DEVICE IN AN ECHO SOUNDING OR SONAR SYSTEM

Abstract: A device in an echo sounding or sonar system is operatively adapted for reducing the adverse consequences of the directionally dependent sensitivety of the transducer (1). The transducer then comprises at least three elements (1A, 1B, 1C, 1D) for converting acoustic input signals into electric output signals. Time measuring means (21, 17, 18) are connected with the transducer (1) and operatively adapted for measuring the time difference between the instants of arrival for a sound wave at at least two of the transducer elements (1A and 1B, 1A and 1C) in at least two planes. The measured time differences are then indications of the angle of incidence for the sound waves in said planes. The device further comprises reference means operatively adapted for producing correction signals corresponding to the measured time differences. Also suitable means (19, 20) are disposed for modifying the signals received by the transducer in such a way that the fully processed signals presented at the indication means (23) of the device exhibit a signal strength essentially independent of the angle of incidence of the sound onto the transducer within compensation range of the device.

Description: Device in an Echo Sounding or Sonar System. The present invention relates to a device in a sonar or echo sounding system which reduces the adverse consequences of the fact that the sensitivity of the transducer is directionally dependent. General background information is e.g. to be found in "Principles of Underwater Sound" by Robert J. Ulrich, published by McGraw Hill Inc. Up to now the directional diagram of the transducer has been a major source of error when presenting targets, in particular a single fish, of echo sounding or sonar. According to the pr-esent invention the uncertainties of presentation caused by the directional diagram are nearly eliminated. This is achieved by the device specified in the main patent claim. An embodiment of the invention is subsequently described with reference to the accompanying drawings, whereon: Figure 1 illustrates a transducer having an ordinary directional diagram. Figure 2 illustrates a directional diagram of a transducer compensated according to the invention. Figure 3 shows schematically a device for providing the compensation according to the invention. Figure 1 illustrates a typical example of a directional diagram of an echo sounding transducer. The sensitivity of the transducer in a given plane is plotted for different directions relative to 0 degrees, which means relative to the direction straight forward, perpendicular to the transducer surface. It may be deduced from the figure that the sensitivity of the transducer is 6 dB lower at the points Al and A2 than at the point AO. This indicates that the sensitivities in the directions Bl and B2 are 6 dB lower than straight forward along the axis of the transducer. Two acoustically equally strong echos incident to the transducer in the directions 0 and Bl, respectively, will produce output signals having a mutual strength difference of 6 dB. If the same transducer, as usual, is used both for trans¬ mission and reception, the directional diagram will be effective twice. The points Al and A2 then represent -12 dB points with respect to AO. When such a transducer is used for fish search, the strength of the echo from one and the same fish will be dependent on the location of the fish in the sound beam. Thus, the output signal strength from the transducer is not only a measure of the size and reflection characteristics of the fish, but also a result of the position of the fish in the " beam. Figure 2 shows in full line the effective diretional diagram of a tranducer in an echo sounding or sonar system according to the invention. The broken line illustrates the corre¬ sponding portion of a normal directional diagram. Between the directions Bl and B2 the sensitivity of the main lobe is approximately constant and equal to the sensitivity along the axis, i.e. at 0 degrees. Outside Bl and B2 the sensitivity is appoximately zero. With such a fish search instrument the strength of the fish echo would be independent of the location of the fish within the aperture angle of the transducer, i.e. between Bl and B2. The signal strength then gives a correct impression of the reflection characteristics of the fish, when in ordinary manner also compensations are made for signal attenuations caused by dispersion and absorption of the transmitted sound in water. In figure 3 it is schematically shown a device providing constant sensitivity independent of the bearing to the target (the fish) . A tranducer 1 is composed of four elements 1A, IB, 1C and ID. Each element 1A, IB, 1C and ID is connected to an amplifier 2, 3, 4 and 5, respectively, which preferably has time controlled gain (TVG) in order to compensate for transmission loss. The output of the amplifier 2 is connected to the inputs of summation amplifiers 6 and 7. The output of the amplifier 3 is connected to the inputs of summation amplifiers 7 and 8. The output of the amplifier 4 is connected to the outputs of summation amplifiers 8 and 9. The output of the amplifier 5 is connected to the outputs of summation amplifiers 6 and 9. The output of each of the summation amplifiers 6, 7, 8 and 9 is connected to an associated zero passage detector 10, 11, 12 and 13, respectively. The outputs of the detectors 10 and 12 are connected to individually associated inputs of an EXCLUSIVE-OR gate 14. The outputs of the detectors 11 and 13 are connected to individually associated inputs of an EXCLUSIVE-OR gate 15. Further, the outputs of the summation amplifiers 6 and 8 are summed in a summation amplifier 16. The output of the gate 14 is connected to a control input of a counter 17. The output of the gate 15 is connected to a control input of a counter 18. The output of the summation amplifier 16 is connected to an analog/digital-converter 19, which in turn is connected to a multiplier 20. A clock pulse generator 21 is connected to the counters 17 and 18. The outputs of the counters 17 and 18 are connected to the address inputs of reference means, i.e. a PROM 22, the data output of which is connected to the multiplier 20. The output of the multiplier 20 is connected to further electronic circuitry 23 for processing and presentation of the signals. The operation is as follows: The transducer elements 1A-D represent four quadrants of the transducer 1. 