A MEANS FOR AND A METHOD OF INITIATING EXPLOSIONS

Abstract: An alternating current generator for use as a shot exploder suitable for firing electric detonators coupled through toroid transformers e.g. 'Magnadet' detonators. The detonating system is energised by means of a signal that is automatically generated at the resonant frequency, thus avoiding the usual time-consuming and dangerous practice involving in-situ determination of inductance and selection of capacitors.

Description: THIS INVENTION relates to a means for and a method of initiating explosions. More particularly, it relates to a means and method utilisable with toroid coupled detonators such as that developed by ICI and marketed under the trade name "Magnadet". Toroid coupled detonators such as that described above are used together with ferrite rings. Each detonator has its own associated ring, with the leading wire from each detonator being threaded several times (typi.cally 4 turns) about its associated ring, to form a secondary circuit. The length of the leading wires is such as to ensure that the rings are situated at the mouth of each blast hole and energy is fed from an exploder to the system via a primary wire which is threaded once only through each ring. With such a system described above, an attractive feature is the frequency selective characteristics of the ferrite rings. Thus, the rings have a band-pass characteristic which effectively attenuates low frequency signals having a frequency below about 10 t-Hz and high frequency signals having a frequency above about 100 kHz. Thus, as the leading wire of each detonator constitutes an isolated closed loop the detonators are substantially immune to stray currents and earth leakage. A problem with such systems is that at frequencies of 15-25 kHz (which is the frequency range in which the best energy transfer is obtained via the ferrite rings) there is a considerable loss of firing energy due to the inductance of the system. The applicant is aware that an attempt has been made to overcome this problem by utilising a series capacitor in an attempt to operate the system in a series resonant mode. However, it will be appreciated, that the inductance of the system will vary in accordance with the number of ferrite ring and associated detonator units utilised, the configuration of the primary wire, and the like. Thus, if a shot exploder is used which generates a detonating signal at a 9Risei -fr quency, then each system will require a series capacitor having a particular capacitance that will result in series resonance at.the fixed frequency. It is thus necessary to measure the inductance of each system in situ, compute the capacitance required, select a suitable capacitor from a stock thereof, and then insert the capacitor in circuit with the system. This procedure is time consuming, dangerous, and requires a stock of capacitors and skilled personnel. Accordingly, the invention provides a means for initiating explosiqns, which includes a power oscillator means for generating an oscillating · electric initiating signal of sufficient power at a variable frequency; a frequency setting means to which the oscillator means is responsive whereby in use, when the power oscillator means is connected to a load it automatically generates a signal at the resonant frequency of the load. The explosion initiating means may be a shot exploder. The shot exploder may then have output connecting means to which the ends of a primary wire forming part of a detonating system as described above are connected. A resonance capacitor may be serially connected with the output connecting means. Further according to the invention there is provided a method of initiating explosions, which includes providing a detonating system that is operable by an oscillating electric signal and which has an undetermined impedance; automatically generating an oscillating initiating signal at the resonant frequency of the system. The invention extends to an initiating means as described, in combination with and connected to an A.C. operable detonating system. Those skilled in the art will appreciate that the operating frequency of oscillators is normally determined by suitable elements or networks. With the shot exploder of the invention, the detonating system itself may, in use, constitute part of the oscillator means. In one embodiment, the power oscillator means may include at least one controllable element and the frequency setting means may include a positive feedback link for controlling operation of the element in accordance with the voltage or current supplied to the detonating system. The, or each, controllable element may be switchable and may conveniently be switchable on and off, such as a transistor. This switchable element is then switched in phase with the initiating signal. Alternatively, the power oscillator