Load cell

A load cell which may be embodied in an arch beam structure or a shear tension link and comprising a flexure area design which requires the machining of only five parallel holes. The five holes comprise a large central hole and four satellite holes to define two opposite load paths, each including two flexure areas which react to a load in opposite tension-compression sense. Strain gage resistors of the foil type are bonded onto the flexure areas and connected into a Wheatstone bridge circuit.

INTRODUCTION 
This invention relates to load cells and particularly to a load cell which 
can be used for a variety of purposes such as single axis tension and 
compression measurement as well as beam loading. The cell is characterized 
by high precision and extreme ease of manufacture due to the simple 
flexure design. 
BACKGROUND OF THE INVENTION 
It is well known to measure forces using a beam type load cell wherein 
first and second spaced rigid end structures are interconnected by a 
plurality of relatively long parallel struts of high modulus of elasticity 
material. These struts tend to flex in the fashion of parallelogram links 
when one of the end structures is fixed to a reference surface and the 
other end structure is loaded. Strain sensitive means such as thin foil 
resistors may be bonded to the struts to monitor strain therein under 
load. The strain sensitive resistors are interconnected into a bridge 
circuit which is calibrated to produce an output voltage proportional to 
load. 
While the device described above is reasonably effective for measuring 
loads in a beam type flexure fashion, the formation of the long parallel 
struts poses an extremely difficult machining problem since it is 
preferable, if not essential, that the entire cell including the spaced 
end structures and the parallel struts be an integral unit; i.e., formed 
from a single piece of solid stock. Moreover, the nature of the struts is 
such that it is relatively difficult to obtain a high precision cell while 
staying within reasonable cost limits. 
BRIEF DESCRIPTION OF THE INVENTION 
It is the principal object of the present invention to provide a load cell 
which is capable of various uses including beam type loading and which is 
both accurate and easy to manufacture. This is accomplished by a simple 
flexure design which minimizes the need for precision machining operations 
and which, in the typical embodiment, can be formed simply by boring a 
plurality of holes or apertures along easily located parallel axes. 
In accordance with the preferred embodiment of the invention, a load cell 
is provided which is characterized by a solid integral body of high 
modulus of elasticity material having longitudinally opposite end portions 
which are typically used to apply the load or affix the device to a 
reference surface such as the case may require. In addition, the body of 
the load cell is so formed as to define a first aperture which is 
typically mediate the end portions and which extends fully between 
opposite faces of the body. In addition, a plurality of second, typically 
smaller, satellite apertures are formed in the body around the first 
aperture and uniformly reversely similarly spaced on opposite sides of the 
longitudinal axis. This arrangement of holes or apertures provides two 
parallel force paths between the end portions, one path on either side of 
the first aperture, and each path having first and second stress 
concentrating flexure areas of greatly reduced cross-sectional area 
bounded by the first aperture and one of the second apertures. 
In the illustrative embodiments of the invention hereinafter described in 
detail, the first aperture is a fairly large through bore drilled between 
opposite plane faces of a solid bar of aluminum stock. The second 
apertures are four in number and are uniformly spaced around and adjacent 
the first aperture such that each of the parallel force paths includes two 
reduced section flexure areas. When loaded as a beam, the flexure areas 
cause parallelogram type deflection of the two end portions thus to load 
two of the flexure areas in tension and two in compression. Strain 
sensitive gage means may be disposed on the flexure areas by convenient 
location on the interior wall of the central aperture so as to measure 
these tension and compression forces. As usual, the strain sensitive means 
are readily connected into a bridge circuit so as to produce an output 
signal which is proportional to load. 
Alternatively, the simple flexure design of the subject invention may be 
incorporated into a single axis tension-compression cell again by forming 
the pattern of holes in the load cell mediate the end portions which, of 
course, are provisioned to accept a load. In addition, the parallelogram 
action is created by a pair of longitudinally spaced and oppositely 
entering slots which are cut transversely through the body so as to 
laterally displace the force application points on the longitudinally 
opposite sides of the flexure area. Again, strain sensitive gage devices 
are placed in the central hole to monitor the tension and compression 
reactions in the flexure areas. The strain sensitive means are in turn 
connected into a bridge circuit to produce a useable output signal. 
