Flexure weighing system

A flexure weighing system comprising a split-beam assembly consisting of two beams centrally connected by a flexure linkage system. The split-beam assembly is also connected to a frame and to mounting studs which connect the outer, load-bearing ends of the beams to a weighing platform. One of the beams is connected to a weight sensor which produces an electrical signal indicative of the movement of the beam to which it is connected and therefore of the weight on the weighing platform which causes that movement. Preferably, flexures are used at all the key pivot points in the system.

FIELD OF THE INVENTION 
This invention generally relates to a split-beam weighing system having 
virtually no friction and very high accuracy and particularly relates to 
such a system utilizing a plurality of flexures at pivot points within the 
system. 
A variety of weighing systems have been developed which utilize such means 
as bearings, linkage pins, knife edges and the like to reduce friction at 
key pivot points in those systems. These systems have not been entirely 
satisfactory due to their lack of sensitivity or their inability to 
function maintenance-free over long periods of continuous usage. A 
flexibly mounted container system including flexural pivot means to 
overcome these deficiencies in the prior art is described in my U.S. Pat. 
No. 4,042,051. The system disclosed in that patent is substantially 
frictionless. Moreover, it has high sensitivity, linearity, accuracy and 
repeatability over a wide range of loads including very heavy loads. 
SUMMARY OF THE INVENTION 
The present invention provides a compact weighing system utilizing flexural 
pivot means or flexures of the type disclosed in U.S. Pat. No. 4,042,051 
and adapted to be used in conjunction with a continuous feed mechanism or 
some other type of metering mechanism. This system achieves high 
sensitivity, linearity of operation, accuracy of weight measurement, 
repeatability and negligible hysteresis. The system is highly resistant to 
corrosion as well as to damage from operator misuse. The invention is 
particularly useful as an integral part of a continuous or batch weigh 
feeder system which operates to accurately measure and feed particulate 
solid ingredients. In many applications, it is essential to operate such 
systems on a continuous or nearly continuous basis and consequently a 
weighing system is needed which requires a minimum amount of adjustment 
and recalibration. The system of the invention provides excellent 
resolution and is virtually maintenance-free as a result of its novel 
design. 
In one embodiment of the invention, the flexure weighing system includes a 
split-beam balance assembly having two halves. Each beam half has a 
load-bearing end and an interconnecting end and consists of a rectangular 
portion and an outwardly extending member rigidly connected thereto. The 
two interconnecting ends are connected together by a flexure linkage 
assembly which connects the outwardly extending members at the center of 
the apparatus. Each flexure comprises two spaced flanges or flange-like 
elements interconnected by a plurality of ribbon-like resilient 
cross-members, so that if the first flange is fixed while torque is 
applied to the second, the second will pivot or rotate with respect to the 
first. One outwardly extending member of a beam half is also connected to 
a weight sensor which translates the vertical movement of the portion of 
the beam half connected to the weight sensor into an electrical signal 
indicative of the weight placed upon the flexure weighing system. One of 
the outwardly extending members may also be connected to a dashpot which 
damps the physical motion of the balance assembly due to outside 
influences such as plant vibration and the like. Additional flexures 
located in the two corners of the rectangular portion of each beam 
furthest from the linkage assembly connect the load-bearing end of each 
beam to two mounting studs which support a weighing platform. The 
rectangular portion of each beam is pivotally connected to the frame of 
the weighing system by two more flexures. 
The weighing operation occurs as follows. When an item to be weighed is 
placed upon the weighing platform, downward force is transmitted through 
the mounting studs to the load-bearing end of each half of the split-beam 
balance assembly. The flexures connecting the mounting studs to each half 
of the split-beam balance assembly "pivot" slightly as the load-bearing 
end of each half moves downward with the result that the downward force on 
each load-bearing end is always applied vertically. This downward force on 
the load-bearing end of each beam half also causes the flexures connecting 
each half to the frame and the two flexures joining the halves at the 
center of the device to "pivot." While the load-bearing end of each half 
moves downwards, the interconnecting end of each half including each 
outwardly extending member rises. This rise is monitored by the weight 
sensor and translated into an electrical signal indicative of the weight 
of the item placed upon the weighing platform. The precise meaning of the 
term "pivot" as used in relation to a flexure will be explained more fully 
below. 
