Abstract:
An electronic module for a detonator, the module comprising an electronic circuit ( 4, 5, 6 ) encapsulated in a mass of hardened resin ( 7 ) in which there terminates at least one inlet conductor ( 8 ) and from which there extend two outlet conductors forming a detonator ignitor line ( 9 ), the mass of resin ( 7 ) being received in a tubular housing ( 1 ) which extends beyond the mass of resin ( 7 ) adjacent to the ignitor line ( 9 ) to define a cavity ( 11 ), the outlet conductors of length greater than the depth of the cavity forming an ignitor line ( 9 ) short circuited outside the mass of resin inside the cavity ( 11 ), which cavity is provided with a removable end cap ( 12 ).

Description:
A standard detonator is usually in the form of a tubular stick filled with a detonating composition of flammable material in which an ignitor device is embedded for the purpose of reacting when powered with electricity. The ignitor device can be a resistance which transforms electricity into heat, two electrodes between which electricity is transformed into an arc passing through the flammable material, or through a suitable dielectric, . . . At one of its ends, the cylindrical stick thus has two electrical conductors enabling it to be connected to an external source of electricity. 
     BACKGROUND OF THE INVENTION 
     Nowadays, the use of pyrotechnics, e.g. in mining or quarrying for destroying a natural obstacle, or for demolishing a structure, . . . requires more and more elaborate blasting plans drawn up and executed by computers and microprocessors for processing data. 
     In this type of application, “electronic delay” detonators are used which include an electronic circuit with a microcontroller enabling information to be exchanged with a central unit controlling blasting and via which an electrical energy storage device is charged, and then discharged into the flammable material with a certain programmed delay. 
     Such electronic delay detonators have been in existence for many years. They are in the form of a one-piece product, the electronic portion being associated on manufacture with the explosive portion. 
     This one-piece characteristic gives rise to major constraints associated with the explosive material of the detonator. Strict regulations govern all the steps in the life of such a product, and in order to be satisfied this requires expensive procedures to be implemented during manufacture, handling, and transport (special packaging). In addition, the transport of such products by air is authorized only with special packaging that is very expensive and that has been approved by the appropriate national authorities. 
     Canadian document No. 2 132 148 published on Mar. 16, 1996 describes a detonator with an electronic pilot made up of two portions that can be manufactured separately and that possess means enabling them to be finally assembled together merely by engaging the explosive portion in the electronic portion. Nevertheless, the device described in that document suffers from numerous drawbacks concerning both manufacture of the electronic pilot and final assembly of the electronic portion with the explosive portion, particularly on site. The electronic module is embedded in a resin which presents at least one outwardly-open cavity with a very small female connector formed in the bottom thereof, and during manufacture it is very difficult to guarantee that the connector will conduct electricity. Furthermore, the nature of the conductors available at the outlets of standard detonators on the market do not make it possible to be sure that the end has been properly plugged into said connector without making adaptations to the conductors, which would require action to be taken on the detonators needing to be performed under the conditions specified by the regulations applicable to the presence of explosives, i.e. conditions that are expensive. 
     Document EP 0 843 807 describes an electronic delay detonator in which the body of the electronic module is assembled with the detonator by means of a snap-fastenable cap without giving other details as to how electrical connections are established which would appear to be possible in the factory only, given the complexity of the elements that are assembled together. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention seeks to remedy those drawbacks, i.e. to provide an electronic module suitable for being fitted to the explosive stick so that the resulting assembly is certain to be operational and so that said assembly can be made simply using any standard detonator on the market. 
     To this end, in a first aspect, the invention provides an electronic module for a detonator, the module comprising an electronic circuit encapsulated in a mass of hardened resin in which there terminates at least one inlet conductor and from which there extend two outlet conductors forming a detonator ignitor line, the mass of resin being received in a tubular housing which extends beyond the mass of resin adjacent to the ignitor line to define a cavity. According to the invention, the outlet conductors are longer than the depth of the cavity and form a short-circuited ignitor line outside the mass of resin inside the cavity, which cavity is provided with a removable end cap. 
     This module which possesses no explosive material can be manufactured, handled, and transported without special precautions, and in any event without it being necessary to satisfy the requirements of regulations concerning explosive materials. 
     In addition, since the output conductors short circuit the ignition line at the outlet from the mass of resin, they constitute a single wire closed on the electronic circuit, which is advantageous in several respects. Firstly, since the outlet of the electronic circuit is short circuited, electrical continuity is established which makes it possible to proceed with various tests during manufacture without it being necessary to close the ignitor circuit. Secondly, the short circuit made at the outlet from the electronic circuit guarantees complete discharge of the capacitance which the electronic circuit includes in conventional manner as means for storing the electrical energy required for igniting the detonator. This guarantee that the energy storage means is completely discharged makes it possible to connect the electronic module to the detonator proper in complete safety. 
