Abstract:
This invention provides a method and electronic circuits for the testing of new initiation devices that use a small transformer whose heating element resistance can not be measured using traditional methods and instruments. A small current testing signal is applied to the primary winding of the detonator through its leg wires that are connected in a certain circuit configuration. The resistance of the secondary winding connected with the bridge wire is then reflected to the primary side and its effects are then observed. By calibrating the relationship of such effects with the value of loop resistance, the loop resistance can he displayed. Since the testing signal is very small in nature for safety considerations, the sampled effects (generally the voltage wave) are amplified. The amplified effects are displayed with analogue or digital meters or other display means.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method and electronic circuitry for measuring the electric resistance of the heating element in a transformer coupled initiator. In a particular example it can be used for measuring the loop resistance, including the bridge wire, in a transformer based electric detonator. 
     BACKGROUND OF THE INVENTION 
     The heating element, such as a bridge wire, of a conventional electric detonator is typically connected directly to the leg wires of the detonator. Its resistance can he measured by connecting the leg wires to a resistance meter (normally called a blasting ohmmeter) which uses much smaller electric currents than normal ohmmeters in the measuring process for safety reasons. However, in a transformer-based detonator, the bridge wire of the detonator is electrically isolated from the detonator&#39;s leg wires. Most often, the small transformer is encapsulated within the header of a detonator, leaving no direct electric access to the bridge wire. Therefore, it is not possible to measure the resistance of the bridge wire directly. That is, the resistance of the closed loop formed by the secondary winding and the bridge wire is isolated from DC in the primary. In use, the initiation energy is transformed from the primary winding to the secondary winding of the transformer via the magnetic linking between the two windings. Therefore, the resistance of the loop formed by the secondary winding and the bridge wire is designed to be in a certain range to receive the right amount of initiation energy so that the detonator can function reliably. If the loop resistance is too low, or too high, such as in the extreme cases of a short circuit or an open circuit, the detonator will fail to initiate. To make sure that the detonator receives the right amount of initiation energy, the bridge wire resistance is often designed to have a certain value with some tolerance. The measured loop resistance is the sum of the bridge wire resistance and the resistance of secondary winding. Since the resistance of the secondary winding is known and is determined by the design of the small transformer, the actual bridge wire resistance is obtained by subtracting the winding resistance from the loop resistance. 
     This apparatus described, and its use concern the method and circuitry for measuring the resistance of a closed loop, without touching the loop. In particular it can be used for checking the resistance of an initiation device which includes the use of two isolated windings. Examples of this kind of detonator are shown in U.S. Pat. No. 3,762,331 to Vlahos and U.S. Pat. No. 4,273,051 to Stratton. My co-pending U.S. patent application No. 08/992412 (assigned to Prime Perforating Systems) discloses the use of an isolated loop detonator in which a combination of different magnetic materials are used, enabling the detonator only to respond to a pre-determined frequency band. FIG. 1 is an illustration of such a detonator indicated generally as  20 . A heating element, in the nature of a bridge wire  22  of detonator  20  has a resistance Rx. It forms a closed loop  24  with secondary winding  26 . For safety considerations, loop  24  is electrically isolated from the primary winding, indicated as  30 . The two windings  26  and  30 , are magnetically coupled by a magnetic material  32  for the transmission of a firing signal having pre-determined characteristics, from leg wires  34 . Since primary winding  30  is isolated from the bridge wire  22 , it is not possible to measure the resistance of bridge wire  22  using a traditional blasting ohmmeter as is practised with conventional electrical detonators. However, the manufacturing process of the detonator requires that bridge wire  22  have a resistance as designed. Too great or too small a resistance may result in a detonator that does not explode as desired. Thus the measuring and monitoring of the actual bridge wire resistance is an important means of quality control. Also, a detonator is checked to assure that it is in good condition before it is used. There is a need for an instrument for measuring the resistance of such detonators. This instrument must not transmit a high energy test signal into the detonator, lest it explode. It must rely on a small signal and amplification. 
