Weighing cell

A weighing cell which is protected against overloading due to mechanical shock and delayed weight readout due to vibration. The weighing cell, which has a platform, a self-contained base and a load cell operatively connected therebetween, includes, in one embodiment, a plurality of posts connected to the base and separated from the platform by a gap to protect against overload forces applied to the non-axial region of the platform. Feet on the base of the weighing cell have an energy absorbing material thereon and are placed outboard of the load cell to provide quick damping of vibrations caused when a load is placed on the platform. The load cell is spaced at a partial gap from the base and platform to protect it from overload forces applied to the axial region of the platform.

BACKGROUND OF THE DISCLOSURE 
This invention relates generally to an apparatus for weighing objects, and 
more particularly, to a weighing cell having protection from overload 
forces applied to the scale's platform and rapid vibrational damping to 
reduce the time that an accurate readout is produced after a load is 
placed thereon. 
In weighing cells, such as the type commonly used to weigh articles for 
determining postage, it is desirable to provide protection against the 
cell being overloaded, particularly by shock loads. Such overloading tends 
to force the cell out of calibration. U.S. Pat. No. 4,107,985 discloses 
one approach to this problem. A parallelogram structure is utilized to 
make the load cell insensitive to, and unaffected by, off-center loading. 
In this type of structure, the bending moment produced by non-axial 
loading is transmitted to restraining arms in the load cell rather than 
through the force measuring element, of sensing beam. Axial over-loading, 
on the other hand, is prevented by the mechanical interfacing of the load 
cell and its base which is separated at a partial gap therefrom. 
More recently, load cells of high accuracy and low cost have been made 
available for application to weighing cells. These devices, called 
single-point load cells, generally have safe overload margins in excess of 
their rated capacities and give accurate weighing of objects regardless of 
where the object is placed on the platform. With the commercial 
introduction of these newer load cell designs, it is possible to replace 
the complicated parallelogram structures formerly used in weighing cells 
with more economical structures which concentrate on shock overload 
protection and the lessening of vibrational damping time. 
U.S. Pat. No. 4,181,001 discloses a representative single point load cell. 
The device disclosed in this patent is primarily directed to eliminating 
the effects of internal axial forces, such as those produced by machining, 
fabrication stresses or temperature gradients, on the sensing beam. It 
seeks to provide better accuracy and linearity, as well as off-center load 
capabilities. This structure isolated the sensing beam from all extraneous 
forces through the configuration of the load cell itself. Only the axial 
forces placed on the load cell find their way to the sensing beam. All 
other forces are funneled away form the sensing beam and to another member 
within the parallelogram structure. 
Accordingly, it is a primary object of the present invention to improve 
weighing cell by providing shock overload protection. 
It is another object of the present invention to lessen the time to dampen 
vibrations produced when an object is placed on the platform of the 
weighing cell. 
It is another object of the present invention to improve axial overload 
protection in weighing cells. 
It is another object of the present invention to improve non-axial overload 
protection in weighing cells. 
SUMMARY OF THE INVENTION 
Briefly stated, and in accordance with the present invention, protection is 
provided to a weighing cell against overloads, particularly those 
resulting from mechanical shocks. In addition, the damping time, after 
which the scale can provide an accurate reading, is shortened. In 
accordance with the present invention, an improved weighing cell is 
protected from overloading, by limiting the force that can be placed on 
the load cell. Overloads applied to the axial region of the scale's 
platform are prevented from acting on the load cell by limiting the amount 
of movement between the load cell and the weighing cell base and/or 
platform. Overloads applied to the non-axial region of the scale's 
platform are prevented from acting on the load cell by limiting: the 
amount of movement between the platform and base, independently of the 
load cell support structure. The time it takes to dampen the vibrations 
created in the weighing cell after a load is placed thereon is shortened 
by damping material located on the base.

While the present invention is described in connection with a preferred 
embodiment and associate method of use thereof, it is to be understood 
that it is not intended to limit the invention to this embodiment and 
method of use. On the contrary, it is intended to cover all alternatives, 
modifications and equivalents as may be included within the spirit and 
scope of the invention as defined by the appended claims. 
DETAILED DESCRIPTION OF THE INVENTION 
Referring more particularly to the drawings, wherein like reference 
numerals have been used throughout to designate like elements, FIGS. 1 and 
2 illustrate schematically one embodiment of the weighing cell apparatus. 
The weighing cell is used in a typical weighing scale environment, such as 
to weigh articles to be mailed. The scale includes a weighing cell which 
functions to receive the articles to be weighed and produce a signal 
corresponding to the weight, a display, such as a sevensegment display, to 
tell the operator the weight of the article and suitable electronics to 
transform the signal from the weighing cell to an operator readable 
display. 
