GB2114295A - Improvements in or relating to ultrasonic transducer arrays - Google Patents

Abstract

An ultrasonic transducer array comprises a plurality of transducer elements in a substantially linear phased array formation (2) and includes means, such as a prism (1) of different acoustic refractive index to that of the surrounding medium, attached to the front of the array (2), whereby a non-linearity may be introduced into the path of ultrasonic energy to or from the array, so as to minimise a non-linear variation of angular resolution with angle associated with the array. The means may alternatively provide a uniform interchannel delay across the array, consisting of a plurality of channels each containing two materials of different acoustic refractive index in varying proportions. "Waveguides" may also be provided to reduce interchannel coupling. <IMAGE>

Description

SPECIFICATION Improvements in or relating to ultrasonic transducer arrays This invention relates to improvements in ultrasonic transducer arrays and it relates especially, though not exclusively, to such arrays as described in our copending application 8202525 of even date herewith and identified by our reference PQ 11505, in which a plurality of transducer elements disposed in a substantially linear phased array formation are coupled by electrical interconnections so as to substantially suppress the main grating lobe which would otherwise be associated with the array.Also described therein are means, consisting of a plane reflector, whereby one half of the linear array of transducer elements may be reflected into the other half, thus substantially removing the main grating lobe associated with the first half of the grating lobe pattern and channelling energy which would otherwise be associated with the main lobe into the second half of the pattern.

Application of ultrasonic transducer arrays to imaging systems is restricted especially where the first order lobe is used for scanning, by an extreme non-linear variation of angular resolution with angle, which is associated with the array.

These non-linearities are derived from two sources; the first arising from the reduction in effective array aperture at scan angles away from the normal to the array which is a substantially unavoidable geometrical effect, and the second occurring because resolution is proportional to frequency and different angles are insonified at different frequencies.

The resulting rapid cotangent variation, produced by the combination of the aperture effect and the frequency dependent effect of the array, requires the attainment of very wide bandwidth and element directivities, and the resulting field will be highly non-linear.

It is known that a uniform interchannel delay may be inserted across the array, thus permitting grating lobes generated by the array to be shifted. If the delay is sufficient to move the first order grating lobes, for the spectral band under consideration, into the opposite half of the array, the non-linearities will substantially cancel. Both the rate of falloff of non-linearity and the amount of shift of the lobes are under the control of the system designer, which permits a substantially improved optimisation of resolution characteristic over the desired field of view for a given bandwidth and directivity.

Formerly electronic implementations have been employed for achieving interchannel delays, but these suffer from the need to access every transducer element individually, to provide bidirectional delays for transmit/receive operation, and to provide a power source, as well as having limited dynamic range.

An object of the present invention is to provide alternative means of reducing the ex treme non-linear variation of angular resolu tion with angle, which is associated with the array, and thereby increasing the range of useful scan angles. Another object of the invention is to provide a substantially im proved manner of achieving interchannel de lays to reduce the non-linearities.

Such improvements are primarily intended to make transducer arrays of this kind more acceptable for most applications, but particu larly where wide and uniform fields of view are required.

One example, in which the improved trans ducer arrays may be of particular use, is target discrimination, wherein, due to the fre quency dependence on angle, the Doppler effect may be employed.

According to the invention, there is pro vided an ultrasonic transducer array compris ing a plurality of transducer elements dis posed in a substantially linear phased array 'formation and including means, associated with said array, whereby a non-linearity may be introduced into the path of ultrasonic en ergy to or from said array, so as to substan tially minimise a non-linear variation of angu lar resolution associated with said array.

The invention will be further described by way of example with reference to the accom panying drawings,-wherein: Figure 1 shows a transducer array incorpo rating one embodiment of the invention, Figure 2 shows a transducer array incorpo rating another embodiment of the invention relating to interchannel delays, Figure 3 shows an arrangement of trans ducer arrays, incorporating the present inven tion, which may be used for target discrimina tion and, Figure 4 shows another arrangement of transducer arrays, incorporating an embodi ment of the invention, which may be used for extreme range underwater surveillance.

Referring now to Fig. 1, a prism 1 of different acoustic refractive index to that of the surrounding medium is attached onto the front of an array 2 of transducer elements, thus providing the desired non-linear angular magnification, which is least when the beam 3 is normal to a prism refracting face 4 and consequently increases the useful scan angle width considerably.

A reflecting surface, which is a feature of the invention described in our aforementioned copending application, is now realised in the present invention as a result of total internal reflection with phase reversal at a side face 5 of the prism, thus producing a virtual image 6 of the array 2. Curvature of the refracting face 4 may be used to create a focussed beam.

The acoustic prism 1 would be conventionally made of plastics material, such as polyure thane, perspex (Registered Trade Mark) or nylon.

For total internal reflection to occur, the prism is required to have a higher refractive index than the surrounding medium, or, alternatively, a plane reflector, such as a metal mirror, may be used by placing it on the side face of the prism.

Another embodiment of the present invention, which is for achieving interchannel delays in a substantially improved manner to that of electronic implementations, is shown in Fig. 2 and will now be described.

Referring to Fig. 2, two materials of different acoustic refractive index are used in varying proportions for each channel 9 attached onto the front of the array 10, one of the materials having a medium 7 of refractive index n1, and the other of the materials having a medium 8 of refractive index n2.