     U.S. Pat. No. 4,482,858 to Plichta discloses a method of testing a kind of electric detonator that has a ferrite core. The method is to use a small capacitor to store a calibrated amount of energy. This energy is discharged to a lead wire which forms the primary winding of the detonator. The value of the capacitor is calculated to ensure that the resulting RLC circuit, with the detonator to be tested, will be overdamped. Then the peak voltage over the primary lead wire is amplified and displayed. As the Plichta claims state, Plichta has “measurement means for effecting a single reading of a single peak value.” It measures a peak value of a discrete pulse, then permits a relatively long time period to elapse. During this time period the pulse decays. After the first test pulse has died, a second test pulse is generated. 
     Since the parameters of the apparatus are determined by the inductance, and the resistance of the bridge wire, and since the apparatus is calibrated by the module of the detonator to be tested, it may tend to be suitable only for testing the detonator for which it is built. It would be advantageous to make a testing meter whose parameters are independent of the detonator to be tested, therefore, it should be flexible to test different designs of transformer-based detonators with minimal adjustments or calibrations. 
     U.S. Pat. No. 4,649,821 to Marshall describes an electrical circuit continuity test apparatus for testing high-energy-discharge circuitry of a firing unit. It includes the use of a transformer structure to sense the capacitance change in the secondary winding. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention there is a method and circuitry for measuring the resistance of a transformer-based detonator. A wave train is generated and applied to a voltage sampling circuit that is configured to obtain the effects of loop resistance change in the secondary winding of the detonator transformer. The signal applied to the sampling circuit is attenuated so that the thermal effects in the detonator are negligible and the measuring process is relatively safe. The sampled voltage is then amplified, rectified and displayed. The loop resistance of the detonator can be read by calibrating the relationship between the loop resistance and amplified voltage. The voltage can be displayed using analogue or digital volt or galvanometers or logical circuits. 
     In another aspect of the present invention there is an apparatus for measuring the loop resistance of a transformer based detonation initiation device, the apparatus comprising a two terminal port for connection to a time varying test signal source. A test port is intermediate the terminals of the two terminal port, for connection to the leads of the transformer based detonation device, to form a circuit path between the two terminals. There is an output signal sensor connected to permit determination of steady state signals in the path, whereby the loop resistance can be deduced from the steady state signals sensed at the output sensor. 
     In an additional feature of that aspect of the invention the apparatus comprises a sampling element in series with the test port. The output signal sensor is connected to sense voltage at the sampling element. In another additional feature of that aspect of the invention the sampling element is a sampling transformer for obtaining a sample voltage isolated from the signal source. The sampling transformer has one winding connected in series with the test port and another winding whence the output signal sensor senses voltage. In an additional feature of that additional feature, there is a charge storage element connected in parallel to the test port and the sampling transformer. In another additional feature of that aspect of the invention, the apparatus includes a current limiter for limiting the current that can flow through the detonation device being tested. In a further additional feature of that additional feature, the current limiter is a resistor. In yet another additional feature of that aspect of the invention, the apparatus is designed for operation at frequencies preferably of at least 1 kHz. 
     In still another aspect of the present invention there is an apparatus for measuring the loop resistance of a transformer based detonation initiation device, the apparatus comprising a signal generator capable of generating a test signal wave train. There is a sampling circuit connected to receive the test signal wave train from the signal generator. The sampling circuit has a test port for receiving the transformer based detonation device in electrical connection therewith. The apparatus has a tap whence the properties of the signal in the sampling circuit can be determined and a signal processor for processing the signal sensed at the tap to yield an indication of said loop resistance. 