The above described system achieves substantially frictionless weighing due 
to its use of flexures at all of the pivot points. This flexure 
arrangement, further details of the invention and its incorporation into a 
weigh feeder system will be discussed in conjunction with the drawings in 
the detailed description which follows.

DETAILED DESCRIPTION 
FIG. 1 shows a weigh feeding system 1 including an auger feed mechanism 10 
and the external details of a flexure weighing system 20. Feed mechanism 
10 includes a hopper 11, a feed chamber 12 connected to the hopper 11, and 
discharge cylinder 14 connected to the feed chamber 12 and containing 
within it a feed auger 13 which is driven by motor 15. Material to be 
metered in a controlled fashion by feed mechanism 10 is placed in hopper 
11. Under the influence of gravity or gravity assisted by mechanical 
agitation, the material flows from the hopper 11 into the feed chamber 12. 
When motor 15 is operating, auger 13 rotates so that material is pushed 
from feed chamber 12 through and then out the end of the discharge 
cylinder 14. 
The flexure weighing system 20 includes a main frame 21, a weighing 
platform 22, removable side panels 23 of which the view shown in FIG. 1 
only shows one and four mounting studs 24 of which the view shown in FIG. 
1 only shows two. The four mounting studs 24 transmit the weight of 
platform 22 and feed mechanism 10 including its contents to the remainder 
of the flexure weighing system 20 which is located behind side panel 23. 
The internal details of the flexure weighing system 20 are shown in FIGS. 
2-9 and will be discussed below. 
In actual operation, a weigh feeding system 1 provides controlled metering 
as follows. The hopper 11 and feed chamber 12 are filled with a material 
to be metered. Motor 15 is activated manually or by an automatic 
controller system and the feed mechanism 10 begins to feed. As the 
material is metered out the end of discharge cylinder 14, the total weight 
of the contents of feed mechanism 10 gradually decreases. Flexure weighing 
system 20 monitors the total weight placed upon weighing platform 22 and 
produces an electrical signal indicative of that weight. 
It is often desirable continuously to provide a constant rate of feed of 
material out the discharge cylinder 14. To achieve that end, the 
electrical signal indicative of the weight upon weighing platform 22, or a 
signal derived therefrom, may be used to modulate the speed of motor 15 
and thus to control the material feed rate. For continuous feeding at a 
constant rate, the system must permit refilling hopper 11 and feed chamber 
12 before they become empty. When hopper 11 is being filled, the total 
weight on weighing platform 22 of flexure weighing system 20 is 
increasing, yet the system must not permit this increasing weight to upset 
the feed rate. Techniques for accomplishing these objectives are disclosed 
in U.S. Pat. Nos. 3,889,848 and 3,967,758. 
FIG. 2 shows a plan view of flexure weighing system 20 taken below the 
weighing platform 22 and the upper portion of the main frame 21, along the 
section line A--A of FIG. 1. In the preferred embodiment shown, flexure 
weighing system 20 includes a split-beam balance assembly 30 having an 
upper beam 31 and a lower beam 32. Beams 31 and 32 are centrally joined by 
a flexure linkage assembly 40 which connects the interconnecting ends of 
beams 31 and 32 and includes upper and lower flexures 41 and 42. Upper 
beam 31 is connected to the main frame 21 by two flexures 25U with one 
flexure 25U connecting beam 31 to each side of frame 21. Similarly, lower 
beam 32 is connected to the main frame 21 by two flexures 25L. These 
flexure connections allow beams 31 and 32 to pivot slightly with respect 
to the main frame when weight is placed upon weighing platform 22. 
