Helical load cell

A helical load cell is disclosed which is capable of accurately measuring axially applied forces, whether the force is applied axially within the diameter of the helical coil or outside of the coil diameter. A pair of strain gage modules are disposed on diametrically opposed surfaces of the coil. Each module includes a pair of transducer elements disposed orthogonally with respect to each other and at forty-five degrees relative to a neutral axis of the coil. The modules are coupled in an additive bridge circuit arrangement. In one embodiment, the modules are disposed along the outside diameter of the coil. In another embodiment, the modules are disposed along the inside diameter of the coil. In yet another embodiment, the modules are dispose on the upper or lower surface of the coil. In yet another embodiment, a second pair of strain gage modules is provided and coupled in a subtractive bridge circuit. The second pair of modules provides information as to the location of the applied load.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates generally to load cells, and more 
specifically to a load cell which is insensitive to the position of the 
loading force. 
BACKGROUND ART 
Load cells are used to provide accurate measurements of compressive or 
tensile forces. Typically, the force creates a strain in the load cell 
which is measured by strain gage transducers. Accurate measurements, 
however, require that the force be applied along an axis which is central 
to the load cell and about which all the transducers are symmetrically 
placed. 
An example of such a load cell which is sensitive to the effects of 
off-axis loading is known as a compression washer. U.S. Pat. No. 4,212,360 
discloses such a load cell (FIGS. 4a and 4b), an example of which is shown 
in a product brochure entitled "Compression Only/Thru Hole Load Washer". 
As the name implies, this type of load cell is configured as a washer so 
that it can be mounted by securing a bolt through the device. 
Other compressive load cells which are less sensitive to off-axis loading 
are characterized by several types. One such load cell employs a shear web 
design disclosed in U.S. Pat. No. 5,461,933. This design consists of a 
pair of concentric rings joined by two or more web members. Transducer 
elements disposed on the webs provide compression and tension force 
measurements. Although this design reduces sensitivity to off-axis loads, 
the web assembly must be accurately machined to tight tolerances. In 
addition, the traducers must be carefully bonded to the walls of the webs 
in a symmetrical manner in order to ensure accuracy in the measurements. 
A ball and socket assembly design serves to maintain the loading force 
along the principle axis and exactly centered on the load cell. Such a 
design is shown in Photo 2 in the article by Clegg entitled "Bonded Foil 
Strain Gauge Force Transducers", Sensors, October 1996, pp. 68-75. As can 
be seen from the photograph, the manufacture of such a device is can be 
quite expensive. 
A shear beam load cell, such as the one disclosed in U.S. Pat. No. 
5,220,971, can be also be used in tension and compression applications. 
The transducer elements are diagonally placed on a machined web in the 
center portion of the device and measure the shear force of the applied 
load. Such devices are expensive to machine and accurate measurements 
depend on consistently centered and axially constrained forces. 
What is needed is an easy to manufacture load cell which can deliver 
accurate measurements and which is insensitive to off-axis loading. It is 
also desirous to construct a load cell which is compact in size. 
SUMMARY OF THE INVENTION 
The present invention is comprised of a helical coil of wire which serves 
to bear an axial load, either a compressive force or a tensile force. The 
helical coil includes two strain gage modules which detect the strain 
caused by the applied axial load. The strain gage modules are located on 
the coil in diametrically opposed relation. 
Each strain gage module consists of a pair transducers, each transducer in 
turn consisting of a plurality of grid elements. The pair of transducers 
are positioned about a neutral axis of the coil in order to reject any 
bending moment perpendicular to that axis which is produced by an off-axis 
load. Each pair of transducers is arranged so that when one transducer is 
subjected to a compressive force, the other is experiences a tensile 
force. Both pairs of transducers are then coupled in a Wheatstone bridge 
arrangement so that bending moments in the plane of the neutral axis, due 
to off-axis loading, are rejected. 
In one embodiment of the present invention, the transducers are positioned 
along the outside diameter of the coil. In another embodiment, the 
transducers are positioned along the insider diameter of the coil. In yet 
another embodiment, the transducers are positioned on an upper or a lower 
surface of the coil. In this embodiment, it is necessary that the coil 
never become fully compressed since doing so will result in crushing of 
the transducers. A compression stop member is provided to ensure that the 
coil does not fully compress. 
