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BACKGROUND OF THE INVENTION
This invention relates generally to pressure sensitive switches and more
particularly concerns switches for the detection of the presence or
absence of a person from a hospital bed, wheelchair, baby carriage or any
other body supporting structure with respect to which it may be useful to
determine the status of occupancy, and to the method of making such
switches.
Pressure sensitive switches presently used in many hospital facilities are
disclosed in previously issued U.S. Pat. Nos. 4,484,043 and 4,565,910. In
these known devices, broad bands of conductive material are used in upper
and lower layers such that contact of any portion of the broad upper band
with any portion of the broad lower band will complete a circuit between
input and output conductors connected to the bands.
While these switches work quite effectively, they are somewhat difficult of
manufacture in a manner providing consistency and predictability of
operation and consequently are sufficiently expensive to limit their broad
use as single use disposable devices. Furthermore, while they work quite
well in sensing the presence or absence of fully grown adults, as lesser
weights are applied to the switch, sensing reliability somewhat
diminishes. The problem of providing a switch having reliable
characteristics over a broad weight range is complicated by the
requirement that the switch must not only reliably close upon the
application of a minimal threshold weight, but must also reliably return
to its open condition upon the removal of that threshold weight. In
present devices, weights resulting in at least 50 to 75 psi are necessary
to achieve desired levels of reliability. In addition, known devices are
of relatively thick profile and stiffness so as to provide adequate
separation of the conductive bands. This results in a diminishing of
patient or occupant comfort and also in a diminishing of the impact of
pressure on its operation, contact more frequently being achieved by a
crimping or kinking of the device than by flex due to pressure. And known
devices are not completely sealed and, therefore, do not provide a fluid
or water impervious device, thus compromising the device's integrity and
consistency of operation. It is therefore an object of this invention to
provide a pressure sensitive switch which will reliably close at
relatively low threshold pressures. It is also an object of this invention
to provide a pressure sensitive switch which will reliably open on release
of the threshold pressure from the switch. It is further an object of this
invention to provide a pressure sensitive switch having gridded conductive
surfaces with a limited matrix of contact points so as to minimize the
possibility of circuit completion in the absence of a relatively low
threshold pressure. It is similarly an object of this invention to provide
a pressure sensitive switch having gridded conductive surfaces with a
limited matrix of contact points separated by a gridded dielectric to
minimize the possibility of circuit completion upon release of the
threshold pressure. Another object of the invention is to provide a
pressure sensitive switch that lends itself to ease in mass production
with minimal impact on predictability and consistency in switch operation.
A further object of this invention is to provide a pressure sensitive
switch of relatively thin and flexible profile so as to maximize patient
comfort and to minimize the possibility of crimping or kinking and
resultant inadvertent contact. And it is an object of this invention to
provide a pressure sensitive switch which is impervious to water and other
fluids.
SUMMARY OF THE INVENTION
In accordance with the invention, a pressure sensitive switch is provided
having upper, middle and lower laminar elongated members. The middle
member has one or more openings which define one or more cavities between
the upper and lower members. A first array of substantially parallel,
spaced-apart electrically conductive bands is fixed to a lower surface of
the upper member and traverses the cavities. A second array of
substantially parallel, spaced-apart electrically conductive bands is
fixed to an upper surface of the lower member and traverses the cavities
and the upper member bands. Selected lower member bands are discretely
connected to an electrical input lead and the other lower member bands are
discretely connected to an electrical output lead. An array of
substantially parallel, spaced-apart dielectric bands is fixed to the
lower member upper surface and traverses the cavities between the first
and second arrays of conductive bands at their alternate overlapping
points, separating the first and second arrays of conductive bands from
making electrical contact with each other in the area of overlap with the
dielectric bands. The upper and lower members are so resiliently flexible
as to permit the overlapping points of the arrays of conductive bands to
close into or open out of electrical contact, except in the area of
overlap with the dielectric bands, upon exertion or removal, respectively,
of a threshold external compressive force to or from the cavities.
