Platform scale with variable capacitance transducer

A platform scale in which the force applied to the platform is transmitted to a capacitance transducer whereby the output of the transducer is correlated to the force on the platform. The transducer includes two parallel square plates spaced apart and insulated from each other. Each plate has four slots, one projecting inwardly from each edge of each plate and each slot being generally parallel to the adjacent edge whereby each plate has four identical cantiliver beams. The free ends of each beam of one plate are connected to but insulated from the free ends of the corresponding beams of the other plate. The space between the plates constitutes the dielectric of the transducer and this space is reduced by an amount virtually correlated to the force applied to the platform so that the output of the transducer is very closely correlated to the force applied to the platform on the scale.

BACKGROUND OF THE INVENTION 
The present invention relates to platform scales and especially to scales 
which commonly are known as bathroom scales. More particularly, the 
invention relates to a platform scale where the force applied to the 
platform changes the capacitance of a capacitance transducer and the 
change in capacitance produces a read-out which indicates the magnitude of 
the force applied to the platform. 
SUMMARY OF THE INVENTION 
The general object of the present invention is to provide in a platform 
scale a capacitance transducer which basically eliminates the problems of 
hysteresis, which is comparatively simple and inexpensive to manufacture 
and assemble, and which has a high degree of linearity between the force 
applied to the platform of the scale and the output of the transducer. 
More specifically, the basic object of the invention is to achieve the 
foregoing through the use of two insulated capacitance plates which are 
identical in size and shape, which are polygonal with all side edges of 
each plate being equal in length, which has a plurality of slots with a 
slot opening through each edge of the plate and generally parallel to the 
adjacent edge to form a plurality of cantilever beams on each plate, and 
by connecting the free ends of the beams of one plate with the free ends 
of the beams on the other plate while insulating the plates from each 
other whereby the space between the plates constitutes the dielectric of 
the transducer and that space changes virutally in a linear relation with 
the force applied to the platform of the scale. 
The invention also resides in the details of the novel construction and 
arrangement of the capacitance transducer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the drawings for purposes of illustration, the invention is 
embodied in a platform scale 10 of the type commonly called a bathroom 
scale. The latter includes a sheet metal base 11 with a generally square 
bottom wall 12 and upstanding side walls 13 connected by short corner 
walls 14. Above the base is a generally square horizontal platform 15 
formed with an integral skirt 16 which projects downwardly and is spaced 
outwardly of the side and corner walls 13 and 14. A force applied to the 
platform is transmitted to a capacitance transducer assembly 17 mounted on 
the base at the center thereof and, for this purpose, the platform is 
supported on the base through the medium of four identical levers 18. One 
lever is fulcrumed on each corner wall 14 by means of a hook position 19 
(FIG. 2) projecting through a hole 20 in the wall. The levers 18 extend 
diagonally inward and their inner end portions 21 bear on the transducer. 
On the underside of the platform at each corner thereof is a bracket 22 
which is rigidly attached to the platform by ears 15a struck down from the 
platform and which includes parallel depending legs 23. A flat loading 
link 24 is carried by each bracket 22 between the legs thereof by ears 25 
which project from the sides of the link and into holes 26 in the legs. 
The underside of each link 24 is formed with a notch 27 which is seated in 
an upwardly opening notch 28 formed in each lever 18 inwardly of the hook 
19. Thus, any force on the platform 15 is transmitted to the levers 18 
through the brackets 22 and the links 24 and causes the levers to be urged 
downwardly and this applies a downward force to the transducer 17. 
The force applied to the transducer 17 changes the capacitance of the 
latter in proportion to the magnitude of the force applied to the platform 
15 and this change in capacitance is used to show the magnitude of the 
force applied to the platform on a display panel 29 which preferably is a 
digital display. As shown schematically in FIG. 9, the force 30 applied to 
the transducer 17 changes the capacitance of the latter and the output of 
the transducer is fed to an RC oscillator circuit 31. The latter circuit 
31 feeds a micro-processor circuit 32 which operates the digital display 
29. Such circuits are well-known in the art and examples are shown in 
Schneider U.S. Pat. No. 3,966,002, Williams U.S. Pat. No. 4,051,721, 
Suzuki et al. U.S. Pat. No. 4,158,396 and Miyoshi et al. U.S. Pat. No. 
4,191,266. 
The present invention contemplates the provision of a novel capacitance 
transducer 17 which has little or no hysteresis effect, which has a high 
degree of linearity between the magnitude of the force applied to the 
platform 15 and the output of the transducer and which is relatively 
inexpensive to manufacture and assemble. To these ends, the transducer 
basically is composed of two spaced parallel plates 33 and 34 which are 
identical and which are polygonal in shape with all sides of each plate 
being equal in length. The plates are formed with a plurality of slots 35 
and 36 equal in number to the sides of the plate with one slot opening 
through each side edge of the plate and being generally parallel to the 
adjacent side edge. Each slot is shorter than the corresponding adjacent 
edge so that the slots form on each plate a plurality of identical fingers 
or cantilever beams 37 and 38 equal in number to the sides of the plate. 
