Load cell with composite end beams having portions with different elastic modulus

A composite load cell utilizes end blocks mounted to the ends of a load cell body to increase the apparent bulk modulus of the end beams on the body. The increased modulus of the end beams improves cornering of the load cell, and further aids in the rejection of moment induced shears, including those due to off-center loading, thereby increasing the usable resolution of the load cell.

FIELD OF THE INVENTION 
The invention is generally directed to a load cell for providing an output 
signal which is indicative of a force applied to the load cell, such as a 
force that is a result of weight, acceleration, pressure, and the like. 
More particularly, the invention is directed to a composite load cell 
having end blocks mounted at the ends of a load cell body to increase the 
apparent bulk modulus of the load cell and thereby improve cornering 
and/or reject moment induced shear effects from the output signal of the 
load cell. 
BACKGROUND OF THE INVENTION 
Different designs of load cells, i.e., devices for measuring applied force 
and outputting signals indicative thereof, are known in the art. Load 
cells may use several different types of force sensors, including strain 
gauges, vibrating strings, force motors, capacitor sensors, resonating 
sensors such as piezoelectric quartz crystals and tuning forks, etc., for 
measuring applied force. 
Many load cell designs transfer an applied force to one or more force 
sensors through a load cell body and other suitable force sensing 
structure. The load cell bodies of many designs typically include a pair 
of relatively stiff and massive end beams which are joined to relatively 
smaller and more flexible connecting beams (e.g., through one or more 
flexures) which extend between the tops and bottoms of the end beams, 
thereby forming a generally rectangular block having an aperture defined 
therein. Typically, force sensing structure including one or more force 
sensors is disposed within this aperture. 
The above-described load cell designs measure force as a function of the 
shear forces applied across the connecting beams as a result of the 
relative deflection of the end beams. By selecting a suitable material and 
design for the load cell body, the force applied to the body may be scaled 
in such a manner as to provide a reasonable range of shear forces for 
measurement by the force sensors (e.g., to provide a load cell with a 
specific capacity). 
The proper operation of a shear responsive load cell relies on the 
assumption that a deflection in the load cell body will be brought about 
through deflection of the connecting beams through their flexures. It is 
generally assumed that the end beams, being stiffer and more massive, will 
not bend or otherwise distort due to an applied force. Otherwise, any 
distortion or bending of the end beams would induce shear that could 
result in inaccurate force calculations, primarily due to what is referred 
to as "moment induced shears." Thus, it is desirable to minimize any 
distortion or bending of the end beams to reduce any extraneous shearing 
resulting from coupling of the applied force. 
It may also be desirable to reject torsional moments which occur as a 
result of off-center loading (i.e., an applied force which is off-axis 
from the load cell). Rejection or compensation of torsional moments 
typically requires the individual flexures joining the connecting beams to 
the end beams to be specifically adjusted (e.g., by removing material from 
the flexures). 
The end beams of conventional load cells are typically made stiffer by 
increasing their size (e.g., by increasing their width). However, I have 
found that the stiffness of the end beams is limited by the inherent bulk 
modulus of the material used for the end beams, and therefore, there is a 
limit to which simple dimensional modifications will stiffen the end 
beams. Consequently, many conventional designs are limited in the degree 
of cornering and moment induced shear rejection that can be obtained, 
thereby limiting the usable resolution of these designs. 
Therefore, a need exists in the art for a load cell having improved 
cornering and rejection of moment induced shears, and which is not limited 
by the bulk modulus of the material in the load cell body. 
SUMMARY OF THE INVENTION 
The invention addresses these and other problems associated with the prior 
art by providing a composite load cell which utilizes end blocks mounted 
to the ends of the load cell body that have a higher modulus than the end 
beams on the load cell body. By securely mounting the load cell body 
through relatively stiffer end blocks, the apparent bulk modulus of the 
end beams increases to improve cornering and better reject moment induced 
shears in the load cell. 
Therefore, in accordance with one aspect of the invention, there is 
provided a force sensing composite load cell which includes a load cell 
body including first and second opposing end beams, the load cell body 
adapted to deflect in response to an applied force; and first and second 
end blocks, respectively mounted to the first and second end beams and 
constructed from a material having a bulk modulus which is greater than 
that of the end beams. 