"Waveguides" 11 are required to reduce interchannel coupling. The effective directivity is then defined by "waveguide" apertures 1 2. A step size 1 3 in the channel 9, defined by the difference in the relative positions of the interface between the two materials, can be made non-linear across the array 10 to provide focussing. The delay depends on the step size 1 3 and on the difference in refractive index, n2 - n1. One of the materials may be an imaging medium.

In a preferred embodiment, medium 7 is sea water and medium 8 is either polyurethane or rubber, which are both low-loss, compliant and produce a large velocity difference with the sea water. The two media are separated by metal or by an air gap.

An advantage of this "waveguide" system of achieving interchannel delays is that the transducer elements of the array need no longer be omnidirectional, thereby permitting monolithic construction techniques.

In Fig. 3 a transmitting transducer array AT and a receiving transducer array AR are shown, the receiving array AR being inclined at an angle f with respect to the plane of the transmitting array AT. The non-linear variations of angular resolution associated with both arrays, AT and AR, have been substantially minimised by incorporating the present invention (not shown in Fig. 3), so that an arrangement of this kind may be used for target discrimination by employing the Doppler effect.

The frequency FT of a signal transmitted in a direction OP is dependent on an angle 0, where 8 is defined by the direction OP with respect to the normal to the plane of the transmitting array AT. If both arrays AR and AT are substantially identically constructed, then the receiving array AR will be sensitive to a frequency FR in the direction OP, where the frequency FR is dependent on the angle (0-0).

No pulse echo signal will therefore be detected in the direction OP, unless there has been a Doppler frequency shift FD, which is defined by the frequency, FR-FT.

As long as the frequency dependence of the angle 0 has been made substantially linear by incorporating the present invention, then targets moving at a particular velocity can be accepted or rejected at any particular value of O. This can be done mechanically or by having a plurality of receiving arrays, each array being inclined at a different value of O with respect to the plane of the transmitting array.

Moreover a configuration of a number of transducer arrays, incorporating the "waveguide" embodiment shown in Fig. 2, may be used in a cluster to provide velocity descrimination in parallel. If the arrays are arranged at different orientations with respect to each other, they can provide coverage of the full velocity range. By channelling the data from each of the arrays into an analyser, the full velocity discriminating sonar image can be built up from a single transmit pulse.

Fig. 4 shows a typical arrangement of transducer arrays T" T2, T3 and T4, in which the "waveguide" structure both along the array and between adjacent arrays has been omitted for clarity. There may be many more elements in each array (typically 100-200 elements for 100 resolvable scan angles) and in the cluster there may well be more arrays.

An array near the centre of the cluster will be used for transmitting so that the sonar will cover a similar range of velocities both towards and away from the array. Alternatively, a separate array of adjustable orientation could be used as transmitter thereby enabling the velocity band to be altered.

Both of these array configurations will support continuous wave and noise insonification, although, unless there are two identical arraystone for transmit and one for the zero doppler shift receive there will be no zero velocity discrimination, as well as no range information. However a moving target will be detected as soon as the receive signal arrives.

As with conventional array sonars, the orthogonal beam profile is defined by the element length. If the target is also to be localised in a vertical plane, whilst still using the same array, then a narrow profile is needed.

This may be provided by having wide profile receivers and one wide and one narrow profile transmitter. Once the target has been located using the wide beam, the narrow beam transmitted is then swept to give vertical information.

In a typical arrangement of arrays produc ing an array sonar of this type, the target may be displayed on a PPI scan with the velocity of the target shown as a variation in colour on the screen.

An application of such configurations of transducer arrays may be in extreme range underwater surveillance with a sector field of view, where the dual transit time even for a single pulse is unacceptable, and they could be used to provide simultaneous target velocity and position information at high frame rates.

An advantage of this system over conventional array sonars, which have to resort to floodlight illumination, is that the transmit and receive beam profiles are multiplied together thereby giving substantially improved sidelobe levels.

To substantially overcome any problems arising from frequency dependent attenuation of the water, which would result in a nonuniform sensitivity across the sector, a range dependent correction similar to a medical TGC (Time Gain Compensation) may be applied.

Claims (9)

1. An ultrasonic transducer array comprising a plurality of transducer elements disposed in a substantially linear phased array formation, and including means, associated with said array, whereby a non-linearity may be introduced into the path of ultrasonic energy to or from said array, so as to substantially minimise a non-linear variation of angular resolution associated with said array.

2. An ultrasonic transducer array, according to claim 1, wherein said means consist of a prism,

3. An ultrasonic transducer array, according to claim 2, wherein said prism has a different refractive index to that of the surrounding medium,

4. An ultrasonic transducer array, according to either of claims 2 or 3 wherein a refracting face of said prism is curved to create a focussed beam.

5. An ultrasonic transducer array, according to claim 1, wherein said means consist of a plurality of channels, each channel containing first and second materials of different acoustic refractive index, the materials being provided in different proportions for different channels to provide a substantially uniform interchannel delay across said array.

6. An ultrasonic transducer array, according to claim 5, wherein "waveguides" are used to substantially reduce interchannel coupling.

7. An ultrasonic transducer array, according to either of claims 5 or 6, wherein a step size defined by the relative positions of the interface between the first and second materials in said channels, can be made nonlinear across said array to provide focussing.

8. An ultrasonic transducer array, according to any of claims 5, 6 or 7, wherein the medium of at least one of said materials is an imaging medium.

9. An ultrasonic transducer array substantially as herein described with reference to the accompanying drawings.