GB2261115A

GB2261115A - Pressure sensitive mat

Info

Publication number: GB2261115A
Application number: GB9222643A
Authority: GB
Inventors: Derek William Jones; Kenneth Lomax
Original assignee: EJA Engineering Ltd
Current assignee: EJA Ltd
Priority date: 1991-10-29
Filing date: 1992-10-28
Publication date: 1993-05-05
Also published as: GB9222643D0

Abstract

A pressure sensitive mat has two electrically conductive plates 1, 2 spaced apart by non conductive compressible separators 3. Located adjacent to the separators 3 are electrically conductive elements 5, eg washers fixed to the lower plate 2 or integral portions of at least one of the plates 1, 2, which extend across a fraction of the gap 6 between the plates 1, 2. The plates 1, 2 are sealed at their edges by a sealing bead 7 and a metal strip 8 is placed adjacent the sealing bead 7. When pressure is applied to the top of the mat above a separator 3, the top plate is deformed and the separator 3 is compressed until the plate 1 contacts the element 5 completing an electrical circuit. When pressure is applied at the edge of the mat the top plate 1 and sealing bead 7 are deformed until the top plate 1 contacts the metal strip 8 completing an electrical circuit. When pressure is applied above the gap 6 away from a separator 3, direct electrical contact is made between the plates 1, 2. <IMAGE>

Description

A PRESSURE SENSITIVE MAT This invention relates to a pressure sensitive mat.

Pressure sensitive mats are used in the field of industrial machine safety equipment to detect the presence of a person or object in the vicinity of a machine when it is in operation. From a safety point of view it is often desirable to turn off a machine when a person is detected in its vicinity. Such mats can also be used for other purposes, for example to detect intruders.

One known pressure sensitive mat comprises a pair of steel plates separated by elastomeric separator pads to define an evacuated air gap, the plates being sealed together along each edge by a sealing bead. The plates are connected to an electrical control circuit remote from the mat. The assembly is encapsulated in a moulded vinyl coating.

When pressure is applied to a mat, for example by a person standing thereon, the top plate deflects and comes into contact with the bottom plate. This has the effect of completing the electrical control circuit, initiating an appropriate response. In this manner the presence of a person can be detected on the mat and the electrical circuit can be used for example to initiate switching action to turn off any machinery in the vicinity of the mat.

The abovedescribed safety mat has disadvantages. If pressure is applied directly over one of the elastomeric separator pads, the pads will deform but will prevent the plates from coming into contact. Thus the mat can fail to detect the presence of a person or object If extreme pressure is applied to an elastomeric separator pad, for example from a fork-lift truck, the pad is deformed to such an extent that it may undergo permanent plastic deformation or delayed elastic recovery.

The sealing bead around the edge of the plates also results in a "dead" area where the mat is not sensitive, the area being larger at the mat corners. This problem is augmented when several mats are joined together, as the non-sensitive edges are adjacent each other.

This is particularly a problem where the corners of four mats meet, giving a large region of non-sensitivity.

It is an object of the present invention to obviate or mitigate the aforesaid disadvantages.

According to the present invention there is provided a pressure sensitive device connectable to an electrical circuit, the device comprising two electrically conductive plates spaced apart at intervals by non-conductive compressible separators such that when the device is not subjected to pressure a gap is formed between the plates, wherein electrically conductive elements are located adjacent the separators, the elements extending across a fraction of said gap such that, when pressure is applied to the device at least one plate is deformed so that electrical contact is made between the two plates either directly or via at least one said element.

Preferably the separators are circular pads. The separators may be surrounded by the said elements or alternatively may be surrounded by the elements. The elements may be in the form of metallic washers or alternatively may be an integral part of one or both of the plates, for example raised formations pressed into one of the plates.

Thus when pressure is applied to the device at a point above a separator, the separator is compressed until the electrically conductive element establishes contact between the opposing plates.

Preferably the separators at each edge of the plates act to seal the plates together. In this case the electrically conductive elements may be in the form of strips lying adjacent the sealing separators.

Preferably the sealing separators are in the form of beading.

Preferably the device is in the form of a floor mat and is coated in a protective compressible layer.

Specific embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a part sectioned perspective view of an undeformed pressure sensitive mat in accordance with the invention; Figure 2 is a pivotally cut-away plan view of one corner of the mat of figure 1; Figure 3 is a sectioned side view, of the mat of figure 1, after deformation; Figure 4 is a part sectioned view of a second embodiment of an undeformed pressure sensitive mat in accordance with the invention; and Figure 5 is a part sectioned view of a third embodiment of an undeformed pressure sensitive mat in accordance with the invention.

Referring to the drawings, there is shown a pressure sensitive mat having two plates 1,2 spaced apart at intervals by separators 3 and covered in a protective coating 4.

