Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – In an ignitor or detonator
Reexamination Certificate
1998-08-24
2001-04-03
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
In an ignitor or detonator
Reexamination Certificate
active
06211682
ABSTRACT:
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'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 t
Blake Cassels & Graydon LLP
Metjahic Safet
Prime Perforating Systems Limited
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