A shear beam, single-point load cell is provided. The load cell includes a shear beam with a pair of shear strain gauge sensors mounted on opposing sides thereof for measuring strain within a small portion of the shear beam. The strain gauges are isolated within sealed pockets from moisture or harsh chemicals. The use of a shear beam, rather than a bending cantilevered beam of conventional load cells, allows the load cell to be sturdier and more robust than conventional load cells, to thereby measure and detect greater amounts of load. The provision of the strain gauges positioned for measuring strain within a small portion of the shear beam allows the load cell to accurately and precisely measure loads regardless of the mounting position of the load on the load cell. Further, the provision of closely adjacent strain gauges renders the load cells relatively immune to thermal gradients inherent in conventional load cells having widely separated strain gauges. The load cell is relatively inexpensive to manufacture and maintain, and yet provides accurate and precise load measurements.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to load cells and, more 
particularly, to a single-point load cell. 
2. Description of Related Art 
A variety of single-point load cells are found in the prior art. A 
single-point load cell is a device adapted for mounting beneath a scale 
platform to measure a weight or load applied to the platform. In contrast 
to multiple load cells, wherein several load cells are mounted at separate 
locations beneath the platform, only one single-point load cell is 
utilized. Thus load is measured at only a single point beneath the scale 
platform, rather than at several points simultaneously. A desired property 
of the single-point load cell is that it be insensitive to bending moments 
produced by random placement of the load anywhere on the scale platform. 
A typical single-point load cell provides a surface for mounting a 
platform, two or more cantilevered beams oriented to bend or strain upon 
the application of a load to the platform, and two or more bending beam 
strain gauges provided on the beams for measuring bending or strain within 
the beams. In the conventional single-point load cell, the bending beam 
strain gauges are spaced widely apart along the cantilevered beams for 
measuring strain at multiple points along the beams. 
A conventional two-beam single-point load cell is shown in FIG. 1. The 
two-beam load cell of FIG. 1 includes a load cell block with a generally 
rectangular opening extending through the block. The opening extends 
almost the entire height of the load cell block, leaving only a thin 
connecting plate or beam at the bottom of the block and a second thin 
connecting plate or beam at the top of the block. Bending beam strain 
gauges 200 are positioned along inside surfaces of the connecting beams 
for measuring strain induced in the connecting beams by a load applied to 
a top surface of the load cell block. As can be seen from FIG. 1, strain 
gauges 200 are widely separated along the inside surfaces of the 
connecting beams. 
A conventional three-beam single-point load cell is shown in FIG. 3. The 
three-beam load cell of FIG. 3 is similar to the two-beam load cell of 
FIG. 2, except that a connecting beam is provided which spans the opening 
of the load cell block. A plurality of bending beam strain gauges 202 are 
provided on opposing top and bottom sides of the connecting beam, rather 
than on the top and bottom connecting plates as in the two-beam load cell 
of FIG. 2. Alternatively, all strain gauges 202 are provided either on the 
top or the bottom of the connecting beam to reduce manufacturing costs. 
However, as with the two-beam load cell of FIG. 2, the strain gauges of 
the three-beam load cell are spaced widely apart along opposing ends of 
the connecting beam. 
In the load cell designs of FIGS. 1 and 2, the strain on each beam varies 
substantially, depending upon the location of the load on the platform 
mounted to the top surface of the load cell. This variation in strain is 
conventionally referred to as an "eccentric" load effect. The strain 
gauges are spaced widely apart in an effort to compensate for the effect 
of an eccentric load. However, eccentric load compensation is difficult to 
achieve with widely separated gauge locations because of variations 
inherent in the structure of the load cell block as the result of normal 
manufacturing tolerances. Often, a secondary means of compensating for 
eccentric load errors is provided. However, such secondary means render 
the load cell more complicated and expensive to manufacture and maintain. 
The connecting beams of the load cells of FIGS. 1 and 2 act as cantilevered 
flexures upon the application of a load to the load cell. As such, the 
connecting beams typically deflect substantially upon the application of a 
large load. Often, such a large deflection is nonlinear, and the load cell 
therefore suffers from nonlinearity hysteresis upon the application of the 
load. 
Thermal gradients within the load cell block can effect the performance of 
the load cell when strain gauges are widely spaced. Such is a particular 
problem when a source of heat affects only some of the strain gauges or 
only a portion of the electronic circuit of the load cell. 
Finally, it is often desirable or necessary to isolate the strain gauges to 
protect the gauges from moisture or harsh chemicals. With a plurality of 
strain gauges widely spaced apart, it is difficult and expensive to 
adequately isolate each individual strain gauge. 
