Load cell

A load cell has a cantilever-type load-sensitive element with an upper beam and a lower beam, each having formed thereon a pair of strain-generating parts which generates a strain corresponding to an applied load. Only one of these two beams has an indented part formed on its surface and strain-detecting elements are attached to its bottom surface at positions corresponding to the strain-generating parts. A moisture-proof sheet is attached to the beam to completely seal the interior of this indented part to protect the strain-detecting elements from humidity and moisture.

BACKGROUND OF THE INVENTION 
This invention relates to load cells used in electronic scales and other 
weighing apparatus and, more particularly, to such load cells having 
improved moisture-proofing means. 
A cantilever-type load cell which is commonly used in electronic scales 
generally comprises a load-sensitive element having an upper beam and a 
lower beam forming a pair. Two strain-generating parts are formed on each 
of these two beams and strain-detecting elements such as strain gauges are 
attached individually to such strain-generating parts on the beams so as 
to measure the tensile and compressive strains at these parts caused by a 
load applied to the load-sensitive element and to thereby determine the 
magnitude of the load itself. 
Moisture-proofing means are sometimes provided to these strain-detecting 
elements of a load cell of this type in order to protect these elements 
against moisture and humidity and to thereby enhance their durability. 
Examples of moisture-proofing method which has been tried include covering 
the elements by a material such as silicone rubber and butyl rubber. Such 
organic polymer materials, however, are moisture-permeable to a certain 
extent because of their molecular structures. Although they do not totally 
lack in moisture-proofing effect, they are not sufficiently effective 
especially when the load cell is used in a very humid or moist 
environment. 
In view of the above, there have been attempts to cover the 
strain-detecting elements with aluminum foil or a metallic moisture-proof 
sheet or cover of a layered structure with a synthetic resin film on 
aluminum foil. Since such sheets and covers are not sufficiently elastic, 
however, they tend to impede the free deformation of the load-sensitive 
element corresponding to the load which is applied thereon. Thus, it is 
now being attempted not to attach these sheets and covers directly on the 
strain-detecting elements but to provide a space therebetween. Japanese 
Utility Model Publication Jikkai 61-30838, for example, discloses a 
load-sensitive element with a pair of upper and lower beams each 
characterized as having an indented part on its surface such that 
load-sensitive elements are placed in these indented parts which are then 
sealed by cover means. Japanese Utility Model Publication Jikkai 
59-183639, on the other hand, discloses a method of providing 
water-proofing putty on such beams above and below the load-sensitive 
element around the positions where strain-detecting elements are to be 
attached such that water-proofing sheets placed thereover can tightly seal 
the strain-detecting elements with a space provided therearound. 
The prior art attempts described above are not satisfactory for several 
reasons. Firstly, it requires additional production processes to provide 
such moisture-proofing means on the surfaces of both upper and lower beams 
for the load-sensitive element. 
This implies an increase in the production cost. Secondly, there still 
remain problems relating to the accuracy of measurement as well as 
moisture-proofing because, even if metallic moisture-proofing sheets and 
covers are attached indirectly by providing a space around each 
strain-detecting element, it is impossible to completely prevent them from 
affecting the strain of the load-sensitive element according to the 
applied load. If these sheets or covers are attached to the beams, their 
effects may be magnified and adversely affect the accuracy of measurement. 
Since moisture-proofing must be effected at two positions, furthermore, 
there is an increased probability of occurrence of a defect which may 
eventually affect the durability of the load cell. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a load cell 
with moisture proofing means. 
It is another object of the invention to provide such a load cell capable 
of measuring weight accurately. 
It is still another object of the invention to provide such a load cell 
which can be manufactured at a relatively lower cost. 
It is a further object of the invention to provide such a load cell with 
improved durability. 
A load cell embodying the present invention with which the above and other 
objects can be achieved may be characterized as being a cantilever-type 
with an upper beam and a lower beam, each of these beams having formed 
thereon a pair of strain-generating parts adapted to generate a strain 
corresponding to an applied load. Either one of these two beams has an 
indented part formed on its surface, the indented part being lower than 
the edge parts therearound. Strain-detecting elements are attached to the 
bottom surface of this indented part at positions corresponding to the 
strain-generating parts and covered by a moisture-proof material. The 
interior of the indented part is completely sealed by a moisture-proof 
sheet which is attached to only small areas of this beam but not to the 
moisture-proof material so as not to significantly interfere with the 
deformation of the cell. 
