Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
1999-06-07
2001-05-01
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
Reexamination Certificate
active
06225565
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to flexible electromagnetic shielding. More specifically, the invention relates to a flexible conductive material and the inclusion of appropriately selected materials of high magnetic permeability. The resulting compound can be extruded as part of the manufacturing process for shielded cables and shielded housings for constituent cable subassemblies.
2. Description of the Prior Art
It is known in the art that sensitive electrical equipment can be affected by Electro-Magnetic Interference (EMI). It is also known in the art that there are several ways to reduce EMI. For example, EMI can be reduced by shielding the electronic equipment by enclosing it in shielded rooms and cabinets, filling any gaps therein with conductive gaskets, and also by shielding cables and cable assemblies connected to the electronic equipment with conductive outer layers.
One example of EMI shielding for rooms and cabinets is disclosed in U.S. Pat. No. 4,992,329 which describes EMI shielding in the form of a laminated sheet. The shielding effect of the sheet is provided by flakes of magnetic amorphous alloy that are deposited between layers of film prior to lamination. Another example is U.S. Pat. No. 4,965,408 which discloses flexible radiation shielding in the form of a laminated sheet. The EMI shielding effect of the sheet is accomplished by laminating a thin metal foil between layers of a flexible outer material.
Examples of conductive elastic gaskets are found in U.S. Pat. Nos. 4,948,922 and 4,937,128 which disclose conductive elastic gaskets used to fill gaps between openings in shielded rooms and cabinets. Both of these patents disclose the use of an elastic material that is electrically conductive in and of itself. Other examples are found in U.S. Pat. Nos. 4,977,295, 4,968,854 and 4,948,922 which disclose conductive elastic gaskets where the elastic material is made conductive through the inclusion of the metallic particles. U.S. Pat. No. 4,966,637 discloses a conductive elastic gasket where the requisite conductivity is provided by an outer wrapping of braided wire.
U.S. Pat. No. 4,920,233 is an example of a special purpose cable which includes a concentric form of Faraday shielding, and incidentally also in alternate embodiments includes a concentric layer of thermoplastic material loaded with ferrite powder. The purpose of that patentee's construction of cabling is to provide a high fidelity music signal transmission media which features consistent phase velocity characteristics over the frequency band of the music, by making the distributed inductance of the cable relatively large. In that patentee's embodiment of
FIGS. 1 and 2
, the distributed inductance is increased by disposing torroidal ferrite sleeves
28
,
FIGS. 1 and 2
, along the cable's axial length. The function of EMI isolation is also present in that patentee's embodiments of
FIGS. 1
,
2
and
6
therein, but in the form of twisted metallic foils strips
34
(
FIGS. 1 and 2
) and
34
A (FIG.
6
), and a surrounding of metallic braiding (
32
,
FIGS. 1 and 2
) and
32
A (FIG.
6
). This results in a design requiring manufacture by multiple manufacturing steps employing multiple types of manufacturing processes, namely, the extrusion of the thermoplastic elements, and the twisting of a metallic jacket and the braiding of another jacket. This multistep and multimode manufacture in turn drives up direct cost of manufacture and also drives up needs for investment in manufacturing machinery. In another of that patentee's embodiments,
FIGS. 6 and 7
, the cable inductance is increased by ferrite powder in an extruded thermoplastic layer
26
A (
FIG. 6
) and
48
(FIG.
7
). These thermoplastic layers are an electrical insulation material. Thus although the ferrite particles provide inductance for purposes of that patentee's invention, the insulation characteristic of the thermoplastic matrix binder of their layers
26
A (
FIG. 6
) and
48
(
FIG. 7
) would result in non-homogeneous electromagnetic leakages in the spaces between the ferrite particles, and would not produce the homogeneous conductivity in all directions (“isotropic”), as required of a Faraday shield. U.S. Pat. No. 4,960,965 discloses a cable of concentric layers where an outer layer of EMI shielding comprises conductive carbon fibers. U.S. Pat. No. 4,769,515 discloses a spirally laminated cable comprising an inner metallic core and a laminated outer layer including metallic foil designed to increase the surface area of the metallic conductor, rather than for the purpose of providing EMI protection.
SUMMARY OF THE INVENTION
Accordingly, it is a general purpose and object of the present invention to provide a flexible cable which integrally incorporates a Faraday shield for providing Electro-Magnetic Interference (EMI) isolation between EMI present in the ambient environment and a conductor of the cable, or vice versa.
Another object is to provide such a Faraday shield which yields economies in its manufacture, including savings as a result of need for fewer types of manufacturing machines, and savings in the form of a concentric construction of less-costly-to-fabricate extrudable layers.
This is accomplished by the present invention by using a conductive elastomer as a matrix binder which is filled with particles of a high permeability iron-based alloy. The conductive property of the matrix binder provides isotrophic conductivity requisite of a Faraday shield.
One illustrative embodiment of a flexible cable unit consists of a single conductive core having thereabout a concentric Faraday EMI shielding structure in accordance with the present invention. The concentric shielding structure consists of a concentric pile of alternating (i) sheaths of a flexible insulating material, such as rubber or polyvinyls chloride (PVC), and (ii) Faraday sheaths of a high permeability ferrous alloy particles loaded in a suitable conductive elastomeric matrix binder material, such as CONSIL manufactured by Technical Wire Products. CONSIL is an extrudeable, cure hardened material which prior to extrusion includes both a flowable resin component and a non-flowable component consisting of resin particles which have undergone a preliminary cure and hardening cycle and are pressure distortable. After the ingredients are mixed, the admixture of the matrix binder and the ferrous alloy particles are extruded the admixture is cure hardened, rendering it capable of providing good homogenous (isotropic) conductivity throughout the material. Further details regarding this matrix binder material are described in U.S. Pat. No. 3,609,104 entitled “Electrically Conductive Gasket and Material Thereof,” specific portions of which are incorporated by reference later herein in the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS section. The sheath next to the central conductive line of the cable, and the outermost sheath, are of insulation materials. The loading of ferrous alloy particles in the conductive set of alternating sheaths is about 75% by volume, and the size of the particles is 10-20 grains per square inch.
Another illustrative embodiment is a form of what is known in industry as a tri-axial cable. It consists of three conductive lines subassemblies, each with a first insulating sheath directly over the core and a second sheath of the aforesaid conductive, elastomeric, matrix binder loaded with high permeability ferrous alloy particles over the first sheath. These three subassemblies sheaths are bundled and covered by five alternating sets of insulator and Faraday sheaths.
REFERENCES:
patent: 3609104 (1971-09-01), Ehrreich et al.
patent: 4499438 (1985-02-01), Cornelius et al.
patent: 4503284 (1985-03-01), Minnick et al.
patent: 4816614 (1989-03-01), Baigrie et al.
patent: 5313017 (1994-05-01), Aldissi
Lall Prithvi C.
McGowan Michael J.
Nguyen Chau N.
Oglo Michael F.
Reichard Dean A.
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