MXPA00004862A - Pressure activated switching device - Google Patents

Abstract

A pressure actuated switching apparatus includes first and second conductive layers and a plurality of discrete spaced apart dots between the first and second conductive layers. The dots serve as a standoff for separating the conductive layers and are fabricated from an insulative, elastomeric polymer foam which can collapse under the application of compressive force applied to the apparatus to allow contact between the conductive layers with minimized dead space. Alternatively, the standoff can include strips of electrically insulative elastomeric polymer foam.

Description

"PRESSURE ACTIVATED SWITCHING DEVICE" REFERENCE TO RELATED REQUESTS r. This is a continuation in part of U.S. Application Serial Number 08/429, 683 filed April 27, 1995, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure-operated switching device for closing or opening an electrical circuit, and particularly with a safety mat for operating and suspending the work of the machinery in response to the movement of personnel towards the mat. 2. Background of the Technique Power-operated electric switchgears are known in the art. Typically, these mat switches are used as mats in the vicinity of machinery to open or close electrical circuits. For example, a floor mat switch that opens an electrical circuit when stepped on can be used as a safety device to shut down the machinery when a person walks into an unsafe area in the vicinity of the machinery. On the contrary, the floor mat switch can be used to close a circuit and thus keep the machinery running only when the person is standing in the safety area. Alternatively, the floor mat switch can be used to sound an alarm when stepped on, or to perform some such function. U.S. Patent No. 4,497,989 issued to Miller discloses an electric mat switch having a pair of outer wear layers, a pair of internal moisture barrier layers between the outer wear layers, and a separating layer between the layers barrier to moisture. U.S. Patent No. 4,661,664 issued to Miller discloses a high sensitivity mat switch that includes external sheets, an open work separator sheet, conductive sheets interposed between the outer sheets on opposite sides of the separator sheet to contact during the flexure through the separator sheet, and a compressible deflection sheet interposed between a conductive sheet and the adjacent outer sheet, the deflection sheet being elastically compressible to protrude through the separator sheet to contact the conductive sheets during the movement of the outer leaves towards each other. U.S. Patent No. 4,845,323 issued to Beggs discloses a flexible touch switch for determining the presence or absence of weight, such as from a person in a bed. U.S. Patent No. 5,019,950 issued to Johnson discloses a combination of night light on the side of the synchronized bed that lights a lamp on the side of the bed when a person steps on a mat adjacent to the bed and connects a synchronizer when the person is off the mat. The synchronizer disconnects the lamp after a predetermined period of time. U.S. Patent Number 5,264,824 issued to Hour discloses an audio-emitting wire mat system. Compressible piezoresistive materials having an electrical resistance that varies according to the degree of compression of the material are also known in the art. These piezoresistive materials are disclosed in U.S. Patent Nos. 5,060,527, 4, 951, 985, and 4,172,216, for example. Even though the aforementioned mats have carried out useful functions, sometimes there is a need for an improved safety mat that can respond not only to the presence of force, but also the amount of force direction applied thereto. Likewise, mat switches that are currently being used frequently suffer from "dead zones". Dead zones are non-reactive areas where an applied force does not result in a switching action. For example, the peripheral area around the edge of conventionally used mats is usually a "dead zone". It would be advantageous to reduce the dead zones in a mat switch.

COMPENDIUM OF THE INVENTION A pressure operated switching device is provided here which includes first and second conductive layers and a plurality of discrete spaced points disposed between the first and second layers. The dots serve as a spacer and are made of electrically insulating elastomeric polymer foam that can be crushed under the application of compressive force applied to the apparatus. The foam of the polymer may be open or closed cell and may be made, for example, of silicone, polyurethane, polyvinyl chloride, and natural or synthetic rubber. The conductive layers can be thin sheets or metal plates for example aluminum, copper or stainless steel. Alternatively, the conductive layers can be made of an elastomerically conductive material. Optionally, a piezoresistive material can be placed between the conductive layers, the piezoresistive layer being separated from the first and / or second conductive layers by a dot layer.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional elevation view of a switching device having a point spacer. Figure 2 is a sectional side view of a switching device using an insulating foam point spacer. Figure 3 is a sectional side view of the switching device of Figure 2, under compression.