In yet a fourth embodiment, a helical coil is outfitted with two pairs of 
strain gage modules. For each pair, the modules are located upon the 
helical coil in diametrically opposed relation. Moreover, each pair of 
modules is orthogonally positioned relative to the other pair. 
One pair of modules is wired in an additive bridge circuit. The other pair 
is wired in a subtractive bridge circuit. This arrangement permits 
accurate load force measurement, while at the same time providing 
information relating to the location of the load force relative to the 
centerline of the coil.

BEST MODE OF CARRYING OUT THE INVENTION 
FIG. 1 shows the various embodiments of the helical load cell 10 in 
accordance with the present invention. A length of wire is wound to form a 
helical coil (a spring) 20. This is preferably done by drawing the wire 
through a die, resulting in a coil consisting of wire having a very 
consistent diameter along its length. Other techniques for forming springs 
are known in the art, however, and the specific method of forming the 
helical coil is not relevant to the practice of the present invention. 
A first embodiment of the helical load cell 10 is shown with respect to 
strain gage modules 80, 82. Each module 80, 82 is mounted to the coil 20 
along a neutral axis of the coil in a diametrically opposed relation to 
the other module. Strain gage module 80 is shown aligned relative to 
neutral axis 40, while strain gage module 82 is aligned with respect to 
neutral axis 42. In the first embodiment, the modules are positioned at an 
outside diameter of the helical coil 20. The module 82 is shown in phantom 
indicating that it is located at the outer periphery of the coil and 
opposite to module 80 along the diameter of the coil. FIG. 1 and the top 
view of FIG. 2A more clearly show this arrangement of strain gage modules 
80, 82 along the outside diameter O.D. in accordance with the invention. 
This placement of the modules 80, 82 along the outer periphery of the coil 
facilitates manufacture of the invention. 
A second embodiment of the helical load cell 10 is also shown in FIG. 1 
with respect to strain gage modules 84, 86. As in the first embodiment, 
each module is mounted to the coil 20 in diametrically opposed relation to 
the other module. Module 84 is aligned relative to a neutral axis 44 of 
the helical coil 20, and module 86 is aligned along neutral axis 46. In 
the second embodiment, however, the modules 84, 86 are mounted along the 
interior diameter of the coil. Thus, module 84 is shown in solid, 
indicating that it is positioned along the interior surface of the coil, 
while module 86 is shown in phantom, being disposed upon the inside 
periphery of the coil at a point opposite to module 84 along the coil's 
inside diameter. FIG. 1 and the top view of FIG. 2B more clearly 
illustrate the arrangement of strain gage modules 84, 86 along the inside 
diameter I.D. of the coil 20. 
Whereas the first two embodiments of the present invention have the strain 
gage modules located either on the outside or inside periphery of the coil 
20, a third embodiment, also shown in FIG. 1, shows the modules 90-93 are 
positioned either upon an upper surface of the helical coil 20 or upon a 
lower surface thereof. In the third embodiment, one of the strain gage 
modules 90, 92 is paired with one of the diametrically opposed strain gage 
modules 91, 93. It can be seen, therefore, that four variations of the 
third embodiment are possible. Thus, in one variation, the diametrically 
opposed modules both are located on a top surface of the coil, such as 
modules 90 and 91. In a second variation, the modules are positioned on a 
bottom surface of the helical coil 20, such as modules 92 and 93. In third 
and fourth variations, the diametrically opposed modules are located on 
opposite surfaces, such as modules 90 and 93, or modules 92 and 91. FIG. 
2C shows these combinations of arrangements as seen from the top. The 
modules 90-93 are illustrated in phantom to indicate that they are 
disposed either on the upper or the lower surface of the coil 20. FIG. 1 
and FIG. 2C also show that the modules 90-93 are aligned relative to their 
respective neutral axes 48, 50, and in diametrically opposed relation 
along a mean diameter M.D. of the coil. 
FIG. 1 illustrates yet a fourth embodiment of the invention. As will be 
explained below, the fourth embodiment of the load cell is capable of 
providing information as to the location of the applied force of an 
off-axis load, in addition to sensing the magnitude of that load without 
sensitivity to its position. Turning then to FIG. 1, strain gage modules 
80-83 are arranged as shown in accordance with the fourth embodiment. 
