shown in Fig. 2.

In principle, it would be possible, to avoid the need for a coupling medium and an input window combination by inserting the transducer head directly into the absorber through a suitably sized aperture. However, due to difficulties in sealing such a system (eg preventing leaks between the transducer head and container wall) and the requirement that all air bubbles are excluded from the absorbing medium, this has been found to be impractical.

Any transducer head which would be damaged by immersion either directly in the absorbing medium, or a coupling medium as previously described, may be protected by covering it with a layer of ultrasonically transparent (but leak-proof) material, such as a thin sheet of plastic. If used, any such protective membrane should be in good acoustic contact with the ultrasoundemitting face of the transducer, with no intervening air gaps. Where a coupling medium is used, the absorbing medium should be free of dissolved gases to avoid cavitation.

After the ultrasound enters the absorbing medium, any surfaces within the container upon which part or all of the beam may impinge should be of high reflection efficiency (for example a thin, air backed layer of metal), but again should be rigid as should the container walls. Ideally, the total number of reflections experienced by the ultrasound beam should be small, since it is inevitable that some energy will be lost to the container at each reflection.

The dimensions of the dosemeter depend on the size and excitation frequency of the transducers whose output is to be measured and should be such that the path length of the beam in the absorber is sufficient for all but a negligible fraction of the ultrasound energy to be adsorbed. In order that this may be achieved within a container of convenient size the ultrasound attenuation coefficient of the absorbing material should be large. The thermal expansion coefficient should also be large, so that the change in volume with the absorption of ultrasound energy will be significant, and should be close to linear over the temperature range within which the dosemeter is to be operated.

The shape of container for the absorbing material is not critical provided that, if a flexible plastic membrane is used as input window, it is situated at a sufficiently different height to the liquid level between the metal plate (as explained above). Figs. 1 and 2 illustrate two possible container shapes. The version shown in Fig. 1 consists of a rectangular box, within which there is a reflector to divert the acoustic energy into the main body of absorbing medium. That shown in Fig. 2 is in the form of a truncated circular cone, so that the volume of absorber is kept to a minimum whilst maintaining the total available path length for the ultrasound to be absorbed in.

(The walls of the cone should be thin, efficient reflectors, and the sides should be angled such that the angle of incidence of the ultrasound beam is greater than the critical angle). Other container shapes may also be envisaged.

The measurement system shown in Figs. 1 and 2 consists of a capacitance-sensitive electrical circuit whose output varies with the amount of dielectric between two metal plates. As the volume of the absorbing medium changes, the level of the dielectric between these plates also changes, thus altering the capacitance and affecting the output signal from the circuit. The magnitude of this signal may therefore be used as a measure of the volume of the absorber.Any suitable capacitance sensitive circuit may be used: for example, an electrical oscillator circuit in which the frequency of operation depends on the capacitance; or an a.c. bridge circuit in which the metal plates form one arm of the bridge (thus any change in capacitance between the plates would affect the balance of the bridge and alter the out-of-balance voltage); or a circuit containing an oscillator of fixed frequency, and a frequency to voltage conversion unit (eg an fto-V integrated circuit with the metal plates as one of the associated components, so that the output voltage varies with the capacitance between the plates.

If the absorbing medium is a suitable fluid then this may be used directly as the dielectric, by submerging the ends of the metal plates in the absorber and constructing the container such that, with the transducer in plate, the only opening is the space between the plates. As the absorber expands, it must therefore rise between the plates. Alternatively, the dielectric could be a different material to the absorber. In this case, the ends of the metal plates would be submerged in a small reservoir of the dielectric, separated from the absorbing medium by a flexible membrane (such as a thin sheet of plastic). The absorbing medium would then be totally enclosed, but able to expand against the membrane and thus cause dielectric to rise from the reservoir and pass up between the plates.

Techniques other than the capacitative system described above can be used to produce an electrical signal related to the absorber volume, such as one or more strain gauges attached to a flexible portion of the container wall, or a pressure transducer submerged in the absorber within a totally enclosed, rigid container. A piezoelectric material can also be used; for example by making one part of the container wall of piezoelectric film.

Whatever volume monitoring system is used (e.g. metal plates with measurement of capacitance; or strain gauges; or pressure transducer; or piezoelectric materials; etc), the electrical signal is amplified (if necessary) and analysed to determine the ultrasonic power output produced by the transducer under investigation. The signal analyser may be, for example, a suitably programmed microprocessor or microcomputer (the analogue signal from the measuring unit first being digitized by an analogue-to-digital converter), and in the present invention is intended to operate by determining the rate of change of the signal over a short period of time.

