GB2251687A - Sonar apparatus - Google Patents

Abstract

Sonar apparatus for a waterborne vessel comprises: a sonar transducer adapted to transmit a wide, shallow beam and to receive echo(es) in a directional manner, the transducer having a plurality of piezo-electric elements 16-21 arranged in a horizontal array, each element being adapted to output a signal at its time of receipt of the echo(es), means 32-34 for driving the transducer for transmission 19 of the beam, and means 10, 38-54 for analysing received echoes as to their range and bearing in accordance with the elapsed time of receipt of each echo at the transducer and the relative times of receipt of the echo at the individual, horizontally arrayed elements of the transducer. <IMAGE>

Description

SONAR APPARATUS The present invention relates to sonar apparatus, particularly though not exclusively for scanning, forwards in particular, from a waterborne craft or vessel for hazards.

Sonar equipment has been widely used for scanning downwards from a vessel, that is to say for detecting the presence of objects beneath the vessel. It has been proposed in International Patent Publication No. WO 84/01833 to scan forwards from a vessel for obstructions with which the vessel is liable to be in collision. Such scanning is particularly useful to small vessel mariners because of the number of flotsam ISO containers now adrift at sea.

Generally these are of neutral buoyancy and represent dangerous hazards. It is also perceived that the present invention will be of assistance in shallow water navigation.

In this specification, the term "hazard" is used to refer to any object, even of a transitory nature such as a wave, from which a sonar reflection or echo is received.

The proposal of the publication mentioned above was to use a plurality of transducers to scan for hazards at discrete beam directions angularly distributed across the heading of the vessel, or indeed to use a single transducer controllable to the different directions.

The object of the present invention is to provide an improved sonar apparatus.

Sonar apparatus for a waterborne vessel according to the invention comprises: a sonar transducer adapted to transmit a wide, shallow beam and to receive echo(es) in a directional manner, the transducer having a plurality of piezo-electric elements arranged in a horizontal array, each element being adapted to output a signal at its time of receipt of the echo(es), means for driving the transducer for transmission of the beam, and means for analysing received echoes as to their range and bearing in accordance with the elapsed time of receipt of each echo at the transducer and the relative times of receipt of the echo at the individual, horizontally arrayed elements of the transducer.

Whilst it is possible to provide separate transmission element(s) in the transducer, preferably the same elements are used for transmission as reception.

In the preferred embodiment, the transducer has six elements arranged in two rows and three columns, to provide the transducer with additional depth facilitating its transmission of a wide, shallow beam. The elements in each column are connected together; whilst the elements in each row are discrete at least for bearing analysis reception.

Preferably the elements are driven at differing amplitudes in the columns in order to reduce side lobes in the beam in plan view.

Preferably, the analysing means is split into a forward unit associated directly with the transducer for outputing a signal for each echo received as to its range and bearing and a processing unit, usually positioned aft, for processing the plurality of signals from the forward unit.

(It should be noted that it is possible for the forward unit to be positioned aft of the processing unit in certain vessels.) Preferably the processing unit is adapted to time each (significant) received echo signal; whilst the forward unit is preferably adpated to output to the processing unit for each echo a pair of signals indicative of the phase difference of the received echo at the individual elements of the transducer and the amplitude of these signals, the processing unit utilizing the amplitude signal for timing.

These signals could be converted to digital signals at the forward unit; but in the preferred embodiment they are analogue signals. To compensate for echoes from farther hazards having smaller amplitudes at the transducer, the forward unit preferably includes a time varying gain amplifier. Further, to discriminate against poor quality signals, the forward unit preferably includes means for comparing the relative phase of the echo signal at one adjacent pair of transducer elements with that at the other pair. The output from the comparing means is applied to the amplitude output signal to block this if the relative phase difference is large, in the case of an insignificant echo.

Normally the transducer will transmit in periodic bursts of several multi-oscillation pulses. In certain modes, the bursts can be more frequent. The processing unit is preferably adapted to count the number of occurrences of echoes from individual hazards, i.e. having the same range and bearing, that is the same phase amplitude signal and same elapsed time from pulse transmission, and to give an indication of a hazard's existence only if a certain number of sequential echoes are received from the hazard.