Preferably, the upper member array is orthogonal to the lower member array
and the bands of the upper and lower member arrays have centerlines
substantially equally spaced. In this arrangement, the overlapping of the
centerlines of the conductive arrays defines a matrix of squares.
Preferably, the bands of dielectric have one edge along the diagonal of
alternate ones of the squares and cover one half of the overlapping
portion of the conductive bands The conductive bands are of substantially
equal width and the dielectric bands are of substantially equal width and
narrower than the conductive bands and preferably one half the diagonal of
the area of the overlapping conductive bands so that, even at those
overlap points partially separated by dielectric, electrical contact is
possible. Such a metricized arrangement has been found to provide most
suitable complements of ease of manufacture and. consistency of operation.
Preferably, the laminar members are of heat stabilized polyester and the
conductive bands are formed of a conductive ink, such as a blend of
graphite/silver ink, screened onto the members.
In making the pressure sensitive switch, one array of substantially
parallel, spaced-apart electrically conductive bands is applied to the
surface of the upper flat flexible member. Another array of substantially
parallel, spaced-apart electrically conductive bands is applied to the
surface of the lower flat flexible member. This array includes a
conductive input lead connected to selected ones of the conductive bands
and a conductive output lead connected to the other conductive bands. An
array of substantially parallel, spaced-apart dielectric bands are also
applied to the surface of the lower member and to the lower member
conductive bands. One or more openings are cut through the middle flat
flexible member. The upper, middle and lower members are laminated
together with the conductive arrays traversing the openings and each other
and the dielectric array diagonally aligned with alternate overlapping
points of the conductive arrays and separating the conductive arrays from
making electrical contact in the area of dielectric overlap.
The thickness of the members and the spacing of the bands of the arrays is
such that the upper and lower members resiliently flexibly permit the
overlapping points of the arrays of conductive bands except in the area of
overlap by the dielectric, to close into or open out of electrical contact
upon exertion or removal, respectively, of the threshold external
compressive force to or from the openings. Preferably the conductive and
dielectric bands are formed by screen painting conductive and dielectric
ink, respectively, on the member, and lamination is accomplished by heat
sealing or adhesive bonding of the polyester members together.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will become apparent upon
reading the following details description and upon reference to the
drawings in which:
FIG. 1 is a bottom plan view of a preferred embodiment of the upper member
of the pressure sensitive switch with a conductive grid applied thereon;
FIG. 2 is a top plan view of a preferred embodiment of the lower member of
the pressure sensitive switch with the input and output conductive grid
applied thereon;
FIG. 3 is a bottom plan view of a preferred embodiment of the dielectric
grid to be applied over the conductive grid and the member illustrated in
FIG. 2;
FIG. 4 is a reversible plan view of a preferred embodiment of the middle
member of the pressure sensitive switch;
FIG. 5 is an exploded plan view of a preferred embodiment of the pressure
sensitive switch illustrating the matrix arrangement of the conductive and
dielectric grids;
FIG. 6 is a sectional view taken along the line 6--6 of FIG. 5; and
FIG. 7 is a bottom plan view of a preferred embodiment of the pressure
sensitive switch.
While the invention will be described in connection with a preferred
embodiment and method, it will be understood that it is not intended to
limit the invention to that embodiment. On the contrary, it is intended to
cover all alternatives, modifications and equivalents as may be included
within the spirit and scope of the invention as defined by the appended
claims.
DETAILED DESCRIPTION OF THE INVENTION
Turning to FIG. 1, a preferred embodiment of an upper member of a pressure
sensitive switch is illustrated. As shown, the upper member 10 consists of
a flat, elongated, substantially rectangular sheet 11 having lengthwise
edges 13 considerably longer than its widthwise edges 15. An array 17 of
conductive bands 19 extends substantially longitudinally on the bottom
face of the sheet 11. Preferably, the conductive bands 19 are parallel, of
equal width and aligned on equally spaced centers. The array 17 extends
substantially across the interior portion of the bottom face of the sheet
11, the length 21 and width 23 of the array 17 leaving a relatively wide
perimeter portion of the bottom surface without any conductive grid.
