EP0109159A1

EP0109159A1 - Pressure sensitive switchmats

Info

Publication number: EP0109159A1
Application number: EP83305906A
Authority: EP
Inventors: Henry Dennis Cronk; Kenneth William Goodfellow; Ronald George Tye
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1982-09-30
Filing date: 1983-09-29
Publication date: 1984-05-23
Also published as: GB2128031B; GB2128031A; EP0109159B1; GB8326135D0; DE3365466D1

Abstract

An electrical switchmat having a normally open switch which is closed under a predetermined minimum load, the switchmat comprising a first metal sheet 3 separated from a load-bearing sheet 2 having an electrically conductive surface by resiliently compressible, non-conductive materials 5 and 7, and a plurality of bridging elements 6 positioned between the sheets such that when at least said predetermined minimum load is applied to the loadbearing sheet 2, the non-conductive material 5 and 7 compresses and one or more of the bridging elements establishes a conductive path between the sheets, the load-bearing sheet 2 being substantially rigid under the intended operating load of the switchmat and the bridging elements 6 being constructed and arranged such that when the load-bearing sheet is subjected to substantial overload, it is supported by the bridging elements 6 thereby preventing damage to the non-conductive materials 5 and 7.

Description

Technical Field

This invention relates to pressure sensitive switchmats and, in particular, to switchmats used for the detection of the presence of pedestrians.
Load sensitive mats incorporating electrical switches are well known and in common use. For example, switchmats are conventionally located adjacent to doors leading into and out of supermarkets, airports and other public places so as to'lie in the path of pedestrians approaching the doors. As a pedestrian approaches a door he steps upon the mat, thereby closing a normally open switch contained in the mat and actuating a mechanism to open the door automatically. ,
Switchmats are also used for the detection of pedestrians in industry where the mats may be used to limit access to dangerous machinery or process controls. Also, switchmats may be used in an electric circuit made part of a normal machine motor control circuit in a manner such that the mat has to experience the load represented by the weight of the operator to allow the machine to start or continue operation. Removal of such. load-, for example by the operator moving away from the machine, opens the motor control circuit and shuts the machine down.

Background Art

Examples of known switchmats are disclosed in U.S. Patent Nos. 3,825,277, 3,722,086, 4,037,069 and 4,105,899 and British Patent Specification Nos. 1,351,911 and 2,064,222. One of the problems associated with the use of any of the known switchmats in industry is that they are often not sufficiently robust to withstand the rigours of an industrial environment for long periods of time. Switchmats used in industry may be subjected to severe overloads; e.g., dropping of a heavy object or passage of a trolley or truck. Furthermore, the switchmats may be subjected to spillage of liquids; e.g., water and oil.
British Patent Specification No. 1,454,805 discloses a pressure sensitive switch construction incorporating means for automatically protecting the switch assembly against damage due to the application of excessive forces. The switchmat essentially comprises a flat sheet of compressible, non-conductive material having an opening therethrough, flexible electrically conductive sheets adhered to each surface of the non-conductive member and bridging means accommodated in said opening which establish a conductive path between the two metal sheets when the switchmat is compressed, and means for limiting the compressive force to which the bridging means may be subjected comprising a relatively incompressible member encircling the bridging member. The whole switch is enclosed in a molded non-conductive sheath of resiliently compressible plastics material.
It'is an aspect of the present invention to provide a switchmat of simple construction which is sufficiently robust and reliable to be used in industrial environments.

Disclosure of. Invention

Therefore, according to the invention, there is provided an electrical switchmat having a normally open switch which is closed under a predetermined minimum load, the switchmat comprising a first metal sheet separated from a load-bearing sheet having an electrically conducting surface by resiliently compressible, non-conductive material, and a plurality of bridging elements positioned between the sheets such that when at least said predetermined minimum load is applied to the load-bearing sheet the non-conductive material compresses and one or more of the bridging elements establishes a conductive path between the sheets, the load bearing sheet being substantially rigid under the intended operating load of the switchmat and the briding elements being constructed and arranged such that when the load- bearing sheet is subjected to substantial overload, it is supported by the bridging elements thereby preventing damage to the non-conductive material.