     Also in preferred manner, the cap of the module of the invention is made in the form of a plug of elastically deformable material and the means for holding the detonator are formed merely by a central orifice in the cap which enables the end of a detonator to be engaged therein by force. The use of a cap made of elastomer makes it possible, while holding the detonator, to ensure that the assembly of the explosive stick with the electronic module is leakproof and provides the electrical connections with protection from one another. Another advantage of an elastomer cap lies in its ability to receive detonators of different diameters in the central orifice merely because it is radially elastic. 
     Still in preferred manner, the cap has a head portion with at least one outside transverse dimension greater than the corresponding inside transverse dimension of the cavity of the tubular housing. In other words, if the tubular housing is cylindrical, then the cap has a head portion of diameter greater than the inside diameter of the cylinder so as to bear against the end of the cylinder via the outwardly-projecting head. Without going beyond the ambit of the invention, the section of the tubular housing could be polygonal. 
     In a second aspect, the invention provides a method of making an electronic delay detonator, the method consisting in assembling a standard detonator to a module for testing the above characteristics, which method consists in separating the housing from the cap, in engaging the detonator in the cap via its end provided with the ignitor conductors, in connecting the conductors of the detonator to the outlet conductors of the electronic circuit, and in replacing the cap fitted with the detonator in the cavity. 
     It will be understood from the description of the method, that the operations involved are extremely simple and can be performed quite safely on the site where the detonators are to be used. 
     Naturally, insofar as the ignition line at the outlet from the electronic circuit is formed by a single looped conductor, it is necessary to cut the loop in order to make the connection with the detonator. 
     Finally, in order to improve the safety of personnel who are to assemble the electronic module and the detonator, in a third aspect, the invention provides tooling for implementing the assembly method when the cap has a head portion as described above, which tooling is constituted by a bell for taking hold of the detonator fitted with the cap, said bell forming a pusher for engaging the cap in the cavity of the electronic module. Thus, an operator who has withdrawn the cap from the electronic module engages the detonator stick in the cap and then engages the assembly in the bell which encloses the detonator in a volume that is designed so that in the event of the detonator exploding in untimely manner, said explosion takes place inside the bell and the hand and forearm of the operator are protected. The operator makes the connections after the cap has been put into place in the bell, and the bell enables the operator to force the cap back into place in the housing containing the electronic circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention appear from the description given below of an embodiment. 
     Reference is made to the accompanying drawings, in which: 
     FIG. 1 is a section view of an electronic module in accordance with the invention; 
     FIG. 2 shows the end of said module for connecting to a detonator; 
     FIG. 3 is a diagrammatic section of the detonator housed in the cap of the module; 
     FIGS. 4 and 5 are respectively a longitudinal section view and an end view of a particular embodiment of the cap of the invention; 
     FIG. 6 is a diagram showing how tooling of the invention is used when the detonator is assembled with the electronic module; 
     FIG. 7 shows a second embodiment of FIG. 6; and 
     FIG. 8 is a fragmentary detail view of the FIG. 7 embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The electronic module shown in FIG. 1 comprises a cylindrical tube  1 , preferably made of metal and open at both ends  2  and  3 . Inside the tube, an electronics card  4  together with its components  5  and an energy-storage capacitor  6  is held inside the tube by means of a hardened block of resin  7 . The resin can be cast into the tube containing the electronic circuit and can be allowed to harden in the tube which then acts as a mold. This block of resin is flush with one end  2  of the tube and it encapsulates the entire electronic circuit with the exception of an inlet conductor  8  and an outlet line  9  for igniting a detonator. The resin block secures the electronic circuit to the tubular housing  1  and serves to seal the circuit. At its end opposite from its end flush with the end  2  of the tube, the resin block is terminated by a surface  10  which is set back from the end  3  of the tube  1 . The portion of the tube which extends between the surface  10  and its end  3  defines a cavity  11  housing the ignitor circuit  9  and constituted by a cavity for receiving a cap  12  made of an elastomer material, and which is therefore elastically deformable. The cap  12  is forced into said cavity  11 . It possesses a head portion  13  of diameter greater than the inside diameter of the tube and serving to come into abutment via a shoulder against the end  3  of the tube. The cap is provided with a central orifice  14  which passes right through it, opening out into the cavity  11  via a portion of smaller diameter. 
     FIG. 3 shows the cap  12  with a detonator  15  forced into its orifice  14 , the detonator having power supply conductors  16  engaged through the narrow portion of the orifice  14 . 