SUMMARY OF THE INVENTION 
In view of the foregoing, it is desirable to provide a single-point load 
cell which is relatively unaffected by thermal gradients, relative immune 
to eccentric load errors, and in which strain gauges can be easily and 
inexpensively isolated from moisture or harsh chemicals. 
Accordingly, it is an object of the invention to provide an improved 
single-point load cell; 
It is another object of the invention to provide a single-point load cell 
having strain gauges closely adjacent; 
It is yet another object of the invention to provide a single-point load 
cell which is relatively immune from eccentric load errors; 
It is yet another object of the invention to provide a single-point load 
cell which does not require extensive secondary eccentric load 
compensation means; 
It is yet another object of the invention to provide a single-point load 
cell which is relatively immune to common manufacture variations and 
tolerances; 
It is yet another object of the invention to provide a single-point load 
cell having sturdier outside flexures to thereby withstand greater 
torsional load than bending beam load cells; 
It is yet another object of the invention to provide a single-point load 
cell wherein strain gauges can be easily and inexpensively sealed; 
It is yet another object of the invention to provide a single-point load 
cell wherein strain gauges are isolated from load mounting areas such that 
the performance of the gauges is relatively immune to mounting influences; 
It is yet another object of the invention to provide a single-point load 
cell which is relatively accurate and precise regardless of the location 
of a load on the load cell mounting surface; and 
It is yet another object of the invention to provide a single-point load 
cell wherein strain gauges are sensitive only to a load rather than to a 
load and a torsional or bending moment induced by the load. 
These and other advantages of the invention are achieved by the provision 
of a single-point load cell comprising: a load bearing block having top, 
bottom, front, and back sides, with the block having an opening extending 
therethrough from front sides and back side; a shear beam integrally 
formed with the block, with the shear beam spanning the opening to connect 
opposing side walls of the opening; and strain sensor means positioned 
with a selected small area on opposing side surfaces of the shear beam for 
detecting principal strains in the selected area within the shear beam. 
The strain sensor beams are thus located on the shear beam in such a 
position that they are isolated from all forces but shear forces. 
In accordance with a preferred embodiment, the shear beam is formed within 
the opening at a position intermediate top and bottom walls of the opening 
and intermediate front and back sides of the load bearing block. Strain 
pockets are formed within opposing side surfaces of the shear beam. The 
strain sensor means comprises two pairs of shear strain gauges, with each 
pair positioned within opposing pockets. Plate members are provided over 
the opposing pockets for enclosing the pockets such that the pockets are 
substantially isolated from moisture and harsh chemicals. 
Thus, the invention provides an improved single-point load cell wherein a 
shear beam technique is used for measuring load rather than a technique 
using one or more bending beams. Further, the invention provides for the 
measurement of strain within the shear beam in a selected small area along 
the shear beam such that the performance of the load cell is relatively 
unaffected by thermal gradients within the load cell. The provision of the 
shear beam allows for the load cell to be sturdier, to thereby withstand a 
greater torsional load than cantilevered beam load cells of the prior art. 
Further, the provision of strain gauge sensors, positioned within pockets 
isolated from mounting areas of the load cell, renders the strain gauges 
relatively immune to mounting influences such that the load cell can 
operate more precisely and accurately.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description is provided to enable any person skilled in the 
art to make and use the invention and sets forth the best modes 
contemplated by the inventor of carrying out his invention. Various 
modifications, however, will remain readily apparent to those skilled in 
the art, since the generic principles of the present invention have been 
defined herein specifically to provide a single-point load cell. 
With reference to FIGS. 3 through 7, a preferred embodiment of the 
invention will be described. A load cell 10 is provided having a load cell 
block 12 with an opening 14 extending through block 12. 
Load cell block 12 includes front and back sides 16 and 18, respectively; 
top and bottom sides 20 and 22, respectively; and ends 24 and 26. Opening 
14 extends from front side 16 through block 12 to rear side 18. Opening 14 
has a generally rectangular cross-section, as shown in the figures. 
Opening 14 is formed by interior side walls 28 and 30, respectively, and 
interior top and bottom walls 32 and 34, respectively. Opening 14 is 
formed within block 12 equidistant from ends 24 and 26 of block 1 and 
equidistant from top and bottom sides 20 and 22 of block 12. 
Opening 14 extends along almost the entire height of block 12 with top and 
bottom interior walls 32 and 34 aligned parallel with and adjacent to 
exterior top and bottom walls 20 and 22, respectively, of block 12. A 
portion of block 12, between interior wall 32 and top wall 20, comprises a 
top plate or connecting beam 36. A portion of block 12 between interior 
wall 34 and bottom wall 22 comprises a lower plate or connecting beam 38. 