According to a further aspect of the invention, a force sensing composite 
load cell is provided which includes a load cell body having first and 
second end beams joined by at least one connecting beam, the end beams 
being adapted to provide a relative deflection due to applied force on the 
load cell body, and being constructed from a material having a first 
elastic modulus; sensing means for sensing shear force applied to the load 
cell body by the relative deflection of the end beams; and first and 
second end blocks, mounted to the end beams and constructed from a 
material having a second elastic modulus, the second modulus being at 
least about 50 percent greater than the first modulus; whereby the 
apparent bulk modulus of the end beams is increased. 
In accordance with an additional aspect of the invention, a scale is 
provided, which includes a base; a force receiving member adapted to 
receive an applied force; a force sensing composite load cell; and 
mounting means for mounting the composite load cell to the base and the 
force receiving member; whereby a force applied to the force receiving 
member is applied across the composite load cell. The composite load cell 
includes a monolithic load cell body having an interior aperture defined 
by first and second end beams joined by flexures to first and second 
connecting beams, the end beams being adapted to provide a relative 
deflection due to applied force on the load cell body and being 
comparatively stiffer than the connecting beams, the load cell body being 
constructed from a material having a first elastic modulus; sensing means, 
disposed within the interior aperture, for sensing effects due to shear 
force applied to the connecting beams through the relative deflection of 
the end beams, the sensing means including a load beam extending from a 
wall of the interior aperture, a cantilevered beam coupled to and 
extending generally parallel to the load beam, and a force sensor coupled 
between the load beam and the cantilevered beam; and first and second end 
blocks, mounted to the end beams and constructed from a material having a 
second elastic modulus which is greater than the first modulus of the end 
beams, for increasing the apparent bulk modulus of the end beams. The 
composite load cell is mounted to the base and the force receiving member 
through the first and second end blocks, respectively. 
These and other advantages and features, which characterize the invention, 
are set forth in the claims annexed hereto and forming a further part 
hereof. However, for a better understanding of the invention, its 
advantages and objectives attained by its use, reference should be made to 
the Drawing, and the following descriptive matter, where a preferred 
embodiment of the invention is described.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning to the Drawing, wherein like numbers denote like parts throughout 
the several views, FIG. 1 shows a preferred composite load cell 30 
including a load cell body 32 having end beams 34 and 36 joined at the 
opposing ends thereof to connecting beams 38 and 40 through a plurality of 
flexures 39, thereby forming an aperture 33 with a force sensing structure 
41 disposed therein. It will be appreciated that different force sensing 
structures may be used in aperture 33 consistent with the invention. 
Load cell body 32 is cooperatively attached to a pair of end blocks 60, 62 
which are mounted to end surfaces 35, 37 of end beams 34, 36, 
respectively, with fasteners 64 extending through apertures 65 and into 
66. The end blocks 60, 62 are constructed of a higher modulus material 
than that of end beams 34, 36 of load cell body 32. 
Principles of Operation 
FIG. 2 is a schematic representation of a typical shear responsive load 
cell 10, shown subjected to a force F that induces a relative deflection 
.DELTA.y between end beams 12 and 16. It will be appreciated that the 
deflection .DELTA.y is exaggerated in this figure for ease of 
illustration. 
In load cell 10, the relative deflection between end beams 12 and 16 (e.g., 
resulting from a force applied perpendicular to top surface 18 of end beam 
16 when end beam 12 is mounted at a fixed position at bottom surface 14) 
results in a shear force, represented by .tau., applied to connecting 
beams 20, 22. The shear force is sensed by the force sensing structure 
(not shown) that is located within aperture 24 and is converted by 
appropriate mechanical and/or electrical means (not shown) into an output 
signal representative of the applied force. It will be appreciated that 
different types of force sensing structures may be used to measure the 
shear forces applied to load cell 10, many of which are discussed below. 
In a shear responsive load cell such as load cell 10, the force is related 
to the deflection by the equation: 
EQU f=k.DELTA.y 
where k is a constant that is unique for each particular load cell. The 
constant k is typically a function of the particular material selected for 
the load cell, as well as the presence of flexures and other structure 
that either increase or decrease the ability of a load cell to deflect 
from an applied force. An increase in the constant k typically increases 
the capacity of the load cell, while a decrease in k typically increases 
the resolving power of a cell. Therefore, the constant k is typically used 
to select a load cell for a particular application to give adequate 
resolution at a desired capacity. 