Preferably, the array 17 of conductive bands 19 will be applied by screen
painting a conductive ink on the bottom surface of the upper member 11.
As shown in FIG. 2, a preferred embodiment of the lower member 30 of the
pressure sensitive switch also consists of a flat, elongated,
substantially rectangular sheet 31, preferably of length 33 and width 35
substantially equal to the length 13 and the width 15 of the upper member
10. An array 37 of conductive bands is applied to the top surface of the
lower member 30, the width of each of the bands of the array 37 preferably
being of equal width with each other and to the conductive bands 19 of the
upper member 10. As shown, the lower member array 37 is preferably
arranged in a width-wise grid orthogonal to the upper member conductive
array 17 on center lines preferably equally displaced as the center lines
of the conductive bands 19 of the upper member 10. Preferably, alternate
ones 39 of the lower member conductive array 37 are discreetly connected
to an electrically conductive input lead 41 while the other bands 43 of
the lower member conductive array 37 are discretely connected to an
electrically conductive output lead 45. Also preferably, the length 47 and
width 49 of the array 37 is substantially the same as the length 21 and
width 23 of the upper member array 19.
Looking now to FIG. 3, a preferred embodiment of a dielectric grid 51 to be
applied over the array 37 of conductive bands 39 and 43 on the lower sheet
31 is illustrated. As shown, the dielectric grid 51 preferably consists of
a plurality substantially parallel and equally spaced apart bands 53 of
dielectric material arranged in a fashion such that each of the dielectric
bands 53 will traverse all of the conductive bands 39 and 43 when the
dielectric grid 51 is applied to the lower sheet 31 and the array 37 of
conductive bands 39 and 43. As shown, and for reasons which will
hereinafter become apparent, the bands 53 are preferably aligned in a
45.degree. angular relationship with respect to the widthwise conductive
bands 39 and 43. The length 55 and width 57 of the dielectric grid 51 is
substantially equal to the length 47 and width 49 of the conductive array
37. The width of each dielectric band 53 is less than the width of the
conductive bands 19, 39 and 43 and preferably one half the diagonal of the
overlapping area of the conductive bands 19, 39 and 43.
Turning now to FIG. 4, a preferred embodiment of a middle member of the
pressure sensitive switch is illustrated. As shown, the middle member 70
consists of a flat, elongated, substantially rectangular sheet 71,
preferably of length 73 and width 75 identical to the lengths 13 and 33
and widths 15 and 35 of the upper and lower sheets 11 and 31. One or more
openings 77, are provided through the middle member 71. The openings 77
are substantially rectangular and arranged in longitudinal alignment
across the middle member 70. The total length 79 of the openings 77 is
substantially equal to the lengths 55 of the dielectric grid 51 or the
lengths 21 and 47 of the arrays 17 and 37 of upper and lower member
conductive bands 19, 39 and 43. Similarly, the width 81 of the openings 77
is substantially equal to the width 57 of the. dielectric grid 51 and the
widths 23 and 49 of the arrays 17 and 37 of upper and lower member
conductive bands 19, 39 and 43.
Looking at FIGS. 5 and 6, the relative alignments of the upper member
conductive bands 19, the dielectric bands 53 and the input and output
conductive bands 39 and 43 when the upper, middle and lower members 10, 70
and 30 are laminarly arranged is shown. In the segment of this arrangement
shown, the conductive bands 19, the dielectric bands 53 and the input and
output bands 39 and 43 are traversing one of the openings 77 in the middle
member 78. As shown, the upper conductive bands 19 and lower conductive
bands 39 and 43 form a matrix of squares while the dielectric bands 53
intersect alternate squares in a diagonal direction. Thus alternate
overlapping portions of the wider upper and lower conductive bands 19, 39
and 43 are partially separated from the possibility of electrical contact
therebetween by the narrower dielectric bands 53, as can best be seen in
FIG. 6. Consequently, in the preferred arrangement, only fifty percent of
the matrix of overlapping points can come into full electrical contact and
the remaining overlapping points can achieve electrical contact over a
maximum of 50 percent of their overlapping area.