Downward force is transmitted from the weighing platform 22 through the 
mounting studs 24 to the outer portions or in other words the load-bearing 
ends of beams 31 and 32. The two left-hand studs 24 are connected to beam 
31 by two flexures 26U. Similarly, the two right-hand studs are connected 
to beam 32 by two flexures 26L. These flexure connections insure that 
force is transmitted to beams 31 and 32 in a vertical direction even 
though the load-bearing ends of the beams 31 and 32 move downwards 
slightly when weight is placed upon weighing platform 22. FIG. 2 also 
shows distances d.sub.1 and d.sub.2 where d.sub.1 is the distance between 
the center of weight sensor 70 and the axis of rotation of flexures 25U 
and where d.sub.2 is the distance between the axis of rotation of flexures 
26U and the axis of rotation of flexures 25U. 
Additional details regarding flexure linkage assembly 40 are found below in 
the discussions of FIGS. 3, 4 and 5. 
FIG. 3 shows in perspective the upper beam 31 and the lower beam 32 without 
bracing members 31A and 32A shown in FIG. 2 and further shows the flexure 
linkage assembly 40. FIG. 3 also shows boxes numbered 25U, 25L, 26U and 
26L which represent schematically the similarly numbered flexures of FIG. 
2. 
Upper beam 31 consists of a rectangular portion 311 which is rigidly 
connected to an outwardly extending member 312. Similarly lower beam 32 
consists of a rectangular portion 321 which is rigidly connected to an 
outwardly extending member 322. The end of member 312 is disposed above 
the end of member 322. These two ends are connected together by flexure 
linkage assembly 40. As is shown in FIG. 4, upper beam 31 is connected 
through member 312 to flexure 41 which is in turn connected to the upper 
portion of mounting plate 43. The lower portion of mounting plate 43 is 
connected to a second flexure 42. Flexure 42 is also connected to member 
322 of lower beam 32 so that a flexible linkage assembly linking beams 31 
and 32 is formed. 
Returning to FIG. 2, upper beam 31 is also connected to a dashpot 50 which 
damps out oscillations of the system caused by outside factors such as 
vibrations from nearby plant machinery. Beam 31 is also connected to a 
weight sensor 70. Vertical movement of beam 31 is monitored by the weight 
sensor 70 and converted into an electrical signal indicative of the 
movement of beam 31 and consequently indicative of the weight upon 
weighing platform 22. This signal may be used to control the speed of 
motor 15. 
FIG. 2 also shows calibration springs 27 and 28 which are connected, in 
compression, between the base of main frame 22 and the outer ends of beams 
31 and 32, respectively. By suitably selecting the spring constant of 
springs 27 and 28, flexure weighing system 20 can be adjusted for the 
weighing of many different ranges of loads so that, for each range of 
loads, the maximum displacement of split-beam assembly 30 corresponds to 
the maximum weight of the range. It should be recognized that the flexures 
in the system have a certain resistance to movement which exists 
independently of the use of calibration springs. For certain applications, 
this resistance may be sufficient so that calibration springs need not be 
used. This is particularly true for small capacity weighing systems. 
Additional counterbalance weights, not shown, may be used to offset or to 
tare all or the bulk of the weight of any feeder mechanism, such as the 
auger feed mechanism 10 of FIG. 1, used in conjunction with the flexure 
weighing system 20. 
The final feature of the flexure weighing system 20 shown in FIG. 2 is the 
electrical connection of weight sensor 70 to circuit means contained 
within electrical housing 90. Housing 90 is sealed so that the circuitry 
contained therein is isolated from the environment of system 20. Housing 
90 is electrically connected to weight sensor 70 by wires contained in and 
protected by a sturdy sealed electrical conduit 91. 
This isolation of the electrical system from the environment of the flexure 
weighing system 20 coupled with the toughness of the flexures, provides 
for a particularly corrosion and degradation-free system. Flexures 25U, 
25L, 26U, 26L, 41 and 42 are typically made of stainless steel. So long as 
they are not flexed beyond their elastic limit and do not become 
permanently twisted, the flexures are unaffected by the weighing process. 
Stops which prevent beams 31 and 32 from moving far enough to flex the 
flexures beyond their elastic limits, can be incorporated to protect the 
system against the effects of an inadvertantly applied excess load. 