The total shear force .tau. applied to load cell 10 is a function of the 
shear due to deflection (.tau..sub..DELTA.y) and the shear due to mounting 
effects (.tau..sub.m), generally according to the equation: 
EQU .tau.=.tau..sub..DELTA.y +.tau..sub.m 
It is desirable to minimize the mounting effect shear .tau..sub.m to 
provide a substantially direct relationship between the shear force .tau. 
applied to the force sensors and the deflection .DELTA.y of the load cell 
body, which is in turn a direct function of the applied force F. 
To minimize the mounting effect shear .tau..sub.m, it is important to 
maximize the stiffness of the end beams. Conventionally, this has been 
done by making the end beams larger. However, as discussed above, the 
ability to increase the stiffness of the end beams by dimensional 
increases is limited by the inherent bulk modulus of the material selected 
for the end beams, and machining difficulties. 
I have found, on the other hand, that the apparent stiffness of the end 
beams may be increased beyond that obtainable through dimensional changes 
by mounting end blocks constructed of a higher modulus material to the end 
beams. As a result, the apparent bulk modulus of the end beam may be 
increased to further reduce the shear forces due to mounting effects, and 
consequently provide a more direct relationship between the applied force 
to the load cell and the shear forces applied to the force sensors. 
Load cell Structure 
Accordingly, preferred composite load cells consistent with the invention 
include end blocks which are mounted at ends of a load cell body for 
increasing the apparent bulk modulus of the end beams on the load cell 
body. As load cell bodies may be made of various materials having 
different properties, it will be appreciated that the particular materials 
and designs for the end blocks will vary depending upon the type and 
design of load cell body to which the blocks are attached. 
One preferred embodiment is shown as load cell body 32 in FIG. 1. This 
design incorporates end beams 34, 36 which are joined by a plurality of 
flexures 39 to connecting beams 38 and 40, thereby forming a generally 
rectangular profile with an interior aperture 33 having force sensing 
structure 41 disposed therein. 
The preferred design of load cell body 32 is used to transmit a force 
applied thereto to force sensing structure 41 for measurement. It will be 
appreciated, however, that several modifications may be made to the design 
of the load cell body consistent with the invention. For example, any 
number of end beams and connecting beams may be used, e.g., as few as one 
of each. Moreover, other beam structures or elements which serve similar 
force transmission functions may be included in preferred embodiments. In 
addition, one or more flexures may be incorporated as desired to modify 
the deflection and force transmission characteristics of the preferred 
load cell body designs. Sensors for sensing shear induced deformations of 
the beams and flexures may be attached to the beams ether internally or 
externally such as strain gages or other displacement sensing devises. 
Returning to FIG. 1, composite load cell 30 incorporates a pair of force 
sensitive resonators 50a and 50b which are placed in tension and 
compression, respectively, by parallel beam structure 41 disposed in 
aperture 33. Various embodiments of this general design, including several 
designs which incorporate only one such resonator, are disclosed generally 
in U.S. Pat. No. 5,313,023 to Thomas H. Johnson and U.S. patent 
application Ser. No. 08/064,834, filed May 19, 1993, by Thomas H. Johnson 
et al., both of which are assigned to the assignee of the present 
invention, and both of which are incorporated by reference herein. 
The force sensing structure in load cell body 32 includes a parallel beam 
structure having a load beam 44 spanning from a base 42 to an opposing 
side of aperture 33. A pair of cantilevered parallel beams 46, 48 are also 
mounted to base 42, and a pair of double ended tuning fork resonators 50a, 
50b are mounted between the load beam 44 and the cantilevered beams 46, 
48, respectively. A relative deflection of end beams 34, 36 applies a 
shear force through connecting beams 38 and 40 which results in a 
deflection of cantilevered beams 46, 48 relative to the load beam 44. The 
deflection of the cantilevered beams places resonators 50a and 50b in 
tension and compression, respectively, thereby altering the resonant 
frequency of each resonator. 
