Abstract:
A protection device for an oscillating power circuit, namely for a demining coil, such device incorporating circuit breaking means positioned between an alternating current supply network and the power circuit, such device wherein it comprises at least one means to measure and analyze the current supplying the power circuit, means able to detect the appearance of a direct current and connected to triggering means ensuring the opening of the circuit breaking means when the alternating current ceases.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The technical scope of the invention is that of protection device for oscillating power circuits. 
     2. Description of the Related Art 
     It is classical to protect a power circuit from excess voltage by providing a circuit breaking organ able to detect the excess voltage, for example a circuit-breaker or else a fuse. 
     However, certain types of circuit and namely oscillating circuits risk being deteriorated by the presence of a direct current. 
     For example, the coils used in electromagnetic mine clearance devices (such as those described namely by patent FR2750204) are sensitive to direct currents which may deteriorate them after a certain period of time, even if these currents are of a voltage level which falls under a given threshold. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is to propose a protection device ensuring protection against the appearance of such direct currents. 
     Thus, the invention relates to a protection device for an oscillating power circuit, namely for a demining coil, such device incorporating circuit breaking means positioned between an alternating current supply network and the power circuit, such device wherein it comprises at least one means to measure and analyze the current supplying the power circuit, means able to detect the appearance of a direct current and connected to triggering means ensuring the opening of the circuit breaking means when the alternating current ceases. 
     The means to measure and analyze the current may comprise a logic module incorporating a retriggerable monostable component whose tripping is controlled periodically by the variation in the current from the supply network, such component wherein the suspension of tripping activates the appearance of an error signal used by the triggering means to cause the opening of the circuit breaking means. 
     The measurement and analysis means may also incorporate a measurement module ensuring an estimation of the value of the current from the supply network, the measurement module incorporating comparator means ensuring the comparison of the measured value with a predetermined threshold, the result of this comparison being used by the triggering means to cause the opening of the circuit breaking means. 
     Advantageously, the triggering means will also be connected to display means enabling the state of the circuit breaking means to be indicated. 
     The invention also applies to the application of such a device to the protection of a demining coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become more apparent from the following description of a particular embodiment, such description being made with reference to the appended drawings, in which: 
         FIG. 1  is a block diagram showing a device according to the invention, 
         FIG. 2  is an organizational flowchart showing in greater detail an embodiment of the device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , a protection device  1  according to the invention incorporates circuit breaking means  2  positioned on a power conductor  3  connected to a direct current supply network R, for example a vehicle network. 
     The circuit breaking means  2  are positioned upstream of a power circuit  4  which here incorporates a power unit  5  linked to a demining coil  6 . 
     A control unit  7  is positioned upstream of the circuit breaking means  2 . It enables the shape of the signals generated by the coil  6  to be piloted and it incorporates a man-machine interface enabling the demining device itself to be piloted. 
     The device  1  also comprises means  8  to measure and analyze the current intended to supply the power circuit  4 . 
     These measurement means  8  are linked to triggering means  9  which enable the opening of the circuit breaking means  2  to be controlled. 
     In accordance with the invention, the measurement means  8  are designed so as to be able to detect the appearance of a direct current in the power conductor  3 . 
     Display means  10  are also linked to the triggering means  9  and enable the open or closed state of the circuit breaking means  2  to be displayed. 
     These display means  10  may, for example, comprise two differently colored lights, one color corresponding to the open position of the circuit breaking means  2  and the other the closed position. 
     It is naturally possible, to ease the implementation of the device, for the display means  10  to be offset onto the control unit  7  or its man-machine interface.  FIG. 1  shows a link  11 , in dotted lines, which illustrates such a link with offset display means  10 . 
       FIG. 2  shows the organic and functional structure of the device  1  in greater detail. 
     The means  8  to measure and analyze the current comprise two different modules: a logic module  12  and a measurement module  13 . 
     The logic module  12  incorporates a retriggerable monostable component  14  (for example a component known under the standardized name of 74HC4538). The logic output of this component alternates with a frequency which is that of the current circulating in the power conductor  3 . 
     This monostable component  14  is linked to a state detector  15  (for example a latch D) which enables a suspension of the tripping of the monostable  14  to be detected. The suspension of such tripping corresponds to the suspension of the oscillation of the current which means that there is no longer any current circulating in the power conductor  3 , or that such current is direct. The state detector  15  in this case supplies a signal error (passage into a logic state  1 , for example), such signal being applied to the triggering means  9  by an OR logic gate  16 . 
     The triggering means  9  thus cause the circuit breaking means  2  to open. 
     The state detector may be made in the form of a specialized electronic component (for example, a component known under the standardized name of 74HC74). It may simply be constituted by a software module incorporated into a computer, such module being programmed so as to detect the suspension of the oscillations at the monostable&#39;s  14  output. Such a computer may furthermore be programmed so as to monitor the shapes of more complex currents. 
     The triggering means  9  may, for example, by constituted by a control circuit to open the circuit breaking means  2 . If the circuit breaking means  2  are constituted by a static switch such as a thyristor, the triggering means  9  may, for example, be the electronic circuit controlling the trigger of said thyristor. 
     The measurement module  13  incorporates means  17  to ensure the measurement of the value of the current circulating in the supply network and the conductor  3 . The current measurement components are well known to the Expert and may be of analogical or digital technology. 
     The measurement module  13  also incorporates a memory or register  18  which incorporates a threshold value. Lastly, the measurement module incorporates a comparator  19  which ensures the comparison of the value measured by means  17  and the threshold contained in the memory  18 . 
     The comparator is designed so as to deliver a signal (for example, a logic state  1 ), such signal being applied to the triggering means  9  by the logic gate  16 . 
     Without departing from the scope of the invention, the memory  18  may be designed in the form of calibrated electronic components incorporated in an analogical comparator  19  made, for example, in the form of a bridge. 
     More simply, a computer may be implemented that ensures the digital comparison of the measured current with a value memorized in the computer. This solution offers the advantage of enabling the easy modification of the threshold by reprogramming. 
     As may be seen, using the invention enables the circuit breaking means  2  to be activated both during the appearance of an excessive intensity level and when the current passes from an alternating state to a direct state. 
     This results in improved protection for those components likely to be deteriorated by a direct current. 
     Note that the device according to the invention also ensures the breaking of the power circuits when the current is suspended. Insulation and protection of the circuit is thereby improved outside of the phase of use. 
     It is also possible for a logic module  12  to be implemented able to detect a direct component over an oscillating signal. For this, a stage comprising a filter to eliminate the alternating component followed by an evaluator measuring the direct component merely needs to be added to the logic module  12  in parallel with means  14  and  15 . This evaluator will incorporate a comparator linked to another memory in which a threshold level for the direct current will be programmed. 
     The circuit breaking means  2  may thus be activated when the current circulating in the control means incorporates a direct component exceeding a predetermined level. 
     The invention has been described with reference to diagrams which do not prejudice any technical means implemented in practice. 
     It is naturally possible for the means constituting the device  1  to be advantageously made in whole or in part using a micro-processor or computer which will incorporate software means firstly to ensure the comparison of the current&#39;s intensity level with a pre-programmed threshold and secondly to enable the suspension of the current&#39;s oscillations to be detected. 
     The invention has been described in its application to the protection of a demining coil. It may naturally also be applied to the protection of other types of electronic circuits likely to be deteriorated by direct current.