INFRA-ACOUSTIC/ELECTRIC FISH FENCE

Abstract: An immaterial fish fence is based on a combination of low frequency mechanical vibrations and synchronously modulated electric fields, where fish approaching the fence, will be given at the same time fear reactions and directional information by the mechanical vibrations, and in addition feel pain due to the electric field. The fish will then turn and swim away. The fence is implemented by means of columns positioned side by side, each comprising a number of low frequency transducers (1) suspended above each other, each column being suspended in a float (4). Each column also has two electrical conductors (2) to which a high voltage can be delivered, and thus synchronized fields of both acoustic and electric type can be generated between and around the columns.

Description: Infraacoustic/electric fish fence The present inventions concerns a fish fence primarily of an immaterial type, based on subsea use of low frequency mechanical (infra acoustic) vibrations in conjunction with an electric field where two such signals are modulated synchronously. The infraacoustic field combined with the electrical field create anxiety reactions in the fish; it will also feel pain caused by the electric field. Usually, a fish fence is material, i.e. sometimes a net is used acting as an obstruction for fish exceeding a certain size. However, in certain cases such a material fence constitutes a problem, e.g. by being a hindrance to other sea traffic, by the fact that the fence is exposed to fouling, as well as by the fact that it is difficult to alter the mesh in order to vary the size of the fish it is desirable to have passing through the fence. Consequently, a different type of fence is needed, for example an immaterial "energy fence", also called "non-physical" fence, of a type to irritate/frighten and cause pain, so as to make the fish turn back when about to swim into the fence. Previously there have been conducted experiments using different forms of both sound and electrical fences, however not in combination. In the majority of the experiments where sound has been used, sound pressure has been used, in the sense that the actual sound pressure is used to frighten away the fish or to make the fish stay behind or inside an imaginary barrier. These experiments to generate sound pressure in water have all been conducted within the hearing range of the fish (i.e. the range 50Hz - approximately 2000 Hz) , audible to fish via otolith and/or swimming bladder, but these methods have proved to be not very effective, for the simple reason that the fish gets used to the sound. Additionally, several experiments have been performed with electrical fences with an aim at establishing an electrical field in the water. Experiments have been conducted with different frequencies and pulse widths in an alternating electric field and, generally, it can be stated that extremely varied results were achieved. The best results were obtained when using a very short pulse width and frequencies of 50Hz. In this case a barrier efficiency of approximately 80% was achieved, but small fish were nevertheless able to swim rather easily through the fence, and larger fish were killed. The reason for these problems is that the fish have no organs to indicate from which direction the electric field comes, this resulting in that the fish, upon entering the electric field, will feel-pain, but will not be able to detect the reason therefor. Usually it will continue swimming into the field, where it will either be killed or may succeed in passing through (sometimes injured) , depending on its physical size. New investigations show that fish exposed to infra¬ acoustic particle movements (both the acceleration and velocity of the water particles are important) can detect such movements almost down to a frequency of 1Hz (experiments have been performed down to 3Hz) with the use of side line organs. In addition to detecting the particle movement (the particle acceleration) , the fish is also able to detect the direction. Fish have such a sensor system as a predator warning, and experiments show that fish exposed to infra acoustic accelerations get spontaneous reactions of fear (under 20Hz) . Experiments with an infraacoustic fence with a dipole type acceleration pattern can produce relatively good results, but the result is nevertheless too dependant on the surroundings, i.e. factors like stress level (traffic), conditions of food/light and threshold of fear (due to possible pedators in the surroundings) . From US patent No. 2.146.105 is previously known electrodes which create electrical dipole fields, but no additional stimulus of the acoustic type to impart fear and direction information to the fish, appears therein. A combined system, however, is known from US patent No. 2.709.984, but this combination relates to light signals and electric influence of the fish from a plurality of electrodes suspended in the water. Correctly, it is stated in the publication that the fish needs a "direction" indicator in additon to the electric shocks sensed, in order to realize in which direction it must swim to avoid the unpleasant effect. However, important differences exist between light and sound regarding such direction indication, as light is a positive stimulus and hence the fish is dependent on training to achieve the desired effect (i.e. the fish must collate a flash of light with a simultaneous electric shock) . However, it would be far more advantageous if an immediate response for the fish could be obtained without any prior