Figure 4 is a perspective view of a switching device having a spacer configured in strips. Figure 5 is a diagram of an electrical circuit for use with the apparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY (S) The terms "insulator", "conductor", "strength" and its related forms are used herein to refer to the electrical properties of the materials described, unless otherwise indicated. The terms "superior," "inferior," "above," and "below," are used in relation to each other. The terms "elastomer" and "elastomeric" are used herein to refer to material that can experience at least 10 percent elastically deformation. Typically, "elastomeric" materials suitable for the purposes described herein include polymeric materials such as polyurethane, plasticized polyvinyl chloride, and synthetic and natural rubbers, and the like. As used herein the term "piezoresistive" refers to a material having an electrical resistance that decreases in response to the safety 80, under operating conditions. The base 81 can be made, for example, of plastic or elastomeric materials. A preferred material for the base is a thermoplastic such as plasticized polyvinyl chloride ("PVC") sheet, which advantageously can be thermally sealed or otherwise bonded to a PVC cover sheet on the edges to achieve a hermetic sealing of the safety mat. The sheet can be, for example, of a thickness of 3.18 millimeters to 6.35 millimeters and can be embossed or flanged. In addition, the base 81 may alternatively be rigid or flexible to accommodate different environments or applications. The conductive layer 82 is a thin metal sheet, or a film applied to the upper part of the base 81. Alternatively, the conductive layer 82 may be a plastic sheet coated with a conductive film. This conductive coating can also be deposited on the base 81 (for example, by painting applied by conductive coating or non-electrolytic deposition). The conductive layer 82, for example, may be a thin sheet of aluminum or copper, which has been adhesively bonded to the base 81. The conductive layer 82 preferably must have a strength that is less than that of the resistance of the piezoresistive material. 84, which will be described below. Typically, the conductive layer 82 has a lateral resistance or edge-to-edge resistance of about 0.001 to about 50f ohms per square. Preferably, the resistance of the conductive layer 82 is less than half that that of the piezoresistive layer 84. More preferably, the resistance of the conductive layer 82 is less than 10 ppr more than that of the piezoresistive layer 84. It is especially preferred that the resistance of the conductive layer 82 is less than 1 percent that of the piezoresistive layer 84. The low relative strength of the conductive layer 82 helps to ensure that the only significant amount of resistance encountered by the current as it passes through of the security mat 80 is in that portion of the cutting path that is perpendicular to the plane of the layers. The conductive face 82 remains stationary relative to the base 81. However, another conductive layer 85, which will be discussed below, resiliently movable when a compression force is applied. The upper conductive layer 85 also has low resistance relative to the piezoresistive material, which is placed between the upper conductive layer 85 and the inferred conductive layer} : 82. In this way, the measured resistance is indicative of the vertical displacement of the conductive layer 85 and the compression of the piezoresistive foam 84, which, in turn, is related to the downward force applied to the device. The lateral position of the descending force, that is, whether the force is applied near the center of the device or near one or the other of the edges, does not significantly affect the measured resistance. The piezoresistive material 84 is preferably a conductive piezoresistive foam comprising a flexible and resilient sheet of a cellular polymeric material having a resistance that changes in relation to the magnitude of the pressure applied thereto. Typically, the piezoresistive foam layer 84 may vary from 1.59 millimeters to about 12.70 millimeters, although other thicknesses may also be used where appropriate. A conductive polymeric foam suitable for use in the present apparatus is disclosed in U.S. Patent No. 5,060,527. Other conductive foams are disclosed in U.S. Patent Nos. 4,951,985 and 4,172,216. Generally, these conductive foams can be open cell foams of which the cell walls are coated with a conductive material. When a force is applied, the piezoresistive foam is compressed and the total resistance is decreased because the resistivity as well as the path of the current are reduced. For example, an uncompressed piezoresistive foam can have a resistance of 100,000 ohms, while when compressed the resistance can drop to 300 ohms. An alternative conductive piezoresistive polymer foam suitable for use in the present invention is an intrinsically conductive expanded polymer cellular foam (ICEP) comprising an expanded polymer with a premixed filler or filler comprising finely divided particles (preferably colloidal). and conductive fibers. Typically, conductive cellular foams comprise a non-conductive expanded foam with an applied conductive coating on the walls of its cells. These foams are limited to open cell foams to allow the inner cells of the foam to receive the conductive coating. An intrinsically conductive expanded foam differs from the above known expanded foams in that the foam matrix is conductive per se. The difficulty in making an intrinsically conductive expanded foam is that particles of the filler or conductive filler that have been premixed in the unexpanded foam, disperse from one another and lose contact with one another as the foam expands , thus creating an open circuit.

