Ammunition and explosives – Igniting devices and systems – Accidental fuse ignition prevention means
Reexamination Certificate
1998-04-15
2001-02-27
Tudor, Harold J. (Department: 3641)
Ammunition and explosives
Igniting devices and systems
Accidental fuse ignition prevention means
C102S202700, C102S202900
Reexamination Certificate
active
06192802
ABSTRACT:
FIELD OF INVENTION
This invention generally relates to an electro-explosive device and, more particularly, to a radio frequency and electrostatic discharge insensitive electro-explosive device having improved firing efficiency.
BACKGROUND OF THE INVENTION
In general, an electro-explosive device (EED) receives electrical energy and initiates a mechanical shock wave and/or an exothermic reaction, such as combustion, deflagration, or detonation. The EED has been used in both commercial and government applications for a variety of purposes, such as to initiate airbags in automobiles or to activate an energy source in an ordnance system. With reference to
FIG. 1
, a typical EED
10
comprises a thin resistive wire or bridgewire
12
suspended between two posts
14
, only one of which is shown. The bridgewire
12
is surrounded by a flammable or explosive compound
18
, commonly referred to as a pyrotechnic mix. To initiate combustion of the pyrotechnic mix
18
, a DC or very low frequency current is supplied through lead wires
16
and posts
14
and then though the bridgewire
12
. The current passing through the bridgewire
12
results in ohmic heating of the bridgewire
12
and, when the bridgewire
12
reaches the ignition temperature of the pyrotechnic mix
18
, the pyrotechnic mix
18
initiates. The pyrotechnic mix
18
is a primary charge which ignites a secondary charge
20
, which in turn ignites a main charge
22
. The EED
10
further comprises various protective elements, such as a sleeve
23
, a plug
24
, and a case
26
.
Although the EED
10
is a well known device, the electromagnetic go environment in which EED's operate has changed dramatically over the past four decades. One change that has occurred in the operating environment for the EED's is that the EED's are being subjected to higher levels of electromagnetic interference (EMI). The necessary operation of high power radar and communication equipment in the proximity of EED's, such as in an aircraft carrier flight deck, has resulted in a typical operating environment that includes high intensity electromagnetic fields. The EED which initiates an airbag in an automobile may be subjected to severe EMI during the normal life-span of the automobile. Thus, EED's are being subjected to high levels of EMI in both military and non-military environments.
The high intensity radio-frequency (RF) fields present a serious EMI problem by coupling electromagnetic energy either through a direct or indirect path to an EED, so as to cause accidental firing. Electromagnetic energy may be coupled directly to the EED when RF radiation is incident on the EED's chassis whereby the EED acts as the load of a receiving antenna. The electromagnetic energy may alternatively be coupled indirectly to the EED when RF induced arcing occurs in the vicinity of the EED and is coupled to the EED, such as through its leads. An RF induced discharge can occur whenever a charge accumulated across an air gap is sufficient to ionize the gas and sustain an ionized channel.
The EED's which are located in the vicinity of intense RF fields, such as naval surface ships, may receive signal components due to rectification of RF radiation. The RF radiation can be rectified, for instance, due to simple metal contact diode action, which is generally caused by corrosion of contacts or incorrectly connected fasteners. The rectified signal may have components that are at much lower frequencies than the source RF radiation and may also contain a DC component, any of which may couple to the EED and cause accidental ignition. The RF radiation may be rectified in many environments in which an EED may be found, including an automotive environment where large currents or voltages are switched very quickly thereby producing high levels of noise.
Another manner in which an EED may be accidentally discharged is by the coupling of an electrostatic discharge (ESD) to the EED. An ESD is characterized as a signal which is of a high voltage and fairly low energy. While the energy of the ESD is usually insufficient to cause any significant ohmic heating of the EED, the high voltage can create a sufficiently large electric field between the input pins of the EED to ignite the pyrotechnic mix.
One approach to protect an EED from EMI is to install one or more passive filters. Several standard types of passive filters exist which can be utilized to attenuate stray RF signals. These filters can usually be classified as either L, Pi, or T types, or as combinations of the three types. The L, Pi, and T type passive filters, which are respectively illustrated in FIGS.
2
(A), (B), and (C), have traditionally been used as a first measure of eliminating EMI problems.
Conventional passive filters being used with EED's, however, have several disadvantages. A conventional filter consists of a combination of inductors, capacitors and/or other lossy elements, such as resistive ferrites. In general, the performance of the filter is directly proportional to the number and size of the elements used in its construction. Thus, a filter can be designed to attenuate a signal to a larger extent if the size of the inductors, capacitors and ferrite sleeves are all increased. Also, a filter having a greater number of stages will generally have an improved performance. The size of the filter, however, is often limited by the amount of available space. As a result, it may not be possible to add a filter to an EED or the filter which can fit within the available space may be ineffective in protecting the EED from EMI.
The filters are usually constructed from standard passive components assembled on a printed circuit board or hard-wired within a metal chassis. A typical example of an RF filter
30
is shown in FIG.
3
(A). The RF filter
30
comprises, inter alia, a ceramic capacitor
32
and a wound torroidal inductor
34
. As shown in FIG.
3
(B), the ceramic capacitor
32
has a plurality of electrode layers
38
separated by a ceramic dielectric material
36
. As should be apparent from FIG.
3
(A), the size of the capacitor
32
and inductor
34
render the filter
30
too large for many applications, such as with weapon systems where space is especially limited. Therefore, a need exists for a small sized EED which is adequately protected from EMI.
In addition to the constraint of available space, the cost of the EED and filter can also limit the size of the filter. The cost of each filter is directly related to the number of capacitors, inductors, and other elements forming the filter. Even though some filters may have only a few components, the cost per unit price in assembling the filter may be relatively high in comparison to the cost of an EED. Thus, with a large scale production of EED's and their associated filters, the overall increase in cost can become quite substantial.
A further disadvantage to passive filters is that they are unable to filter out many low frequency signals which can cause accidental firing of the EED. Because the signal for firing an EED is a DC signal, the conventional filters are designed to freely transmit DC and other low frequency signals. These filters, therefore, are unable to attenuate the low frequency signals due to rectification of RF signals as well as other low frequency or DC signals.
Even with a filter that can effectively filter many types of EMI, the EED is not completely safe from accidental firing. In a conventional filter system, the filter and EED are essentially two separate components. With reference to
FIG. 4
, a non-propagating magnetic field B may induce an emf via closed loop induction. The emf is proportional to wAB, where B=moH, A is the cross-sectional area, and w is the frequency of the magnetic field B.
The EED can be further protected from EMI by shielding. The shielding of an EED, however, is effective only if construction of a barrier and operational procedures can guarantee the integrity of the shielding structure. When a large number of EED's are manufactured, it becomes likely
Auburn University
Tudor Harold J.
Womble Carlyle Sandridge & Rice PLLC
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