This uniform distribution of full and partial contact points in spaced
apart relationship affords the control necessary to assure that
appropriate applications of threshold pressure to the cavity portions of
the switch will consistently cause completion of the switching circuit and
also that removal of or lack of such an appropriate threshold pressure
will reliably cause the circuit not to be complete.
In making the pressure sensitive switch, the conductive grids are screen
painted onto their respective members. Preferably, the upper and lower
members 10 and 30 will be 5 mil heat stabilized polyester and the
conductive bands 19, 39 and 43 will be formed by use of a suitable
conductive ink such as 50/50 graphite/silver blend. The input and output
leads 41 and 45 of the lower member conductive bands 39 and 43 are screen
painted simultaneously with the conductive bands 39 and 43. After the
conductive ink has been screened onto the lower sheets 31, a dielectric
ink can be used to screen the dielectric array 51 over the lower member
conductive array 37. Preferably, a plurality of arrays 37 can be screened
onto a single sheet which can then be cut into a number of sheets 31. The
openings 77 are die cut into the middle member 70 which will preferably be
formed of 10 mil sheet such as a 7 mil polyester film with a 1 1/2mil
adhesive on each side thereof if adhesive bonding is used to accomplish
lamination. The upper, middle and lower members 10, 70 and 30 are then
laminated together, as by heat sealing or adhesively bonding the middle
member 70 between the upper and lower members 10 and 30. FIG. 7
illustrates the upper member 10 of FIG. 1 and the lower member 30 of FIG.
2 with the dielectric array 51 of FIG. 3 superimposed thereon laminated to
the middle member 70 of FIG. 3 using a clear polyester for the upper,
middle and lower members 10, 70 and 30. The polyester need not necessarily
be clear. As shown, the input lead 41 and the output lead 45 are extended
externally of the switch to a plug 90 for connection of the switch to an
appropriate electrical power and control unit (not shown).
The device as shown is in an elongated rectangular configuration with
conductive arrays 17 and 37 in orthogonal arrangement and dielectric array
51 intersecting the orthogonal conductive arrays 17 and 37 at alternate
diagonals. However, while this arrangement is preferred, it is not
necessary that the conductive arrays 17 and 37 be in orthogonal
relationship to each other or that they be on equally spaced centers.
Depending on the particular application involved, it is necessary only
that a limited matrix of full and partial contact points be established so
as to provide the consistency of operation desired for given threshold
pressures. It has been found that, for operation at a desired threshold
pressure of approximately 2 pounds per square inch, a switch approximately
3.5 inches wide by 29 inches long with 5 mil heat stabilized polyester
upper and lower members 10 and 30 and a 10 mil polyester middle member 70
with three openings 77 each 2 inches by 8 inches and spaced 1/2inch apart
and inset 2 inches from the ends of the device and 3/4inches from the
sides of the device is a very workable structure. In this arrangement,
upper and lower member conductive grid bands 19, 39 and 43 of 0.09 inches
on 0.18 inch centers with 0.06 inch wide dielectric bands 53 on 0.26 inch
centers using 50/50 graphic/silver blend conductive ink for the conductive
bands 19, 39 and 43 is an optimum arrangement.
Thus, it is apparent that there has been provided, in accordance with the
invention, a pressure sensitive switch and method for making the switch
that fully satisfies the objects, aims and advantages set forth above.
While the invention has been described in conjunction with specific
embodiments and methods, it is evident that many alternatives,
modifications and variations will be apparent to those skilled in the art
and in light of the foregoing description. Accordingly, it is intended to
embrace all such alternatives, modifications and variations as fall within
the spirit of the appended claims.
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