means may include an amplifier. One form of feedback linking means may include a transformer whose primary current is the initiating signal current and whose secondary current controls the said switchable element or elements or amplifier. An alternative form of feedback linking may be a direct link whereby the initiating signal current directly controls the switchable element or elements. A still further feedback link may include a detector which senses the current in the output circuit and an amplifier responsive to the detector providing a current output in phase with the initiating signal current to control the switchable element or elements. The transformer - coupled and direct links are advantageous for low power requirements. The detector and amplifier link is advantageous for higher power requirements. The explosion initiating means may also advantageously include a voltage setting means to provide a predetermined firing current into any fixed load within the operational impedance range of the power oscillator. As a further feature, an auxiliary inductance may be provided for reducing the resonant range. This auxiliary inductance may be in series with the connecting means. The power oscillator means may be D.C. operable and the voltage setting means may then include a controllable voltage supply means for supplying to the power oscillator a variable voltage D.C. supply and a sensing means for sensing the magnitude of current supplied, in use, to the detonating system, the voltage supply means being responsive to the sensing means. The shot exploder may also include a timing means such that a detonating signal is supplied for a predetermined period of time. The shot exploder of the invention makes it unnecessary first to determine the inductance of a detonating system and then to compensate therefor by means of a resonance capacitor to obtain a predetermined resonant frequency. Thus, the detonating system in energised by means of a signal that is automatically generated at the resonant frequency. The invention is now described, by way of examples with reference to the accompanying drawings, wherein all like components are similarly referenced and in which: Figure 1 shows schematically a detonating system of the type with which a shot exploder in accordance with the invention is used; Figure 2 shows an equivalent circuit of the detonating system; Figures 3 and 4 show two circuit diagrams of power oscillators used with a shot exploder of the invention utilising a transformer coupled feedback link. Figure 5 shows the circuit diagram of a shot exploder in accordance with the invention, incorporating the circuit of Figure 4. Figure 6 shows a circuit diagram of an alternative power oscillator utilising a direct feedback link. Figure 7 shows a circuit diagram of a further alternative power oscillator utilising a current detector and amplifier feedback link. Figure 8 shows a circuit diagram including a further oscillator for use in association with a power oscillator in a shot exploder to give a pre-set output voltage. Figure 9 shows a circuit diagram of a shot exploder n accordance with the invention, incorporating the circuits of Figures 7 and 8. Reference is initially made to Figure 1. Shown therein generally by reference numeral 10 is a detonating arrangement. The detonating arrangement 10 comprises a shot exploder 12 connected to a detonating system 14. The detonating system 14 comprises a number of detonating modules 16. Each detonating module 16 comprises a standard electric detonator 18 which is coupled with a ferrite ring 20 by means of a loop of leading wire 22. As shown, each leading wire 22 is wound a few times around its ferrite ring 20. The detonating system 14 further comprises a firing cable 24 and a primary wire loop 26, the latter being passed through the ferrite rings 20. Further as shown, one end of the firing cable 24 is connected to the shot exploder 12 and the other end to the primary wire loop 26. Referring now to Figure 2, an equivalent circuit diagram of the detonating arrangement 10 is shown. Thus, the firing cable 24 and primary wire loop 26 are represented by an inductance 28 and a resistance 30 whereas the detonating modules 16, as referred back to the primary loop 26, are represented by a resistance 32 and an inductance 34. The inductance 28 typically has a value of 60-600 pH and the resistance 30 has a value of 5-10 ohm. Similarly, the resistance 32 has a value of N x 0,125 ohm where N is the number of detonators and the inductance 34 has a value of N x 2,5 H. As indicated earlier, the ferrite rings 20 are frequency selective and have an optimal energy transfer characteristic in the frequency range of 15-25 kHz. It will thus further be appreciated that at these frequencies the inductive characteristic of the detonating system 14 is significant. In order to