Cantilever support beam assembly for a load cell and the like

An improved cantilever support beam assembly for a load cell and the like having a longitudinally extending support beam with two openings extending through the sidewalls of the beam. The openings are independent from one another and separated by a web. Strain gages, are mounted on the concave portions of the openings which are adjacent one another.

BACKGROUND OF THE INVENTION 
This invention relates to an improved cantilever support beam assembly for 
a load cell and the like. Many weighing scales today use a group of 
electric strain gages to electronically determine the weight of an object 
placed upon the scale platform. The strain gages are mounted on the 
support beam of a load cell and connected to form a bridge circuit which 
measures variations in load upon the support beam. When there is no load 
upon the support beam the circuit is balanced. However, when a load is 
placed upon the beam, strains are set up which are sensed by the strain 
gages. The strain gages convert the strains into electric signals which 
place the bridge circuitry in an unbalanced state. If theload cell is 
properly designed, the amount of imbalance in the circuit is proportional 
to the weight of the object. If the load cell is not designed properly, 
the margin of error in the circuit will be great and as a result, the load 
cell readings will be inaccurate. Oftentimes, this error is due to beam 
design and the location of the strain gages. 
It is therefore an object of the present invention to provide a support 
beam assembly for a load cell which has reduced error and increased 
accuracy. Other objects and advantages will become apparent from a review 
of the following specification and claims. 
SUMMARY OF THE INVENTION 
This invention relates to an improved cantilever support beam assembly for 
a load cell such as is commonly used in weighing scales. The assembly 
consists of a longitudinally extending support beam which is rigidly 
attached at one end. Extending through the sidewalls of the beam are two 
openings. The area of the beam between two the openings further defines a 
central region or vertical web. When a load is placed along the beam, the 
strains on the beam are measured by strain gages. In the preferrred 
embodiment four strain gages, two per opening, which are mounted one above 
the other on the portions of the concave surfaces of the openings adjacent 
the vertical web. 
The circuitry for the strain gages is adjusted electrically to be balanced 
when there is no load on the cell. When a load is placed upon the cell, 
the design of the beam and the location of the gages yield a very accurate 
reading of the force that the load is exerting upon the load cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A support beam assembly 10 for a load cell is shown rigidly attached to a 
rigid support member 11 in FIG. 1. The assembly 10 consists of support 
beam 12 and four strain gages 14, 6, 18 and 20 which are connected in a 
wheatstone bridge circuit (not shown). The support beam 12 is a 
longitudinally extending member which as shown in FIG. 2 has a rectangular 
cross-section. The beam 12 can be made from any elastic material with 
metal being the most commonly used material of construction. In the 
embodiment shown in FIG. 1, the support beam material is aluminum. 
Load cells such as the present one use strain gages to measure the strains 
that load W places upon the beam 12. With no load applied to the load 
cell, the bridge circuit containing the strain gages is electronically 
balanced to give a zero reading. When a load is applied at some point 
along the beam, strains are set up in the load cell which are sensed by 
the strain gages and place the bridge circuitry in a state of imbalance. 
The amount of strain that the load cell senses is in turn representative 
of the weight of the load in a properly designed load cell. The key to the 
precision of applicant's invention lies in the design of the support beam 
and the location of the strain gages. 
Referring to FIG. 1, two openings 22 and 24 are drilled completely through 
the sidewalls of support beam 12. In FIG. 1, the two openings are 
cylindrical holes but other non-symmetrical openings have been found to 
work equally well. Cylinders were used because they happen to be the 
easiest shape to machine through the support beam 12. The axes of 
cylindrical holes 22 and 24 are perpendicular to the longitudinal axis 26 
of support beam 12. Cylindrical hole 22 has a radius r and cylindrical 
hole 24 has a radius r'. In FIG. 1, r is equal to r' but this is not 
necessary for effective operation of the present invention. 