Fasteners 39 connect the free ends of the beams 37 on the plate 33 to the 
free ends of the corresponding beams 38 on the plate 34 and the plates are 
insulated from each other so that the space between the plates constitutes 
the dielectric of the transducer. Thus, when a force is applied to the 
platform 15, the levers 18 apply a corresponding force 30 to the plate 34 
and the latter force bends the beams 37 and 38 to change the spacing 
between the plates 33 and 34. This change in spacing is virtually 
proportional to the magnitude of the force applied to the platform and, 
accordingly, the output of the transducer also is virtually proportional 
to that force. 
In the form shown in FIGS. 1 through 7, the plates 33 and 34 are square and 
disposed in spaced horizontal planes with the plate 33 being beneath the 
plate 34. An upstanding bracket 40 (FIG. 2) is rigidly attached to the 
bottom wall 12 of the base 11 at the center thereof by ears 12a struck up 
from the bottom wall. The bracket includes a support part 41 with an 
upwardly facing horizontal surface 42. The latter is the support for the 
lower plate 33 which is centered relative to the base by a cylindrical 
boss 43 which projects up through a hole 44 in the center of the plate 33 
and which is centered relative to the base 11. The edges of the plates and 
the slots 35 and 36 are straight so that the beams 37 and 38 are 
rectangular in shape. The fastening elements 39 are rivets which project 
through holes 45 (FIG. 4) larger than the diameter of the rivets and 
formed in the free end portions of the beams. To insulate the lower plate 
33 from the upper plate 34 and to establish the initial spacing of the 
plates, a cylindrical spacer 46 of a non-conductive material such as 
plastic surrounds each rivet 39 (FIG. 4) and is formed with axial 
shoulders 47 and 48 which abut the plates 33 and 34 respectively. Beyond 
the shoulders are reduced end portions which fit fairly snugly in the 
holes 45 in the beams. The ends of the rivets are upset to form heads 49 
and 50 and non-conductive washers 51 are disposed between the heads and 
the plates. Thus, the rivets 39, the spacers 46 and the washers 51 rigidly 
connect each corresponding pair of beams 37 and 38 on the plates 33 and 34 
and also electrically insulate the plates from each other. As a result, 
the plates are those of the capacitance transducer 17 with the space 
between them being the dielectric of the transducer. 
In order that the levers 18 apply the force on the platform 15 centrally to 
the upper plate 34, a casting 52 having a downwardly facing surface 53 is 
disposed above the upper plate which is engaged by the surface 53 and the 
casting is centered relative to the upper plate by a cylindrical boss 54. 
The latter is coaxial with the boss 43 and projects down into a hole 55 in 
the center of the upper plate 34. The upper side of the casting includes a 
peripheral flange 56 and the inner end portions 21 of the levers are 
fingers which bear against the top surface of the casting 52 inside the 
flange 56 so that the levers exert a force 30 proportional to the force 
applied to the platform. The force 30 bends the beams 37 and 38 and 
thereby changes the spacing between the plates 33 and 34 to a degree which 
change is almost exactly proportional to the force applied to the platform 
15. The range of the scale is illustrated in FIGS. 5, 6 and 7 in which 
FIG. 5 shows zero deflection for no force on the platform, FIG. 6 shows an 
intermediate deflection for a force within the range of the scale and FIG. 
7 shows the maximum range of the scale where the plates 33 and 34 engage 
each other. In practice, it has been found that the deviation of linearity 
between the force applied to the platform and the output of the transducer 
17 is 0.0017 percent. For most practical applications, this accuracy is 
more than adequate but, in any event, even this small inaccuracy may be 
factored out by the micro-processor circuit 32. 
The modification shown in FIG. 8 reduces the deviation from straight 
linearity even more and, in this form, the corresponding parts are 
identified by the same but primed reference numerals. Basically, the 
structure of the transducer 17' is the same as that of the transducer 17 
except that edges of the plates have shallow indentations 57 between the 
ends of the adjacent slots and the slots are reversely indented as 
indicated at 58 to provide the center of each beam with a necked-down 
portion 59. Depending upon the accuracy desired, this form may reduce the 
function of the micro-processor circuit 32 to eliminate whatever small 
deviation in linearity as may exist. 
With the invention as illustrated in the drawings and described above, the 
linearity of the transducer 17 and the force applied to the platform are 
almost equal. Moreover, the transducer is virtually void of hysteresis 
effects. At the same time, the transducer is simple and inexpensive to 
manufacture and assemble.