The plates 1,2 are flat, rectangular in plan and are made of a suitable electrically conductive material such as steel. The separators 3 are circular pads manufactured from a compressible non-conductive material such as an elastomeric polymer. A conductive metal washer 5 is fixed to the lower plate 2 around each separator 3 by electrically conductive adhesive such that it is in electrical contact with the plate 2. The washer 5 is smaller in thickness than the uncompressed separator pad 3, only extending across part of an air gap 6 defined between the plates. The plates 1,2 are sealed together at the periphery by a sealing bead 7 which also acts as a separator. A steel strip 8 is fixed to the lower plate 2 by electrically conductive adhesive, adjacent the sealing bead 7.The steel strip 8 is of smaller thickness than the sealing bead 7 and hence only extends across part of the air gap 6.

The assembly as abovedescribed is covered in the protective compressible coating 4 which may be moulded vinyl. Part of the coating 4 is not shown in Fig. 1 to expose one edge of the plates.

The plates 1,2 form normally open contacts of an electrical control circuit (not shown) which is positioned remote from the mat.

Closing of the contacts i.e. electrical contact between the plates 1,2, causes the electrical circuit to be completed. This can initiate a switching action to turn off any surrounding machinery.

When pressure is applied to the top surface of the mat (as shown by the arrows F in figure 3) the top plate 1 deforms. If the pressure is applied above an air gap 6 the top plate 1 contacts the lower plate 2 directly to complete the electrical circuit as described.

If pressure is applied above a separator pad 3, the pad 3 is compressed as a result of the deformation of the top plate 1 until the plate 1 contacts the washer 5, completing the electrical circuit. The washers 5 also act as stops to prevent the separator pads 3 deforming beyond their elastic range, thereby avoiding permanent deformation.

The same principle is in action when pressure is applied above the edges of the mat, as the sealing bead 7 deforms until the top plate 1 contacts the steel strip 8. The steel strip thus ensures the mat is sensitive right up to its edges.

Alternative forms of the same invention are shown in Figures 4 and 5. In figure 4 the lower plate 2 is press formed to have a raised annulus 10 which performs the same function as the washer described above. The annulus 10 facilitates the location of the separation pad 3 by providing for a central recess in which the pad fits snugly. The lower plate can be press formed by, for example, a CNC machine.

Figure 5 shows a further embodiment of the present invention.

The lower plate 2 is again press formed, but the raised portion 10 is this time in the form of a filled-in circle. The separator pad 3 is in the form of an annulus dimensioned such that it fits neatly over the raised portion 11, the arrangement again facilitating the location of the separator pad 3.

In both figures 4 and 5 the separator pad is deformable as described above to allow the top plate to contact the raised portion of the lower plate to complete the electrical circuit.

It is to be appreciated that both or either of the plates could be press formed in the mannner described to provide the same effect.

It is to be appreciated that the application of the invention as described is not limited to use in industrial safety switching. Such pressure sensitive mats may be used in for example any security application to activate an alarm when an intruder is detected.

Claims

1. A pressure sensitive device connectable to an electrical circuit, the device comprising two electrically conductive plates spaced apart at intervals by non-conductive compressible separators such that when the device is not subjected to pressure a gap is formed between the plates, wherein electrically conductive elements are located adjacent the separators, the elements extending across a fraction of said gap such that, when pressure is applied to the device at least one plate is deformed so that electrical contact is made between the two plates either directly or via at least one said element.

2. A pressure sensitive device according to claim 1 wherein the separators are circular pads.

3. A pressure sensitive device according to claim 1 or 2 wherein the separators are each surrounded by an electrically conductive element.

4. A pressure sensitive device according to claim 1, wherein the electrically conductive elements are each surrounded by a separator.

5. A pressure sensitive device according to any preceding claim wherein the electrically conductive elements are metallic washers.

6. A pressure sensitive device according to any one of claism 1 to 4 wherein the electrically conductive elements are integral raised portions of at least one of said plates.

7. A pressure sensitive device according to any preceding claim wherein the edges of said plates have sealing separators which act to seal the plates together.

8. A pressure sensitive device according to claim 7 wherein electrically conductive edge elements are positioned adjacent the sealing separators.

9. A pressure sensitive device according to claim 8 wherein the edge elements are in the form of metallic strips.

10. A pressure sensitive device according to claim 8 wherein the edge elements are formed from integral raised portions in at least one of said plates.

11. A pressure sensitive device according to claims 7 to 10 wherein the sealing separators are in the form of beading.

12. A pressure sensitive device according to any preceding claim wherein the device is in the form of a floor mat and is coated in a protective compressible layer.

13. A pressure sensitive device substantially as hereinbefore described with reference to the accompanying drawings.