When a load is applied to a load cell thus structured, tensile and 
compressive strains generated correspondingly to the load at the 
strain-generating parts are detected by the strain-detecting elements and 
the load is calculated on the basis of these detected strains. Since the 
strain-detecting elements are all attached to the bottom surface of the 
indented part and the interior of this indented part is completely sealed 
by a moisture-proof sheet, these elements are well protected against 
humidity and moisture. In particular, since this indented part and the 
moisture-proof sheet are provided to only one of the beams, both the 
production cost of the load cell and the interfering effect of the 
moisture-proof sheet on the free deformation of the load cell are 
significantly reduced. Moreover, since a plurality of strain-detecting 
elements are protected against humidity and moisture by a single 
moisture-proofing means, occurrence of problems related to durability 
caused by defective moisture-proofing can also be reduced significantly.

DETAILED DESCRIPTION OF THE INVENTION 
With reference simultaneously to FIGS. 1, 2 and 3, numeral 1 indicates a 
load cell embodying the present invention. Numeral 10 indicates a 
load-sensitive element which serves as the main body of this load cell 1 
and has a hollow, rectangular configuration with a rigid fixed portion 11 
on the left-hand side (with reference to FIG. 1), a rigid movable portion 
12 on the right-hand side (with reference also to FIG. 1) and a pair of 
upper and lower beams 13 and 14 between these two rigid portions 11 and 
12. If this load cell 1 is incorporated in an electronic scale, the rigid 
fixed portion 11 might be affixed to the main structure 3 of the scale by 
a bracket 2 and a tray 4 to the rigid movable portion 12 by another 
bracket 3. At both ends of the upper and lower beams 13 and 14 where these 
beams 13 and 14 join the rigid portions 11 and 12, the beam widths are 
reduced by the formation of semicircular cutouts on the inner sides of the 
beams 13 and 14 to define strain-generating parts 13a, 13b, 14a and 14b 
as shown in FIG. 1. If a load is applied and the rigid movable portion 12 
is thereby displaced downward with respect to the rigid fixed portion 11, 
tensile strains are generated on the surfaces of the strain-generating 
part 13a at the junction between the upper beam 13 and the rigid fixed 
portion 11 and the part 14b at the junction between the lower beam 14 and 
the rigid movable portion 12, while compressive strains are similarly 
generated at the strain-generating parts 13b and 14a respectively at the 
junction between the upper beam 13 and the rigid movable portion 12 and at 
the junction between the lower beam 14 and the rigid fixed portion 11. 
The upper surface of the upper beam 13 is provided with an indented part 15 
which includes both the strain-generating parts 13a and 13b and is lower 
than its surrounding sections. On the bottom surface of this indented part 
15, there are two strain gauges 21.sub.1 and 21.sub.2 attached at the 
left-hand strain-generating part 13a and two others 21.sub.3 and 21.sub.4 
at the right-hand strain-generating part 13b. In other words, a total of 
four strain gauges 21 is attached to the top of the load-sensitive element 
10 on the bottom surface of the indented part 15. Lead lines (not shown) 
connected individually to these strain gauges 21 are passed through 
throughholes 16 and taken out through both side surfaces of the rigid 
fixed portion 11. These throughholes 16 are eventually filled with a 
moisture-proof filler material after these lead lines are passed 
therethrough. 
As shown in FIGS. 3 and 5, the longitudinal side edges 10a of the entire 
top surface of the load-sensitive element 10 inclusive of the rigid 
portions 11 and 12 and the upper beam 13 are rounded. With reference next 
to FIG. 1, the thickness x of the upper beam 13 at the strain-generating 
parts 13a and 13b as measured from the bottom surface of the indented part 
15 is made somewhat smaller than the thickness y of the lower beam 14 at 
the strain-generating parts 14a and 14b. This is in order to make the 
rigidity of the upper beam 13 at the parts 13a and 13b equal to that of 
the lower beam 14 at the parts 14a and 14b because the longitudinally 
extending edge parts 10b on both sides of the indented part 15 serve as 
rigidity-enhancing beams stretched between the rigid fixed portion 11 and 
the rigid movable portion 12. 
The four strain gauges 21 form a Wheatstone bridge circuit as shown in FIG. 