Surprisingly, the combination of finely divided conductive particles with conductive fibers allows the filler or conductive filler to be premixed into the resin prior to expansion without loss of conductive capacity when the resin subsequently expands. The conductive filler or filler material may comprise an effective amount of conductive powder combined with an effective amount of conductive fiber. By "effective amount" is meant an amount sufficient to maintain electrical conduction after expansion of the foam matrix. The conductive powder may be of powdered metals such as copper, silver, nickel, gold and the like, or pulverized coal such as carbon black and powdered graphite. The particle size of the conductive powder typically varies in diameters from about 0.1 to about 300 microns. The conductive fibers can be metal fibers or, preferably, graphite, and typically range from about 2.54 millimeters to about 12.70 millimeters in length. Typically the amount, of the conductive powder, ranges from about 15 percent to about 80 percent by weight of the total composition. The conductive fibers typically range from about 0.01 percent to about 10 percent by weight of the total composition.

The intrinsically conductive foam can be made according to the procedure described in Example 1 which is presented below. With respect to the Example, the silicone resin may be capable of being obtained from Dow Corning Company under the designation of silicone resin SILASTIC ™ s5370. The graphite pigment can be obtained as Asbury Graphite A60. The carbon black pigment can be obtained as Shawingigan Carbon Black. The graphite fibers can be obtained as graphite fibers from Hercules Magnamite Type A. A significant advantage of the intrinsically conductive foam is that it can be a closed cell foam.

EXAMPLE 1 108 grams of silicone resin were mixed with a filler or filler consisting of 40 grams of graphite pigment, 0.4 gram of carbon black pigment, 3.0 grams of 6.35 millimeter graphite fibers. After the filler or filler was dispersed in the resin, 6.0 grams of a foam-forming catalyst was stirred into the mixture. The mixture was emptied into a mold and allowed to foam and gel to form a piezoresistive elastomeric polymeric foam having a sheet strength of approximately 50K ohms / square.

The pre-foamed silicone resin can be thinned with a solvent, such as methylethyl ketone to reduce the viscosity. The polymer usually forms a "thin layer" when it forms foam and gels. The thin layer decreases the sensitivity of the piezoresistive sheet because the thin layer usually has a high strength value that is less affected by compression. Optionally, a fabric can be wrapped around the mold where the pre-foamed resin is melted. After the resin has been foamed and gelled, the fabric can be pulled away from the polymer, thereby removing the thin layer and exposing the polymer cells for greater sensitivity. When loaded, that is, when a mechanical force is applied to it, the strength of a piezoresistive foam decreases in a manner that is reproducible. That is, the same load applied repeatedly in a constant manner provides the same resistance values. Also, it is preferred that the cellular foam present little or no hysteresis resistance. That is, the measured resistance of the conductive foam for a specific amount of the compression displacement is essentially the same whether the resistance is measured when the foam is being compressed or expanded. Advantageously, the piezoresistive foam layer 14 achieves non-sparking switching of the apparatus, which provides a greater margin of safety in environments with flammable gases or vapors present. The cover sheet 86 is a non-conductive layer 86 which is preferably elastomeric (but which may alternatively be subtle but not elastomeric). The above comments regarding the negligible resistivity of the conductive layer 82 relative to that of the piezoresistive foam are also applied to the conductive layer 85. The conductive cover 85 can be deposited on the upper non-conductive layer 86 in order to form an assembly of cover 89 with a lower elastomeric conductive surface. For example, the deposited layer 85 can also be a polymeric elastomer or a coating containing filler or filler material such as finely powdered metal or carbon to make it conductive. A suitable conductive layer for use in the present invention is disclosed in U.S. Patent No. 5,069,527, herein incorporated by reference in its entirety. An elastomeric conductive layer 85 can be fabricated with the conductive powder and fibers as described above with respect to the intrinsically conductive expanded polymer foam; with the exception that the polymer matrix for the conductive layer 85 does not need to be cellular. Preferably, an elastomeric silicone is used as the matrix as indicated in Example 2.