eliminate the inductive effect the shot exploder 12 incorporates a series capacitor 36 which is of a suitable value so that when used with detonating systems 14 of a specified type the series resonant circuit formed thereby has a resonant frequency between 15 and 25 kHz. Refering now to Figure 3, shown therein is a power oscillator arrangement 38 which is connected to the detonating system 14. In Figure 3, the inductances and resistances shown in Figure 2 have been lumped together to provide an inductance 40 and a resistance 42. The oscillator arrangement 38 further has an auto-trasformer 44 and a feedback transformer 46. The auto-transformer 44 is serially connected with the detonating system 14 via the primary winding 46.1 of the feedback transformer 46 and the resonance capacitor 36. At the heart of the oscillator arrangement 38 is a transistor 48 which is controlled by a feedback loop from the secondary winding 46.2 of the feedback transformer 46. In order to protect the baseemitter junction of the transistor 48 a reverse polarity freewheeling diode 50 is provided. An energy storage capacitor 52 is also provided. It will be appreciated that the oscillator arrangement 38 is self-tuning in that it will generate an osci-llating signal at the resonant frequency of the circuit formed by the auto-transformer 44, the feedback transformer 46, the resonance capacitor 36 and the detonating system 14. Thus, in operation, once the oscillator arrangement 38 is triggered (as is indicated below with reference to Figure 5) the transistor 48 is switched on and current starts to flow through the primary winding 46.1. The polarity of the secondary winding 46.2 is chosen such that positive feedback to the transistor 48 is provided. Thus the transistor 48 remains switched on while the output current flows in the original direction. When current flow reverses the transformer 46 turns the transistor 48 off. With the next reversal of current polarity, to the original direction, the transistor 48 is switched on again and the process is repeated. The positive feedback signal applied to the switching transistor 48 is proportional to the load current and is always in phase with it. The oscillator arrangement 38 accordingly generates a signal at the resonant frequency of the load, providing the inductance of the load circuit is within reasonable limits (say 50 rH to 1 mH). Referring to Figure 4 an alternative oscillator arrangement 38.1 is shown. This arrangement 38.1 is similar to the arrangement 38 of Figure 3, except that two transistors 48 are used in a push-pull configuration. The various components shown in Figure 4 are similarly referenced to those in Figure 3. As the operation of the circuit shown in Figure 4 will be self-evident to those skilled in the art if reference is made to Figure 3, it will not be described further. Although the auto-transformers 44 produce a squarewave output voltage signal, the current in the firing loop is sinusoidal as known from the theory of resonant circuits. The firing current therefore contains a low proportion of harmonic frequencies. This is a very useful feature of the exploder although the harmonics consume the exploder output power, they are attenuated by the ferrite rings and by the inductance of the detonator leading wires and therefore they contribute very little to the transfer of energy to the detonators. Reference is now made to Figure 5. Shown therein is a circuit diagram of a shot exploder 54 in accordance with the invention. The shot exploder 54 has output terminals 56 to which a detonating system such as that described earlier and referred to by reference number 14 may be connected. The shot exploder 54 also has a power oscillator arrangement 38.1 similar to that shown in Figure 4 and similarly referenced. However, the auto transformer 44 is a step-up transformer which provides an output signal of about 115 volt peak with a supply voltage of about 35 volts. A controlling triac 58 is also provided in series with the secondary winding 46.2. It will be appreciated by those skilled in the art that when the triac 58 is switched on its associated transistor 48 will be triggered thereby starting up the oscillator arrangement 38.1. Further, whilst the triac 58 is energized the oscillator arrangement 38.1 is enabled. The shot exploder 54 further has a rechargeable battery 60 and a key-operated switch 62. In the position shown in Figure 5r the switch 62 is off and the exploder 54 is inoperative. When the switch 62 is closed a storage capacitor 64 is charged. The voltage across the capacitor 64 is monitored by a level detector 66 which provides an output signal when the voltage across the capacitor 64 is at a specified value (35 volts). The level detector 66 operates a timer 68 which supplies an output signal of about 4,SmS duration. The output signal of the timer 68 energises a