Weighing scale transducer

A transducer system for a weighing scale having a flexure mode crystal resonator includes a parallelogram linkage for supporting the load platform of the scale, a mounting structure for mounting the crystal resonator between two pivotally connected mounting arms, and a coupling assembly for coupling force from the parallelogram linkage to one arm of the mounting structure.

BACKGROUND OF THE INVENTION 
This invention relates to scales for measuring the weight of an object, and 
more particularly to transducers for converting the weight of an object 
into an electrical signal property indicative of the weight of the object. 
Transducers including flexure mode piezoelectric crystal resonators have 
been developed which are capable of converting a force applied to the 
crystal resonator into an electrical signal property representative of the 
applied force. One such resonator is shown in EerNisse U.S. Pat. No. 
4,215,570 entitled "Miniature Quartz Resonator Force Transducer". "Flexure 
mode" refers to the particular mode of oscillation of the resonator. This 
mode is characterized by oscillatory flexural bending of relatively long, 
slender crystal members. Resonators having this mode of oscillation are 
typically relatively delicate structures which are prone to break is 
subjected to mechanical shock or to loading which applies any significant 
bending, torsion, or shear force to the resonator member. The resonator is 
strongest and most accurate when loaded as a column in pure tension or 
compression. 
The flexure mode crystal resonator need not be piezoelectric, but may 
alternatively be a non-piezoelectric crystal structure (e.g., a silicon 
crystal) driven in the flexure mode by piezoelectric material (e.g., zinc 
oxide) associated with the crystal. Although in the preferred embodiment 
of the present invention the resonator is a piezoelectric crystal, it will 
be understood that the term "flexure mode crystal resonator" as used 
herein and in the appended claims includes both flexure mode piezoelectric 
crystal resonators and piezoelectrically driven flexure mode crystal 
resonators. 
A problem which is relatively unique to weighing scales is that the line of 
action of the force due to gravity of the object to be weighed cannot be 
predicted with certainty. Unlike devices such as pressure transducers and 
accelerometers in which the line of action of the force to be measured is 
always the same, the line of action of the force in a weighing scale 
depends on where the user places the object to be weighed on the load 
platform of the scale. It is of the utmost importance to the proper and 
satisfactory operation of a weighing scale that the indicated weight of 
the object not depend on its location on the load platform. In addition, 
in a scale employing a flexure mode crystal resonator of the type 
mentioned above, the resonator must be protected from the bending, 
torsion, or shear force components which typically result from off-center 
loading of the scale. These force components not only increase the risk of 
resonator breakage, they also subject the resonator to load components 
which reduce the accuracy of the transducer. 
Others have developed weighing scale transducers including thickness shear 
mode (as distinguished from flexure mode) piezoelectric crystal 
resonators. The crystal resonators in those transducers were relatively 
large and robust quartz crystal discs about 1.5 cm in diameter (see, for 
example, Walker U.S. Pat. No. 4,130,624). These transducers included (1) 
two vertically spaced, substantially parallel flexure assemblies for 
supporting the load platform of the scale, (2) a thickness shear mode 
crystal resonator disc mounted for loading by a compression force applied 
at two diametrically opposite points on the edge of the disc, and (3) a 
coupling assembly for applying vertical force from the flexure assemblies 
to the edge of the resonator disc. The coupling assembly included pivotal 
bearings for preventing application of moments from the flexure assemblies 
to the resonator disc. The coupling assembly also included, between the 
pivotal bearings and the resonator disc, a second system of vertically 
spaced, substantially parallel flexure assemblies for insuring vertical 
alignment of the coupling assembly. In addition to the mechanical 
differences between those transducers and the transducers of the present 
invention, there was no suggestion in that work of employing much more 
fragile flexure mode crystal resonators or even of a structure capable of 
mounting such a flexure mode resonator. 
In view of the foregoing, it is an object of this invention to provide a 
weighing scale transducer employing a flexure mode crystal resonator. 
It is a more particular object of this invention to provide structures for 
assuring that the flexure mode crystal resonator element in a weighing 
scale of the type mentioned above is subject only to axial loading and is 
prevented from receiving any significant bending, torsion, or shear 
loading regardless of the location of the object to be weighed on the load 
platform of the scale. 