Of course, may other uses or embodiments of the invention are possible. The 
details, as well as the various features and advantage of the present 
invention may be best understood by a reading of the following 
specification in which two embodiments of the invention are fully 
described in such concise detail as to enable those skilled in the art to 
make and use these as well as other embodiments of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
Looking now to FIGS. 1 through 4, the invention is first shown to be 
embodied in an arch beam load cell 10 for measuring loads such as weights 
and mechanical forces and comprising a solid body 12 of high modulus of 
elasticity material such as aluminum, steel or other machinable and 
dimensionally stable material. Body 12 is of longitudinally elongated 
shape having a generally rectangular cross-section and is machined to 
define first and second longitudinally opposite end portions 14 and 16 and 
a central stress concentrating flexure portion 18. These portions of the 
body 12 are defined by a large central aperture 20 which is bored and 
finished through the body 12 between the major plane faces thereof and of 
such diameter as to substantially reduce the load-bearing cross-sectional 
area of the body in the vicinity of the aperture 20. Four additional 
apertures 22, 24, 26 and 28 are bored and machined into the body 12, the 
axes of these additional apertures being parallel to that of the aperture 
20 and being arranged uniformly and reversely similarly with respect to 
the longitudinal axis of the body 12. In the illustrated case, four 
apertures are disposed at 90.degree. intervals around the aperture 20 and 
on equal radii such that the apertures 22 and 24 lie above the 
longitudinal center line. These additional apertures all open to the 
laterally opposite edges of the body 12 so as to create, in combination 
with the aperture 20, two parallel load paths from one longitudinal end 
portion to the other and on opposite ends of the aperture 20. The upper 
load path, for example, includes two flexure areas of greatly reduced 
cross-section where stress concentration occurs. These flexure areas 
include the reduced sectional portion of the body between apertures 20 and 
24 and between apertures 20 and 22. Similar but transversely opposite 
flexure areas exist in the body between apertures 20 and 28 and again 
between apertures 20 and 26. 
To measure the strain in the flexure areas, variable resistance foil-type 
strain gages 30, 32, 34 and 36 are insulatively bonded to the interior 
surface of the aperture 24 on the flexure areas as previously defined. 
These strain gages are connected into the well-known Wheatstone bridge 
circuit shown in FIG. 4 with strain gages 30 and 32 forming one leg of the 
bridge and strain gages 34 and 36 forming the other leg of the bridge. A 
voltage source 38 is connected across both legs of the bridge and a 
voltage responsive meter 40 is connected to the midpoints of the two legs 
so as to produce an output reading which is proportional to resistance 
changes in the two legs due to the imposition of a load as hereinafter 
described in greater detail. The strain gage resistors are all of the same 
nominal value in the typical case and the bridge circuit is set to produce 
a zero reading when there is no load imposed upon the arch beam load cell 
10. 
Load cell 10 is further provided with facilities for receiving a load in 
the fashion of a beam by securing end portion 16 to a reference surface 
and securing end portion 14 to the loading mechanism. To this end, a 
tapped hole is formed transversely through end portion 14 between the 
laterally opposite edges of the body 12 such that a load support rod may 
be threaded into the body and a load applied along the axis of the 
threaded hole 42. Threaded or tapped holes 44 and 46 are formed in the 
opposite end portion 16 to receive machine screws or the like for securing 
the load cell to a reference surface such as a scale frame, a laboratory 
table or the like. End portion 16 may be integrally machined or separately 
jacketed to produce the reinforcement sleeve which is apparent in the 
drawings so as to facilitate securement to the reference or support 
structure. A longitudinally extending surface slot 48 is formed in the 
jacket along with drilled holes 50 and 52 to provide a convenient and 
protected path for the connecting wires of the bridge circuit, it being 
necessary to interface the bridge circuit strain gages with the voltage 
source 38 and the voltage meter 40 at some point at least relatively 
remote from the transducer itself. 
In a typical operation, the loading of the end portion 14 with a vertical 
load such as depending weight causes a parallelogram type transverse 
shifting of end portion 14 relative to end portion 16 with the upper and 
lower force paths deflecting in the manner of parallel beams or struts as 
indicated by the phantom lines in FIG. 2. The strain which is concentrated 
in the flexure areas is such as to produce tension at the flexure areas 
associated with gages 32 and 36 and compression at the flexure areas 
associated with gages 30 and 34. The tension forces at or in strain gages 
32 and 36 causes a resistance change in one direction, for example a 
resistance increase, in gages 32 and 36 while the compression forces at 
the other flexure areas produces a resistance change in the opposite 
direction, for example a decrease in resistance, in gages 30 and 34. Thus, 
the total resistance of each leg of the bridge circuit may remain 
substantially constant but the values of the individual resistors in the 
two legs change in an inverse fashion so that the potentials at the bridge 
points A and B swing in opposite directions. In the example given, the 
potential at point A tends to decrease while the potential at point B 
tends to increase and the resulting potential difference between points A 
and B produces a voltage reading across the meter 40. The bridge circuit 
is preferably calibrated so that the reading on meter 40 is directly 
related to the magnitude of the imposed load. 