learning process. The present invention tries to achieve an effective solution to the problem regarding the generation of an immaterial barrier in water in order to stop the approach of the fish, where the sound pressure and the intensity of the electric field are not of great importance, but on the contrary infraacoustic particle acceleration in conjunction with a very short and intense electric field modulated synchronously with the infraacoustic field. There is no learning problematics involved as the fish already from the first try will swim away from the electric field. It appears namely that when the fish enters the infra¬ acoustic acceleration vector field, it exhibits a fear reaction, and at the same time it feels pain due to a voltage drop across the body as a result of the electric field. Since the fish is able to detect the acoustic field (which is aligned in parallel with the electric one, see details later) , together with the direction thereof, pains provided by electricity will be perceived as associated with the acoustic field and the direction-found source of the field, and the fish will turn and swim away from what it considers to be a position which is the cause of the pain. The invention is implemented by a fish fence based on under water use of low frequency mechanical vibrations combined with an additional stimulus, and is particularly characterized by a plurality of electromagnetic low frequency transducers for mechanical vibrations, suspended in vertical columns beneath floats positioned in regular rows in or below the water surface, this transducer configuration constituting a grid, and a corresponding plurality of pairs of parallel- connected electric conductors, each " conductor in a pair being positioned close to and along each respective side of a vertical transducer column, said further stimulus being an electric field from said conductors, modulated in synchronism with the mechanical vibrations from the transducers, as also stated in appended patent claim 1. The transducers establish the acoustic field, and the electrodes on each side of the transducers establish the electric field. More specific embodiments of the invention will appear from the associated dependent patent claims. The transducers in each column (together with the electrodes) are arranged to oscillate in phase with each other and in opposite phase with the nearest neighbouring column in the row (it is also possibe to use the alternative that the transducers in a column oscillate in the opposite phase with the next nearest neighbouring column, see below) . In this way an acoustic field is raised together with an electric field as a row of single dipole configurations in the water between the transducer columns. In a preferred embodiment of the invention, the transducers are connected to a drive voltage generator the time behaviour of which is adapted by a Fourier series to provide maximum water particle acceleration in the area betwen the transducer columns. At the same time as the generator generates a pulse, it also generates a synchronizing pulse to a high voltage generator so that it delivers a powerful electric pulse but with a much shorter duration than the acoustic pulse. This means that the acoustic and electric pulses are modulated synchronously. The fundamental frequencies for both types of pulse are in the infra level, i.e. below 20Hz. The voltage level and the width of the electric pulse depend on the fish type, the size of the fish and the water conductivity (see below) . The acoustic generator may preferably be adapted to generate drive voltages for the transducers by: i) providing a sine curve with the topical fundamental frequency, ii) providing a number of odd harmonic curves with the sine curve as a basis, for example the first, third, fifth and seventh harmonic, and i ' ii) adding these odd harmonics to the sine curve with scaling coefficients chosen from a knowledge of physical parameters of the transducers, in order to create a periodic drive voltage curve with a characteristic time behaviour. The characteristic time behaviour of the drive voltage applied to each transducer results in an approximately square shaped time function for the dynamic pressure in the water, and thus the highest possible particle velocity. Preferably the generator is equipped to deliver the applied drive voltage as well as the same applied drive voltage phase, to every other column in the row, respectively. The synchronization takes place with the aid of a simple square pulse placed at the highest level of the acoustic pulse (at the leading edge thereof, which square phase at the same time triggers the electric pulse. As an example the grid can either be one-or two- dimensional, the one-dimensional grid comprising columns with only one transducer in each column, and the two-dimensional case comprising columns with two or more transducers in each column. In a special embodiment of the fish fence a regular additional row of columns with only transducers and no electrodes can be used, i.e. this additional row is solely an infraacoustic row producing its own dipole fields. Preferably, the transducer columns in the first row and in the additional row are placed in two parallel lines, in a view from above, and in such a manner that each column in a row is placed at the summit of an equilateral triangle where two neighbouring columns in the other row constitute the end points of the triangle base line, all in a view from above. The reason for a main row and an additional row (from the front, where the fish is to be barred) is