Detailed Description of Invention

In its simplest form a switchmat in accordance with the invention consists of two rigid metal sheets, at least one of which is load-bearing, separated by an insulating elastomer; e.g., by strips of elastomer positioned around the perimeter of the sheets, each metal sheet being connected to part of an electrical circuit. The switchmat additionally comprises bridging elements positioned between the metal plates such that when the elastomer is compressed a conductive path.between the metal sheets is established by the bridging elements. By suitable selection and positioning of the bridging elements and the elastomers, the mat may be constructed to operate under a predetermined minimum load: e.g., for detecting the presence of pedestrians the mat should be sensitive to a pressure .of about 0.44 kg/cm2, this being equivalent to a load of 20 kg applied over the area of an average footstep, and being substantially less than the load imposed by an adult standing or stepping on the mat. The bridging elements used in the switchmats of the invention form an electrical contact between the two metal sheets when the mat is compressed and are sufficiently robust to support the load-bearing sheet when under substantial overload; e.g., at least five times the normal load to which the mat is subjected, thereby preventing the resiliently compressible, elastomeric, non-conductive material from being subjected to severe strain.
The load-bearing metal sheet used in the switchmat of the invention is substantially rigid; i.e., it has the most limited flexibility, under normal use and may be substantially non-deformable. Most metal sheets of moderate area; e.g., 1 m²will be subject to slight bowing when a heavy load; e.g., a pedestrian, is applied to the middle of the sheet unless the whole area of sheet is adequately supported. In the switchmats of the invention a limited amount of flexibility or bowing may be allowed since this will increase the sensitivity of the central areas of the switchmat; however, such flexibility is not essential and the mode of operation of the switchmat of the invention always relies upon compression of the non-conductive material separating the metal sheets. The second metal sheet is preferably non-deformable under the operating conditions either by virtue of its own thickness and strength or by the provision of support means; e.g., a reinforcing plate, or the surface to which the switchmat is applied.
By the term "load-bearing" in the practice of the present invention, it is required that the load- bearing metal sheet contributes less than 50% of the deformation necessary to complete the electrical contact.
That is, at least 50% of the deformation is caused by compression of the resiliently compressible, non-conductive material. Preferably, at least 75% of the deformation occurs in this non-conductive material, and most preferably at least 90 or 95% of the deformation occurs in the resiliently compressible, non-conductive material.
In one embodiment of the invention the bridging elements comprise protrusions which are raised from the surface of one or both of the metal sheets. The protrusions . are preferably arranged at regular intervals over the whole area of the metal sheet and may be formed by punching indentations from the outermost surface of the sheet. The protrusions are raised to a height and are of sufficient number such that when the mat is loaded the upper metal sheet is supported on the lower metal sheet by the protrusions thereby preventing the non-conductive material from being compressed to the point where damage occurs.
In accordance with a second embodiment of the invention, the bridging elements take the form of inserts of electrically conductive material which are positioned between the two metal sheets. These inserts may conveniently comprise metal spheres; e.g., ball bearings or other conveniently shaped particles; e.g., barrel-shaped bodies of metal or resin which has been made conducting by the incorporation of conductive particles; e.g., of metal. Such elements need not be welded or adhered to one of the two metal sheets but each element may be conveniently confined to a desired area by forming a closed cell between the metal sheets with portions of compressible non-conductive elastomeric material.
In practice it has been found that good, reliable electric contact between the bridging elements and metal sheet is obtained when the bridging elements have a curved profile which is presented towards the metal sheet.
The invention will now be described with reference to the accompanying drawings, in which:

Figure 1 represents a perspective view, partly in section, of a portion of a switchmat in accordance with the invention.
Figure 2 represents a plan view of the switchmat of Figure 1 with the top metal sheet removed.
Figure 3 represents a gasket suitable for use in the switchmat of Figures 1 and 2.
Figure 4 represents a perspective view, partly in section, of a portion of a further switchmat in accordance with the invention.
Figure 5 represents a gasket suitable for use in the switchmat of Figure 4.