light emitting diode 70 and also energises the triac 58 which thereby triggers the oscillator arrangement 38.1 and enables it for the 4,5mS. With a detonating system connected across the output terminals 56 an oscillating signal at resonant frequency is then supplied to the detonating system which initiates the detonators of the system. It will be noted that the battery 60 may also be charged via the output terminals 56, a unidirectional charging link being provided by diodes 72 and resistor 74. Referring now to Figure 6, a further embodiment of a power oscillator arragnement 38.2 is shown, employing a direct feedback link. The arrangement 38.2 is similar to the arrangement 38.1 of Figure 4 except that the transformer 44.1 has an isolated secondary winding 44.2, and the load current passes directly to the transistors 48.1 and 48.2 instead of through the feedback transformer 46 of Figure 4. The oscillator arrangement 38.2 is self-tuning to the resonant frequency of the series circuit formed by the transformer 44.1, capacitance 36, inductance 76, detonating system 14, and the direct feedback link to transistors 48.1 and 48.2. The operation of the arrangement 38.2 is similar to that of the arrangement shown in Figure 4, except that the load current directly controls the switchable elements 48.1 and 48.2, instead of via the feedback transformer 46 of Figure 4. The polarity of the secondary winding 44.2 of transformer 44.1 is chosen such that positive feedback to the switchable elements 48. and 48.2 is provided. Free wheeling diodes 75.3 and 75.4 allow a safe rundown of system energy if the option of stopping the signal after a predetermined time is taken. Reference is now made to Figure 7, which shows a power oscillator arrangement wherein a detector and amplifier circuit 79 supplies the necessary positive feedback signal to transistors 48. It will be appreciated that when a series tuned circuit is driven at its resonant frequency, the resulting current is in phase with the drive voltage. For a square wave drive voltage the current therefore crosses zero at the instant the drive voltage changes polarity. The voltage across series rcsistor 78 is a measure of the current in the series tuned circuit. My monitoring the voltage across resistor 78, and causing the respective drives to transistors 48 to reverse at the instant the said voltage crosses zero, the drive will be oscillating at the resonant frequency. The detection and amplification of the feedback voltage across resistor 78 is carried out by the zero-crossing detector and amplifier circuit 79. To compensate for propagation delays in circuit 79, a small series inductor 77 is included to advance the feedback voltage signal relative to the current in the tuned circuit. If the circuit 79 is polarity dependent, the polarity of secondary winding 44.2 frill need to be defined. It will be appreciated that the circuits of Figures 3 to 7 will supply the detonating system 14 with a firing current that will vary depending on the load. However, it is generally desirable that the firing current be above a certain specified minimum level in order to minimise the delay time spread of delay detonators. The circuit of Figure 8 is designed to preset the output voltage of the circuits of Figures 3 to 7 according to the value of the load, thereby giving a constant output current above the specified minimum level. In the circuits of Figures 3 to 7 the secondary voltage of the transformers 44 and 44.1 within their linear range (ingoring losses), will be given by E = t V STG (1) where E is the secondary voltage, t is the transformer turns ratio, and VSTG is the voltage on capacitor 52. Referring to Figure 7, the series circuit 84, to which the voltage E is applied to initiate detonating system 14, comprises capacitance 36, inductance 76, detonating system 14, inductance 77 and resistance 78. As the series circuit 84 operates in a series resonant mode, the load impedance as seen by an applied voltage e will appear resistive and will conform to the basic electrical equation e = ir .................... (1) Where e is the applied votage i is the resulting current, and r is the load resistance. The total resistance RT of the series circuit 84 (as seen by the applied voltage e) is the sum of the resistances included in the said series circuit (42 and 78), the resistive losses of the reactive components in the said series circuit at the driven frequency (36, 76 and 77) and the resistive losses in the transformer 44.1 Thus, from equation (2) the current is produced in resistance RT when voltage e is applied is riven by the equation i = 1 x e (3) RT In particular, from equations (1) and (3), if the capacitor 52 is charged to a value VSTG the power oscillator output voltage will be tVsTG, and the resulting load current I is given by the equation 1=1 x t VsTG (4) T The oscillator