     When the cap  12  is withdrawn from the cavity  11 , it is possible to deploy the ignitor circuit that is coiled therein in the form of a single wire, and to cut said wire so as to define two conductors  9   a  and  9   b  of length greater than the axial depth of the cavity  11 , for connection to the conductors  16  of the detonator  15 . 
     The conductors  9   a  and  9   b  are connected to the conductors  16  by means of conventional connectors  17  (see FIG. 4) which are in widespread use, particularly in the field of telephony. These are connectors which act like staples bridging together the conductors through their insulation by applying pressure and encapsulating the resulting connection in a semisolid substance to keep it sealed. Such a connection is easy to make since it is made outside the cavity  11 , given that the wires  9   a  and  9   b  are of sufficient length for their free ends to be beyond the end of the tubular housing. 
     In a preferred procedure for making an electronic delay detonator of the invention, prior to connecting the conductors  9   a  and  9   b  to the conductors  16 , and after the detonator  15  has been engaged in the cap  12 , the cap is placed in a handling bell  18  by engaging the head  13  of the cap into the opening of the bell which is appropriately dimensioned for this purpose. The inside volume  19  of the bell constitutes a volume in which the gases generated by a detonator that has exploded in untimely manner can expand in the unlikely event of the capacitor  6  of the electronic circuit still being charged at the moment a connection is made between the detonator and said circuit. The bell  18  thus provides effective protection for an operator handling it by means of a handle  20  situated opposite from the mouth of the bell. The handle can be of any shape, such as the shape of a screwdriver handle or of a doorknob. Once the connection has been made, the operator can reinsert the cap  12  in the cavity  11  by using the handling bell, the plugging of the tube  1  leaving sufficient space empty above the surface  10  of the resin block  7  for receiving the conductors  9   a ,  9   b , and  16  and also the connectors  17 . Provision is made for the residual space between the resin block and the cap  12  to be large enough so that the effect of the pressure that exists therein after the cap has been forced into engagement is not great enough to overcome the friction forces holding the cap in the tube and the friction forces holding the detonator  15  in the cap. 
     Nevertheless, in order to avoid leaving too great a volume which would lengthen the device pointlessly, a cap  12  can be provided of the kind shown in FIGS. 5 and 6, i.e. a cap whose thickness is hollowed out by three housings  21  partitioned by spacers  22  connecting an inner jacket  23  defining the orifice  14  to an outer jacket  24  which comes into contact with the tube  1 . Another way of limiting the amount of excess pressure established inside the cavity  11  when the cap is engaged in the open end of the tube  1  consists in providing longitudinal fluting  12   a  on the engagement portion of the cap, thereby providing an air vent during most of the stroke whereby the cap is engaged in the cavity  11 . 
     FIG. 7 shows some of the same elements as described with reference to the preceding figures. In this case, the handling bell  18  is formed by a metal enclosure  21  having an end wall  22  whose center is pierced and shaped around the pierced hole into a centering shape  23  corresponding to that of the top of the head  13  of the cap  12 . The inside volume of the enclosure  12  is filled with a relatively rigid cellular material  24  which possesses a housing  24   a  in register with the end wall  22  for receiving the detonator  15  as a friction fit, and extending away from the end wall  22  in the form of a portion  25  outside the enclosure  21  and constituting a handle for manipulating the bell  18 . This mass of foam is preferably enclosed in a film  26  of plastics material, e.g. a heat-shrink material which serves to hold the cellular material. The advantage of this disposition lies mainly in the cellular material forming an effective trap for detonator fragments or debris that would result from an untimely explosion of the detonator. The enclosure  21  can comprise two portions (a box having an end wall plus a lid screwed around the root of the handle  25 ) so as to make the portion made of cellular material easily interchangeable. 
     Finally, FIG. 8 shows a bell  18  which, for example, can be mounted on the mandrel of a tool for forcing the cap into the electronic module, which tool is in the form of a sensitive hand press with fixed tooling for holding the module surmounted by a moving vertical column fitted with a support mandrel for the bell. This disposition is mainly for use in making up detonators and ignitor modules in a workshop. The feature shown here lies in the cellular material being structured as two adjacent blocks meeting in a plane  27  that does not contain the axis of the detonator  15 . Thus, if an untimely explosion of the detonator were to occur, the fragments would remain trapped in the cellular material while the gas of the explosion would escape from the bell rearwards through the contact plane  27  by causing the blocks of cellular material (foam) to part as represented by dashed lines in the figure. For this purpose, the rear portion of the metal enclosure  21  has an opening  28  of section almost equal to the inside section of the enclosure so as to avoid reducing the flow section for the explosion gases.