The weighing cell portion of the scale includes a platform to receive and 
hold the articles to be weighed, a base member supportable to mechanical 
ground and a load cell adapted to convert the force of the load placed on 
the platform into a usable output such as an electrical signal. The term 
"mechanical ground" means any suitable surface or support means such as a 
table, which will hold the weighing scale while in use. It should be 
sufficently stable so as not to materially affect the accuracy of the 
weighing process. Mechanical ground is also used to refer to any position 
of the scale which has no relative motion to the table. 
In weighing cells, such as the one described herein, the weight is 
ultimately determined by a load cell which creates a difference of voltage 
corresponding to the weight of the object on the platform. Although any 
suitable load cell can be used, load cells typically contain a strain gage 
in the form of a fine wire arranged in a pattern and cemented to a piece 
of metal that will be subjected to physical strain. The fine wire will 
have some small amount of resistance when the metal to which it is 
cemented is not under strain. When the metal is distorted, the attached 
wire, due to its elasticity properties, will be stretched. This, in turn, 
reduces the wire diameter and increases its length so that the resistance 
within the wire is changed. The differences in resistance are converted to 
changes in voltage through the use of a basic bridge circuit. The wire, 
itself, rather than the piece of metal, is the strain gage. This 
configuration is commonly used in modern load cells of high accuracy. 
The load cell produces a difference in voltage due to the basic bridge 
circuit employed and this voltage is applied through signal conditioning 
electronics to any suitable monitoring device which forms a display to the 
operator of the weight of the article. Present day scales frequently 
employ microprocessors which, in turn, drive digital display devices for 
quick and accurate display of the weight. 
FIGS. 1 and 2 are directed to the weighing cell, per se. The weighing cell 
2, shown in both figures, contains a load cell sub-assembly 1, a weighing 
platform 3 and base member 4. Base member 4 contains any suitable support 
members 5, such as the adjustable feet 6 shown in the figures. The purpose 
of the feet is to fix the weighing cell to mechanical ground, such as the 
top surface of a desk or table, while the scale is being used to weigh 
articles. 
The four feet, 6, shown in this embodiment are located outboard of the load 
cell 1 and supporting structure thereof. The feet can be placed at any 
convenient location on the base to render support for it. It is 
preferable, however, to locate the feet outboard of the load cell and 
supporting structure, as in the figures. The load cell and supporting 
structure are located entirely within the area between feet 6. The 
supporting structure should be a rigid member between the load cell and 
the feet. 
The feet are made of or covered with a suitable energy absorbing material. 
This material serves to dampen oscillations of various types in the scale 
system, the most critical being the equivalent mass spring system 
consisting of the load cell as a torsion spring and the combined weight of 
the weighing scale and object weight. Any suitable energy absorbing 
material can be used for this purpose. One type of material found to be 
useful is "E-A-R Isodamp C-1002", an energy absorbing meterial 
manufactured by the E-A-R Corporation. "E-A-R Isodamp" is a trademark of 
E-A-R Corporation of Indianapolis, Ind. Feet 6 are also adjustable so that 
the weighing cell can be placed in a secure and level position relative to 
the surface upon which it rests. 
The base member 4 has attached to it spacers 7 which, in turn, have lower 
adapter plate 8 and upper plate 9 physically attached thereto by bolt and 
nut assemblies 10. Load cell 1 is placed adjacent upper adapter plate 9 
and separated therefrom by lower shim 11. bolt and nut assemblies 12 
rigidly connect the load cell, shim and upper adapter plate 9. On top of 
load cell 1 is a platform 3 suitable for supporting objects to be weighed. 
The objects are placed on the top surface of the platform by the operator 
for this purpose. Weighing platform 3 has fixedly connected to it upper 
mating plate 13 which, in turn, is secured to upper shim 14 and load cell 
1 by screw and nut assmeblies 15. Load cell 1 is thereby separated from 
upper mating plate 13 and upper plate 9 by a partial gap. The gap is 
produced by the placement of shims 14 and 11, respectively, and is 
"partial" because the gap extends only in those areas outside of the 
shims. 
Through this structure it can be seen that all of the components, including 
load cell 1, described above and located between platform 3 and feet 6 are 
securely attached. These components make up the portion of the weighing 
cell which enables an electrical signal to be produced by the load cell 
corresponding to the weight of an object placed on the platform. When an 
object is placed on platform 3, the force of its weight acts on platform 3 
and against upper mating plate 13 which, in turn, acts on shim 14 and onto 
load cell 1. Load cell 1 is, in turn, supported by feet 6 connected 
through base 4, spacers 7, adapter plates 8 and 9 and shim 11. When a 
force is placed on platform 3, the force is transmitted down to the load 
cell while shim 11, through its support elements including feet 6, is 
supported at mechanical ground. This arrangement causes load cell 1 to 
deflect and produce a signal corresponding to the weight on the scale. 