SUMMARY OF THE INVENTION 
These and other objects of the invention are accomplished in accordance 
with the principles of the invention by providing a weighing scale 
transducer in which the load platform is supported for vertical movement 
by two vertically spaced, substantially parallel flexure assemblies; a 
flexure mode crystal resonator is mounted between two pivotally connected 
arms of a mounting structure; and a coupler applies, from the flexure 
assemblies to the mounting structure, substantially only force aligned 
with the vertical axis. The flexure assemblies largely resolve into a 
vertical force any off-center loading of the load platform. The coupler 
further attenuates any non-vertical force components. And the mounting 
structure substantially absorbs any remaining non-vertical force 
components which are transmitted by the preceding elements so that the 
crystal resonator is highly insulated from such force components. 
Further features of the invention, its nature and various advantages will 
be more apparent from the accompanying drawing and the following detailed 
description of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
A typical weighing scale which may include the transducer system of this 
invention is shown in FIG. 1. This scale includes a housing 10, a load 
platform 12 mounted for limited vertical movement above housing 10, 
control buttons 14 and 16 (e.g., an on/off control and a tare control), 
and an electronic digital display 18. The scale is turned on and tared, if 
necessary, by operation of controls 14 and 16. An object to be weighed is 
then placed anywhere on load platform 12, and the weight of the object is 
displayed by digital display 18. The particular scale shown in FIG. 1 is 
merely illustrative of the weighing scales in which the transducer system 
of this invention can be employed. Apart from the transducer system 
described in detail below, the construction and operation of the scale 
form no part of the present invention and can be conventional. 
As best seen in FIG. 3, load platform 12 rests on a shock absorbing 
assembly 20, which includes an annulus of resilient shock absorbing 
material 22 (e.g., conventional high compression polyether urethane 
cellular foam) sandwiched between upper and lower metal subplatforms 24 
and 26 (see also FIG. 5). The components of shock absorbing assembly 20 
are held together by any suitable means such as an adhesive. Shock 
absorbing assembly 20 absorbs at least a major portion of any transient 
force which is suddenly applied to load platform 12 as the result, for 
example, of dropping an object on the platform. 
Lower subplatform 26 is connected to flexure arm post assembly 30 by screws 
28. Post assembly 30 includes vertical post 32 and horizontal arm 34 which 
are held together by screw 36 (see also FIG. 4). Post assembly 30 is 
supported for limited vertical movement by upper and lower flexure arm 
assemblies 40 and 50 (see also FIGS. 2 and 4). Each flexure arm assembly 
includes a substantially rigid planar member 42, 52, which in plan view is 
substantially triangular. Each of members 42, 52 is connected, adjacent 
one of the vertices of its triangular periphery, to the upper or lower end 
of vertical post 32. Member 42 is thus connected to the upper end of post 
32 by flexure 44, and member 52 is thus connected to the lower end of post 
32 by flexure 54. Each of members 42, 52 is similarly connected adjacent 
its two remaining peripheral vertices to the fixed and rigid frame 60 of 
the scale. These connections are provided by flexures 46 and 48 in the 
case of member 42, and by flexures 56 and 58 in the case of member 52. 
Each of flexures 44, 46, 48, 54, 56, and 58 is a planar piece of 
resiliently flexible metal (e.g., beryllium copper). Each flexure is 
substantially coplanar with the associated rigid member 42, 52, and each 
flexure is flexible substantially only perpendicular to the plane of the 
rigid member. Accordingly, flexure assemblies 40 and 50, together with 
post assembly 30 and the vertical portion of base 60, comprise a 
parallelogram linkage which substantially constrains post assembly 30 to 
vertical movement in the plane of the paper in FIG. 3. 
The two flexures on the fixed side of each of flexure assemblies 40 and 50 
(i.e., flexures 46 and 48 in the case of assembly 40, and flexures 56 and 
58 in the case of assembly 50) are horizontally spaced from one another by 
a sufficient distance to substantially prevent twisting of the 
parallelogram linkage (i.e., motion out of the plane of the paper in FIG. 