Separating the two cylindrical holes 22 and 24 is a central region or 
vertical web 30. This web 30 must be present between the holes 22 and 24 
to achieve the objects of the present invention. The parameters defining 
this vertical web 30 are important to the precision of the load cell 
according to the present invention. Vertical web 30 has an area of minimum 
thickness l at or near the longitudinal axis 26 of support beam 12. The 
distance l between the cylindrical holes 22 and 24, as measured along the 
longitudinal axis 26 of support beam 12, should always be less than the 
length of the smaller of the radii r and r'. Similarly, each distance l' 
should also be less than the length of the smaller of the radii r and r'. 
l' is the measure of the distance between the point on the top and bottom 
of each of the openings 22 and 24 which is closest to the respective top 
28 and bottom 29 of support beam 12. (When either of the openings are 
non-cylindrical, the distances l and l' should be less than the larger 
width of either of the two openings 22 and 24). Given these parameters, 
the vertical web 30 somewhat resembles a pair of triangles with their 
apexes meeting at an area of minimum thickness at or near the longitudinal 
axis 26 of support beam 12. 
Each cylindrical hole 22 and 24, can be divided into four quadrants marked 
A, B, C and D, respectively. With respect to holes 22 and 24, quadrants A 
and B are located adjacent vertical web 30, while quandrants C and D are 
located opposite the vertical web 30. Also note that quadrants A and D lie 
above the longitudinal axis 26 of support beam 12 while quadrants B and C 
lie below the longitudinal axis 26. 
A strain gage is located in each quadrant of each hole 22 and 24 which is 
adjacent the vertical web 30. Thus, in each hole a strain gage will lie 
above and below the area of minimum thickness located at the longitudinal 
axis 26. Strain gage 14 is located above the longitudinal axis 26 in 
quadrant A of cylindrical hole 22. As can be seen in FIG. 1, strain gage 
14 is mounted to the concave surface of quadrant A adjacent the vertical 
web 30. Strain gage 16 is mounted in quadrant B, on the concave surface 
which is adjacent the vertical web 30 in cylindrical hole 22 and below the 
area of minimum thickness. Strain gages 18 and 20 are mounted in a similar 
fashion on the concave surfaces of cylinder 24 in quadrants A and B, 
respectively. 
When a load W is applied along the beam 12, it causes strains in the beam. 
Testing has indicated that the design of the vertical web region 30 only 
causes it to see strains resulting form the load W. Other strains caused 
by bending and twisting of the beam 12 are not sensed by this web 30 which 
means that gage creep is kept to a minimum. This is because the web region 
30 resembles a free body. The forces due to the bending simply stretch the 
top part of the web 30 and compress the bottom part without changing the 
shape of the portion of the web where the gages 14, 16, 18 and 20 are 
located. Any strains which are introduced into the gage area due to 
bending or twisting will be very small. As a result, the imbalance in the 
strain gage circuitry and the resultant output signal will essentially be 
a function only of the actual force the load W is exerting upon the load 
cell. What a beam, according to the present invention, actually senses is 
thus the parallel movement of one end of the beam versus the other. This 
parallel movement causes the two "triangles" of the vertical web 30 to 
rotate horizontally in different directions. It is this movement induced 
strain that the gages sense and which is in turn a measure of the force of 
the load W. 
Once the cantilever support beam assembly 10 is completed, the strain gages 
14, 16, 18 and 20 are connected in a bridge circuit and the circuit is 
then balanced electronically while the support beam 12 is at rest with no 
load applied. With the beam balanced, a linearity test was run using 
varying loads from 0 pounds to 8 pounds in two pound increments. The 
results of this test are shown in Table I below. 
TABLE I 
______________________________________ 
Linearity Test 
Load Pounds Reading Linear Value 
______________________________________ 
0 0 0 
2 1081 1081 
4 2161 2161 
6 3243 3243 
8 4324 4324 
6 3243/4 3243 
4 2162 2162 
2 1081 1081 
0 0 0 
______________________________________ 
From Table I it can be seen that the load cell using the improved 
cantilever support beam assembly gives accurate linear results. For 
example, when a 2 pound load is applied, a reading of 1081 is recorded. 