     In an additional feature of that aspect of the invention the signal generator has a test signal stabilizer. In another additional feature of that aspect of the invention the test signal stabilizer includes a voltage regulator. In still another additional feature of that aspect of the invention the test signal stabilizer includes a signal generator such as a timer. In yet another additional feature of that aspect of the invention the apparatus has a design frequency that is preferably at least 1 kHz. In a further additional feature of that aspect of the invention the apparatus includes a current limiter for limiting current flow in the sampling circuit when a transformer based detonation device is being tested. In a further still additional feature of that aspect of the invention the apparatus further comprises an under-voltage warning circuit. 
     In an additional feature of that aspect of the invention the sampling circuit includes a sampling transformer having a primary winding in series with the test port, and the tap is operatively connected to a secondary winding of the sampling transformer. In again another additional feature of that aspect of the invention the signal processor includes an amplifier responsive to the signal sensed at the tap. In a still further additional feature of that aspect of the invention the apparatus has a display having a first state indicating too low a loop resistance, a second state indicating too high a loop resistance, and a third state indicating loop resistance within specification. 
     In a further aspect of the present invention there is a method of determining the loop resistance of a transformer based detonation initiation device. The method comprises establishing the transformer based detonation transfer device to be tested in connection with a test port of a sampling circuit of a testing apparatus, exposing the sampling circuit, and the device to be tested, to a known wave train, sensing the steady state signal flowing in the sampling circuit, and determining from the sensed signal whether the loop resistance is within a specified range. 
     In an additional feature of that method aspect of the invention, the step of sensing includes monitoring a secondary winding of a sampling transformer whose primary winding is connected in series with the test port. In another additional feature of that aspect of the invention the method further comprises the step of generating the wave train, having a timed signal at a frequency preferably of at least 1 kHz. In a further additional feature of that aspect of the invention the step of generating the wave train includes providing a DC voltage to a voltage regulator, providing a low voltage warning circuit for warning of low voltage, and inverting a DC voltage output from the voltage regulator to yield the wave train. In yet another additional feature of that aspect of the invention the method includes providing a current limiter to limit current flow in the sampling circuit. In still another additional feature of that aspect of the invention the step of sensing is followed by the steps of amplifying the sensed signal and rectifying the amplified signal. In a yet further additional feature of that aspect of the invention the step of determining includes comparing the rectified signal with a high reference and with a low reference, and displaying a too high indication when the loop resistance is too high, displaying a too low indication when the loop resistance is too low, and displaying an acceptable indication when the loop resistance is within specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many of the advantages of the present invention will be apparent as the invention becomes better understood by reference to the following detailed description with the accompanying drawings, wherein: 
     FIG. 1 is an illustration of the structure of a transformer-based detonator, having an unknown resistance to be measured. 
     FIG. 2 shows a an example of a first, power supply, portion of an example embodiment of a circuit for measuring the unknown resistance of FIG. 1 according to the principles of the present invention. 
     FIG. 3 shows an example of a second, sampling and amplifying, portion of the example embodiment of a circuit for measuring the unknown resistance of FIG. 1, for connection to the first, power supply portion of FIG.  2 . 