The processing unit is preferably adjustable as to the count of echo occurrences required for a hazard indication to be given. In particular, the processing unit can be adjusted as to hazard count for different sea states. In rough weather, many more echoes will be received from waves.

The hazard indication is preferably in the form of a visual display of range and bearing. Conveniently, the bearing is displayed in both an analogue and a digital form, whilst the range is in digital form. An audio indication of hazard presence can also be given.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a side view of a vessel equipped with sonar apparatus of the invention and of the apparatus' beam shape; Figure 2 is a plan view of the vessel and beam shape of Figure 1; Figure 3 is a side view of a transducer assembly of the apparatus; Figure 4 is a front view of the transducer assembly; Figure 5 is a block diagram of a forward unit of the apparatus; Figure 6 is a block diagram of a processing unit; and Figure 7 is a plan view of a control panel and an indication display of the apparatus.

The vessel 1 shown in Figure 1 has a transducer assembly 2 mounted amidships, forward of its pair of fin keels 3. It is well protected in this area. The sonar beam 4 from a transducer 5 within the transducer assembly 2 is propagated substantially horizontally, along the water surface 6, with a downwards inclination of 40 only. Use of such a shallow beam is important in avoiding echoes from the bottom 7. As described below, the apparatus can be adjusted as to its range in accordance with the depth of water.

Certain deviation from a 40 beam is possible.

As shown in Figure 2, the beam has a nominal beam spread horizontally of 200 to both sides of dead ahead.

Again certain deviation is possible.

Turning now to Figures 3 and 4, the transducer assembly 2 has a moulded after fairing 11, to the front of which is secured a spherical housing 12 of accousticly transparent material. Within the housing 12 is mounted, on a gimbal 13 in oil 14 of the accoustic density of water, the transducer 5. This comprises six tall thin piezo-electric elements 16,17,18,19,20,21. They are set in polyurethane 22, backed by polystyrene foam (not shown) and contained in an open fronted steel case 23 via which they are supported in the gimbal 13. The arrangement is such that the elements are always substantially upright, even during pitch and roll of the vessel, but constrained to face forwards of the vessel.

The individual elements are 50mm high, 5mm wide and adapted to operate at 200kHz. The inter-element spacing is imam, giving a horizontal pitch between the elements of 6mm.

Elements 16,17,18 are in a top row; whilst elements 19,20,21 are in a bottom row. This arrangement of the elements adapts the transducer to propagate a beam of the above described beam shape. It should be noted that the wavelength of a 200kHz accoustic oscillation in water is approximately 7.5mm, i.e. of the same order as the horiziontal pitch of the elements. This enables the transducer and the detection circuitry to measure the bearing 8 with respect to the vessel's heading of an echo incident on the transducer, from, for instance, a hazard 9 shown in Figure 1.

From a processing unit 10 shown in outline in Figure 1, a forward unit 24, mounted behind the gimbal 13 and shown in block form in Figure 5, receives Transmit control signals on line 31. These signals typically will be of a 200 microsecond duration at a period of 0.4 seconds for 2 seconds every 30 seconds. In other words, the Transmit signals will be in 2 second bursts with an inter-burst period of 30 seconds and having five 200 microsecond pulses at a 0.4 second period within each burst.

The Transmit signals are applied to gate 32 to allow a 200kHz excitation signal from an oscillator 33 to pass to a transmission amplifier 34. A 300 volt drive signal from the amplifier is passed through an array of back to back diodes 35 to the transducer elements. The full voltage is passed to the central elements 17,20, whilst the voltage applied to the edge elements 16,18,19,21 is halved by series resistors 36. The effect of these is to substantially eliminate side lobes 37, see Figure 2, on the beam, which would otherwise be expected and decrease the forwards directiveness of the beam and its usefulness for navigation in channels.