Strain gage modules 80, 82 are shown in diametrically opposed relation, as 
are strain gage modules 81, 83. In addition, the module pair 80/82 is 
shown orthogonally positioned relative to the module pair 81/83, although 
this is not necessary. The top view of FIG. 2D more clearly shows the 
arrangement of the strain gage modules 80-83 in accordance with the fourth 
embodiment. 
Referring once again to FIG. 1, additional detail of the strain gage 
modules used in each of the three above-described embodiments will be 
discussed. Consider the strain gage module 80. As can be seen, the module 
80 consists of a first transducer 60 and a second transducer 62, shown in 
the figure in enlarged format for clarity. Each of the transducers 60, 62 
consists of a plurality of grid elements 61, 63, respectively. The grid 
elements are the active portion of a transducer, the resistance of which 
changes with an applied force. Transducer technology is a well understood 
art so that operation of the transducers requires no further discussion in 
connection to the practice of the invention. 
The first transducer 60 is positioned on the helical coil 20 symmetrically 
about the neutral axis 40. That is, as much of the transducer 60 lies 
above the neutral axis 40 as does below the neutral axis. In addition, the 
transducer 60 is oriented so that its grid elements 61 lie at 
substantially forty-five degrees relative to the neutral axis. The second 
transducer 62 also is positioned symmetrically about the neutral axis of 
the helical coil 20 and proximate the first transducer. The second 
transducer is oriented so that its grid elements 63 lie at a forty-five 
degree angle relative to the axis 40 and at a ninety degree angle relative 
to the grid elements 61 of the first transducer 60, as shown in FIG. 1. 
Each of the strain gage modules 80-93 are constructed in a manner similar 
to strain gage module 80. Each module consists of two transducers aligned 
to their respective neutral axes and to each other as shown for module 80. 
The transducers which comprise the strain gage modules of the invention may 
be mounted simply by gluing them to the surface of the helical coil 20 
with an appropriate glue such as an epoxy compound. Alternatively, thin 
film transducers may be used, which are sputtered onto the surface of the 
coil. This method lends itself to high volume automated processes. Another 
type of transducer involves silicon transducers bonded to the coil. In 
this process, the transducers are fabricated out of silicon in wafer form, 
not unlike the fabrication of silicon integrated circuits. The silicon 
chip transducer is then bonded to the metal surface of the coil and wires 
are attached by wire bonding techniques similar to those used in 
integrated circuit manufacture. Silicon transducers are on the order of 
fifty times more sensitive than their metal-based counterparts. The gage 
factor of a transducer is defined as the ratio of the change in its 
resistance to the change in its length, multiplied by 1.times.10.sup.6. 
For metal transducers, the gage factor is typically 2, while the gage 
factor for a typical silicon transducer is 100. 
It is clear that any one of a number of types of transducers can be used, 
and any of a number of methods for mounting the devices to a coil are 
known. The particular device type and mounting method are dictated largely 
by desired performance characteristics and manufacturing costs. It is 
noted, therefore, that the specific type of transducer and the mounting 
method is not relevant to the practice of the present invention. 
When attaching the transducers to the coil, the coil (spring) first is 
fully compressed to a solid cylinder. This pre-stresses the coil, changing 
the yield point so that the new yield point of the material is reached 
only when the coil is fully compressed. Thus, transducers mounted to the 
coil will not be damaged when the coil becomes fully compressed, since the 
material of the coil will not exceed its yield point. The transducers are 
mounted after the coil is pre-stressed and allowed to return to its 
uncoiled state. Alternatively, the transducers may be mounted to the coils 
when the coils are in the fully compressed condition. For larger coils, 
however, the machinery used to fully compress such coils makes it 
impractical to mount the transducers. Thus, the former method is 
preferred. 
