Abstract:
A process is provided for transmitting a signal, without material connection, through a conducting wall which may be a ship&#39;s hull, to external user apparatus. 
     A current supplied by a generator and whose intensity is proportional to the amplitude of the signal to be transmitted is applied between two points on the hull. The magnetic field variations generated by the passage of the current are detected by a sensor on the other side of the hull and, suitably amplified, are used for controlling an apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The invention relates to a process for signal transmission without electric connection through a conducting wall, which may for example be the hull of a ship, and a device for implementing same. 
     The process of the invention may be used for example in the field of ultra-sonic remote detection. 
     2. Description of the Prior Art 
     Sonar systems generally comprise transmitter-receiver transducers disposed inside the hull of ships and connected to a signal generator by electric conductors passing through the walls of the ship. Preferably, these transducers are housed in shaped bodies fixed to the hull. The installation of the different elements of an ultra-sonic remote detection system on board a ship requires the system to be adapted at least to allow signal transmission cables to pass through the hull to the ultra-sonic transmission-reception means. Such adaptation, which is costly, for it very often requires placing the boat in a dry dock, is not always desirable especially when the ultra-sonic remote detection system is only installed temporarily, for the requirements of a particular application. 
     The signal transmission process of the invention allows a boat to be very easily equipped with an acoustic remote detection system, without requiring modification of the hull. 
     SUMMARY OF THE INVENTION 
     The process of the invention comprises the application to the conducting wall, on one side thereof, of an electric current dependent on the signal to be transmitted and the detection, on the other side of the wall, of the magnetic field generated by the current flow and proportional to the amplitude thereof. 
     In one particular embodiment the current generated is applied between distinct points on the hull of a ship on one side thereof and the detection of the magnetic field is carried out on the other side, on the outside for example. 
     Since transmission of the signal is effected without material connection, the control of the apparatus fixed to the outside of the hull of a ship for example is facilitated. If, furthermore, the external apparatus are fixed magnetically, any system using these apparatus and in particular any ultra-sonic pulse remote detection system may be rapidly installed temporarily on the hull of a ship. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the process and of the device for implementing same will appear from reading the description of a particular embodiment described by way of non limiting example with reference to the accompanying drawings in which: 
     FIG. 1 shows schematically the transmission assembly; and 
     FIG. 2 shows schematically an AC current generator producing the electric current which is caused to flow in the conducting wall. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, it can be seen that the process of the invention consists essentially in causing an AC current i to flow between two distinct locations A and B on the metal hull 1 of a boat, said AC current being produced by a current generator 2 and being proportional at all times to a signal V which is applied to its input. An electric circuit F connects the points of application A and B to the terminals of generator 2. Since the electrical resistance of water is very much greater than that of the hull and since this latter is relatively isolated from water by protective coats of point, by oxidation and/or deposits or sediments, the largest part of the electric current i flows in the metal sheets of the hull between the two points of application A and B and this current generates a magnetic field H outside of the hull. 
     On the outside of the hull is disposed a user apparatus. Preferably this apparatus is contained in a shaped body 3 fixed to the hull be magnetic means, for example, so as to be able to fit it and remove it rapidly. The apparatus includes for example an electro-acoustic transducer 4 connected a transmitter 6, which is fed with electricity by an electric current source 14. 
     The magnetic field H generated by the flow of 10 current in the hull is detected by a magnetic sensor 5, such as a coil or a Hall effect device or magneto-resistive probe the output signal of which is applied to the transmitter 6, which amplifies it before applying it to the transducer 4. 
     The signal v applied to the current generator 2 is, in this embodiment, an AC electric voltage of given frequency f and duration Δt. 
     The current generator 2 as shown in FIG. 2, comprises two amplifiers 7, 8 whose outputs are connected respectively to the points of application A and B on the hull of the boat on the one hand, and to two switches 9, 10, on the other hand, which switches are adapted in the closed position to put points A and B at the positive voltage V+ delivered by an electric current source 13. 
     These switches 9, 10 are preferably of the electronic type, such as a field effect transistor, and operate to close when a signal of logic level 1 is applied to the control input thereof. 
     The input signal v is applied to the inverter input of amplifier 7 and to the input of a shaping element 11 of the Schmitt trigger type. The logic signal available at the output of this shaping element 11 is applied to the control input C of switch 10 and to the input of a logic inverter 12, the output of this inverter 12 being connected to the control input C of switch 9. The output of amplifier 7 is also connected to the inverter input of amplifier 8. 
     The current generator 2 operates in the following way: with a periodic signal applied to the inverter input of amplifier 7, 
     the positive half period of the input signal V is transformed by trigger 11 into an electric voltage of logic level 1 which causes switch 10 to close and, inverted by the logic inverter 12, causes switch 9 to open. Since the electric voltage source 13 is at that time connected &#34;through the conductive circuit from point B to point A on the hull&#34; to the output of amplifier 7 and since the internal resistance of amplifier 7 varies as a function of input signal v, a current proportional to this voltage passes from point B and to point A through the hull of the boat. In this case, the output of the amplifier 8 will have a positive polarity due to the double inversion of the input signal v by the amplifiers 7 and 8. 
     the negative half period of the input signal v which follows is transformed by trigger 11 into a signal of logic level 0, which causes switch 10 to open and, inverted by the logic inverter 12, causes switch 9 to close. Since the electric current source 13 is at that time connected &#34;through the conductive circuit from point A to point B on the hull&#34;; to the output of amplifier 8 and since the internal resistance of amplifier 8 varies as a function of input signal v, a current proportional to this voltage will pass from point A to point B, in a direction opposite that of the current passing through the hull during the preceding half period of the signal. In this case, the output of the amplifier 7 will have a positive polarity due to inversion of the input signal by the amplifier 7. 
     The variable magnetic field H, proportional to the instantaneous amplitude of the oscillating current passing between points A and B on the hull, is picked up by the magnetic sensor 5 (FIG. 1) and, suitably amplified by the transmitter 6, is applied to the electroacoustic transducer 4. 
     The device described may be used for example for implementing the method of remote acoustic measurement described in published French patent application No. 2 519 424 comprising the successive transmission from three electro-acoustic transducers contained in shaped bodies, fixed at three different locations on the hull of a boat, of three acoustic pulses, reception thereof by a transducer-receiver disposed on an immersed fish towed at the end of a cable and transmission to the boat through said cable of the telemetric measurements effected from these impulses. In this case, the currents proportional to the signals to be transmitted are applied successively between three pairs of points A, B at distinct locations on the hull, in the vicinity of the shaped bodies containing the three electro-acoustic transducers. 
     The described device may also be used in combination with an echo-sounding system or a Doppler effect navigational apparatus where the echos of the pulses transmitted from a transducer, such as the transducer 4 and back scattered by the sea bed or the water mass, are received by the same transducer. In this case, the signals picked up are suitably amplified and a current generator similar to generator 2 is also housed in the shaped body 3 and is adapted to apply to the wall of the boat a proportional electric current which generates a variable magnetic field picked up inside the hull in the manner already described. 
     The process of the invention may also be used for transmitting a movement without mechanical connection by making points A and B where the current is applied moveable. The magnetic field variations resulting from the movement of current lines A-B are detected on the other side of the hull and used for reproducing a movement similar to the one which gave rise to the magnetic field variations. 
     By moving points A and B over a face of the wall along a circumference, a motor for example may be caused to rotate for actuating different members.