For reception of echoes, the elements are connected in pairs across the transducer, i.e. 16,19; 17,20; 18,21, to receiver pre-amplifiers 38,39,40. The effect of the back to back diodes 35 is to isolate the elements as regards received signals. The output from the the pre-amplifiers is applied to a summing amplifier 41; whose output is applied to a time varying gain amplifier 42 under the control of the processing unit 10. Thus the received echo signals from the transducer elements are all summed and amplified in accordance with the time elapsed from the instant when the present (i.e. immediately preceding) transmission pulse was initiated by the processing unit 10, to compensate for reduced echo strength from more distant hazards. The output, an analogue AC signal, from the amplifier 42 is passed to a detector 43 for conversion to a DC voltage signal.

The output from the pre-amplifiers 38,39,40 is applied via respective limiters 44,45,46 to a pair of phase measurement circuits 47,48. The first 47 of these compares the phase difference between the received echo signals at the transducer elements 16,19 and 17,20. It will be appreciated that an echo returning to the transducer at any bearing other than straight ahead will arrive at different times at the horizontally spaced elements, and thus will give rise to the signals from such elements having different phases. The separation of the elements, the frequency of transmission/reception and the speed of sound in water are matched to provide that no more than 3600 of phase difference is present between adjacent elements. The second phase measurement circuit 48 compares the phase difference between the received echo signals at the transducer elements 17,20 and 18,21.For good quality echo signals, the two circuits 47,48 will output identical signals. For poor quality signals of uncertain phase, the outputs will differ.

Such signals are likely to come from indistinct/distant hazards and can generally be ignored.

The outputs from the two phase measurement circuits 47,48 on lines 49,50 are applied to a difference squared and threshold circuit 51. This circuit is adapted to give a constant voltage output on line 52, if the square of the difference between the signals on the lines 49,50 is less than a certain threshold, corresponding to a good quality signal. If the difference squared exceeds the threshold, the output is zero on line 52. The DC received signal amplitude signal from the detector 43 and the signal on the line 52 are applied to a multiplier 53. This, via a further detector circuit 54 passes a DC voltage indicative of the amplitude of good quality received signals to the processing unit 10 on line 55. The phase signal, also a DC voltage, on line 49 is passed from the first phase measurement circuit 47 to the processing unit 10 on line 56.

The processing unit 10, as shown in Figure 6, comprises a central processor 60, connected to a clock 61, a keypad 62, an LCD display 63, audio and visual alarms 64,65 and an analogue to digital convertor 66. The central processor is an appropriately programmed micro-computer. Its programme will not be described in detail beyond that it provides the functional requirements of the apparatus as described herein. Via the clock, it times the bursts of transmission and it times the elapsed time at which echoes are received after transmission. The elapsed time, phase and amplitude of each echo are memorized. The stored echo data is compared with new data at each transmission. Where an echo is received from a hazard which has given a previous echo, this is recognised by similarity of phase and elapsed time and a "count" parameter associated with each echo's memorized data is increased by one.On the other hand, if no echo is received in the case of a previously noted hazard, its count is decreased by one on each transmission.

If the count reaches zero, the hazard's data is deleted.

When the count reaches a certain threshold, for instance seven, the presence of a detected hazard is indicated on the LCD display 63, in terms of an analogue indication 70 of bearing, computed from the phase of the received echo signal. The range 71 of the hazard is also displayed. In accordance parameters such as range and bearing, the audio and visual alarms 64,65 may also be triggered. Where more hazards than can be displayed are detected, only those of highest quality, e.g. echo amplitude are displayed. Further where a number of hazards of similar range and bearing are detected, such as from a breakwater, they are displayed as a single hazard and the alarms triggered as appropriate.

The processing unit 10 can be pre-set via the keypad 62 to take account of the present sea state. In rough conditions, many more echoes will be received from waves and thus hazards must reach a higher count before being regarded as real and having their presence displayed. For inshore navigation, the range at which an excessive number of echoes from the sea bottom will be ignored is set, with the result that all hazards having an elapsed time longer than that corresponding to the setting are ignored. In a particular man-over-board mode, every echo is displayed to enable the navigator to assess the bearing of the man-over-board for himself.

The invention is not intended to be restricted to the details of the above described embodiment. For instance, the hazards may be assessed in a different statistical manner. The apparatus may be associated with other equipment such as a compass to give a grid bearing as opposed to a bearing with respect to the ship's head.