EXAMPLE 2 A filling material or conductive filler was made of 60 grams of graphite pigment (Asbury Graphite A60), 0.4 gram of carbon black (Shawingigan Black A), 5.0 / grams of 6.35 millimeter graphite fibers (Hercules Magna ite Type A) ). This filler or filler was dispersed in 108.0 grams of the silicone elastomer (SLYGARD ™ 182, silicone elastomer resin). A catalyst was then added and the mixture melted in a mold and allowed to cure. The result was an elastomeric silicone film that has sheet strength of approximately 10 ohms / square.

Alternatively, the cover assembly 89 may be flexible without being elastomeric and may comprise a metallized polymer sheet such as an aluminized MYLAR® polymer film, the aluminum coating providing the conductive layer 85. In still another alternative, the cover assembly 89 may consist of an upper layer 86 of flexible polymeric resin, either elastomeric or only flexible, and a continuous layer 85 of a thin layer of metal. Preferably, the top layer 86 is a plasticized PVC sheet that can be thermally sealed or otherwise bonded (for example by solvent welding) to a PVC base 81. The advantage of using a continuous thin sheet layer is the greater conductivity of the metallic sheet in comparison with the polymers that become conductors by means of the mixture of conductive components. The aforementioned layers are assembled or assembled with conductive wires and are connected individually respectively with the conductive layers 82 and 85. The wires are connected with a power supply and form part of the electric switching circuit. See, for example, Figure 5 which will be discussed below. As a further modification, the conductive layer 85 may comprise a composite of a conductive elastomeric polymer bonded to a thin sheet of segmented metal or a thin sheet of flanged metal. The slits in the segmented thin sheet (or in the flanged thin sheet) allow the elastomeric elongation of the conductive layer 82 while providing the high conductivity of the metal through the majority of the conductive layer 82. The points 83a and 87a are placed respectively in order to define a layer and can be applied to the conductive layers 82 and 85, as the upper and / or lower surface of the piezoresistive material, for example, by depositing a fluid insulator (eg, a synthetic polymer) through a screen , thus allowing the dot pattern formed in this way to harden or cure. Points 83a and / or 87a can be placed as a regularized pattern or, alternatively, they can be randomly formed. When used in conjunction with a piezoresistive foam layer 84, the points 83a and 87a may optionally be fabricated from a relatively incompressible material such as a solid non-cellular material. For example, the material to be used to manufacture the spacer points 83a and 87a can be a polymer (eg, methacrylate polymers, polycarbonates, polyurethane or polyolefins) dissolved in a solvent and applied to the conductive layers 82 and / or 85 as a liquid viscous. The solvent is then allowed to evaporate, thus leaving the polymer spots deposited. Alternatively, points 83a and 87a can be deposited as a catalyzed resin that is cured under the influence of an energy source (e.g., heat, or ultraviolet light). Silicones, polyurethanes, rubbers, and epoxy resins are preferred as materials for making points 83a and E ¿87a. The points 83a and 87a are preferably hemispherical but can be manufactured in any shape and are preferably from about 4.77 millimeters to about 6.35 millimeters in height. Other smaller or larger dimensions can be selected appropriate for the desired application. The dimensions provided herein are only for exemplification of one of the many appropriate size scales. The amount of deflection force required to switch the device 80 depends at least in part on the height of the points. The edges of the mat switch 80 are preferably sealed, for example, by thermal sealing. The active surface for the drive extends very close to the shore with little area area dead. Alternatively, the points 83a and 87a can be manufactured from an electrically insulating elastomeric polymer foam. For example, silicone resin without a filler or conductive filler can made in a cellular polymeric material by the addition of a foaming agent. Various other known materials and methods for foaming can alternatively be used. For example, the cellular polymeric material can be foamed rubber (natural or synthetic), polyurethane or plasticized PVC. The foaming agents within the resin systems can be dissolved gases, low boiling temperature liquids, and chemical blowing agents that decompose or react with other components of the pre-foamed polymer composition to form a gas. The formation of the gas within the plastic matrix forms the cells of the resulting foam. The dead space is the area of the mat switch where the upper and lower electrodes can not make contact. The use of a spacer comprising the plurality of discrete discrete points is advantageous since it greatly reduces the amount of dead space in a mat switch. The use of an insulating elastomeric foam to manufacture the stitches still further reduces the total dead space by reducing the dead space around the individual stitches. Typically, the foam density of the uncompressed polymer can vary from about one pound per cubic foot ("pcf") to about 20 pcf. The gap space as a percentage of the total volume can vary from less than about 30 percent to more than 90 percent. Consequently, the foam points are crushed under the force of a weight that is applied to the mat switch, and its volume is correspondingly reduced. The electrodes are brought into contact with each other without having to bend abruptly around the points. The greater the density (and corresponding smaller gap space) the greater the foam strength and its compressive strength. Generally, a density of 2 pcf to 15 pcf is