Looking now to FIGS. 5 and 6, the invention is shown embodied in a second 
load cell 54 which is configured to measure loads having components which 
lie along the axis X--X. Like load cell 10, the cell 54 is machined to 
define longitudinally opposite end portions 56 and 58 and a central 
portion 60. Within the central portion 60 is machined a large central 
aperture 62 abounded by four uniformly spaced parallel apertures 64, 66, 
68 and 70 of lesser diameter. In addition, two longitudinally spaced holes 
or apertures 72 and 74 are drilled or machined into the body of cell 54 
along the measurement axis and, like the other apertures, extending fully 
between the opposite plane faces of the load cell 54. Load cell 54 is 
further machined so as to define a first slot 76 extending between one 
lateral edge of the cell and the aperture 72 and a second slot 78 
extending between the opposite lateral edge and aperture 74. The load cell 
body is further slotted so that apertures 64 and 66 open to aperture 72 
and apertures 68 and 70 open to aperture 74. It will also be noted from 
the cross-sectional view of FIG. 6 in combination with the perspective 
view of FIG. 5 that the satellite aperture 64, 66, 68 and 70 open to the 
central aperture 62 except at the center line of the load cell 54 thus to 
define four stress concentrating flexure areas on which are insulatively 
bonded strain gage transducers represented by transducer 80 in FIG. 5. As 
was the case with respect to load cell 10, there are four such strain gage 
transducers so disposed at 90.degree. intervals and an equal radii around 
the interior surface of the aperture 62 and connected into the Wheatstone 
bridge circuit of FIG. 4 to monitor strain in the load cell 54 as a result 
of a tensile or compression load imposed along the axis X--X. It will be 
seen that the transverse slots 76 and 78 together with the basic 
five-aperture flexure design in the central portion 60 of the load cell 54 
result in the production or definition of two parallel load paths, each 
having two flexure areas to produce a strain pattern which is similar or 
identical to the strain pattern realized by the load cell 10 of FIG. 1 
through 3 when subjected to beam type loading. In other words, two 
diagonally opposite strain gages vary in one sense while the remaining 
diagonally opposite strain gages vary in the other sense when the load is 
applied. In the illustrated case, the load cell 80, along with the 
diagonally opposite load cell adjacent aperture 70, would experience a 
compression force if the load cell 54 were loaded in tension along the 
axis X--X. Conversely, the strain gages associated with the flexure areas 
adjacent aperture 66 and 70 would be loaded in tension thus to produce the 
variations in resistance necessary to produce the bridge output reading 
previously described with reference to FIG. 4. 
The transducer 54 includes two longitudinally opposite tapped holes 81 and 
82 into which load-applying rods or the like may be inserted. 
Referring now to FIG. 7 there is shown a weigh scale structure 100 
embodying the invention and adapted to provide an electronic signal 
representing the mass or weight W applied to an input frame 102 by way of 
a support post 104. As will be noted from inspection of FIG. 7 the 
structure 100 incorporates the basic five-hole flexure design into a 
structure which is designed for resistance to structural warpage due to 
off-axis loads and similar factors. 
Structure 100 comprises, in addition to the input frame member 102, an arch 
beam 106 having flexure apertures 132 and 134 formed therein and being 
integrally attached to a rigid support frame 108. Frame 108 is 
substantially rectangular, comprising relatively heavy vertical end 
structures 109 and 111 interconnected by relatively thin spring sections 
110 and 112. In addition to being integral with the righthand portion 109 
of the frame 108 as shown in FIG. 7, the arch beam 106 is also integrally 
connected with the lefthand frame section by a hinge 114 which is reduced 
down to a minimum cross-section to avoid distortion in the end of beam 106 
when a load is applied via frame 102 and structure 111. The 
anti-distortion effect is further enhanced by the aperture 116 which is 
milled or otherwise formed in the portion 111 of the support frame 
adjacent the flexure area 114. The input frame member 102 is mechanically 
secured to the frame 108 by means of machine screws 118 which extend 
through holes 120 in the frame 102 and into tapped holes 122 in the 
lefthand portion of the frame 108. 
The structure 100 further comprises a support frame member 124 which is 
L-shaped in design and which is mechanically connected to the right side 
of the support frame 108 by means of machine screws 126 which extend 
through the holes 128 in the frame member 124 and enter tapped holes 130 
in the frame member 108. The member 124 rests on and is connected to a 
base, not shown. 
Looking to the main flexure area of beam 106, the large central aperture 
132 is adjoined by the satellite apertures 134 in the fashion previously 
described with reference to FIGS. 1 and 2 to provide the five-hole 
flexure. The apertures 132 and 134 are preferably located closer to the 
end of the beam 106 adjacent frame 102 than the opposite end. The reduced 
cross-section areas between the apertures 134 and the aperture 132 are the 
locations at which the strain measurement is taken and to this end the 
strain gage resistors 136 are bonded to the flexure areas and connected 
into a bridge circuit as previously described. 