4 such that an output voltage e is obtained from a specified input voltage 
E according to the changes in resistance of the individual strain gauges 
21. With reference next to FIG. 2, the strain gauges 21.sub.1 and 21.sub.2 
proximal to the rigid fixed portion 11 experience tensile strains and the 
strain gauges 21.sub.3 and 21.sub.4 proximal to the rigid movable portion 
12 experience compressive strains when a load is applied to the load cell 
1. Let us assume now that these strain gauges 21 are connected as shown in 
FIG. 4. If the initial resistance values of the strain gauges 21.sub.1 
-21.sub.4 are respectively R.sub.10, R.sub.20, R.sub.30 and R.sub.40, 
their changes caused by strain are respectively .DELTA.R.sub.1, 
.DELTA.R.sub.2, .DELTA.R.sub.3 and .DELTA.R.sub.4 and their resistance 
values after the changes are respectively R.sub.1, R.sub.2, R.sub.3 and 
R.sub.4, 
EQU R.sub.1 =R.sub.1O +.DELTA.R.sub.1 
EQU R.sub.2 =R.sub.20 +.DELTA.R.sub.2 
EQU R.sub.3 =R.sub.30 +.DELTA.R.sub.3 
EQU R.sub.4 =R.sub.40 +.DELTA.R.sub.4 
and the Wheatstone bridge formula provides 
EQU e=(.DELTA.R.sub.1 /R.sub.10 +.DELTA.R.sub.2 /R.sub.20 -.DELTA.R.sub.3 
/R.sub.30 -.DELTA.R.sub.4 /R.sub.40) x R.sub.10 R.sub.20 E/(R.sub.10 
+R.sub.20).sup.2. 
If R.sub.10 =R.sub.20 =R.sub.30 =R.sub.40 =R in particular, .DELTA.R.sub.1 
=.DELTA.R.sub.2 =-.DELTA.R.sub.3 =-.DELTA.R.sub.4 =.DELTA.R and we obtain 
EQU e=(.DELTA.R/R)E. 
In other words, the output voltage e is proportional to the fractional 
change (.DELTA.R/R) in the resistance of each strain gauge 21. 
The load cell 1 is further provided with moisture-proofing means for 
protecting its strain gauges 21 from humidity and moisture. As shown 
enlarged in FIG. 6, not only are the strain gauges 21, which is attached 
to the bottom surface of the indented part 15, covered by a 
moisture-proofing material 22 such as silicone rubber or butyl rubber 
which is applied thereon but a small piece of a polymer film such as 
polytetrafluoroethylene (Teflon) or a composite metallic foil-polymer film 
formed with metallic foil (for example, of aluminum) over a moisture-proof 
polymer film of polyester, polypropylene, polyethylene or the like 
(hereinafter generally referred to as a covering member 23 whether the 
polymer film is laminated or not laminated with a metallic foil) is 
attached to this moisture-proofing material 22 from above. The indented 
part 15 itself is tightly sealed by a moisture-proof sheet 24 of a 
composite metallic foil-polymer film of the type described above which is 
attached to the top surface of the load-sensitive element 10 by applying 
an adhesive only on the frame-shaped area shown shaded in FIG. 5. 
Described more in detail with reference to FIG. 5, the adhesive is applied 
only on the frame-shaped area which surrounds the indented part 15 and 
includes top sections X.sub.1 on the side surfaces of the load-sensitive 
element 10 and edge sections X.sub.2 extending transversely in the 
direction of the width over the top surface of the load-sensitive element 
10 along the transverse side edges of the indented part 15. In other 
words, this moisture-proof sheet 24 is not adhesively attached to the 
aforementioned longitudinally extending edge parts 10b on both sides of 
the indented part 15. 
Let us assume next that the load cell 1 described above is being used in an 
electronic scale as shown in FIG. 1. When an article to be weighed is 
placed on the tray 4 and its weight is transmitted through the bracket 3 
to the rigid movable portion 12 at one edge of the load-sensitive element 
10, this rigid movable portion 12 is displaced downward with respect to 
the rigid fixed portion 11 on the opposite edge part, causing tensile 
strains on the surface at the strain-generating part 13a of the upper beam 
13 proximal to the rigid fixed portion 11 and compressive strains on the 
surface at the other strain-generating part 13b of the upper beam 13 
proximal to the rigid movable portion 12. The resistance values of the 
individual strain gauges 21 are thereby changed and a voltage e indicative 
of the load (that is, the weight of the article being weighed) is 
outputted from the Wheatstone bridge circuit shown in FIG. 4. The desired 
weight value can be calculated from the measured value of this output 
voltage e. 