preferred. This particularity, that is to say, the foamable foam points, can advantageously also be provided to the mat switches having two electrodes separated only by a spacer. For example, now referring to Figure 2, the mat switch 90 includes an insulating cover sheet 91 and a base 95, an upper electrode layer 92 in contact with the cover sheet 91, a lower electrode layer 94 in contact with the base 95, and a spacer composed of a plurality of points 93 of electrically insulating polymeric foam and placed between the upper and lower electrode layers 92 and 94. The sheet 91 covered with the electrode layer 92 may correspond in materials and Methods of fabricating the cover assembly 89 with the non-conductive layer 86 and the conductive layer 85 and the base 95 with the electrode layer 94 may correspond to the base 81 with the conductive layer 82. The polymer foam may either be a Open cell foam or closed cells can be manufactured from materials described above with respect to points 83a and 87a. Both the cover sheet 91 and the base 95 are optionally made, for example of PVC, and are preferably joined around their periphery to form a watertight and / or air tight seal. The upper and lower electrode plates 92 and 94 both fabricate from a sheet of an electrically conductive material, for example, a thin sheet of metal, a sheet, a resin coating filled with a conductive particulate material. The electrode layers 92 and 94 typically vary in thickness from about .254 millimeter to about .762 millimeter, although any thickness of the appropriate metal layer can be used for the purposes described above. The plates of the electrode 92 and 94 can optionally be made, for example, of aluminum, copper, nickel, a thin sheet of stainless steel or a conductive plastic film. Referring now to Figure 3, when a force F is applied to the mat switch 90, the spacing points 93 are crushed to less than 50 percent of their original height and volume, preferably, fjk 20 percent of their height and original volume, especially preferably less than 5 percent of its original height and volume. Accordingly, the upper electrode layer 92 flexes under the compressive force and is placed in intimate contact with the lower electrode layer 94 leaving a minimal dead space around the periphery of the points 93. When the The force is removed, the spacers resiliently return to their original configuration and the mat switch 90 returns to the position as shown in Figure 2. Referring now to Figure 4, shows an alternative embodiment of the safety mat switching device. The safety mat 90a includes a base 95a with a lower electrode layer? 94a fixed to the isama, and a sheet 91a of insulating cover with the upper electrode layer 92a fixed to the same. The spacer comprises a plurality of separate insulating polymer foam strips 93a placed between the electrode layers 92a and 94a. The materials and dimensions of the base insulating cover sheet 91a, and the layers of the electrode 92a and 94a may correspond to the respective components of the mode 90 of the security mat that is described above. The spacer 93a of insulating resilient polymer foam can be made of the same material as described above with respect to points 83a and 87a. Alternatively, a layer of piezoresistive foam can optionally be incorporated in a safety mat switching device 90a and placed between the spacer layer 93a and one or other of the electrode layers 92a and 94a. In still another alternative, a combination of both strips 93a and points 87a can be used as the spacer layer. Referring now to Figure 5, a circuit 50 is shown in which any of the mat switches of the present invention can be used to operate a relay. The circuit 50 is energized by a direct current source, i.e., a battery 51, which provides a direct current voltage Vo ranging from about 12 to 48 volts, preferably from 24 to 36 volts. The safety mat A can be any of the embodiments of the invention described above. The potentiometer R] _ can vary from 1,000 ohms to approximately 10,000 ohms and provide a calibration resistance. The resistor R2 has a fixed resistance of approximately 1,000 ohms up to approximately 10,000 ohms. The transistors Qi and Q2 provide amplification to the signal of the security mat A in order to operate the relay K. The relay K 5 is used to close or open the electric circuit where the machinery M to be controlled works. Capacitor C] _ varies from approximately 0.01 microfarad to 0.1 microfarad and is provided to suppress noise. K can be replaced with a device of regulated supply to measure the force in A. This would require the adjustment of the ratio of R_ and A (compression versus force) pa to push the transistors Qi and Q2 towards their scale of linear amplification. This circuit represents an example of the way in which the mat. Many other circuits can be used including the use of triacs. The present invention can be used in many applications other than machine safety mats. For example, the invention can be used for Intrusion detection, load displacement detection, shock duplication, athletic targets (e.g., baseball, karate, boxing, etc.), sensing devices on human limbs to provide computer intelligence for prosthetic control, feedback for virtual reality presentations, mattress covers to monitor heat (especially for use in hospitals or to send signals of sudden infant death syndrome heart arrest), toys, assistive devices for the blind, computer input devices , boat auxiliaries, keyboards, analog button switches, weighing scales and the like. It will be understood that different modifications can be made to the modality disclosed herein. Therefore, the foregoing description should not be construed as limiting but only as exemplifications of the preferred embodiments. Those skilled in the art may present other modifications within the scope and spirit of the appended claims hereto.