Compressing the coil to a solid cylinder results in a load cell that no 
longer measures, but is rigid and quite strong; as strong as a true 
cylinder with the same cross sectional area. As an example, a load cell 
rated at 5,000 pounds can be made from 0.625 inch diameter wire wound to a 
two inch diameter coil. The coil reaches a solid at about 10,000 pounds of 
axial force. No further measurement is possible, but the load cell is then 
able to withstand additional load to approximately 100,000 pounds without 
any damage to the transducers disposed on the coil. This is due to the 
fact that the yield point of the metal comprising the grid elements of the 
transducers, in .mu.in/in of strain, is nearly the same as that of the 
material in the coil, and preloading to the solid condition assures that 
the strain cannot subsequently be exceeded. Prior art load cells only 
allow two to three times the rated load force before permanent damage 
occurs. A helical load cell manufactured in accordance with the present 
invention allows up to twenty times overload without any damage. The stops 
are built-in and are effectively present when the spring has been 
compressed to a solid. 
FIG. 3 shows the forces experienced by the helical coil 20 due to a load F 
applied at an arbitrary off-center position. Consider first, the special 
case wherein the loading force is applied along the central axis of the 
coil 20. The maximum shear stress .tau..sub.max experienced along the 
outside diameter of the coil is: 
##EQU1## 
where: .tau..sub.max is the maximum shear force at the outside diameter 
surface of the coil, 
F is the applied force, 
A is the cross sectional area of the coil 
T is the resisting torsional force, 
r is the radius of the wire comprising the coil, and 
J is the polar area moment of inertia of the wire. 
The maximum shear force experienced along the inside diameter of the coil 
is: 
##EQU2## 
where the terms are as defined above. 
Since the applied force is at the center of the coil, the torsional force, 
T, is equal to F.times.D/2, where D is the mean diameter of the coil. 
In the generalized case, however, the applied force may be arbitrarily 
located, such as shown in FIG. 3. In this case, the torsion term 
##EQU3## 
varies depending upon the location of the applied force on the coil, and 
thus the shear stress .tau. will vary. Consider the arrangement in FIG. 3. 
Strain gage modules 80, 82 are positioned along the outside diameter of 
the helical coil 20. A loading force F is applied off-axis relative to the 
coil, at a-units left of center along the Y-axis and c-units off center 
along the X-axis. 
Since the loading force is applied off-axis, a bending moment M about the 
Y-axis is created, as shown in FIG. 3. Recall from the above discussion, 
however, that each transducer 80a, 80b of the strain gage module 80 is 
symmetrically aligned along a neutral axis 40 of the coil. By so doing, 
one half of the active grid elements are located above the axis, and the 
other half of the grid elements are below the axis. A bending moment M, as 
shown in FIG. 3 applies tension force on the elements below the axis and a 
compressive force on the elements above the axis. The resistance changes 
experienced by the grid elements above and below the axis are equal and 
opposite, and thus the total resistance change experienced by the entire 
transducer due to the bending moment M is substantially equal to zero. 
Therefore, the maximum shear equation needs not account for the bending 
moment due to an off-axis load, since the strain gage modules are not 
sensitive to such a force. 
Continuing with FIG. 3 then, the shear force .tau..sub.80 experienced by 
strain gage module 80 is: 
##EQU4## 
where the torsion force T is F.times.a. 
Similarly, the shear force .tau..sub.82 experienced by the diametrically 
opposed strain gage module 82 is: 
##EQU5## 
where the torsion force T is F.times.b. 
If the outputs of the two strain gage modules are summed, then: 
##EQU6## 
Since (a+b) is equal to the mean diameter D of the coil 20, then: 
##EQU7## 
Thus, the total shear .tau..sub.TOTAL measured by the two diametrically 
opposed, outside diameter strain gages 80, 82 is directly proportional to 
the loading force F, where the constant of proportionality k is: 
##EQU8## 
Since the proportionality constant k consists only of the physical 
parameters of the coil, the total shear .tau..sub.TOTAL is completely 
independent of the position of the load. Thus, loading anywhere along the 
top surface of the coil produces the same .tau..sub.TOTAL. Moreover, a 
loading force applied outside the diameter of the coil will result in the 
same .tau..sub.TOTAL irrespective of the location of the load. This can be 
shown by a similar analytical treatment of the applied load and the forces 
resulting therefrom. 
Recall from Eq. (2) that the inside shear is computed by the addition of 
the force terms, resulting in a greater range of measurable shear force. 