In the meantime, since the strain gauges 21 contained inside the indented 
part 15 are covered by a moisture-proof material 22 such as silicone 
rubber and the indented part 15 itself is sealed by a moisture-proof sheet 
24 of a metallic foil-polymer film of the type described above, these 
strain gauges 21 may be said to be doubly protected from external humidity 
and moisture, providing improved durability even if the load cell is used 
in a particularly humid and moist environment. Since the strain gauges 21 
are all inside the single indented part 15 on the upper beam 13 according 
to the present invention, only a single moisture-proofing means is 
required to protect all the strain gauges 21 together. In other words, 
moisture-proofing is effected more efficiently according to the present 
invention than if strain gauges were attached to both the upper and lower 
beams. 
Generally, when a moisture-proof sheet is attached to cover the 
strain-generating parts of a beam, the sheet tends to interfere with the 
free deformation of the load-sensitive element so as to adversely affect 
the accuracy of measurement of a load by the load cell. According to the 
present invention, however, since the strain gauges 21 are disposed only 
on the upper beam 13 and the moisture-proof sheet 24 is attached only to 
the upper beam 13, the total effect of the aforementioned interference 
with the free deformation of is significantly less than if moisture-proof 
sheets are attached to both the upper and lower beams. As a result, 
strains are obtained more accurately corresponding to the load at the 
strain-generating parts 13a, 13b, 14a and 14b according to the present 
invention. 
Moreover, since a space is provided between the strain gauges 21 on the 
bottom surface of the indented part 15 and the moisture-proof sheet 24 
sealing the interior of the indented part 15 and since this moisture-proof 
sheets 24 is attached by an adhesive only through the sections X.sub.1 and 
X.sub.2 shown in FIG. 5 and not adhesively attached to the longitudinally 
extending edge parts 10b, the interference with the free deformation of 
the load-sensitive element 10 is further reduced. 
Next will be examined the effect of the moisture-proof sheet 24 on the 
generation of strain at the strain-generating parts, or on the change in 
the resistance of each strain gauge, if it were attached to the top 
surface of the beam. Since the moisture-proof sheet 24, thus attached, 
would have the effect of impeding the generation of strain, the 
relationship between the resistance of the strain gauges 21 before and 
after the change due to an applied load would be rewritten as follows: 
EQU R.sub.1 =R.sub.10 +(.DELTA.R.sub.1 -.DELTA.r.sub.1) 
EQU R.sub.2 =R.sub.20 +(.DELTA.R.sub.2 -.DELTA.r.sub.2) 
EQU R.sub.3 =R.sub.30 -(.DELTA.R.sub.3 -.DELTA.r.sub.3) 
EQU R.sub.4 =R.sub.40 -(.DELTA.R.sub.4 -.DELTA.r.sub.4) 
where the changes in resistance of the strain gauges 21.sub.1 -21.sub.4 due 
to the moisture-proof sheet 24 are written respectively as .DELTA.r.sub.1, 
.DELTA.r.sub.2, .DELTA.r.sub.3 and .DELTA.r.sub.4. If .DELTA.r.sub.1 
=.DELTA.r.sub.2 =.DELTA.r.sub.3 =.DELTA.r.sub.4 =.DELTA.r, we obtain, 
similarly as before, 
EQU e=(.DELTA.R-.DELTA.r)E/R. 
According to the present invention, however, the moisture-proof sheet 24 is 
attached to the top surface of the load-sensitive element 10 only through 
relatively small sections (X.sub.2). Thus, the effect of the 
moisture-proof sheet 24 can be reduced from the expression given above and 
the accuracy of measurement by the load cell 1 can be maintained at a high 
level. 
If a load is applied eccentrically, the strain gauges 21.sub.1 -21.sub.4 
all experience a tensile strain. In such a situation, the changes in 
resistance of the strain gauges caused by these strains and the effect of 
the moisture-proof sheet 24 tend to cancel each other. Accordingly, it is 
advantageous to have all strain gauges attached to one of the beams from 
the point of view of accuracy of measurement in the case of an 
eccentrically applied load. 
There will next be presented the results of tests performed both on sample 
load cells embodying the present invention as described above and 
comparison samples. For FIGS. 7 and 8, use was made as comparison samples 
those which are similar to the samples embodying the present invention 
except their strain gauges are covered only by silicone rubber or the like 
and there is no moisture-proof sheet of the kind shown at 24 in FIG. 6. 