This rate of change may then be used to identify the ultrasound from the transducer by previously calibrating the unit using known power levels (supplied by calibrated ultrasonic transducers, or from an electrical heater situated within the absorbing medium).

Such a system would not be restricted solely to the determination of ultrasound power output from a transducer, but could clearly be used as a measuring device in other applications involving a change in shape or volume of an enclosed material. For example, by replacing the degassed absorbing medium required in the dosemeter with a liquid containing dissolved gas the device could be used for the measurement of cavitation. A further use which may be envisaged is in the precise measurement of pressure, or rates of change thereof. This requires some portion of an otherwise rigid container to be coupled to the pressure changes and capable of movement in such a way that these changes are transmitted to the (incompressible) fluid within the container.

The ultrasound dosemeter can also be made to provide an indication of the ultrasonic intensity distribution or of the temperature distribution within the absorbing medium, by constructing a suitable array of temperature and/or acoustic pressure or intensity sensors within the container.

Several sensors (thermocouples, for examples) are positioned axially, laterally or in any other one, two or three dimensional arrangement. (Clearly the sensors should not absorb sufficient acoustic energy to significantly affect operation as a dosemeter). The output from the sensors is amplified and passed to a computer (with multiplexing, if necessary) for analysis of the signals and production of a plot based on this data. Linear or two dimensional plots of variations in temperature or ultrasonic intensity distribution within the absorber can thus be obtained. (The data for the two dimensional plots may be derived either from a two dimensional array of discrete thermocouples or from a grid composed of parallel wires of one type of metal in one direction and parallel wires of a different metal in the orthogonal direction. The wires are in contact at each crossing point, thus creating a two dimensional thermocouple grid.

Claims (14)

1. A method of measuring energy adsorption of a body which expands in response to an input of energy, which comprises causing said body to dispiace liquid and measuring the liquid displacement so caused, converting said measurement into an electrical signal, and analysing said signal.

2. A method according to Claim 1 wherein the energy is ultrasound energy and the liquid, at least in part, is a dielectric.

3. A method according to Claim 2, wherein the body is a bubble-free liquid dielectric contained in a rigid container.

4. A method according to Claim 3 wherein the measurement of the liquid displacement is effected either by a pair of dielectric plates having their ends disposed in a dielectric liquid or by a pair of concentric cylinders similarly vertically arranged.

5. A method according to Claims 1 to 3, wherein the dielectric liquid is separated from the body by means of a flexible diaphragm.

6. A method according to any of Claims 3 to 5 wherein the rigid container comprises a deflector plate to change the direction of the ultrasound energy.

7. A method according to Claim 1, wherein the rigid body is provided with a spaced array of energy sensing elements within the liquid, said elements each providing an output which is analysed to provide a plot of the energy imparted to each part of the rigid container.

8. An energy adsorption measuring device comprising a body expansible in response to an input of energy, a liquid displaceably responsive to said expansion, and means for providing an output signal in response to movements of said liquid.

9. A device according to Claim 8 wherein the energy is ultrasound energy and the liquid, at least in part, is a dielectric.

10. A device according to Claim 9 wherein the body is a bubble-free liquid dielectric contained in a rigid container.

11. A device according to Claim 10, wherein the measurement of liquid displacement is effected either by a pair of dielectric plates having their ends disposed in the dielectric liquid, or by a pair of concentric dielectric cylinders spaced apart, and similarly arranged.

1 2. A device according to Claims 8 to 10 wherein the dielectric liquid is separated from the body by means of a flexible diaphragm.

1 3. A device according to any of claims 10 to 12 wherein the rigid container comprises a deflector plate to change the direction of ultrasound energy.

14. A device according to Claim 8 wherein the rigid body is provided with a spaced array of energy sensing elements within the liquid, said elements each providing an output which is analysed to provide a plot of the energy imparted within the rigid container.

1 5. An energy adsorption measuring device substantially as hereinbefore set forth with reference to and as illustrated in Figs. 1 or 2 of the accompanying drawings.

1 6. A method of measuring energy adsorption of a body which expands substantially as hereinbefore set forth with reference to, and as illustrated in either of Figs. 1 and 2 of the accompanying drawings.