Because of the increased dynamic range, the inside diameter strain gage 
modules 84, 86 shown in FIG. 1 are more sensitive to loading forces than 
are the outside diameter mounted modules. However, placement of 
transducers along the inside periphery of the coil is more difficult to 
achieve and in some applications may subject the transducers to risk of 
damage, as will be explained below. Nonetheless, inside diameter modules 
may be applicable under certain conditions and certainly fall within the 
scope of embodiments contemplated for the present invention. 
It is further noted, in connection with the embodiments of the invention 
involving the surface mounted strain gages 90-93 shown in FIG. 1, that the 
transducers thereof are not subject to the direct shear component, namely 
F/A, leaving only the torsional shear to F be measured. A shortcoming of 
this embodiment of the invention, applicable to all four variations, is 
that positioning of the transducers on the upper or lower surface of the 
coil subjects them to damage when the coil is fully compressed to a solid, 
i.e. with no gaps between the wires. 
Referring again to FIG. 3 and to Eqs. 3 and 4, if the difference between 
the measured shear forces .tau..sub.80 and .tau..sub.82 is computed, then: 
##EQU9## 
Thus, the difference .tau..sub.DIFF reflects the distance x from the 
centerline A of the coil 20 to the applied load force. 
As shown in FIG. 3, the distance x measured by the strain gage modules 80, 
82 is the distance between the centerline of the coil and a line which is 
perpendicular to the line between the modules and which passes through the 
applied force F. It can be seen from Eqs. 6 and 8, therefore, that a 
helical load coil can be constructed which is both insensitive to off-axis 
loads and capable of providing information as to the location of such 
loads with respect to the distance from the centerline of the coil. 
Returning to FIG. 1 then, such a configuration is shown with respect to 
strain gages 80-83. As will be explained below, strain gages 80 and 82 
which provide force measurements that are independent of the location of 
the load are coupled in a conventional bridge configuration. Strain gages 
81 and 83 are coupled in a modified bridge circuit to provide a signal 
that is a function of the distance of the loading force from the 
centerline of the coil. 
Turning now to FIGS. 4A-4C, the bridge circuits used to connect the strain 
gage modules will now be described. Although the figures are illustrated 
using the outside diameter-mounted strain gage modules 80, 82, the same 
bridge circuitry is applicable to the second and third embodiments, namely 
the inside diameter-mounted modules 84, 86 and the top/bottom 
surface-mounted modules 90-93, all as shown in FIG. 1. 
Referring to the strain gage modules 80, 82 shown in FIG. 4A, recall that 
the transducers in each of the transducer pairs 60A/62A and 60B/62B, 
respectively comprising the two modules, are orthogonally oriented 
relative to each other. As such, when one transducer, say 60A, is subject 
to a compressive force the other transducer, 62A is subject to an equal 
and opposite tensile force. This is represented in FIGS. 4A and 4B by the 
T and C reference letters. The particular transducers which are in 
tension, however, will depend upon whether the coil is in tension or 
compression and whether the coil has been wound in a right or left hand 
sense. 
FIG. 4A shows a first bridge circuit arrangement wherein the oppositely 
stressed transducers of each of the modules are coupled together. Thus, an 
end 100A and 106A of each of transducers 60A and 62B are coupled together 
and in turn coupled to a voltage supply, and an end 102A and 104A of each 
of transducers 62A and 60B are coupled together and in turn coupled to 
ground. The remaining ends 100B, 102B of transducers 60A, 62A are coupled 
together and serve as a V.sup.+.sub.out output reference. Similarly, two 
ends 104B, 106B of transducers 60B, 62B are coupled together and serve as 
a V.sup.-.sub.out output reference. The circuit diagram 112 of FIG. 4A 
shows that the interconnections form a traditional Wheatstone bridge, 
where the resistive elements represent the transducers 60A-62B. 
FIG. 4B shows a second bridge circuit arrangement. The interconnections 
among the transducers 60A-62B are identical to those shown in FIG. 4A. 
However, the voltage supply and ground connections are interchanged with 
the V.sup.+.sub.out and V.sup.-.sub.out connections. The accompanying 
circuit diagram 212 illustrates the resulting bridge circuit. 