Referring to Figure 1, the switchmat comprises a foundation metal plate 1; e.g.., 2mm thick aluminum, having a series of protrusions 4 at regular intervals throughout its area. The protrusions may conveniently be formed by punching indentations from beneath and may be raised to a height of about 3 to 4 mm above the surface of the metal plate. Around the perimeter of the foundation plate 1 are fixed strips of compressible non-conductive material 5; e.g., strips of closed cell foamed neoprene rubber 1.5 cm wide x 0.45 cm thick. If desired, flanges 3 may be provided along one or more of the sides or edges for fixing the mat to the floor. A top plate 2 is fixed to the perimeter wall of non-conductive material 5. The top plate may comprise 2 mm base thickness aluminum "treadplate" which has a raised 5 bar pattern embossed upon its upper surface and is intended to act as the walking surface for a gedestrian. Electrical connections may be made directly to the two plates 1 and 5. The switchmat is designed to work at very low voltages since electrically live parts are exposed to the user. The switchmat will be sensitive to loads less than 20 kg, this being sufficient to compress the rubber wall and permit contact between the protrusions 4 and the top plate 2, and will therefore easily detect the pressure exerted by a person^ts foot should he step on the mat. The use of a 2 mm base thickness "treadplate" renders the mat resistant to damage, and overloads are accommodated by carrying the weight of the top plate and its load on the protrusions 4. By ensuring that protrusions are provided immediately adjacent to the edges, total edge sensitivity is achieved. It is readily possible for the protrusions to be formed in the top plate instead of the bottom plate or in both plates without interfering . with the operation of the switchmat.
The switchmats may be constructed of materials other than aluminum sheets or plates; for example, in the interests of hygiene, the food industry may require the use of stainless steel. The metal sheets may be constructed of any of the metals or combination of metals commonly available in the engineering industry. The practical advantages of aluminum, availability in convenient sizes, lightness, cost, pleasing appearance without painting and sufficient strength, make it a convenient and desirable material in most cases. It is well known that aluminum readily forms a layer of oxide upon its surface, which inhibits further corrosion. This oxide is also a good insulator but this apparent drawback in the electrical properties may readily be overcome by shaping the bridging elements to rupture the oxide layer.
The construction of the switchmat described with reference to Figures 1 and 2 above provides a robust and reliable switchmat of moderate size which may be_' used in an industrial environment, which is not susceptible to significant contamination by liquids.
If a large switchmat area is required; e.g., the span of the top plate is greater than 400 mm, then the weight of the plate may cause it to sag in the middle. In order to avoid any danger of inadvertent contact, it is desirable to introduce additional supports of compressible non-conductive material; e.g., foamed neoprene strips, at intervals of about 300 mm to sufficiently support the top plates.
It has been found that when the metal plates are ·aluminum, the protrusions should be sufficiently pointed at their apex to enable them to pierce the oxide layer on the surface of the plates when contact is made. In practice, the punching of rounded indentations from the opposite side of the sheet provides protrusions having the desired contact points. To further increase the reliability, the protrusions should be placed at intervals such that the weight of the pedestrian's foot at any point shall exert enough pressure on the point to ensure good electrical connection. It has been found empirically that spacings of between 50 and 150 mm, preferably 60 to 120 mm, are satisfactory for the aluminum sheets described above, with the most satisfactory performance being achieved at 75 mm. Above 120 mm a reduction of sensitivity becomes apparent when a load is placed at the mid-point between contacts, the sensitivity rapidly decreasing when spacings of 150 mm are exceeded. If the spacing of the protrusions is too close on the aluminium sheet, the individual contact pressure of each protrusion may not be sufficient to pierce the oxide layer.
The use of a closed cell foamed neoprene rubber strip around the perimeter of the plates as the compressible non-conductive material provides-the interior of the switchmat with some protection against contaminants. Suitable closed cell foam neoprene strip is available from C.B. Frost & Company Ltd. under the trademark "Neontrice". The neoprene strip may be affixed to both metal plates with an oil and water resistant adhesive; e.g., a vinyl adhesive such as that commercially available from 3M UK PLC under the trademark "Scotchgrip". However, the inevitable movement in the switchmat as it is loaded and unloaded may cause "breathing" through small gaps in the edge seal. According to one embodiment of the invention, each protrusion is further protected with a gasket of closed cell neoprene rubber in the form of a square or disc of material with a hole in the center to accommodate the protrusion. An example of a circular gasket is shown in Figure 3. Typical dimensions for the circular gasket are 25 mm diameter with a central hole of 5 mm diameter and a thickness equal to that of the perimeter wall 5. Other shapes may be used providing the width of the wall protecting the protrusions is sufficiently thick to resist distortion and consequent failure. The use of such a gasket, which may be conveniently affixed with a contact adhesive, provides a second barrier to the ingress of contaminants to the contact area with a consequent substantial increase in reliability, and may also serve as a support for the top. plate in mats with a large span.
Figure 4 illustrates an alternative switchmat in accordance with the invention in which the bridging elements are provided separately and mechanical working of the foundation plate in order to provide the protrusions is avoided. Figure 4 uses identical reference symbols to Figure 1. The foundation plate 1 comprises a plain metal sheet and the bridging elements are provided by a series of metal balls 8 which are enclosed within the inner space of gaskets 7 which are similar to those shown in Figure 3. The diameter of the balls is less than the thickness of the perimeter wall and gasket, generally about 1 to 1.5 mm smaller. The internal diameter of the gasket is larger than the diameter of the balls, generally about 1.5 to 3 times larger, in order to allow the balls to float within the gasket cavity, thus presenting fresh contact surfaces from time to time and generally reducing wear. The spherical shape of-the balls presents a high pressure contact to each plate as the switchmat is loaded, acting as a conductive bridge between the two plates. As in the case of the protrusions, the spacing of the ball contact points is preferably between 50 mm and 150 mm. Preferably the balls are constructed of stainless steel and such balls are freely available in commerce in a variety of sizes.