that at the same time it is possible to use hydroacoustic detectors (of the echo sounder or sonar type) which detect fish approaching too near the fence. Usually, only the acoustic part is switched on, i.e. only the transducers are in operation, the electric part of the system being inactive. When the fish approaches and enters the first row, it is detected at the same time and the electric part of the main row at the back is activated. When the fish enters row 1, i.e. the additional row, the swimming speed is reduced, and it will show signs of fear. When it enters row 2, i.e. the main row, it will also experience pain from the electric pulse. The signal level in row 2 is preferably 3 dB higher than in row 1, i.e. the double. By using "empty" areas, the two rows can be kept acoustically separated, so that the one only to a small extent influences the other. A more detailed description of the invention will be given by means of explanatory and non-limitative examples, referring to the appended drawings: figure 1 shows a fence grid with transducers and electrodes seen respectively from the side and from above, indications of the acoustic and electric fields also being shown, figure 2 shows a fence grid seen from above, with additional row consisting of acoustic transducers alone, figure 3 shows a section through an example of an electro¬ magnetic low frequency transducer, figure 4 shows what happens when a fish enters a combined acoustic/electric field (AE field) , figure 5 shows overlapping AE fields with every second pulse, and figure 6 shows curve shapes for acoustic and electric pulse. Figure 1, part A, shows an example of a fish fence according to the present invention. In the example four vertical columns are shown, each with three electromagnetic low frequency transducers l adapted to receive signals from a not shown generator and distribution unit. Additionally, the figure shows in part A the current electrodes 2, connection boxes 3 for current electrodes, floats 4 and anchoring on the bottom 5. In part B of the same figure dipole-shaped vector fields are shown between transducers and between electrodes, where a dark arrow 7 indicates the acceleration vectors and the lighter arrow 8 indicates electric field vectors which are mainly parallel to the acoustic vectors. Around each transducer will appear pressure rings which are largest in one certain direction. Measurements show that the dipole field is relatively narrow, for example at a distance of 2 metres between each column the dipole field will possibly extend 4-5 metres to each side, while a very intense field arises between the columns. Figure 2 shows a fish fence with an additional row in front of the main row, where the additional row is solely an acoustic row, while the main row is an acoustic/electric row. In front of both rows there is a hydroacoustic detector which is either of the sonar type or consists of an echo sounder. Eventually, the fish will enter the zone in front of the additional row where it is detected, and consequently the electric field in the main row will be activated. The reason that the electric field is not activated all the time is that the power consumption can be reduced, and that it is desirable to reduce the danger for mammals and humans who might enter the field area. Figure 3 shows a possible construction of a suitable transducer, with a membrane 11 beating up and down and constituting the bottom of the transducer. As it appears from figures 1 and 2, this will create strong particle velocity fields between the columns, as shown with dark arrows. Additionally, the transducer has a top mounted fixing means 14 for cable and suspension, electronic circuitry (amplifier) 13, and a pressure compensated housing so that the air 15 inside the transducer has the same pressure as the water. An electromagnetic motor 12 drives the piston/membrane 11. Figure 4 shows a picture of what happens when a fish 24 enters a parallel acoustic/electric field. The field is produced from a transducer 1 and an electrode 2 (corresponding elements can be found on the right hand side, but are not marked by any reference numeral) , these elements establishing the AE fields 25. The fish 23 will then with its side line organs sense the direction of the acoustic part of the field 25, and at the same time feel pain and obtain muscle contractions due to the voltage drop 24 (ΔV) across the fish, again due to the electric field lines penetrating the fish body. The bigger the fish 23, the larger the voltage drop 24 across the fish. This indicates that the voltage, frequency and pulse width must be adjusted to the size of the fish. Again, by changing these parameters, the fence can be adjusted to be effective for fish above a certain size. Furthermore, the intensity increases as one approaches the centre line between the transducers. Around the transducers pressure circles 6 will be created. Figure 5 shows very simply how to establish a relatively homogenous AE field by using overlapping fields, i.e. every other transducer operate in pairs, and each of these pairs activate every other pulse. This means that transducer columns A and C cooperate in opposite phase and thereafter transducer columns B and D operate in opposite phase, and this takes place in successive order, every other time. The ideal frequency is 5-7 Hz for each pair, the total frequency from two pairs then being 10-14 Hz. Figure 6 shows the time function for the acoustic and electric pulses, for a main row and an additional row. Frequency and width of the pulses are indicated, but not their level.