In both of the above bridge circuits 112, 212, similarly stressed 
transducers are coupled as opposing arms of the bridge; thus, the T.sub.1 
and T.sub.2 transducers are on opposite arms of the bridge, as are the 
C.sub.1 and C.sub.2 transducers. The circuits 112, 212 are common bridge 
configurations for effectively summing together the transducer outputs. 
These circuits therefore each produces a signal indicative of the sum of 
the detected shear forces in accordance with Eq. 6. 
The bridge outputs V.sup.+.sub.out, V.sup.-.sub.out are fed into processing 
circuitry 120. In the preferred embodiment, the processing circuitry is a 
differential amplifier wherein its differential inputs receive the bridge 
outputs. The output of the differential amplifier can then be digitized to 
drive a visual display 122 to provide a digital readout. 
Turn now to FIG. 4C for a description of a bridge circuit used in the 
fourth embodiment of the present invention. Recall that this embodiment 
involves the use of four strain gage modules 80-83 arranged on the helical 
coil as illustrated in FIG. 1. Strain gage modules 80, 82 are coupled in 
an additive manner utilizing a conventional bridge circuit, such as those 
shown in FIGS. 4A and 4B. Strain gage modules 81, 83, however, are coupled 
in a modified bridge circuit. 
Such a circuit is shown in FIG. 4C. The similarly stressed transducers 60A, 
60B of the respective modules 81, 83 are coupled together and provide 
V.sup.+.sub.out, as are transducers 62A, 62B which provide 
V.sup.-.sub.out. In addition, terminals 300B, 302B of transducers 60A, 62A 
are coupled to V.sub.supply, while terminals 304B, 306B of transducers 
60B, 62B are coupled to ground. The equivalent circuit is shown by circuit 
diagram 312. 
It can be seen that the similarly stressed transducers of each module 81, 
83 are coupled as adjacent arms of the bridge; thus T.sub.1 and T.sub.2 
form adjacent arms, and C.sub.1 and C.sub.2 form adjacent arms. In this 
circuit arrangement, the transducer outputs are combined in a subtractive 
manner. 
A peculiar aspect of the subtractive bridge circuit 312 is that it does not 
sense the presence of on-axis loads. In the presence of an on-axis load, 
the strain gage modules 81, 83 are subject to the same loading force; thus 
T.sub.1 and T.sub.2 will change by the same amount, and C.sub.1 and 
C.sub.2 will change by the same amount. The differential output 
V.sup.+.sub.out, V.sup.-.sub.out will therefore remain unchanged. Thus, if 
the circuit is null-balanced when there is no load, i.e. the differential 
output is zero volts, the circuit will remain balanced in the presence of 
an on-axis load. This is consistent with Eq. 8 (and FIG. 3) where x=0 for 
an on-axis load. 
Thus, a helical load cell constructed in accordance with the 
above-described fourth embodiment, provides: (1) a signal proportional to 
the loading force F, irrespective of its location, produced by strain gage 
modules 80, 82; and (2) a signal proportional to the loading force times 
the distance of the force from the centerline of the coil, produced by 
strain gage modules 81, 83. Eqs. 6 and 8 show that these two signals are 
readily combined to derive the location x of an off-axis load, while at 
the same time providing an accurate measurement of the load irrespective 
of its location. 
A few additional observations are provided in connection with the 
subtractively coupled strain gages; e.g. 81, 83 shown in FIG. 1. First, 
although FIG. 1 shows the second pair of strain gage modules 81, 83 to be 
mounted in a fashion as to be perpendicular to the first pair of strain 
gage modules, this is not necessary. As already noted above, the second 
pair of modules may be positioned at any angle relative to the first pair 
of modules. 
Second, FIG. 1 shows the second pair of modules 81, 83 to be mounted along 
the outside diameter O.D. of the coil 20. It is noted that the modules may 
be mounted along the inside diameter I.D. of the coil as well. The 
operation of the invention is unaffected whether both pairs of strain 
gages are mounted along the outside diameter, along the inside diameter, 
or whether one pair is mounted on the outside diameter and the other pair 
is mounted on the inside diameter. 