When the load is increased threefold to six pounds, the recorded reading 
is also increased threefold to a value of 3243. This indicates that a 
support beam assembly according to the present invention yields very 
accurate results which are directly proportional to the load applied. 
During testing of the present invention, strain gages were placed at 
various positions about the interior surfaces of holes 22 and 24 in an 
attempt to determine where the strain levels were constant, which meant 
very accurate readings, and where the strain levels were varying to an 
unacceptable degree. From the testing it was determined that the strain 
gages must be located adjacent the vertical web 30 for accurate load 
readings. Strain gages which are positioned further away from the web 30 
than the four points marked E in FIG. 1 will have strain levels that vary 
with the load position. This is unacceptable in load cell design. With the 
present invention, accurate readings are given irrespective of the 
position of the load W along the length of the beam 12 beyond the holes 22 
and 24. 
Another common location for strain gages is on the exterior surfaces of the 
thin sections marked G in FIG. 1. See for example, Kawai et al., U.S. Pat. 
No. 4,212,197 and Jacobson et al., U.S. Pat. No. 4,146,100, wherein the 
strain gages are placed on the upper and lower exterior surfaces of a 
longitudinally extending beam. Stresses are present at these positions and 
the magnitude of these stresses is dependent upon both the thickness of 
the section G and upon the application point of the load W. These stresses 
are sensed by the strain gages and can cause the load cell to give 
inaccurate readings. In addition, when the strain gages are separated by 
large distances, as they are when positioned on the exterior surfaces of 
the beam 12, temperature gradients greatly affect the responses of the 
strain gages which in turn affect the overall accuracy of the load cell. 
With applicant's invention the strain gages 14, 16, 18 and 20 are located 
in close proximity to one another about the vertical web 30. As a result, 
the temperature gradients are much smaller and thus the error in the 
readings due to temperature is negligible. 
Other embodiments of the present invention were also tested and found to 
work well so long as there existed the vertical web region 30 and the 
strain gages were placed adjacent the vertical web in positions above and 
below the area of minimum thickness. FIG. 2 shows a support beam 112 
similar to the support beam 12 in FIG. 1 except for the location of the 
strain gages 114, 116, 118 and 120. In FIG. 2, all four strain gages 114, 
116, 118 and 120 are located in just one of the openings, cylindrical hole 
122. Strain gages 114 and 116 are positioned side by side above the area 
of minimum thickness and adjacent the vertical web 30 in quadrant A. 
Strain gages 118 and 120 are positioned side by side below the area of 
minimum thickness and adjacent the vertical web 30 in quadrant B. With 
this configuration the strain gages still sense the parallel movement of 
one end of the beam 112 versus the other which causes the two "triangles" 
of web 30 to rotate horizontally in different directions. As a result, 
accurate readings of the load W are also achieved irrespective of the load 
W's location along the beam 112. 
FIG. 3 shows a beam 212 which is similar to beam 12 in FIG. 1 except for 
the concave recesses 231, 232, 233 and 234 in the top and bottom surfaces 
228 and 229 directly above and below cylindrical holes 222 and 224. By 
placing the recesses in the beam 212, the sections l' are made thinner 
thus making the vertical web 230 more independent. 
Finally, FIG. 4 shows a cylindrically shaped support beam 312 with an area 
of reduced cross-section 313. Within this area of reduced cross-section 
313, there are two cylindrical holes 322 and 324 with a vertical web 330 
and strain gages 314, 316, 318 and 320 positioned just as those shown in 
FIG. 1. Here again, because of the vertical web 330 and the placement of 
the strain gages 314, 316, 318 and 320 adjacent the web 30 accurate 
readings of the load W are achieved irrespective of the location of the 
load W along beam 312. 
The foregoing demonstrates that the present invention yields a cantilever 
support beam assembly for a load cell and the like with improved accuracy. 
Having thus described the present invention in detail and with reference 
to the accompanying drawings, it should be understood that various 
modifications and changes may be made without departing from the spirit 
and scope of the following claims.