Load cell 1 can be any suitable type presently available. An example of 
such a load cell presently available is the Platform Sensor sold by Revere 
Corporation of American of Wallingford, Connecticut. In this instance, the 
load cell is a single-point-type cell which produces an accurate signal 
corresponding to the weight on the platform 3 regardless of where the 
weight is placed on the platform. Suitable electrical leads (not shown) on 
load cell 1 are fed out of the weighing cell to signal conditioning 
electronics. The signal conditioning electronics typically consists of a 
preamplifier to increase the weighing cell's small output changes and a 
low-pass filter to stabilize these outputs in the presence of electrical 
noise and ground vibration. 
Platform 3, upper mating plate 13, adapters 8 and 9, support 7, and base 
member 4 can be made of any suitable material such as steel, which has the 
strength and stiffness to support the weights expected on the scale. 
However, alternate materials such as aluminum, as well as others, can be 
used for this purpose. Shims 11 and 14 can be made of any suitable 
material able to withstand the pressure placed on the upper and lower 
portions of the load cell 1 during the weighing process. One suitable 
material for the shims is brass. The thickness of the shims is selected to 
maintain a pre-determined partial gap between the adjacent surfaces. The 
thickness must be greater than the distance the platform moves when 
weighing an object at the scale's rated capacity, but small enough to 
enable the gap to close before the load cell maximum overload capability 
is exceeded. 
The purpose of the shims is to provide a mechanical interface between upper 
mating plate 13 and load cell 1 and upper adapter plate 9 and load cell 1 
when an overload force is applied to the axial region of the platform. The 
"axial region of the platform" means that region directly over the load 
cell. For instance, assuming one were to view the platform from directly 
above and trace on the platform itself the outline of the load cell lying 
thereunder, the tracing would substantially define the "axial" region of 
the platform. "Overload" means a force applied to the platform which, if 
the invention were not used, would cause the scale to exceed its intended 
operational capacity. An example of this would be when a heavy object to 
be weighed is inadvertantly dropped by the operator onto the platform from 
a point directly above the platform. 
Assuming that the weighing cell is designed to accommodate objects up to 70 
pounds, there may be those situations in which a much greater weight is 
placed on the weighing cell and load cell. This greater weight may be 
applied to the platform by the force of an object having much greater 
weight, or by the force of an object dropped onto the platform from a 
distance above it. This latter condition, overload produced by shock, will 
produce a peak force much greater on the weighing cell than the same 
object placed directly on the platform. 
The load cell, although it may have a safe overload margin, must be 
protected against overload forces applied to the axial region of the 
platform at some pre-determined force value in excess of that produced by 
70 pounds. Assuming, hypothetically, that the load cell is designed to 
have a safe overload margin up to 100 pounds before its calibration is 
permanently disturbed, and thus placed out of calibration, the thickness 
of shims 11 and 14 are made to allow upper mating plate 13 and upper 
adapter 9 to make physical contact with the load cell surfaces when a 
force equivalent to an object of between 70 and 100 pounds is placed on 
the platform. 
The partial gap produced by shims 11 and 14 is sufficient to allow 
mechanical interfacing between the mating plate and upper adapter plates 
with the load cell at some predetermined point of force equivalent to that 
produced by an object between 70 and 100 pounds. Shim 11, and the partial 
gap produced thereby, is intended to provide a mechanical interface and 
overload protection primarily for forces applied to the right side of the 
platform in FIG. 1. Shim 14, and the partial gap produced thereby, is 
intended to perform the same function primarily for forces applied to the 
left side of the platform in FIG. 1. An equally effective method of 
achieveing a suitable protective gap size is with adjusting screws. The 
shims can be replaced with spacer blocks large enough to accommodate the 
largest gap desired. The adjustable screw can be placed on the load cell 
or member on the other side of the gap and adjusted in height according to 
operating conditions. If the screw were located on the load cell, the size 
of the gap would be measured between the screw head and upper adapter 
plate. 
A further overload protection device exists in the weighing cell in the 
embodiment of posts 16 which are secured to base 4. The height of the 
posts are adjustable by the interaction of threaded portions 17 and 
locking nuts 18. Any suitable number of posts can be provided in a 
particular scale. In a rectangular shaped platform, four posts are 
preferrable. The posts act in a manner on the weighing cell that is 
completely independent of the load cell itself. 
In the apparatus shown in FIGS. 1 and 2, there are 4 posts, one near each 
corner of the weighing platform. The posts are designed, after adjustment, 
to be fixed relative to base 4. There is a gap "A" between the underside 
of platform 3 and the top of the posts. The top of the posts act as 
bearing surfaces upon which bosses 19 on the underside of the platform can 
rest in an overload condition. 