A portion of block 12 between interior side wall 12 and end wall 24 
comprises a first, generally square, end block 40. Likewise, a portion of 
block 12 between interior side wall 30 and end wall 16 comprises a second, 
generally square, end block 42. 
A top mounting surface 44 is formed along top surface 20 of end block 40. 
Likewise, a lower load mounting surface 46 is formed along bottom surface 
22 of end block 42. Both mounting surfaces 44 and 46 include a plurality, 
preferably four, mounting bores, generally denoted 48. Mounting bores 48 
allow a load platform (shown only in FIG. 7) to be securely mounted to 
load cell 10 over mounting surfaces 44, 46. Alternatively, although not 
shown, bores can be provided on ends 24 and 26 of load cell 10 for 
receiving a mounting bracket. 
A shear beam 50 spans opening 14, connecting interior side walls 28 and 30. 
Shear beam 50 is integrally formed with load cell block 12. Shear beam 50 
includes opposing side surfaces 52 and 54 which extend parallel to block 
side walls 16 and 18, respectively. 
A pair of shear pockets 56 and 58 are formed within opposing side walls 52 
and 54 of shear beam 50, respectively. Pockets 56 and 58 are formed within 
the shear beam 50 equidistant from interior side walls 28 and 30. Pockets 
56 and 58 each extend almost halfway through shear beam 50, leaving only a 
relatively thin vertical shear plate 60. Shear plate 60 includes parallel 
opposing side walls 62 and 64 within opposing shear pockets 56 and 58, 
respectively. 
A pair of shear strain gauge sensors 66 are centrally positioned along 
surface 62 of shear plate 60. A second pair of shear strain gauge sensors 
68 are centrally positioned along opposing surface 64 of shear plate 60 
within pocket 58. Shear strain gauge sensor pairs 66 and 68 can comprise 
any conventional gauge for measuring shear strain within shear plate 60, 
such as by means of detecting electrical resistance, capacitance, or 
inductance changes, or by detecting piezoelectric and magnetostriction 
effects. 
Thus, two pairs of shear strain gauge sensors are positioned along opposing 
sides of shear plate 60 for measuring strain within shear plate 60. Thus, 
strain is measured in a small area of shear beam 50 occupied by sensors 
56, 58. This area is desirably selected so as to isolate sensors 56, 58 
from all but shear forces. In the embodiment shown, the sensors are 
centered on shear beam 50 to achieve this purpose. The provision of strain 
gauge pairs 66 and 68 positioned for measuring strain at a single point 
substantially eliminates thermal gradient errors inherent in load cells 
having strain gauge sensors spaced widely apart. 
Since shear plate 60 is oriented along a central longitudinal axis of the 
load cell, shear plate 60 is sensitive only to loads and is not sensitive 
to moments. Consequently, load cell 10 is somewhat less sensitive to 
common manufacturing variations and tolerances affecting load cells of the 
prior art having widely spaced sensors. Further, for an equal load, shear 
plate 60 deflects less than a cantilevered beam of a load cell of the 
prior art. Consequently, load cell 10 can be constructed to be sturdier 
and more robust than load cells of the prior art, and can thereby 
withstand more torsional loads than previous load cells. Further, since 
shear plate 60 deflects less than cantilevered beams of the prior art, 
load cell 10 can withstand greater loads without producing inelastic 
deflection within strain plate 60. Thus, load cell 10 is more immune to 
nonlinear effects and hysteresis effects than load cells of the prior art. 
Thus, the provision of strain gauge sensor pairs 66 and 68 positioned along 
a shear plate 60, rather than along bending beams of load cells of the 
prior art, allows load cell 12 to be of substantially sturdier 
construction than load cells of the prior art. To this end, shear beam 50 
includes fluted end portions 70 and 72. Further, plate members 36 and 38 
are somewhat wider and sturdier than bending plates of the prior art 
shown, for example, in FIGS. 1 and 2. 
Shear pockets 56 and 58 are sealed by sealing plates 74 and 76, 
respectively, shown in phantom lines in the drawings. Sealing plates 74 
and 76 completely seal the interior of shear pockets 56 and 58 from 
moisture, harsh chemicals, etc. Sealing plates 74 and 76 are preferably 
constructed of the same material as load cell block 12, and are mounted to 
shear beam 60 by any conventional means such as, for example, welding. 
A flexible printed circuit board 80 is mounted to a lower interior side 
wall 82 of pocket 56. Printed circuit board 80, described in greater 
detail below with reference to FIG. 6, includes bridge circuit wiring 
necessary for processing electrical signals received from strain gauges 66 
and 68. 