Using suitable controller electronics (not shown), these resonators may be 
driven to oscillate at their respective resonant frequencies, whereby the 
frequency output signals therefrom may be converted to digital form. The 
value of the force applied to the load cell body may be obtained by taking 
the difference of the outputs, since the respective resonant frequencies 
of the resonators will react oppositely to an applied force. By taking the 
difference of the outputs, many common mode effects, such as due to 
environmental effects, will be rejected by the subtraction operation. 
It has been found that this particular design provides extremely high 
resolution with reliable rejection of many environmental interferences. 
Many modifications may be made to this embodiment, including several 
modifications disclosed in the aforementioned references, including using 
only one resonator, sealing the aperture with a sealant to protect the 
force sensors from environmental effects, coating the wire leads from the 
force sensors with a dampening material, etc. 
An alternative composite load cell design 80 is shown in FIG. 5, whereby a 
load cell body 81 includes a pair of end beams 82, 84 joined by connecting 
beams 86 and 88, and having an aperture 89 which includes a load beam 90 
spanning across the aperture. One or more strain gauges 92 are mounted on 
load beam 90 to measure the shear induced on the load beam through the 
load cell body. The design of load cell body 81 is one of several suitable 
embodiments disclosed in U.S. Pat. No. 5,336,854 issued to Thomas H. 
Johnson, which is assigned to the assignee of the present invention and is 
incorporated by reference herein to the extent necessary to support this 
disclosure. Any of the embodiments shown in this reference may be used in 
a composite load cell consistent with the invention. 
Beyond the particular embodiments disclosed herein, the principles of the 
invention may be applied to any shear responsive load cell design which 
relies upon relatively stiff end beams for rejecting moment induced shears 
and/or torsional moments. For example, different strain gauge designs such 
as shear beams, flexored bending beams (e.g., by Toledo, Revere, BLH, and 
others), vibrating string designs such as those used to sense shear force 
(e.g., by K-Tron and others), force motor designs such as those used to 
sense shear force (e.g., by Sartorius, Metler, A&D, Bizerba, and others), 
capacitive coupling designs such as those used to sense shear force (e.g., 
by Setra and others), piezoelectric crystal designs such as those used to 
sense shear force and tuning fork designs such as those used to sense 
shear force (e.g., by Ishida and others) are examples of suitable load 
cell bodies which may be used consistent with the invention. 
The preferred load cell bodies may be constructed of many different 
materials. The load cell bodies may be constructed of a homogeneous 
material, or alternatively, may include different components formed of 
different materials. 
Preferred compositions for the preferred load cell bodies include metals 
such as, for example, elemental metals and metal alloys. Metal compounds 
including aluminum and its alloys such as 2024-T3, 7075-T6, and 1100; 
copper and its alloys including ASTM B147, ASTM B145, and ASTM B146; zinc 
and its alloys including ASTM A40A, and ASTM AC41A, as well as any other 
metals that are known to provide a lightweight structure having good 
resilience to the forces intended to be sensed by the cell, may be used. 
Most preferably, metals such as aluminum and its oxides may be used in 
forming the load cell bodies of the invention but almost any structural 
material which lends itself to manufacturability may be used. 
The load cell body may also be made from polymer or composite systems which 
provide uniform material characteristics (e.g., modulus, temperature 
sensitivity, expansion characteristics, etc.). Plastics such as 
polyamides, polyamide-imides, polyvinyl chloride, polyethylene, propylene, 
polycarbonates, aminoplasts such as melamine resins, cast epoxy resins, 
cast acrylics, cast fluoroplastics, phenolics, polyacrylonitriles, cast 
polyurethanes, cast polyesters or polyolefins, synthetic or natural rubber 
polymers and copolymers such as silicones; ceramics or ceramic composites 
such as silicone dioxide; composites such as carbon graphite composites; 
and cellulosic products; or mixtures of any of these compounds, may be 
used. 
Different materials may be selected based upon the desired capacity, 
resolution, and/or manufacturability requirements of the finished 
assembly. It has been found that most of these materials typically have an 
elastic modulus of between 10 and 30 million psi. 
Returning to FIG. 1, composite load cell 30 includes a pair of end blocks 
60, 62 mounted to end surfaces 35, 37 of load cell body 32. 