Claims: P A T E N T C L A I M S 1. A fish fence to be used under water based on low frequency mechanical vibrations in combination with a further stimulus, c h a r a c t e r i z e d b y a plurality of electromagnetic low frequency transducers for mechanical vibrations, suspended in vertical columns beneath floats positioned in regular rows in or below the water surface, this transducer configuration constituting a grid, and a corresponding plurality of pairs of parallel-connected electric conductors, each conductor in a pair being positioned close to and along each respective side of a vertical transducer column, said further stimulus being an electric field from said conductors, modulated in synchronism with the mechanical vibrations from the transducers. 2. A fish fence according to claim 1, c h a r a c t e r i z e d i n that the transducers in a column are arranged to oscillate in phase with each other and in opposite phase with the transducers in the nearest neighbouring column in the row, thereby to set up vibration fields as a row of single dipole configurations in the water between the transducer columns. 3. A fish fence according to claim 1 or 2, c h a r a c t e r i z e d i n that each transducer is suspended for a downward acting movement of the vibrating membrane of the transducer. 4. A fish fence according to one of the previous claims, c h a r a c t e r i z e d i n that the transducers are connected to a generator for drive voltages, the time behaviour of which is adapted by means of a Fourier analysis to give maximized water particle velocity in the area betweeen the transducer columns. 5. A fish fence according to claim 4, c h a r a c t e r i z e d i n that the generator is adapted to generate drive voltages for the transducers by I) providing a sinusoidal curve with the topical fundamental frequency, II) providing a number of odd harmonic curves with the fundamental sinusoidal curve as a basis, for 1 example the first, third, fifth and seventh harmonic curves, and III) adding these odd harmonics to said sinusoidal curves with scaling coefficients chosen from a knowledge of physical parameters of the transduers, to create a periodic drive voltage curve with a characteristic time behaviour, said characteristic time behaviour of the applied drive voltage on each transducer causing an approximate squarέwave time function for the dynamic pressure in the water outside the transducer membranes, thereby providing maximum particle acceleration in the water and hence the highest possible particle velocity. 6. A fish fence according to claim 5, c h a r a c t e r i z e d i n that the generator is adapted to deliver said applied drive voltage and the same applied drive voltage in opposite phase to respectively every other column in the row. 7. A fish fence according to one of the previous claims, c h a r a c t e r i z e d i n that the grid is in one or two dimensions, the one-dimensional case including columns with only one transducer in each, and the two-dimensional case including columns with two or more transducers in each column. 8. A fish fence according to one of the previous claims, c h a r a c t e r i z e d b y a regular additional row of transducer columns without associated electrode pairs, said transducers being driven in a corresponding manner and from the same generator, said additional ro * in addition to providing its own vibrating dipole fields also being adapted to generate further dipole fields by cooperating with the first row, in the area between the two rows, said two rows together forming a three-dimensional grid. 9. A fish fence according to claim 8, c h a r a c t e r i z e d i n that the transducer columns in the first row and in the additional row are laid out in two parallel lines when viewed from above, and in such a manner that each column in one row is positioned at the summit of an equilateral triangle where two neighbouring columns in the other row constitute the end points of the triangle base line, all viewed from above. 10. A fish fence according to one of the previous claims, c h a r a c t e r i z e d in that the electric conductors are positioned in such a manner and have a voltage source arranged in such a manner that the electric field and the mechanical velocity vector field during synchronous operation have substantially the same direction at any point in the water at a distance from the columns/electrodes and in the areas therebetween. 11. A fish fence according to claim 10, c h a r a c t e r i z e d i n that the electric field is constituted by a row of single dipole configurations in the water between the transducer/conductor columns. 12. A fish fence according to one of the previous claims, c h a r a c t e r i z e d i n that the electric field is separately switchable, i.e. independent of the mechanical vibrations, as a response on detection of incoming fish. 13. A fish fence according to one of the previous claims, c h a r a c t e r i z e d i n that the electric field is active in a short timespan at the leading edge of each periodic mechanical pulse from the transducers, a typical pulse width being 25-75 ms at frequencies of 10-3 Hz, the mentioned timespan typically being 150-500 μs .

Inventors: OESTLIE LARS, OESTLIE, LARS
Applicants: OESTLIE LARS, OESTLIE, LARS
Application year: 1992
Main classification: A01K79
IPC codes: A01K3, A01K63, A01K79