3) in response to any off-center loading of load platform 12. The 
broad-based triangular shape of members 42 and 52 also contributes to the 
resistance of the parallelogram linkage to twisting due to off-center 
loading. 
Although in the preferred embodiment shown in the drawing, flexure 
assemblies 40 and 50 are each attached to frame 60 by two horizontally 
spaced flexures, other mounting arrangements are possible and are within 
the scope of the invention. For example, one long continuous mounting 
flexure could be used for mounting each member 42, 52; or three or more 
horizontally spaced flexures could be used. 
The amount of vertical movement permitted by post assembly 30 is preferably 
limited by stop assembly 70. This assembly includes stop support 72 
mounted on scale frame 60 and having two transverse arms 74 and 76 which 
respectively project under and over a portion of horizontal arm 34. Arm 34 
carries set screw 82, the lower end of which contacts stop surface 84 on 
arm 74 to limit downward travel of post assembly 30. Similarly, arm 76 
carries set screw 86, the lower end of which contacts stop surface 88 on 
horizontal arm 34 to limit upward movement of post assembly 30. Stop 
assembly 70 prevents excessive force, due either to large mechanical shock 
or an overload applied to load platform 12, from being transmitted to the 
flexure mode crystal resonator described below. 
The end of horizontal arm 34 remote from vertical post 32 is connected to 
coupling assembly 90. This assembly includes diaphragm element 100 and 
push rod element 110. Diaphragm element 100 includes adapter 102 which is 
threaded into the end portion of horizontal arm 34. Set screw 104 is used 
to lock adapter 102 in place in the desired position. The lower end of 
adapter 102 carries resilient metal diaphragm 106, which acts as a spring 
between post assembly 30 and push rod element 110 to allow greater 
vertical motion of post assembly 30 than is correspondingly produced in 
push rod element 110. This allows mechanical stop assembly 70 to be used 
for post assembly 30, even though the maximum operational displacement of 
the resonator crystal is very small (e.g., approximately 0.0002 inch). The 
spring effect produced by diaphragm 106 amplifies this resonator crystal 
displacement considerably so that the corresponding operational 
displacement of post assembly 30 may be ten times greater (e.g., 
approximately 0.002 to 0.003 inch). Mechanical stops can be set to control 
this amount of post assembly displacement. 
Diaphragm 106 has a centrally located aperture 108 for receiving and 
seating the upper end of push rod assembly 110 (see also FIG. 6). Push rod 
assembly 110 includes vertically aligned metal rod 112 having a socket in 
each end. A sapphire disc 114 is seated in the bottom of each aperture, 
and a stainless steel ball 116 is also contained in each socket so that 
the ball bears on the associated sapphire disc. A system of apertures 118 
in rod 112 facilitates assembly of discs 114 in rod 112 by preventing 
entrapment of air behind discs 114. Coupling assembly 90, and especially 
push rod element 110, transmits force aligned with rod 112 from post 
assembly 30 to mounting structure 120 described below, but substantially 
prevents the transmission of any moments about balls 116. Discs 114 and 
balls 116 provide substantially frictionless pivotal bearings at each end 
of push rod element 110. Accordingly, coupling assembly 90 helps to 
further protect the crystal resonator from spurious force components. 
The lower end of push rod element 110 bears on an interior portion of 
mounting structure 120. This structure is preferably an integral (i.e., 
one-piece) element of metal which includes several arms or bars pivotally 
connected together by flexural or "living" hinges in the material of the 
structure. Aluminum and beryllium copper are preferred materials for 
mounting structure 120. Although other configurations for the mounting 
structure are within the scope of the invention (several alternatives 
being shown in FIGS. 8-10), in the particular embodiment shown in FIGS. 