Other shaped pieces of electrically conducting material may be used in place of the balls; e.g., barrel-shaped pieces. of resin which have been made conducting by the incorporation of conductive particles. However, such bridging members must be sufficiently robust to carry the load presented to the top plate of the switchmat under conditions of overload.
In order to facilitate the rapid assembly of switchmats incorporating separate bridging elements, e.g., balls, the rubber gasket may be fashioned in the form of a perforated strip as illustrated in Figure 5. The strip may be formed of closed cell foam neoprene rubber, in which closely spaced holes are punched. The mode of application of such a strip, which may be coated with pressure sensitive adhesive on both sides, is to lay it in strips parallel to an edge at 50 to 150 mm intervals across the foundation plate and to insert the bridging elements into the punched holes at the desired intervals of 50 to 150 mm along the length of the strip. The plurality of holes in the strip reduces its resistance to compression by loading and thus maintains the sensitivity of the switchmat. Exemplary dimensions for strip gaskets are as follows:
These gaskets are suitable for use with bridging elements comprising metal spheres of 4 mm diameter. The length of the strip is dependent upon the size of the switchmat to be constructed.
An alternative method of constructing a switchmat using individual bridging elements is to lay upon the foundation plate a punched sheet or grid of elastomer having many spaces and to insert the bridging elements into the spaces at the desired intervals. This method of construction may provide the perimeter wall of compressible material as well as the internal gaskets. However, it is important to ensure that the presence of such a large amount of elastomeric material does not increase the support given to the top plate to such an extent that the switchmat's sensitivity is impaired.
In certain applications it may be necessary that the lower plate of the switchmat be totally insulated from the environment; e.g., when the switchmat is to be installed on a metal gantry or the like. 'Insulation may readily be effected by the presence of a sheet of insulating material positioned beneath the lower foundation plate. The insulating sheet may comprise plywood, plastics material or any other insulating material. The insulating sheet may be constructed to possess a high load-bearing capacity, thus the lower conducting sheet may be fashioned with less regard for its load-bearing capability as the main loading will be transmitted to the insulating sheet. In such a case, the lower conducting sheet may be thin or perforated to some extent. When perforations are used, they must be of a smaller diameter than any individual bridging elements used for contact purposes in order to ensure that electrical contact will be effected.
Although switchmats may be made so small as to contain only one bridging element, in practice the area of such mats will be sufficient to require a plurality of bridging elements. Practical dimensions of switchmats of the type described above range from 100 mm wide strips up to rectangles 2.4 m x 1.22 m which is the size of the largest aluminum sheet currently readily available from stockholders in the United Kingdom. However, large switchmats are difficult to handle and may suffer damage in transit unless reinforced, and accordingly it is often desirable to utilize smaller switchmats; e.g., 1 m x 1.2 m and cover large areas of floor by laying several smaller mats edge to edge and electrically connecting them in series. The smaller mats may be handled by one person and are sufficiently rigid to need protection only at the edges during transit.
The thickness of a typical mat is about 9 mm which is similar to some domestic floor coverings and thus the mat does not present a significant trip hazard.
Whilst the dimensions specified above have found practical utility and the mats are constructed from materials which are readily commercially available, it will be appreciated that the specific dimensions of the switchmat may be varied according to the materials used and the particular application of the mat. In general it has been found that the height of the bridging elements, either in the form of protrusions or independent elements; e.g., balls, should represent from 25% to 95% of the thickness of the elastomer separating the metal sheets when in its relaxed mode. Preferably the height of the bridging elements represents 75 to 80% of this thickness.
The electrical system connected to the switchmats of the invention may be of any desired type requiring a "make" contact lat low voltage; e.g., a.5 volt system controlling T.T.L. logic circuits, or a 12 volt relay may be directly connected to the mat. When the metal conductors are directly accessible by the user, it is essential that the voltage present upon the mats be low, and that the suitable earthing (grounding) precautions be taken. The electrical conducting wires may conveniently be affixed to the metal plates with screws or rivets and commonly a double connection is made to each plate so that the four-wire system with continuity loops may be employed. An example of a control system commercially available for use with switchmats is the "Safety Switchmat System" control manufactured by 3M UK PLC.
It is essential that the switchmats remain reliable throughout their life particularly when they are required to act as a safety device in industrial locations. Switchmats in accordance with the invention have been rigorously tested to determine their performance. Satisfactory electrical performance is considered to be a resistance of less than 1 OHM across the mat when contact is made.
Switchmats of the invention have withstood the impact of 75 kg at 2.5 m/sec transmitted through a circular steel plate approximately 45 cm² for 1000 cycles at the same point upon the mat without significant loss of performance, although the aluminum tread plate was visibly marked. With the impact speed reduced to a few centimeters per second, a closer representation to a foot step, 4,000 cycles produced no visible or measurable effect upon the mat's performance.
Another switchmat, in accordance with the invention, which possessed internal gaskets around the bridging elements, was submerged in ordinary tap water for 16 days and operated from time to time. After a continuous .period of 16 days it failed due to the ingress of water.
To test the shear strength between the aluminum top plate and foundation plate of a switchmat, a motor car was driven onto the switchmat and stopped with its rear driven wheels upon the mat. The car was then accelerated away. No deterioration in the appearance or performance of the mat was noted.
Switchmats constructed using metal balls as the bridging elements as illustrated in Figure 4 have exhibited a sensitivity for a 1 m mat, of 10 to 12 kg at the center, and the sensitivity of 15 to 16 kg at an edge rising to 18 to 20 kg at an extreme corner, the loads being distributed over a circular area of about 45 cm² to simulate a footstep. This sensitivity is more than adequate to detect the step of a pedestrian on any portion of the mat.
While indentations and ball bearings have been used individually in the examples, these bridging elements may be combined as necessary in one mat.
The mats, although designed for use on an essentially horizontal floor, will function equally well at any angle to the horizontal, up to and including inversion.