shown in Figure 8 generates a search current in the series circuit 84 for determining the supply voltage required for the power oscillators of Figs. 3 to 7 to deliver the necessary firing current. This oscillator selftunes in a similar manner to that described for Figure 7. If the output voltage of the search current generator is a fixed portion of the power oscillator output voltage, from equation (3) the resulting search current will be the same fixed proportion of the expected firing current. Denoting that proportion as 1, then from equation (4) S I x 1 = 1 x t VsTG xl (5) SR S T where tVSTG x 1 is the oscillator output voltage -S and the resulting search current is I x 1 -S It will be apparent that the value of S must be such that the current is insufficient to initiate the detonating system 14. It will be seen from equations (4) and (5) that when the search current reaches a value of If x I, where If is S the desired firing current to initiate the detonating system 14, the value of VSTG' the voltage on capacitance 52, will be sufficient for a power oscillator of any of Figures 3 to 7 to initiate the detonating system. In the circuit of Fig. 8 circuit 83 is the search current generator which also measures the amplitude of the search current produced. Circuit 82 is a charging control circuit interposed between an energy source 81 and the energy storage capacitance 52. Circuit 82 will include a switchable element such as a transistor to enable the charging current to be stopped at the required VsTG in response to a signal from circuit 83. Circuit 85 is a firing control circuit which is also responsive to circuit 83, to provide the triggering signal for a power oscillator of any one of Figures 3 to 7. Indicator 86 shows when the exploder is ready to fire, and indicator 87 shows that the load is outside the specified range of the exploder 12. The operation of the circuit of Figure 8 begins with the connection of the series circuit 84 to the output of the search current generator 83. A resistance 80 is included in series with the said output to simulate the resistive losses in the transformer 44.1 of Figure 7, and also to modify the relation between the expected firing current If and the total resistance of the series circuit R. The modification is arranged to allow for the fact that while an approximately constant current is drawn from capacitance 52 when firing, resulting in an approximately constant rate of voltage decay, the percentage rate of voltage decay is greater when the initial voltage is lower. The percentage rate of firing current decay is therefore greater when the load resistance is low. Low resistance loads are therefore given a higher initial firing current than high resistance loads for constant impluse energy. The exploder energy source 81 is then connected via the charging control circuit 82 to the energy storage capacitance 52. As the voltage on capacitance 52 increases, the output voltage of the search current generator 83 also increases as indicated in equation (5) above. When the amplitude of the search current reaches a value of If x I, further charging of capacitance 52 is S prevented by a trip signal from circuit 83 to circuit 82. Circuit 85 is simultaneously signalled by circuit 83 that the exploder is ready to fire., and indicator 86 is energised. In the circuit of Fig. 9 switch 88 is a safety switch (shown in the normal or safe position) whereby the capacitance 52 is discharged via resistance 91. Firing switches 89 and 90 are ganged, and are shown in the normal or test position. In the operational sequence for firing a detonating system 14, switch 88 is operated and the part of the shot exploder circuit as shown in Figure 8 is completed. Capacitor 52 will start charging from the energy source 81 (which may be a hand cranked generator), via the charging control circuit 82. Concurrently, the search current generator 83 will apply an alternating voltage, self-tuning to the resonant frequency of the series circuit 84, having an amplitude proportional to the instantaneous voltage on capacitor 52. When the resulting search current is of the required amplitude, circuit 83 will simultaneously signal circuit 82 to prevent further charging, signal circuit 85 that the exploder is ready to fire, and energise indicator 86. Switch 88 should remain operated, and switches 89 and 90 should not b operated. The part of the shot exploder circuit from Figure 7 is now completed. Circuit 85 triggers circuit 79 to start the firing sequence as described for Figure 7, and can stop the sequence after a predetermined time if required. Every shot exploder has specified limits to the load impedance into which it is capable of firing the required initiating current. Thus, in a shot exploder of this invention the circuit component ratings dictate the maximum allowable voltage on capacitance 52, and