     FIG. 4 shows an example of a third, output, portion of the example embodiment of a circuit for measuring the unknown resistance of FIG. 1, for connection to the second, sampling and amplifying, portion of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, loop  24  containing bridge wire  22  with resistance Rx, is electrically isolated from, but magnetically coupled with, the primary winding  30 . Only alternating signals (AC signals or DC Pulses) can reach the loop  24  to be measured. When a signal is applied to primary winding  30  through leg wires  34 , the resistance of the loop to be measured, loop  24 , is reflected to the primary side, yielding some electrical effects. The signal applied to detonator  20  must be sufficiently small that heat generated by bridge wire  22  is negligible to avoid safety problems during the measuring process of detonator  20 . The obtained effects are then simplified and displayed. 
     A resistance measuring apparatus is indicated generally as  50 . Part of this apparatus is shown in each of FIGS. 2,  3  and  4 . In FIG. 2, a power supply for the preferred embodiment is indicated generally as  60 . The source voltage Vs can be obtained with a 9V instrument battery  62 . S 1  is a power switch of the circuitry. When the power is switched on, LED 1  comes on and R 1  is connected in series with LED 1  to limit the current running through LED 1 . If the current rating of LED 1  is 2 mA, R 1  should be 4-5 Kohms. A voltage regulator  64  is used so that when the voltage of battery  62  drops, the voltage output of power supply  60  to the other circuitry, to be described, still remains at a predetermined value. In the embodiment, voltage regulator  64  is a  78 L 05 , the capacitors C 1  and C 2  are 1 micro Farad each. Voltage regulator  64  can stabilize the output voltage Vrs only if the source voltage Vs is in a certain level. When Vs drops below that level, the output of the regulator Vrs will drop too, and will consequently influence the measuring accuracy of apparatus  50 . The circuitry on the right side of  60 , is designed to monitor the voltage change. Diode D 1 , resistor R 2  and zener diode D 2  sample the voltage from the output end of voltage regulator  64 ; resistors R 3  and R 4  sample the voltage Vs from battery  62 . The operational amplifier OP-AMP 1  functions as a voltage comparator. When source voltage Vs drops below a predetermined level, OP-AMP 1  outputs a current through zener diode D 3 , and resistor R 5  to the base of transistor Q 1 . Then the transistor conducts, and current runs through R 6  and LED 2 . LED 2  comes on, giving a low battery voltage warning to remind the operator to change battery  62 . In the example illustrated, D 1  is 1N4148, D 2  and D 3  are 2.4 V zener diodes. R 2  and R 3  are 10 Kohms and 70 Kohms, respectively, R 4  is a 100 Kohm potentiamer. R 5  and R 6  are 2 Kohms each. The transistor Q 1  is 2N2222. 
     An alternating or DC pulse signal is needed for the measuring process. This signal can be a sinusoidal, triangular or square wave and can be generated by a number of alternative circuits. In the embodiment of the illustrative example shown in FIG. 3, a signal generator is indicated generally as  70 . It uses a 555 timer  72 , drawing power at Vrs to generate a square wave. Resistors R 6 x and R 7  are 1 Kohm and 35 Kohm, respectively, capacitors C 3  and C 5  are 1 micro Farad each. The output square wave is coupled by the capacitor C 4  (4.7 micro Farad in the embodiment) to a voltage sampling circuit indicated generally as  80 . The output signal frequency of signal generator  70  is approximately 2 kHz. 
     As shown in FIG. 3, voltage sampling circuit  80  has a pair of transformers X 1  and X 2 . Transformer X 1  is the small transformer of detonator  20 . Rx is, as noted above, the resistance of bridge wire  22  in loop  24  whose measurement is sought. Leg wires  34  of Transformer X 1  are connected into sampling circuit  80  at a test port P 3 , thus placing primary winding  30  in series with primary winding  82  of Transformer X 2  between resistor R 8  and a ground,  84 . Resistor R 8  is connected to capacitor C 4  of signal generator  70 , the connection defining a first terminal  86  of sampling circuit  80 . A further capacitor C 6  is connected in parallel with primary windings  30  and  82  between R 8  and ground  84 . The connection with ground  84  defines a second terminal  88  of sampling circuit  80 . Terminals  86  and  88  define a first two-terminal port, P 1 , for connection to a time varying source, namely the output square wave at capacitor C 4 , noted above, relative to ground  84 . 
     Transformer X 2  is mounted on a circuit board with resistor R 8 , and capacitor C 6 . It is comparable in size with Transformer X 1 . The Transformer X 2  is used to isolate the sampled voltage, Vx 1 , over capacitor C 7  from the signal source. The voltage Vx 1  is obtained as stated below. 
     Transformer X 1  has primary and secondary windings of N 1  and N 2  turns respectively. When a signal is applied to X 1 , the loop resistance Rx is reflected to the primary side according to the following relationship: 
     