Third, it is observed that another pair of strain gage modules (not shown) 
can be utilized in a subtractive manner to obtain the location of the 
center of gravity of the applied load. Recall that a subtractively coupled 
pair of modules, such as modules 81, 83, provides a force measurement that 
is a function of the distance between the centerline of the coil and a 
line which is perpendicular to the line between the modules and which 
passes through the applied force (see FIG. 3). Thus, if two pairs of 
strain gage modules are mounted to the coil, each pair arranged orthogonal 
to the other pair and each pair coupled as a subtractive bridge circuit, 
then X and Y locations (of a plane defined by the two pairs of strain gage 
modules) of the center of gravity of the applied load can be computed and 
the location of the center of gravity determined. 
Various applications of a helical load cell constructed in accordance with 
the present invention will now be described with reference to FIGS. 5A-5C 
and 6-10. FIG. 5A shows a load bearing platform 302 having a rod 304 
coupled thereto which slidably fits into a tube member 306 anchored to a 
base member 308. The rod and tube portions fit through the center of a 
helical coil 20 which supports the platform 302. In this configuration, 
the strain gage modules 80, 82 are located along the periphery of the 
outside diameter of the coil, since the inside diameter-mounted strain 
gages would likely be crushed between the inside periphery of the coil and 
the rod 304. FIG. 5B shows a configuration which permits the use of inside 
diameter strain gage modules 84, 86. In this configuration, the platform 
302 includes an inner tube member 310 into which the coil 20 fits. The 
inner tube 310 slidably fits into an outer tube member 312 which is 
anchored to the base 308. 
The configurations of FIGS. 5A and 5B are also adaptable to the surface 
mounted embodiments of the helical load cell, where the transducers are 
located on the upper and/or lower surfaces of the coil 20 (see FIG. 1). 
The length(s) of the rod 302 and/or tube members 306, 310, 312 can be 
extended so that the coil cannot reach a fully compressed condition. In 
this way, the surface mounted transducers will avoid being crushed by a 
compressive load which would fully compress the coil. 
FIG. 5C shows the use of linear ball bearings 314 placed in the tube 306 to 
provide lateral support in high side loading applications. The linear ball 
bearings serve to minimize frictional forces between the rod 304 and the 
tube 306. In this case, the vertical force component F.sub.v is measured, 
while the horizontal force component F.sub.h is absorbed by the bearings. 
An application for such a high side-force load is in the construction of a 
hydraulic clamp used for lifting an object whose weight needs to be 
measured. An example would be a garbage container clamp where the 
horizontal clamping force which grips the container can be separated from 
the weight of the container. 
Turn now to FIG. 6 for another use of the helical load cell 10 of the 
invention. In chemical processing situations, corrosive chemicals 
sometimes need to be transported, stored and utilized. This is 
particularly true in the manufacture of semiconductor integrated circuits. 
One chemical utilized in this field is hydrofluoric acid HF used for 
etching semiconductor wafers. HF is highly reactive and is used hot. The 
tanks and tubing used to fill and drain them are usually made of Teflon or 
other nonreactive material, since any reactive substances will corrode, 
fail, and contaminate the acid. Commercially available level sensors exist 
which are used within the tank to monitor the level. However, such sensors 
are quite expensive. The helical load cell of the present invention 
provides a viable alternative to such sensors. 
As can be seen in FIG. 6, a chemical tank 402 having mounting brackets 404 
is supported by helical load cells 10 upon a base 406 and held in place by 
retaining nuts and bolts 408. The weight of the tank and its contents are 
then easily determined by monitoring the output of the load cells 10. 
Since the helical load cells are insensitive to loading conditions, 
existing chemical tanks can easily be re-fitted without expensive 
modifications. 
Refer to FIG. 7 for another application of the present invention. Many 
vehicles have suspension systems which utilize a coiled spring as opposed 
to leaf springs. FIG. 7 shows that such coil springs 502 can be readily 
converted to a helical load cell by placing transducers on them in 
accordance with the present invention. This is especially useful in the 
case of race cars having suspensions consisting of wound helical springs 
at all four wheels. Converting such springs to helical load cells allows 
real-time measurements to be taken by an on-board data acquisition system, 
allowing the race team mechanics to optimize the car's handling 
characteristics. 
