GB2173953A - Electrical switch and device for use therein - Google Patents

Abstract

An electrical switch comprising two spaced electrical conductors and a restrained bridging member comprising an electrically conductive spring member which is held in a compressed state by frangible restraining means 25 alone positioned between the conductors when said restraining means 25 are broken by a predetermined minimum load being applied to the switch the bridging member moves towards its relaxed state thereby completing a conductive path between said conductors eg by engagement thereof by ends 24, which conductive path is maintained after removal of said load. The switch may be used as a manual fire alarm switch, a limit of travel switch on a crane, or an overload warning switch on a pressure sensitive switchmat. <IMAGE>

Description

SPECIFICATION Electrical switch and device for use therein This invention relatesto an electrical switch and in particularto an electrical switch which is activated by a predetermined load and when activated, corn pietes an electrically conductive path which cannot subsequently be brokenwithout dismantl i ng the switch. The invention also relatesto a restrained bridging member suitable for use such as an electrical switch.

Load sensitive mats incorporating electrical switches are well known and are generally referred to as "switchmats". For example, switchmats a re conventionally located adjacent to doors leading into an out of supermarkets, airports and other public places so as to lie in the path of pedestrians approaching doors. As a pedestrian approaches a door he steps upon the mat, thereby closing a normally open switch contained in the mat so as to actuate a mechanism for opening the door automatically. Switch mats are also used in industrial environments where they may be used to limit access to dangerous machinery or control the operation of machinery. In particular, a switch mat may be employed to actuate a safety circuit prevention operation of a machine when a person enters a dangerous zone.

Examples of known pressure-sensitive switches and switchmats are disclosed in British Patent Speci fication Nos. 392936, 1 862,1 209564,1 351 911,1 358006, 1 174,1 454 1 454805,2064222,2083858 and 2 088 637, United States Patent Specification Nos. 1 775 755,2951 921,3 722 086,3 812 313,3 825 277,4 037 069 and 4105899, French Patent Specification Nos. 1 416 570 and 2431178 and German Offenlegungsschrift No.2148760.

Switchmats generally comprise first and second electrically conductive members which are normally planar, separated by non-conductive material and optionally include one or more intermediate conductive elements positioned between the first and second conductive members. The switchmat is normally constructed such that there is no conductive path between the first and second electrically conductive members, i.e. the switch is open, and when the switch mat is subjected to load the first and second electrically conductive members move towards each other, by compression of the non-conductive material and/or bowing of one or both ofthe electrically conductive members, such that a conductive path is established betweenthe first and second electrically conductive members optionally via intermediate conductive elements thereby closing the switch.When the loading is removed the first and second electrically conductive members return to their spaced position.

One ofthe problems associated with the use of many ofthe known switch mats in industry is that they are often not sufficiently robust to withstand the rigours of an industrial environment for long periods oftime.Theswitchmats used in industry may be subjected to severe overloads, e.g. dropping of a heavy object or passage of a trolley or truck. Such physical abuse of a switchmat may result in complete failure of the switch mechanism. There is a demand in industry for switchmats to " "fa i Isafe" such that when the switch mechanism of the switchmat is damaged, the switchmatwill be rendered inoperative with the switch permanently in the closed position, i.e. a permanent conductive path will be established between the first and second conductive elements thereby completing the electrical safety circuit in which the mat is integrated.

Ourcopending British Patent Application No.

8413419 (Publication No. 2140625A) discloses a normalLy-open pressure responsive switch mat comprising first and second electrically conductive members separated by non-conductive material and optionally oneormore intermediate electrically conductive elements positioned between said first and second electrically conductive members, constructed and arranged such that when the switchmat is subjected to an acutation load there is a relative movement betweensaid first and second electricallyconductive members to complete an electrically conductive path therebetween optionallyviatheintermediate electrical conductive elements, characterised in that the switchmat includes a safety switch comprising a bridging member positioned between said first and second electrically conductive members but in normal conditionsnotcompleting an electrically conductive path therebetween, the bridging membercomprising an electrically conductive spring member which is held in a compressed state by frangible restraining means such that when the switch mat is subjected to a predetermined minimum overload the frangible restraining means are broken allowing the bridging memberto move towards its relaxed mode thereby completing an electrically conductive path between the first and second electrically conductive members which path is maintained after removal of said overload.

The safety switch is activated by a predetermined load being exerted upon the restraining means, causing the restraining means to break thereby permitting the bridging member to spring across the gap between the two electrical conductors so completing an electrical circuit. The switching action cannot be reversed without dismantling the device and accordingly the switch is particularly suitable as a failsafe device. For example, the switch may be constructed in such a fashion that dismantling will not be practical atthe site of use thereby preventing interference with the circuitry by unauthorised persons.

The bridging member may take any desired form providing that when the restraining means is broken the bridging member will spring from its compressed state towards its relaxed mode to complete a path between the conductors.

Whilstthe bridging member may be held in its compressed state by the restraining means compressing the bridging member against one ofthe electrical conductors it is preferred that the restraining means alone is responsiblefor compression of the bridging member.