For the tests for comparing the changes in moisture-proof characteristics 
of these samples, one cycle is defined as a first period of 12 hours 
during which samples are left under a temperature-humidity condition of 
25.degree. C. and 95%, followed by a second period of 12 hours during 
which they are left under another condition of 55.degree. C. and 95%, and 
the samples were made to undergo several cycles. FIG. 7 relates to the 
rate of change in the span, that is, the difference in the output between 
the no-load situation and when a load of 15 Kg is applied as compared to 
its initial value, and FIG. 8 relates to the rate of change in the 
zero-point, that is, the output value under a no-load condition as 
compared to its initial value. These experimental results clearly show 
that the rates of change in span and zero-point are significantly smaller 
with the load cell samples according to the present invention than with 
prior art samples. It is also noteworthy that the fluctuations in the 
measured values among the tested samples are also smaller in the case of 
those according to the present invention. 
FIGS. 9-11 are the results of tests for showing the effects of not 
attaching the moisture-proof sheet to the top surface of the 
load-sensitive element. For this purpose, use was made as comparison 
samples those which are similar to the samples embodying the present 
invention except the moisture-proof sheet (similar to that shown at 34 in 
FIG. 6) was attached to the entire contacting (top) surface of the 
load-sensitive body. FIG. 9 relates to creep tests wherein the input 
voltage was 12 V, a load of 15 Kg was applied for 30 minutes and the 
difference between the output value under a no-load condition and that 
with the load of 15 Kg after the test (the span) was compared with the 
initial value. FIG. 10 relates to zero-return tests wherein the zero point 
after a load was applied under the same conditions as above and its change 
from the initial value was observed. FIG. 11 relates to non-linearity 
tests wherein a straight line was drawn between the output value under a 
no-load condition and the output when a load of 15 Kg was applied and the 
maximum deviation from this straight line among the output values when the 
applied load was 3 Kg, 6 Kg, 9 Kg and 12 Kg was obtained by taking the 
effects of hysteresis into consideration. FIGS. 9-11 show that the effects 
of not attaching the moisture-proof sheet directly to the top surface of 
the load-sensitive element are clearly visible in all of these tests. 
As explained above with reference to FIGS. 3 and 5, the longitudinally 
extending edge parts 10b are rounded. This is for preventing the 
moisture-proof sheet 24 from becoming damaged by a sharp edge. The purpose 
of the covering member 23 shown in FIG. 6 and made of a Teflon sheet or a 
piece of a metallic foil-polymer film of the type described above is to 
prevent the moisture-proof material 22, which is usually sticky, from 
becoming attached by its own stickiness to the moisture-proof sheet 24 
disposed thereabove. It is further to be noted, as shown in FIG. 6, that 
the strain gauges 21 are not attached exactly at the strain-generating 
parts 13a and 13b defined as the parts where the thickness of the upper 
beam is the smallest. Instead, the strain gauges 21 are slightly displaced 
from the strain-generating parts 13a and 13b by a specified distance Z 
towards the edge of the load-sensitive element 10. This is because, in the 
case of a load cell having strain gauges attached to only one beam on one 
side of the its load-sensitive element, a linear relationship between 
applied load and output voltage is not obtained if the strain gauges are 
disposed exactly at the strain-generating parts, while a good linearity 
relationship can be obtained if the strain gauges are attached, as shown 
in FIG. 6, slightly displaced from the strain-generating parts towards the 
nearer edge of the load-sensitive element. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and many modifications and variations are possible in light of 
the above teaching. For example, although the strain gauges 21 and the 
means for protecting them against humidity and moisture are all provided 
on the upper surface of the upper beam 13 according to the embodiment 
described above, they may be provided instead on the lower surface of the 
lower beam 14. Alternatively, semicircular indentations for providing 
strain-generating parts may be formed on the upper surface of the upper 
beam 13 and the lower surface of the lower beam 14. With strain-generating 
parts thus formed, strain gauges 21 and means for their protection may be 
attached to the lower surface of the upper beam 13 or the upper surface of 
the lower beam 14 although such alternatives are not illustrated. Another 
effective variation to the embodiment described above is to enclose dry 
air or nitrogen gas inside the indented part 15 sealed by the 
moisture-proof sheet 24. Moreover, semiconductor gauges comprising 
amorphous silicon or elastic surface wave oscillator gauges may be used as 
strain-detecting elements instead of strain gauges. Still further, 
although FIG. 2 shows two strain gauges 21 at each of the two 
strain-generating parts 13a and 13b on the upper beam 13, it is possible 
to attach only one strain gauge at each of two strain-generating parts on 
either the upper beam 13 or the lower beam 14. By using two resistors of 
fixed resistance together with these two strain gauges in a Wheatstone 
bridge circuit as shown in FIG. 4, it is possible to obtain the magnitude 
of a load similarly from the output voltage. In summary, any such 
modifications and variations that may be apparent to a person skilled in 
the art are intended to be included within the scope of this invention.