Blocks 60 and 62 are preferably a similar profile as that of end beams 34, 
36, whereby when they are mounted to body 32, the end blocks generally 
form an extension of the load cell body. For example, end blocks 60 and 62 
include mating surfaces 61, 63 which generally follow the contours of end 
surfaces 35, 37 to provide a substantially flush junction therebetween, 
and which preferably match up along at least three edges of the end 
surfaces. 
The longitudinal width of blocks 60, 62 (i.e., the distance in which they 
extend from end surfaces 35, 37) is preferably at least 50% of the lateral 
width of the end beams, more preferably at least 100% of the lateral 
width. 
It is preferable to mount blocks 60, 62 such that they are substantially 
flush with surfaces 35 and 37 on load cell body 32. The blocks are 
preferably secured to load cell body 32 through a plurality of fasteners 
such as bolts 64 which extend through apertures 65 in end blocks 60, 62 
and which engage threaded apertures 66 that extend into load cell body 32. 
It will be appreciated that fasteners 64 may also be countersunk to 
provide flush end surfaces for blocks 60, 62. 
It will be appreciated that apertures 65 and 66 may be formed in the end 
blocks and the load cell body by different known means such as drilling or 
milling. Moreover, for some materials such as plastics, castable metals, 
and composites, the apertures may be formed concurrently with the molding 
or casting of the end blocks or load cell body. 
Different manners of mounting the end blocks to the load cell body may also 
be used in the alternative. For example, other fasteners, such as other 
numbers and types of bolts, screws, and/or pins may be used. In addition, 
different mounting methods such as adhesives, welding, soldering, etc., 
which provide a secure junction between the blocks and the load cell body, 
may also be used. 
End blocks 60, 62 may be constructed of any number of materials, including 
any of those listed above for the load cell body. However, the particular 
material chosen for end blocks 60, 62 will vary depending upon the 
construction of the load cell body, given that the blocks should have a 
higher modulus than that of the load cell body. In the preferred 
embodiment, the elastic modulus of the end blocks is preferably at least 
50% greater than the elastic modulus of the load cell body. 
For example, for an aluminum load cell (with an elastic modulus of about 10 
million psi), it has been found that end blocks constructed of steel (with 
an elastic modulus of about 30 million psi) provide a sufficient increase 
in the apparent bulk modulus of the load cell body to substantially 
improve the cornering and rejection of moment induced shears. Similarly, 
for load cell bodies constructed of various composite materials, it is 
believed that aluminum or its oxides may be suitable for use as end 
blocks. 
Several additional modifications may be made to end blocks 60, 62 
consistent with the invention. For example, the end blocks may be 
constructed to be integral with brackets or other supporting or mounting 
hardware, such as brackets for interconnecting the load cell with the base 
or platform of a scale. For example, in load cell 100 shown in FIG. 6, end 
blocks 102 and 104 have integral brackets 103 and 105, respectively, for 
mounting load cell body 101 between a base 107 and platter 109, e.g., with 
fasteners 106. For sensing other types of forces, other suitable mounting 
hardware and other structures may also be used. 
In addition, it may be possible to increase the apparent modulus of the end 
blocks in various known manners, such as by spring loading or clamping the 
blocks to increase the stiffness thereof. It will be appreciated that 
other modifications may be made to the preferred end blocks consistent 
with the invention. 
The design of composite load cell 30 provides several advantages. For 
example, as discussed above, the preferred end block designs provide 
improved rejection of moment induced shears by increasing the apparent 
modulus of the end beams of the load cell body. This results in improved 
resolution for the composite load cell 30. Moreover, the improved 
rejection of moments may even enable lower modulus materials to be used in 
load cell bodies where they were previously not usable due to limitations 
imposed by the inherent properties of the material. For example, it may be 
possible to use less expensive materials and/or less expensive 
manufacturing processes to reduce the cost and/or complexity of a 
composite load cell. 
The preferred designs also improve cornering to reject torsional moments 
due to off-center loads. As described above, many conventional designs 
require filing or other adjustments to individual flexures on the 
connecting beams to improve the cornering and correct for transverse or 
longitudinal moments due to off-center loading. It has been found that the 
stiffer end blocks also reduce these torsional moments, which may reduce 
or eliminate the custom adjustment of individual flexures, thereby 
resulting in lower manufacturing costs and complexity. 