       
         R APPARENT =(N1/N2) 2 Rx 
       
     
     For example, if the winding ratio N1/N2 is 2 and the loop resistance is 1 ohm, the apparent resistance seen from the primary side of the detonator is 4 ohms. When a constant wave train signal from signal generator  70  is applied, the electric current Ix running in the circle formed by X 1 , X 2  and C 6  is a function of the loop resistance Rx. The voltage Vx 1  across the small capacitor C 7  connected to the secondary winding of X 2  is in turn a function of Ix. The effects of Rx are measured by observing the voltage changes of Vx 1  at an output port P 2 . In the stated sampling circuit, Vx 1  changes inversely with Rx, That is, when Rx is short circuited, Vx 1  has the maximum value, and if RX is infinite, as in the case of an open circuit, Vx 1  has the minimum value. Resistor R 8  is used to limit the maximum possible electric current that may run through detonator  20 . R 8  is chosen so that the signal is strong enough for sampling but the thermal effects generated by the current running through Rx are not large. In the embodiment described, R 8  has a value of 500 ohms, dictating that the maximum current running in the circle of X 1 , X 2  and C 6  is about SmA (assuming that the duty cycle of the signal to be 0.5). Capacitors C 6  and C 7  are 4.7 micro Farad each. The detonator transformer X 1  has a ratio of 50:25 and the winding ratio for X 2  if 10:5 (the core of X 2  may be a Mn-Zn ferrite core having an outer diameter of about 10 mm). Since the heat generated by the current is proportional to the square of the current, therefore, the smaller the signal current, the safer the measuring process. Given that in blasting ohmmeters the testing current used is generally in the order of 50 mA, the preferred embodiment illustrated may well provide improved safety for the measuring process. Smaller signal currents (for example, less than 1 mA) are possible by increasing the value of R 8 . When lower testing current amperage is used, the sampled voltage will also be low, this can be compensated for by increasing the gain of the amplifiers so that the same magnitude of voltage can be displayed. Due to the small signal used, the magnitude of the sampled voltage Vx 1  is normally in the order of 20-80 mV. It is amplified so that it can be displayed and observed more easily. An amplifier, in the form of amplifying circuit  90  of FIG. 3 shows circuitry for amplifying Vx 1 . Two operational amplifiers, OP-AMP 2  and OP-AMP 3 , form a differential amplifier. The signal is further amplified by OP-AMP 4 . In the embodiment illustrated, a single supply is used for the operational amplifiers and they are not biased. Therefore, the signal is half-wave amplified. In the embodiment illustrated, resistors R 9  and R 10  are equal and have resistance of 10 Kohms. R 11  and R 12  are 50 Kohms each; R 13  and R 14  are 20 Kohms each and R 16  is 220 Kohms. R 15  is typically 10 Mohms. The amplified signal from the output of OP-AMP 4  is half-wave signal. The value of the resistors from R 9  to R 16  are chosen so that where Rx=0, and Vx 1  has its maximum amplitude, the output from OP-AMP 4  is close to saturation. 
     The half-wave signal from OP-AMP 4  is now easily detectable. For the convenience of display, it can be rectified. A circuit is shown generally as  100  in FIG.  3 . The amplified signal from amplifying circuit  90  signal being rectified by diode D 4 , to give a stable DC voltage Vx 2  over the capacitor C 8 . In the example shown D 4  is 1N4148 diode, CS is 0.47 micro Farad and R 17  is a bleeder resistor for CS having a value of 10 mega-ohms. Now Vx 2  is ready for display. Analog or digital volt meters or galvanometers can be used to display the value of Vx 2  that corresponds to the Rx values. 
     Once a detonator has been designed, the bridge wire resistance should take a certain value with a given tolerance. For a transformer-based detonator, if the loop resistance falls within a given value, it will be accepted. Otherwise, it will be rejected as an off-specification product, whether the resistance is lower or higher. Also, it may be of interest to know the reason why the detonator is off-specification. For example, if a batch of detonators tested all tend to have a too low, or too high, resistance, there may be a problem with the manufacturing process and equipment, that should be corrected. Therefore, the range within which the loop resistance falls, instead of the actual value of the resistance, may be of more concern. The display circuits indicated generally as  110  and  120  of FIG. 4 are designed to address this concern. 
     Circuits  110  and  120  form a three-state display circuit. In circuit  110 , two operational amplifiers OP-AMP 5  and OP-AMP 6  are used to form a double-limit voltage comparator. The upper limit of voltage V H  for comparison is obtained by adjusting the potentiometer POT 1 . V H  should correspond to the minimum loop resistance that is acceptable. Similarly, the lower limit of voltage V L  for comparison is obtained by adjusting potentiometer POT 2  and V L  corresponds to the maximum acceptable loop resistance. The voltage to be displayed Vx 2  is connected to the inverting input terminals of the two operating amplifiers, as shown. 
     The outputs Va from OP-AMP 5  and Vb from OP-AMP 6  are determined by the actual value of Vx 2  compared to V H  and V L . In circuit  120  of FIG. 4, the four CMOS NAND gates are used for logical operations of the results before they are displayed. 
     GateA reverts Vb to give an output Vb. C 9  and R 18  form an oscillation circuit with Gate B. The circuit is so designed that when the resistance measured in loop  24  is smaller than the lower limit, LED 3  comes on but LED 4  does not. When the resistance in loop  24  is higher than the high limit, LED 4  comes on but LED 3  does not. When the resistance is in the right range (RL≦Rx≦RH), LED 3  and LED 4  will turn on and off alternatively. Table  1  is the logical truth table of the three-state display circuit. In the embodiment shown, C 9  is 1 micro Farad. R 18  is 560 Kohms, giving an oscillating frequency of approximately of 2 Hz. R 19  and R 20  are 1 mega-ohms each. R 21  and R 22  are 1.5 Kohms each. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Logical truth table of the three-state display circuit 
               