1A is fore starboard, IB is fore port, 1C is aft starboard and ID is aft port. A sound wave which is not incident along the axis of the transducer and striking all four elements at the same time, would arrive at the various elements with a certain time difference dependent on the angle of incidence. The transducer elements 1A-D are utilized for determining the angle of incidence for the sound with respect to two mutually perpendicular planes normal to the transducer surface. The angle in the fore-and-aft direction is determined by the time difference between the fore elements 1A, IB and the aft transducer elements 1C, ID, whereas the angle in the trans¬ verse direction is determined by the time difference between the transducer elements 1A, 1C and the elements IB, ID. The sound waves which strike the elements 1A-D are converted to electrical signals, which are amplified in the associated amplifiers 2-5. The output signals from the amplifiers 2 and 5 are added in the amplifier 6. Thus, the output signal from the summation amplifier 6 is an indication of the sound waves incident on the starboard side of the transducer. The output signals from the amplifiers 3 and 4 are added in the summation amplifier 8. Thus, the output signal from the amplifier 8 is an indication of the sound waves incident on the port side of the transducer 1. In corresponding way the signals from the amplifiers 4 and 5 are added, so that the output signal from the summation amplifier 9 is an indication of the sound waves incident on the aft part of the transducer 1. The output signals from the amplifiers 2 and 3 are added, and the output signal from the amplifier 7 is an indication of the sound waves incident on the fore part of the transducer 1. The signals from the starboard and port halves of the transducer 1 are added in the summation transducer 16 and represent the sum total of the signals received by entire transducer. In the zero passage detector 10 the starboard signal from the summation amplifier 6 is detected. The output signal from the detector 10 enters one input of the EXCLUSIVE-OR gate 14. From the zero passage detector 12 comes the corresponding signal derived from the port half of the transducer and enters the other input of the EXCLUSIVE-OR gate. The time difference between the arrivals of the sound wave at the starboard and port parts, respectively, of the transducer provides different incidents of zero passage. The duration of the output signal from the gate 14 is dependent on how long time the first zero passage detector has produced output signal before also the other zero passage detector starts producing output signal. As the output signal from the gate 14 is dependent on the time difference between the arrival of the sound wave at the port and starboard halves, respectively, of the transducer 1, this output signal is also an indication of the angle of the wavefront in the transverse direction. In corresponding manner the output signal from the EXCLUSIVE- OR gate 15 is an indication of the angle of the wavefront in the fore-and-aft direction. The signal from the gate 14 controls the counter 17, which counts the clock pulses from the clock 21 as long as the signal from the gate 14 lasts. Thus, the count reached by the counter is an indication of the transverse angle of the wavefront. Correspondingly the counter 18 counts clock pulses which indicate the fore-and-aft angle. The counts in the counters 17 and 18 now indicate the components of the angle of incidence in transverse and fore- and-aft directions, respectively, and this angle of incidence is unambiguously defined within one quadrant. As the transducer is symmetric, a determination of the quadrant in which the angle of incidence resides is not necessary in order to compensate for the varying sensitivity of the transducer 1. Said counts in the counters 17 and 18 are parts of addresses of storage locations in PROM 22. Thus, for each angle defined by the counters 17 and 18, there exists a corresponding address in the data store of PROM 22. The content of each address of the data store, i.e. the data in the PROM 22, indicates the correction factor to be used for the particular angle of incident, in order that the strength of the output signal be independent of the angle of incidence of the sound. In the multiplier 20 the output data from PROM 22 are multiplied by the digitalized output signal from the A/D- converter 19. Thereby the output signal from the multiplier is an indication of the size of the fish, independent of the angle of incidence of the sound wave. The data in the PROM 22 are calculated on the basis of the measured directional diagram of the transducer 1. Thus, the accuracy of these data is only dependent on the accuracy of the measurements and the storage capacity of the data store. If it is desirable to know the quadrant in which the fish is located, it would be necessary to determine which transducer half, i.e. starboard or port, fore or aft, receives the wavefront first.

Claims: Patent Claim. In an encho sounding or sonar system a device comprising: - a transducer (1) having at least three elements for converting an acoustic input signal into an electric output signal , - time measuring means (21, 17, 18) connected to the transducer (1) and operatively adapted for measuring the time interval between the instants of arrival of a sound wave at at least two of said elements (1A and IB, 1A and 1C) in at least two planes, by which the measured time intervals are indications of the angle of incidence of said sound wave, c h a r a c t e r i z e d b y a reference device (22) which is operatively adapted to produce a correction signal dependent on the measured time intervals, as well as a device or modifying the received signal in accordance with the produced correction signal by the reference device (22), in such a manner that the resulting signal strength is approximately independent of the angle of incident of the sound within the compensated range.

Inventors: BODHOLT HELGE, KRISTENSEN HOGNE FINN, SOLLI HAAKON, BODHOLT, HELGE, KRISTENSEN, HOGNE, FINN, SOLLI, HAAKON
Applicants: SIMRAD SUBSEA A S, SIMRAD SUBSEA A/S
Application year: 1985
Main classification: G01S7
IPC codes: G01S3, G01S15