The present invention provides an electrical switch containing such a preferred form of bridging element.

Therefore according to the present invention there is provided an electrical switch comprising two electrical conductors in spaced relationship and a restrained bridging member comprising an electrical lyconductivespring member which is held in a cumprnssedstate by frangible restraining means alone positioned therebetween such that when said restraining means are broken by a predetermined minimum load being appliedtotheswitch the bridging membermovestowards its relaxed mode thereby completing a conductive path between said conductors which conductive path is maintained after removal of said load.

Forexample,the bridging member may comprise one or moreturns of a helical spring which may be held in a compressed state by a sleeve, collaror loop of restraining means compressing the turns together, In practice, it has been foundthat 1.3 turns of a helical spring, the overlapping portions ofthe turns being compressed together by a sleeve or bead of frangible material, provides an effective bridging member particularly when each end ofthe spring wire is bent respectively up and down to be at right angles to the plane ofthe contact surface ofthe conductors. It is desirable thatthe tips ofthe bridging memberwhich are intended to contactthe surface ofthe conductors are pointed in order to penetrate any oxide or dirt which mayform upon the surface.Itwill readily be appreciated thatthe bridging member can take many otherforms, e.g. a simple U-shape, and other examples will be described with referencetothe drawings.

The restraining means may conveniently comprise a collar or bead of frangible material, e.g. Bakelite. The restraining means secures the bridging member in a compressed or restrained state until it is broken by application thereto of a load greaterthan its breaking strength. Upon breaking, the bridging memberisfree to spring towards its relaxed mode thereby completing and maintaining a path between the conductors.

Switchmats incorporatin restrained bridging members are disclosed in detail in ourcopending British PatentApplication No.8413419 (Publication No.

2140625A).

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 represents a compressed bridging element in accordance with the invention, Figure 2 represents a diagram of a modified form of bridging memberfor use in a safety switch of the invention, Figures 3A to 3D represent further variants of a bridging member for use in a safety switch in accordance with the invention, and Figures 4A and 4B represent a furthervariant of a bridging memberfora safety switch in accordance with the invention.

Figure 1 illustrates the compressed bridging member of a safety switch in accordance with the invention.

The bridging member, generally shown at 8, comprises a helix 10 of 1.3 turns of spring wire such that in the relaxed mode the ends ofthe spring are separated by a distance between the two conductive plates of a switchmat, i.e. greater than the height ofthe strips of non-conductive compressible material of Figures 1,2 and 4. The helix 10 is restrained in a compressed mode by a frangible collar or bead 12 which may be formed of glass, ceramics material or brittle plastics material.

The ends ofthe helix 10 are sprung apart in the horizontal plane and each passed through the res training meansl2in oppositedirectionssothatthey lie side byside within the restraining means 12. The ends are then relaxed inthe horizontal plane but constrained in the vertical plane. The restraining means 12 is prevented from moving aiong the length of the spring wire by friction, the curvature and the bent up ends l4ofthewire.

In use the safety switch is maintained in a horizontal position and thetwo conductors6 and the restrained bridging member8 is not able to bridge the distance between the two conductors. Upon application of a load upoon one ofthe conductors sufficientto sandwich the restraining means betweerrttre conductorsthe restraining means 12 is shattered between the two conductors,the ends ofthe helix 10 will no longer be constrained and will spring apartto foorm an electrical bridge between the two conductors thereby actuating the switch. The bridging member will continue to bridge the conductors thereafter whether the compression force is removed from the mat or not.

Forexample, in the switchmatshown in Figure 4of British PatentApplicationSerial No.21 40625Athe bridging member may take the place of one ofthe metal balls 6. In normal operation,the balls may make slight indentations in the aluminium plates upon application of a load, e.g. a pedestrian, but insufficient to ailowthe gap between the plates to close to the point where the restraining means is broken. In conditions of severe overload, the balls may be driven into the aluminium plates to such an extent that normal operational sensitivity is impaired, i.e. the weight of a person would not close the switch.In such circumstances, the gap between the plates will be closed to such an extentthatthe restraining means would be shattered thereby ailowing the bridging memberto spring to its relaxed position to permanently short-circuit the aluminium plates. Thus, the mat will therebyfail putting machinery into a safe condition.

In practice it is often desirableto ensurethata restrained bridging member used in a switch ofthe invention has two or more ball switches in close proximity in orderto prevent accidental breakage of the restraining means caused by a sudden localised load, e.g. should an operator drop a tool.

In a typical switch mat, which has 6 mm separations between its plates, a helical spring bridging switch is contained in an elastomeric ring 12.5 mm inside diameter.The helix (Figure 2) has an 11 mm outside diameter, a fixing lug 5 mm long by 2 mm wide, and in the relaxed mode the ends ofthe helix are at least 7 mm apart in the vertical plane. The helix is made from spring wire 0.45 mm diameter, and the ends are bent upwards and downwards to give "spikes" of approximately 1 mm.A suitable glass bead for use with this helix is in the forum of a tube 3 mm long,2 mm outside diameter, with a wall thickness of 0.35 mm. Such a bead required a load of approximately 1.3 keg radially applied to cause itto shatter and release the spring.