             
          
           
               
                 RX value 
                 Vx2 value 
                 Va 
                 Vb 
                 {overscore (Vb)} 
                 LED3 
                 LED4 
               
               
                   
               
               
                 RX &lt; RL 
                 VX2 &gt; VH 
                 0 
                 0 
                 1 
                 1 
                 0 
               
               
                 RL &lt;= RX &lt;= VH 
                 VL &lt;= VX2 &lt;= 
                 1 
                 0 
                 1 
                 1* 
                 1* 
               
               
                   
                 VH 
               
               
                 RX &gt; VH 
                 VX &lt; VL 
                 1 
                 1 
                 0 
                 0 
                 1 
               
               
                   
               
               
                 *In this state, LED3 and LED4 come on alternatively at a frequency determined by C9 and R18.  
               
             
          
         
       
     
     In the method and circuitry illustrated, the parameters of the testing circuitry are independent of the detonator to be tested. For a certain design of the detonator, the inductance, as well as the winding and bridge wire resistance each fall in a given range. A testing instrument made according to the circuitry illustrated is adaptable to change in the parameters of the detonator to be tested. The testing apparatus can be adapted to test a detonator of a different design (new designs of a transformer-based detonator may vary in primary and secondary winding turns, the material of the transformer core as well as the size of the core, actual bridge wire resistance, and so on) by changing the calibration of the instrument using the new design of the detonator. In the case of a three-state display as described the apparatus can be adapted for testing a different type of detonator by changing the values of lower and upper limits V L  and V H  for the double-limit voltage comparator. 
     The preferred embodiment described above has been provided to elucidate the method and circuitry of the present invention. Modifications are possible without departing from the spirit and scope of the principles of the present invention. For example, a dual power supply can be used in place of a single supply. Varied signal generators may be used and varied sampling circuits and display means are also possible as mentioned early in the text. The principles of the invention are not limited by this embodiment but only by the claims appended hereto.