The purpose of the posts is to protect the load cell from overload forces 
and particularly shock over-loading, applied to the non-axial regions of 
the platform. Under normal operating conditions; that is, when the force 
of the object being weighed is within the range of the scale, the platform 
does not close gap "A" between the underside of the platform and the top 
of the posts. However, when an overload force is applied to the non-axial 
region of the platform and the rated capacity of the scale is exceeded, 
the underside of the platform bottoms out on one or more of the posts 
thereby protecting the load cell from the overload. The term "non-axial 
region of the platform" means the region of the platform outside the 
"axial region" defined previously. Overload forces applied to the 
non-axial region of the platform, particularly those applied near the edge 
of the platform that is paralled to the long dimension of the load cell, 
can be very damaging to the load cell. An example is when a heavy object 
to be weighed is inadvertantly dropped or thrown near the side of the 
platform. Shims 11 and 14, and the associated partial gaps, do not 
necessarily protect the load cell from such non-axial overloading because 
the force is applied to the load cell in such a way that the load cell 
tends to twist. Damage can be done to the load cell by such twisting 
before the partial gap closes. 
The size of gap "A" is determined as a function of how much overloading 
will be tolerated before the platform bottoms out and is a function of the 
capacity of the scale. The gap is preferably about twice the distance the 
platform moves when weighing an object at the scale's rated capacity. The 
posts 16, through thread portions 17 and lock nuts 18, can be adjusted to 
any suitable gap intended for "A". The underside of platform 3 is shown as 
having four bosses 19 which come into contact with the top of the post in 
the overload condition. The posts can be made of any suitable material 
which will take the stress placed on the scale by overload conditions. The 
posts act on the platform, in the overload condition, without reliance on 
the load cell and are attached directly to base 4. 
The particular scale shown in FIGS. 1 and 2 has a capacity of 70 pounds and 
the load cell has an accurate operating range to 100 pounds. A typical 
safe overload limit for a 100 pound load cell is in the 150 to 200 pounds 
range. The weighing cell is designed to have an overload condition occur 
when a force equivalent to approximately 120 pounds is placed on it. In 
this example, the following specifications can be applied to the weighing 
cell. The shim thickness for shims 11 and 14 should be in the range of 
0.015 to 0.020 inches. Gap "A" between the posts 16 and bosses 19 should 
be in the range of 0.060 to 0.100 inches. Shim thicknesses are a function 
of the full scale displacement of the load cell and will vary for load 
cells made by different manufacturers. Gap "A" is a function of the 
torsional rigidity of the load cell about an axis through its long 
dimension. This will also vary for load cells made by different 
manufacturers. 
This scale, although rated at 70 pounds, will be able to safely withstand a 
force equivalent to a 120 pound weight on the platform without 
deleteriously affect the load cell. When a force equivalent to a weight 
greater than 120 pounds is placed on the platform, the load cell will be 
protected from overload forces applied at both axial and non-axial regions 
of the platform by shims 11 and 14 and by gap "A" and posts 16, 
respectively, depending upon how and where the overload is applied. If the 
overload force is applied substantially over the load cell; i.e., in the 
axial region, it is likely that the partial gaps produced by shims 11 and 
14 will be closed thereby making a mechanical interface between the lower 
surface of upper mating plate 13 and the top of load cell 1 and/or the 
upper surface of adapter plate 9 and the bottom side of the load cell. 
However, if the overload force is applied substantially outside the load 
cell region; i.e., in the non-axial region and this force is equivalent to 
a weight greater than 120 pounds, gap "A" will be closed thereby providing 
a mechanical interface between the platform and the base independently of 
the load cell. It is pointed out that either the partial gaps produced by 
the shims or gap "A" is free to function in the case of a particular 
overload placed on the platform and these may work in combination to 
protect the weighing cell from overload. The sizes of the partial gaps 
produced by the shims and gap "A" are to be chosen so that normal, 
non-overload weighing on the weighing cell can be accomplished without the 
mechanical interfacing described above. 
The use of energy absorbing feet 6 minimize the delay in weighing time 
otherwise observed when an object is placed on the platform. The placement 
of a weight on the side of the platform, in particular, produces a long 
duration oscillation of the weighing cell structure. The energy absorbing 
feet serve to quickly dampen these oscillations and substantially shorten 
the time to reduce the vibrations to the point of making an accurate 
measurement of weight. The special feet are placed outboard of the 
weighing cell in a position where their energy absorbing properties serve 
to dampen the oscillations most effectively. Without the energy absorbing 
feet, typical delays observed are 3 to 12 seconds for heavier objects. 
With their use, suitable damping occurs in about one second. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications in the structural 
and functional features of the weighing cell can be devised by those 
skilled in the art without departing from the invention. Accordingly, the 
present invention is intended to embrace all such alternatives, 
modifications and variations and fall within the spirit and scope of the 
appended claims.