FIG. 8 shows yet another application of a helical load cell 10. Stem 
casters 500 are wheels which can be added to any container to make that 
container portable. FIG. 8 shows that a helical load cell 10 can be easily 
added to a caster and the combination 510 placed on a dumpster 530, for 
example. The addition of a digital readout device 540 effectively and 
inexpensively converts that dumpster to a scale. This is especially useful 
for dumpsters which hold recyclable materials. The scale can be used to 
weigh each load placed in the dumpster for the purpose of paying the 
contributor. 
FIG. 9 shows that a helical load cell 10 can operate in tension as well as 
measure compressive loads. The transducers 60, 62 which comprise each of 
the strain gage modules 80, 82 behave in the same manner under tensile 
loads as under compressive loads. 
FIG. 10A shows an application of the helical load cell in connection with 
automobile seats. Considerable attention is being devoted to the 
development of airbag systems for automobiles which are inherently safer 
than current systems. 
A problem that has been identified is that the explosive force of 
deployment required to safely arrest the motion of a large adult may not 
be appropriate for adults of diminutive stature or children. Much 
discussion has centered around the creation of "smart" airbag systems that 
would moderate or inhibit airbag deployment based on the weight of the 
occupant of the vehicle seat protected by the airbag. Currently, no 
suitable technology exists to accurately measure the seat occupant's 
weight at a reasonable cost. 
FIGS. 10A and 10B show a low cost weight sensor for a vehicle seat 
utilizing the helical load cell of the present invention. The system 600 
is compatible with the form factor and adjustment parameters of standard 
auto seats. A helical load cell 10 is placed between a base plate or frame 
604 of the seat and a seat plate 602 which holds a foam pad 606. The 
helical load cell 10 replaces a sheet metal framework which is used in 
current seat systems to couple the plate 602 to the base 604. The base is 
free to move on rollers 608 to permit adjustment of the seat. The load 
cell 10 is oriented so that the axis of the coil portion 20 is vertical, 
thus measuring only vertical forces and not horizontal forces. The entire 
assembly is then covered in fabric. 
Since the helical load cell is capable of measuring the total downward 
force present on the seat plate, regardless of the manner or load 
distribution of that weight, the occupant size and seating orientation has 
no affect on the measured weight. Deflection of the seat under load, and 
the compliance of the seat to forces away from the center of the spring 
may be adjusted by the thickness of the wire, the diameter of the coil, 
and the total number of turns in the load cell. For example, a coil made 
from 5/8 inch diameter wire wound to a diameter of two inches results in a 
load cell which deflects about 0.05 inches when impressed with a 200 pound 
force. With two active turns of the wire, this coil also appears to have 
sufficient stiffness so as not to have excessive compliance when the seat 
is loaded off-center, such as when the occupant is sitting on the front 
edge of the seat. 
Another embodiment of the automobile seat incorporates a helical load cell 
20 having an arrangement of four strain gage modules, such as the 
arrangement of modules 80-83 shown in FIG. 1. The top view of FIG. 10B 
shows the orientation of the strain gage modules. The modules 80, 82 are 
arranged laterally, facing the side doors of the automobile. These strain 
gage modules are coupled in an additive type of bridge circuit, such as 
those shown in FIGS. 4A and 4B. These modules provide a measure of the 
weight of the seat occupant, independently of how the occupant is seated. 
The modules 81, 83 are arranged front to rear, and are coupled in a 
subtractive type of bridge, such as shown in FIG. 4C. These modules 
provide a signal that is combined with the signal provided by modules 80, 
82 to give a measure of how far forward or rearward the occupant is seated 
relative to the central axis A of the helical load cell 20. 
This is an especially useful aspect of the helical load cell as used in an 
automobile seat. Airbag safety systems can be greatly enhanced to provide 
additional safety in connection with children's car seats. An automobile 
seat enhanced with a helical load cell in accordance with the fourth 
embodiment of the invention can detect whether the child's car seat is 
facing forward or rearward. At the same time, an accurate measurement of 
the weight of the child and the child's car seat is provided, even though 
the car seat and child are not centered over the central axis of the 
helical load coil; i.e. the child and the seat represent an off-axis load 
to the load coil.