Claims

1. An electrical switchmat having a normally open switch which is closed under a predetermined load, the switchmat comprising a first metal sheet separated from a load-bearing metal sheet having an electrically conductive surface by resiliently compressible, non-conductive material, and a plurality of bridging elements positioned between the metal sheets such that when at least said predetermined minimum load is applied to the load-bearing metal sheet the non-conductive material compresses and one or more of the bridging elements establishes a conductive path between the metal sheets characterized in that the load-bearing metal sheets are substantially rigid under the intended operating load of the switchmat and the bridging elements are constructed and arranged such that when the load-bearing metal sheet is subjected to substantial overload, it is supported by the bridging elements thereby preventing damage to the non-conductive material.

2. A switchmat as claimed in claim 1, characterized in that the bridging elements comprise protrusions on the surface of one or both metal sheets.

3. A switchmat as claimed in claim 1 or claim 2, characterized in that the bridging elements comprise bodies of electrically conducting material.

4. A switchmat as claimed in claim 3, characterized in that the bodies are stainless steel spheres.

5. A switchmat as claimed in any preceding claim, characterized in that the height of the bridging elements represents from 25 to 95% of the thickness of the compressible non-conductive material when in its relaxed mode.

6. A switchmat as claimed in any preceding claim, in which the bridging elements are spaced from 50 to 150 mm apart.

7. A switchmat as claimed in any preceding claim, in which each bridging element is surrounded by a gasket of compressible non-conductive material, which gasket is affixed to both sheets.

8. A switchmat as claimed in any preceding claim, characterized in that the compressible non-conductive material separating the sheets seals the perimeter of the sheets.

9. A switchmat as claimed in any preceding claim, characterized in that the metal sheets are aluminum sheets.

10. A switchmat as claimed in any preceding claim, characterized in that the resiliently compressible non-conductive material comprises a closed cell foamed neoprene rubber.