Claims (22)

CLAIMS:

1. Sonar apparatus comprising: a sonar transducer adapted to transmit a wide, shallow beam and to receive echo(es) in a directional manner, the transducer having a plurality of piezo-electric elements arranged in a horizontal array, each element being adapted to output a signal at its time of receipt of the echo(es), means for driving the transducer for transmission of the beam, and means for analysing received echoes as to their range and bearing in accordance with the elapsed time of receipt of each echo at the transducer and the relative times of receipt of the echo at the individual, horizontally arrayed elements of the transducer.

2. Sonar apparatus as claimed in claim 1, wherein the piezo-electric elements are provided as transmission ones and reception ones.

3. Sonar apparatus as claimed in claim 1, wherein the same piezo-electric elements are arranged for transmission and reception.

4. Sonar apparatus as claimed in claim 1, claim 2 or claim 3, wherein the elements are arranged in a plurality of rows and a plurality of columns, to provide the transducer with additional depth facilitating its transmission of a wide, shallow beam.

5. Sonar apparatus as claimed in claim 4, wherein the elements in each column are connected together.

6. Sonar apparatus as claimed in claim 4 or claim 5, wherein the elements in each row are discrete at least for bearing analysis reception.

7. Sonar apparatus as claimed in claim 4, claim 5 or claim 6, wherein the transducer driving means is adapted to drive the elements at differing amplitudes in the columns in order to reduce side lobes in the beam in plan view.

8. Sonar apparatus as claimed in any preceding claim, wherein the analysing means is split into a forward unit associated directly with the transducer for outputing a signal for each echo received as to its range and bearing and a processing unit, usually positioned aft, for processing the plurality of signals from the forward unit.

9. Sonar apparatus as claimed in any preceding claim, wherein the processing unit is adapted to time each (significant) received echo signal.

10. Sonar apparatus as claimed in claim 9, wherein the forward unit is adpated to output to the processing unit for each echo a pair of signals indicative of the phase difference of the received echo at the individual elements of the transducer and the amplitude of these signals, the processing unit being adapted to utilize the amplitude signal for timing.

11. Sonar apparatus as claimed in claim 10, wherein the forward unit is adapted to convert the pairs of signals to digital signals.

12. Sonar apparatus as claimed in claim 10, wherein the forward unit is adapted to output the pairs of signals as analogue signals.

13. Sonar apparatus as claimed in any one of claims 8 to 12, wherein the claim forward unit includes a time varying gain amplifier for amplifying received signals, to compensate for echoes from farther hazards having smaller amplitudes at the transducer.

14. Sonar apparatus as claimed in any one of claims 8 to 13, wherein the forward unit includes means for comparing the relative phase of the echo signal at one pair of transducer elements with that at another pair to discriminate against poor quality signals.

15. Sonar apparatus as claimed in claim 14,wherein the output from the comparing means is applied to the amplitude output signal to block this if the relative phase difference is large, in the case of an insignificant echo.

16. Sonar apparatus as claimed in any one of claims 8 to 15, wherein the transducer is adapted and arranged transmit in periodic bursts of several multi-oscillation pulses.

17. Sonar apparatus as claimed in claim 17, wherein the processing unit is adapted to count the number of occurrences of echoes from individual hazards and to give an indication of a hazard's existence only if a certain number of sequential echoes are received from the hazard.

18. Sonar apparatus as claimed in claim 17, wherein the processing unit is adjustable as to the count of echo occurrences required for a hazard indication to be given.

19. Sonar apparatus as claimed in claim 18, wherein the processing unit can be adjusted as to hazard count in accordance with the sea state input to the processing unit.

20. Sonar apparatus as claimed in any preceding claim, including a visual display of hazard range and bearing for hazard indication.

21. Sonar apparatus as claimed in any preceding claim, wherein the transducer is mounted on a gimbal, the arrangement being such that the elements are always substantially upright, even during pitch and roll of a vessel, but constrained to face forwards.

22. Sonar apparatus substantially as hereinbefore described with reference to the accompanying drawings.