The weighing operation of flexure weighing system 20 may be briefly 
summarized as follows. Weight is placed on weighing platform 22. Downward 
force is transmitted through mounting studs 24 and flexures 26U and 26L to 
the load-bearing ends of beams 31 and 32. As a result of this downward 
force upon the ends of beams 31 and 32, flexures 25U rotate slightly 
counter-clockwise and flexures 25L rotate slightly clockwise as 
illustrated by arrows in FIG. 5 located near those flexures. The other 
arrows shown in FIG. 5 indicate the directions of rotations of the 
flexures they are associated with. The load-bearing ends of beams 31 and 
32 therefore lower slightly. As this occurs, flexures 26U and 26L also 
rotate slightly. Also, the linkage flexures 41 and 42 rotate slightly as 
the interconnected ends of beams 31 and 32 rise. The amount of rise 
(d.sub.rise), the quantity measured by weight sensor 70 and translated by 
it into an electrical signal indicative of the weight, is mathematically 
related to the amount by which the load-bearing end of the beam 31 lowers 
(d.sub.lowered) as follows: d.sub.rise =(d.sub.1 
/d.sub.2).times.d.sub.lowered where d.sub.1 and d.sub.2, it will be 
recalled, are respectively the distance between the center of weight 
sensor 70 and the axis of rotation of flexures 25U and the distance 
between the axes of rotation of flexures 26U and 25U. Weight sensor 70 
produces an electrical signal which is indicative of the weight on the 
weighing platform 22. 
Further details of flexure linkage assembly 40 are discussed in the context 
of FIG. 5 which shows a side elevation view along section line B--B of 
FIG. 2. As discussed above, upper beam 31 is connected to flexure 41. 
Lower beam 32 is connected to flexure 42. Both flexures 41 and 42 are 
connected to mounting plate 43. In the preferred embodiment, these 
connections are made using nuts and bolts; however, other methods of 
connection may be used. Where nuts and bolts are used to join two surfaces 
either one or both of which is not machined flat, shims are used so that 
the flexures do not become warped when the bolts are fully tightened. The 
mounting plate 43 preferably has four horizontal slots 44 and four 
vertical slots 45 (only three vertical slots are visible in FIG. 5) cut 
through it. These slots 44 and 45 are used for adjusting flexure weighing 
system 20 when it is first installed. These adjustments in most cases will 
only need to be made once. 
The slots 44 are used to adjust the horizontal distance d.sub.4 from the 
axial center of flexure 42 to the axial center of the two flexures 25L so 
that it is substantially equal to the horizontal distance d.sub.3 from the 
axial center of flexure 41 to the axial center of the two flexures 25U. 
When d.sub.4 is equal to d.sub.3, the flexure weighing system 20 will be 
insensitive to whether weight is placed to the left or right or the front 
or back of weighing platform 22. The slots 45 are used to level the 
split-beam assembly 30. 
FIGS. 6 and 7 illustrate what is meant when it is said that the flexures 
25U, 25L, 26U, 26L, 41 and 42 rotate, flex or pivot. FIG. 6 shows a single 
flexure 100 in the no-load position. This flexure 100 consists of two 
flanges or flange-like elements 101 and 102 and four interconnecting 
cross-members 103, 104, 105 and 106. The cross-members 103-106 preferably 
comprise elongated ribbon-like members which are all substantially 
identical. The connection between each cross-member and the flanges 101 
and 102 is made by welding. Cross-members 103-106 are preferably 
fabricated from stainless steel selected from the group consisting of AISI 
types 410, 420 and 440, tempered to a hardness of between about 42 to 
about 55 Rockwell C. Preferably, the flanges 101 and 102 are fabricated 
from AISI type 304 stainless steel. Two of the cross-members, for example 
103 and 105, are disposed in a first plane and the other two 104 and 106 
are disposed in a second plane preferably perpendicular to the first. The 
four cross-members 103-106 are symmetrically disposed about a pivot axis 
C--C extending perpendicular to the flanges 101 and 102 and passing 
through their centers. FIG. 6 also shows holes such as hole 107 for 
mounting flexure 100. It will be appreciated that when one flange 101 is 
rigidly mounted to a frame and torque is applied to the other flange the 
second flange rotates with respect to the first. This rotation is 
illustrated by Fig. 7. 