hence the maximum allowable load resistance. Also, the maximum and minimum values of oscillating current frequency to which the detonating system 14 will respond efficiently and/or to which the shot exploder circuitry can respond, will dictate the minimum and maximum values of load inductance 40 that can be tolerated, given the values of capacitance 36 and inductances 76 to 77. The resonant frequency of the series circuit 84 is given approximately by the equation EMI19.1 where L is the total inductance of the said series circuit, the sum of inductances 40, 76 and 77, and C is capacitance 36. In a shot exploder according to the invention incorporating the circuit of Fig. 9, preliminary measurements of the search current are used to ensure that the separate limits of load resistance and inductance are not exceeded. If any limit is exceeded, circuit 83 will signal that the load is outside the specified range, that the exploder is not ready to fire, and indicator 87 is energised The exploder is therefore inhibited from firing even if switches 89 and 90 are operated in an attempt to fire. A shot exploder according to the circuit of Figure 9 will therefore provide a constant current firing output into a detonating system of undetermined impedance. The firing circuit components are thus protected from overload, and the exploder is efficient in the use of energy from its energy source. The shot exploder circuit of Fig. 9 also provides a time-delay self-discharge mechanism to prevent a partially or fully charged exploder from remaining in that state any longer than necessary.

Claims: CLAIMS 1. A means for initiating explosions which includes a power oscillator means (48.1, 48.2, 79) for generating an oscillating electric initiating signal of sufficient power at a variable frequency, characterised in that a frequency setting means (46, 38, 77, 78, 79) is provided to which the oscillator means is responsive whereby, in use, when the power oscillator means is connected to .a load it automatically generates a signal at the resonant frequency of the load. 2. An explosion initiating means as claimed in Claim 1, characterised in that the frequency setting means (38, 46, 77, 78, 79) includes a resonance capacitor serially connected with an output connecting means (56). 3. An explosion initiating means as claimed in Claim 2, characterised in that the power oscillator means includes an amplifier means (79) and the frequency setting means (77, 78, 79) is connected in a positive feedback manner to the amplifier means (79). 4. An explosion initiating means as claimed in Claim 2, characterised in that the power oscillator means includes a switchable element {48.1, 48.2) and the frequency setting means (77, 78, 79) is controllably connected with the switchable element to switch it, in use, in phase with the initiating signal. 5. An explosion initiating means as claimed in Claim 3 ar Claim 4, characterised in that the frequency setting means includes a transformer (46) having a primary winding (46.1) and a secondary winding (46.2) the primary winding being serially connected with the output connecting means (56), and the secondary winding being connected with the amplifier means (79) or the switchable element (38). 6. An explosion initiating means as claimed in Claim 2, characterised in that the power oscillator means includes an output transformer (44). 7. An explosion initiating means as claimed in any one of the preceding claims, which includes a voltage setting means (82) to provide, in use, a predetermined initiating current to a detonating system (32, 34) connected thereto, the detonating system having any resistive impedance within a predetermined operational range. 8. An explosion initiating means as claimed in any one of the preceding claims, which includes a timing means (68, 85) for providing that the initiating signal is supplied for a predetermined period of time. 9. An explosion initiating means as claimed in Claim 3 or Claim 4, characterised in that the frequency setting means includes a feedback voltage generating impedance (77, 78) in series with the output connecting means (56) and a zerocrossing detector and amplifier unit (79), the voltage across the said impedance being supplied to the detector and amplifier unit, the detector and amplifier unit being connected with the amplifier means (79) or the switchable element (38). 10. A method of initiating explosions, which includes providing a detonating system that is operable by an oscillating electric signal and which has an undetermined impedance, and automatically generating an oscillating initiating signal at the resonant frequency of the system.

Inventors: GELLER, JOHN MICHAEL EWEN, PLICHTA, BOHUMIL MARIA JAN, WILSON, JOHN PURDIE
Applicants: IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1981
Main classification: F42C15
IPC codes: F41A19