Itwill beapparentthatthe bridging member described above will only be suitable for operation in a horizontal plane unless it is secured to one of the conductors. There is a remote possibilitythat a particle of shattered restraining means may lodge between the heLix and one of the conductors prevent ingconduction between the conductors Figure 2 illustrates a modified bridging member in which the basic helix shape 20 of the spring wire is distorted by theaddition of a reverse bend 22 in the natural shape at a point opposite-the overlap 23.The ends of the wire 24are bent upwards and downwards respectively bya lengtlrslightly greater than the thickness of the wall of the frangible restraining means 25. The reverse bend 22 is usedas a fixing lug and may be secured to one of the conductorsmaintaining the plane ofthecon strained-helix parallel to the surface of the conductor.

This may be conveniently achieved in the switchmat illustrated in Figures 1,2 and 4 of British Patent Application Serial No. 2140625A by inserting the lug undertheedge ofthe insulating strips 5 orgasketso thatthe helix liesflat againstthe surface of one of the conductors. This enables the switch tabe operated in any attitude. The ends ofthe spring wire are preferably sharpened so that good metal to metal contact is ensured upon actuation of the switch. Furthermore, the length of the bent portions is sufficientto prevent particles of shattered restraining means from dis abling the switch.

Figures3Ato 3D illustrate further variants of bridging members suitable for use in the invention which may be punched from metal sheet, e.g.

phosphor bronze, beryllium copper strip, etc., formed into shape, and, if required, tempered in akiln. Each of these designs include two arms 30 which are held in the restrained mode in a horizontal plane by the restraining means 32. Upon breaking the restraining means, the ends 30 spring apartto contact with each conductor. The bridging members may include a lug 34for securing the bridging memberwithin the switch. Alternatively, the bridging member may be secured buy a portion of its circumference. The bridging members illustrated in Figures 3Ato 3D have the advantage that they may be mass produced cheaply by use of an automatic mechanical press.

It is not necessary forthe arms 30 ofthe bridging memberstooverlapandeach may be inserted intothe end ofthe restraining means, e.g. a glass bead, resting against each other in the centre ofthe aperature of the bead. The bead may be prevented from moving by tapering the ends of the arms 30.

Figures 4A and 4B represent a further bridging elementfor use in the invention in the compressed and relaxed modes respectively, the same reference numerals being used as in Figure 7. The bridging element is in the form of a wire spring having a substantially rectangular form and dimensioned to provide a broad base to prevent twisting ofthe bridging member in use.

in addition to its use in switch mats the electrical safety switch may be incorporated into afire alarm circuit, the switch being mountedvertically on a wall orthe like and being operated by a person compress ing the switch by hand. In such a construction the insulating material separating the two conductors comprises a relatively soft compressible material, e.g.

afoamed elastomer. Upon shatteringthe restraining means the switch will complete the alarm circuit which could not be reset except by replacement of the bridging means.

The electrical switch ofthe invention may also be used asa "limit of travel" switch by mounting it upon a frameworkof a machine which has moving parts, e.g. a crane. Ihe switch is positioned such that if unacceptable or unauthorised movement occurs the switch is compressed thereby breaking the restraining means and activatingthe switch whicb would complete an alarm or disablingcircuit.

Claims (9)

1. An electrical switch cromprising two electrical conductors in spaced relationship and a restrained bridging member comprising anelectrically conductivespring memberwhich is held in acompressed state by frangible restraining means alone positioned therebetween such that when said restraining means are broken by a predetermined minimum load being applied to the switch the bridging member moves towards its relaxed mode thereby completing a conductive path between said conductors which conductive path is maintained after removal of said load.

2. A switch as claimed in Claim 1, characterised in that the spring member comprises at least oneturn of spring wire, adjacentturns being compressed together by frangible restraining means,the ends of the spring wire being bent respectively up and down at right angles to the plane of the compressed spring.

3. A switch member as claimed in Claim 2, characterised in that the bridging member is a helical spring obtained by pressing from a sheet material.

4. Aswitch member as claimed in Claim 2, characterised in that the restraining means comprises a collar or bead.

5. Aswitch asclaimed in Claim 1 substantiallyand herein described with reference to the accompanying drawings.

6. A restrained bridging member suitable for use in a switch as claimed in any preceding claim comprising an electrically conductive spring member which is held in a compressed state by frangible restraining means alone.

7. A bridging member as claimed in Claim 6, characterised in that the spring member comprises at least one turn of spring wire, adjacentturns being compressed together byfrangible restraining means, the ends of the spring wire being bent respectively up and down at right angles to the planeofthe compressed spring.

8. A bridging member as claimed in Claim 6, characterised inthatthe bridging member isa helical spring obtained by pressing from a sheet material.

9. A bridging member as claimed in Claim 6, chaaracterised in that the restraining means comprises a collar or bead.

1Q Abridging memberas claimed in Claim 6 substantially as herein described with reference to the accompanying drawings.