Load cell Mounting 
Depending upon the particular application, the preferred composite load 
cells may be mounted in several ways to detect and generate an output 
signal representative of the applied force on the load cell. For example, 
as shown in FIG. 3, composite load cell 30 may be interposed between the 
base 71 and platter (or force receiving member) 72 of a scale 70 for the 
purposes of measuring the weight of an object placed upon platter 72. In 
this embodiment, composite load cell 30 is mounted to base 71 along the 
bottom surface of end block 60 through a spacer 74. Similarly, composite 
load cell 30 is mounted to platter 72 with a spacer 74 interposed between 
the platter and the top surface of end block 62. Spacers 74 are preferably 
constructed of a shock resistant material such as phenolic to dampen 
vibrations and protect composite load cell 30 from sudden jarring forces. 
As shown in FIG. 1, the interconnection between the end blocks and the 
scale components may be made through a pair of threaded fasteners 76 which 
engage threaded apertures 67 formed in each block. Other manners of 
fastening the end blocks to the platter and base of scale 70 may also be 
used, such as any of the mechanisms described above for attaching the end 
blocks to load cell body 32. 
As is also shown in FIG. 3, a pair of stops such as bolts 78 or other 
suitable protruding members may also be affixed to the base 71 and/or the 
platter 72 to prevent damage to the force sensing structure within load 
cell 30 due to excessive deflection of the load cell body. In addition, 
strips 79 constructed of phenolic or a similar material may be installed 
opposite stops 78 to prevent damage to the load cell. In addition, any 
suitable brackets or alternative fasteners may be used consistent with the 
invention. 
FIG. 4 shows an alternate manner of mounting a load cell 30' in a scale 
70'. In this embodiment, apertures 67 are formed in the top surfaces of 
blocks 60 and 62'. Block 60 is in turn mounted through a spacer 77 to 
platter 72, while block 62' is mounted to base 71 through a spacer 78 
mounted to bracket 73. Both blocks are mounted to their respective members 
through the use of fasteners 76 engaging the threaded apertures 67 in the 
blocks. Moreover, bracket 73 includes an aperture for permitting passage 
of spacer 77 therethrough. Stops 75 and strip 79 may be used to protect 
the load cell from excessive deflection. 
Composite load cell 30 may be mounted in several alternative ways to that 
shown in FIGS. 3 and 4. For example, load cell 30 may be mounted across 
the bottom surfaces of blocks 60, 62. Also, load cell 30 may be mounted 
using the end surfaces of one or both of blocks 60, 62. Moreover, other 
types of mounting configurations, such as cantilevered or fulcrumed 
configurations, and other types of force receiving members and other 
mounting hardware, may also be used, depending upon the particular type of 
force which is being measured, e.g. pressure, acceleration, weight, 
torque, etc. 
Applications 
Composite load cells consistent with the invention may be constructed to 
measure various forces, such as due to weight, acceleration, torque, 
pressure, and the like. Moreover, several applications may exist for each 
category of force. Therefore, it will be appreciated that the use of the 
preferred composite load cell in a weight sensing environment is provided 
merely for illustration, and in no way should be construed as limiting the 
invention. 
For example, the preferred load cells may be used in different weighing 
applications, such as counting scales, laboratory scales, scanning scales, 
postal scales, etc. One type of scale suitable for use with the preferred 
load cells is the PC-810 Counting Scale, manufactured by Weigh-Tronix, 
Inc., the assignee of the present invention. 
FIG. 7 shows one preferred scale 110 which includes composite load cell 30 
with load cell body 32 mounted between base 114 and platter 112 through 
end blocks 60, 62. Shroud 116 is disposed over platter 112 for providing a 
working surface on which objects may be placed for measurement, as well as 
for ornamental purposes. A load cell controller 31 is also mounted to load 
cell 30. 
The above discussion, examples and embodiments illustrate our current 
understanding of the invention. However, since many variations of the 
invention can be made without departing from the spirit and scope of the 
invention, the invention resides wholly in the claims hereafter appended.