A coaxial sensor cable 84 extends from an interior side wall 86 of pocket 
58 through load block member 40 to end surface 24. Coaxial sensor cable 84 
allows printed circuit board 80 to be electrically connected with an 
external sensor or detector (not shown). 
With reference to FIG. 6, the wiring of load cell 10 will now be described 
in greater detail. 
Strain sensor pair 66 includes two individual sensors 66.sub.1 and 66.sub.2 
oriented along opposing axes for detecting strain changes within shear 
plate 60. Likewise, strain gauge pair 68 comprises individual strain 
gauges 68.sub.1 and 68.sub.2 oriented along opposing axes on strain plate 
60. Together, strain gauges 66.sub.1, 66.sub.2, 68.sub.1, and 68.sub.2 are 
connected to form a bridge circuit 100. Bridge circuit 100 is connected to 
printed circuit board 80 via input feed wires 102 and 104, and is 
connected to coaxial sensor cable 84 via output lines 106 and 108. Printed 
circuit board 80 is connected to coaxial sensor cable 84 via input lines 
110 and 112. 
An input excitation voltage is applied to bridge circuit 100 via printed 
circuit board 80. Printed circuit board 80 includes component resistors 
for temperature compensation and output adjustment. Specifically, printed 
circuit board 80 includes respective pairs of modulus compensation and 
shunt trim resistors 118; 114 and 120; 116. The modulus compensation 
resistors 118, 120 compensate for the temperature-sensitive modulus of 
elasticity of the load cell material. For example, the temperature 
coefficient of the modulus of elasticity of steel varies .about.2% per 
100.degree. F. The modulus compensation resistors may be nickel or balco 
(iron/nickel alloy), which decreases the load cell output by 2% per 
100.degree. F. to make the output signal of the load cell temperature 
independent. Shunt trim resistors 114, 116 perform a fine adjustment of 
the modulus temperature compensation. 
Also included are a pair of span trim resistors 122 and 124. These 
resistors are added after measuring the load cell output in order to 
adjust the final output to compensate for manufacturing tolerance induced 
variations. 
Finally, printed circuit board 80 includes an input trim resistor 126 
connecting input lines 110 and 112. Input trim resistor 126 may be 
included if it is desired to maintain the input resistance equal to that 
of bridge circuit 100 alone, e.g. 350 ohms, etc. 
The thus-processed input excitation signal is applied to bridge circuit 
100, which operates to produce an output voltage in response to strain 
changes in shear plate 60 caused by a load applied to load cell 10. Bridge 
circuit 100 is configured such that, when no load is applied, a zero 
balance voltage is output to coaxial sensor cable 84 via output feed lines 
106 and 108. 
Thus, the electrical circuit of FIG. 6 includes electrical resistance 
sensors which detect strain within shear plate 60. As noted above, 
however, alternative shear gauges can be provided which detect strain via 
another electrical property such as inductance or capacitance. In such 
case, the electric circuit of FIG. 6 would, of course, be adapted to 
process the corresponding electrical signals output from the individual 
sensor pairs. 
Referring to FIG. 7, load cell 10 is shown mounted for use within a scale 
platform 90. Platform 90 includes an upper flat surface 92 upon which 
loads can be applied. As a consequence of the design of load cell 10, 
discussed above, a load can be applied anywhere on surface 92 with little 
or no resulting variation in the output of load cell 10. Preferably, load 
cell 10 is bolted to the inside of platform 90 via mounting bore holes 48. 
Thus, the invention provides a load cell 10 capable of accurately detecting 
and measuring loads regardless of the position upon which the load is 
mounted to the load cell. The invention achieves this result without the 
need for additional and expensive secondary compensation means required 
for compensating eccentric load errors inherent in prior art load cell 
designs. 
Further, the provision of strain gauge pairs 66 and 68, positioned closely 
adjacent for measuring strain within a small area of shear beam 50, allows 
load cell 10 to operate accurately and precisely despite any thermal 
gradient within load cell block 12. 
Finally, the provision of shear beam plate 60 allows load cell block 12 to 
be constructed of a sturdier or more robust design to enable load cell 10 
to accurately and precisely detect and measure a greater load than 
measurable by load cells of the prior art. 
Those skilled in the art will appreciate that various adaptations and 
modifications of the just-described preferred embodiment can be configured 
without departing from the scope and spirit of the invention. Therefore, 
it is to be understood that, within the scope of the appended claims, the 
invention may be practiced other than as specifically described herein.