FIG. 7 shows an end view of flexure 100 looking along the axis of rotation 
C--C when flange 102 has rotated from its initial orientation through an 
angle 109 due to application of torque. The cross-members 103-106 are 
twisted substantially uniformly throughout their length so that the 
orientation of their ends adjacent flange 102 are indicated by solid lines 
103-106 and at flange 101 by broken lines 103'-106'. When the torque is 
removed, the resilience of cross-members 103-106 will cause them to return 
flange 102 to its neutral position. In the preferred embodiment, the angle 
of rotation 109 is less than or equal to three degrees; however, larger 
angles of rotation are possible as long as the steel for the flexures is 
selected so that the steel does not become permanently deformed when 
maximum rotation occurs. 
Flexures of suitable size for any desired application can be constructed 
according to the discussion above or according to the specification of 
U.S. Pat. No. 4,042,051, and flexures so constructed are suitable for use 
as any of the flexures 25U, 25L, 26U, 26L, 41 and 42 of flexure weighing 
system 20. Alternatively, for smaller flexure weighing systems 20, a 
prefabricated flexure such as Model No. 5032-400 manufactured by the 
Bendix Corporation has been found satisfactory. It will be appreciated by 
those skilled in the art that any suitable flexure may be used in the 
flexure weighing system of the invention. 
Weight sensor 70 shown in FIG. 8 includes an induction coil 71 in a 
dustproof housing 72 which is attached to the base of main frame 21, an 
armature 73 which fits into the center of induction coil 71, a collar 74, 
and a locking nut 75 which fits onto a threaded upper portion of armature 
73. The armature 73 connects to beam 31 by way of brackets 76 and the 
locking nut 75. Locking nut 75 and the threaded upper portion of armature 
73 allow for the adjustment of weight sensor 70 by simply loosening 
adjusting nut 75, adjusting the vertical position of the armature 73 and 
then tightening nut 75 again. The armature 73 does arc slightly in its 
travel within the induction coil 71 but, when properly adjusted by 
adjustment of mounting brackets 76, armature 73 will not touch the inside 
of the coil over the full range of operation of flexure weighing system 
20. A common load cell might be used, rather than the inductive coil 
arrangement shown in FIG. 8, as weight sensor 70. However, most load cells 
provide resolution of only 1 part in 40,000 while the inductive coil 
arrangement discussed above is capable of providing resolution of 1 part 
in 1,048,000. 
Dashpot 50 shown in FIG. 9 damps out disturbances due to operation of 
nearby equipment or normal plant vibration. Dashpot 50 consists of a body 
51 connected to the base of main frame 21, a cover and gasket unit 52, a 
piston 53 attached to one end of a plunger shaft 54, an adjustable nut 55 
for adjusting the amount of damping produced, and a corrosion-resistant 
ball joint 56 connecting the other end of plunger shaft 54 to a bracket 57 
affixed to beam 31. Dashpot body 51, typically cylindrical, is partly 
filled with oil, and only a small clearance is provided between piston 53 
and the inner wall of body 51. To insure that any movement of shaft 54 
will be substantially vertical so that no weighing inaccuracies will be 
caused by interference between piston 53 and the inner wall of body 51, 
ball joint 56 prevents arcing of shaft 54. 
The above discussion of flexure weighing system 20 describes a preferred 
embodiment of a split-beam balance system having ten flexures. It is 
apparent that a greater number of flexures can be used (for example, two 
or more flexures may be added to flexure linkage assembly 40) or that 
fewer flexures might be used (for example, studs 24 might be connected to 
beams 31 and 32 using a ball joint connection similar to that described 
for connecting dashpot 50 to beam 31). A reduction in the number of 
flexures used would of course generate a certain amount of friction in the 
weighing system since the flexures described are friction-free. Any 
reduction in the number of flexures would also tend to increase the 
susceptibility of the system to an adverse operating environment, for 
example, a knife edge pivot or a ball bearing is more susceptible to grit 
and dust from the workplace than is a flexure.