Claims (9)

CLAIMS;

1. A pressurized switching apparatus comprising: (a) first and second conductive electrode layers, at least one of the first and second electrode conductive layers being movable in response to the application of a mechanical force thereto from a first position of the open circuit to a second position wherein at least a portion of the first conductor electrode layer is in electrical contact with at least one / portion of the second layer of the conductive electrode; each layer of the conductive electrode is electrically connected to a respective terminal of a power source to maintain the first and second conductive electrode layers at different electrical potentials with respect to each other in at least the first position of the open circuit; and (b) a spacer including a plurality of discrete spaced strips of an electrically insulating elastomeric polymer foam placed between the first and second conductive electrode layers and resiliently urging the first and second conductive electrode layers toward the first position of the circuit open. The pressure operated switching apparatus of claim 1, further comprising an insulating cover sheet bonded to one side of the first conductive electrode layer and an electrically insulating base bonded to one side of the second conductive electrode layer. 3. A pressurized switching apparatus comprising: a) first and second conductive layers; b) a / spacer including a plurality of discrete spaced strips of an electrically insulating elastomeric polymer foam positioned between the first and second conductive layers; and c) a layer of compressible piezoresistive material wherein the plurality of discrete spaced strips of the electrically insulating elastomeric polymer foam comprises a first layer of laterally spaced foam strips placed between the compressible piezoresistive material and at least one of the first and second compressible material. conductive layers. 4. The pressure operated switching apparatus of claim 1, wherein the electrically insulating elastomeric polymer foam is an open cell foam. The pressure operated switching apparatus of claim 1, wherein the electrically insulating elastomeric polymer foam is a closed cell foam. The pressure operated switching apparatus of claim 1, wherein the strip is movable in response to pressure between an initial configuration having a first volume and a compressed configuration having a second volume that is less than 50 percent from that of the first volume. The pressure operated switching apparatus of claim 1, wherein the spacer further includes a plurality of discrete spaced points of electrically insulating elastomeric polymer foam. The pressure-operated switching device of claim 1, wherein the strips spaced apart from the spacer are parallel to one another and positioned to define a single spacer layer in contact with both the first and second conductive electrode layers. The pressurized switching apparatus of claim 1, wherein the strips each have a height of at least 4.77 millimeters.