In use, the frame 102 is mechanically connected to the left side of the 
frame 108 as seen in FIGS. 7 and the frame member 124 is mechanically 
connected to the right side or reference side of the frame 108 by means of 
the machine screws 126. The post 104 carries a pan (not shown) or other 
flat structure of considerable area so as to receive objects for precision 
weight measurement. The imposition of a load W onto the pan imposes the 
same load on the frame member 102. The load is thus transmitted directly 
to the lefthand side 111 of the frame 108. Because the portion 111 of the 
frame is connected with the righthand side 109 only by way of the thin 
spring hinge section 110 and 112, (in addition to the arch beam 106) the 
load, for all practical purposes, acts directly upon the lefthand side of 
the arch beam 106 to cause strain in the flexure area as previously 
described. Should the weight be placed off-center on the pan or other 
receiving structure, the resulting moment on the input frame structure 102 
is resisted by the hinges 110 and 112 since, although they are extremely 
resilient in response to pure vertical loads, they offer substantial 
resistance to torsional warpage as well as longitudinal loads. Thus, the 
hinge structures 110 and 112, in addition to the flexure are or hinge 114, 
resist error producing warpage in the arch beam and result in the pure 
shear load caused by the weight W on the left side of the beam and the 
necessary reaction or support of equal magnitude and opposite direction in 
the right side of the frame 108 as will be apparent to those skilled in 
engineering mechanics. 
It should be apparent from the foregoing description of the structure 100 
of FIG. 7 that even greater resistance to warpage may be achieved by 
widening the frame 108 out into a box-like structure having end plates in 
place of the vertical end portions of the frame 108 and having the corners 
of these plates longitudinally horizontally interconnected by thin hinge 
sections, two at the top and two at the bottom in place of the hinge 
sections 110 and 112. However, the substantially uniplanar structure of 
FIG. 7 has the advantage of easy machinability and thus may be preferred 
over such a three-dimensional structure. 
To fabricate the device of FIG. 7 it is clear that a duplicating type mill 
may be employed with a template representing half of the frame structure 
108 since it is reversely similar or symmetrical about a horizontal center 
line. In the fabricatio process, a single block or plate of machinable 
material such as aluminum or steel is preferably shaped by boring the 
apertures 132, 134, 122, and 130. Thereafter, the balance of the machining 
takes place thus to form the thin hinge sections 110, 112, and 114. 
Aperture 116 may be milled out either before or after the main milling 
operation. 
FIG. 8 discloses a second weigh scale structure 200 which embodies the 
invention and which is also adapted to provide an output signal responsive 
to the magnitude of a weight W imposed thereon. Structure 200 comprises a 
ring 202 of high modulus of elasticity material such as aluminum or steel 
and having four of the five-hole flexure designs machined therein at equal 
90.degree. intervals as shown. Each of the five-hole flexure designs is 
provided with strain gage resistors in the manner previously described, 
such resistors being interconnected into a parallel bridge arrangement to 
provide a single reading as will be apparent to those skilled in the art. 
The load is applied across the upper surfaces 204 of the ring 202 at 
diametrically spaced points. The load reaction or support force is applied 
across the bottom surface of the ring by means of support stand portions 
206 and 208 at diametrically spaced points. The diametrically spaced 
points for the imposition of the load are on a diameter which is disposed 
at 90.degree. to the diameter for the reaction force and all load areas 
are precisely mediate the five-hole flexure areas as shown. Accordingly, 
the load, which may be a weight on a pan or the like is applied directly 
to the ring causing elastic structural deformation therein. This 
deformation causes variation in the resistive values of the strain gages 
in the flexure areas thus producing a reading proportional to the weight 
W. 
It can be seen that the structure 202 is easily machined and thus provides 
an extremely convenient and inexpensive structure for the precision 
weighing of various items. The invention may be implemented in still 
further forms and for still further uses and applications and accordingly 
the foregoing descriptions are not to be considered as exhaustive of the 
various possibilities. 
It will be noted that in all cases the satellite apertures on each side of 
the longitudinal centerline of the five-hole flexure pattern are isolated 
from one another; e.g., aperture 22 is isolated from aperture 24 by the 
cuts extending to the nearest external surface. The same is true of 
apertures 134 in the beam 106. Where an exterior surface is not available 
because the pattern occurs in a large surface area not near a boundary, 
long slots may be formed so as to span the distance between (and slightly 
beyond) adjacent pairs of satellite apertures and the cuts may then be 
made from the apertures to the slots to provide the isolation. 
It is to be understood that the invention herein is believed to lie in the 
basic five-hole flexure design which is illustratively embodied in the 
load cells 10 and 54 and is not limited to the specific geometric 
configuration of those cells, although such specific details may in 
themselves comprise invention. Accordingly, the basic design may be 
embodied in a great variety of cells of varying physical configuration and 
varying practical application.