2-4 mounting structure 120 is essentially a four bar linkage having four 
co-planar bars or arms 122, 124, 126, and 128 connected together in a 
closed circuit by flexural hinges 132, 134, 136, and 138. Arm 122 is 
rigidly mounted to scale base 60 by bolt 142. The portions of arm 122 to 
the left and right of bolt 142 as viewed in FIG. 3 are cantilevered above 
scale base 60. This cantilevering, especially the cantilevering of arm 122 
to the right of flexural hinge 132 as viewed in FIG. 3, helps to insulate 
mounting structure 120 from spurious forces due, for example, to 
distortion of base 60. Arm 124 is pivotally connected to arm 122 by 
flexural hinge 132, which, like the other flexural hinges, is formed by 
milling out or boring out material on each side of the hinge to leave a 
thin strip or neck of material connecting arms 122 and 124. The pivotal 
axis of hinge 132, like the pivotal axes of all the other hinges, is 
perpendicular to the plane defined by the longitudinal axes of the arms 
joined by the hinge. The elastic limit of the flexural hinges is never 
exceeded during operation of the apparatus. Thus mounting structure 120 
tends to resiliently return to its initial condition when whatever load 
has been placed on the scale is removed. 
Continuing with the discussion of mounting structure 120, arm 124 is 
pivotally connected to arm 126 by flexural hinge 134. Arm 126 is pivotally 
connected to arm 128 by flexural hinge 136. And arm 128 is pivotally 
connected back to arm 122 by flexural hinge 138. Push rod element 110 
passes freely through aperture 144 in flexural hinge 136, and the lower 
ball 116 of push rod element 110 seats in recess 146 in mounting structure 
arm 126. 
The arrangement of elements in mounting structure 120 is such that when 
push rod element 110 moves vertically downward in response to the weight 
of an object on load platform 12, arm 126 also moves vertically downward. 
This causes arm 124 to pivot counterclockwise (as viewed in FIG. 3) about 
hinge 132. Arm 128 also pivots clockwise about hinge 138. Arm 122, of 
course, remains stationary at all times. The arms of mounting structure 
120 in the embodiment of FIG. 3 are arranged in such a way that they 
effectively constitute a so-called Watt linkage. This means that for all 
operational movements of the elements in the linkage, the point at which 
the lower end of push rod element 110 bears on arm 126 is constrained to 
move only vertically. Arm 126 as a whole may pivot slightly about this 
point, but the motion of this point is linear (not arcuate) and vertical. 
This helps keep push rod element 110 vertically aligned at all times, 
which contributes to insulating the crystal resonator from spurious load 
components and to linearizing the response of the transducer to loads of 
various magnitudes. 
Arms 122 and 124 include portions which extend substantially parallel to 
one another (to the right of hinge 132 as viewed in FIG. 3) and which are 
vertically spaced apart. Flexure mode crystal resonator transducer 150 is 
mounted between the ends of arms 122 and 124 remote from hinge 132. 
Although other known flexure mode crystal resonator configurations are 
possible, a particularly preferred configuration is shown in the 
above-mentioned EerNisse U.S. Pat. No. 4,215,570. As shown in simplified 
form in FIG. 7, this type of resonator includes two laterally spaced, 
substantially parallel tines 152, 154 connected together at both ends by 
mounting tabs 156, 158. Typically, each tine is about 0.333 inch long, 
0.008 inch thick, and 0.01 inch wide. Electrodes (not shown herein but 
shown in the EerNisse patent) are mounted on the surface of the crystal 
for coupling electrical signals to the resonator in the conventional 
manner. The tines of the resonator oscillate in the flexure mode in the 
plane of the paper in FIG. 7. Mounting tabs 156 and 158 are respectively 
attached to the ends of arms 122 and 124 by any suitable means such as an 
adhesive. Accordingly, when an object to be weighed is placed on load 
platform 12, flexure arm post assembly 30, coupling assembly 90, and 
mounting structure arm 126 all move vertically downward in response to the 
gravitational force on the object. This pivots mounting structure arm 124 
counterclockwise, which places the tines 152, 154 of resonator 150 in 
tension. The tension force thus applied to the resonator tines is 
proportional to the weight of the object, and it produces a proportional 
change in the frequency of oscillation of the resonator tines and 
therefore in the electrical output signal of the transducer. The scale 
determines the weight of the object on the scale from this change in 
transducer output. 
The use of relatively broad pivotal connections or hinges 132, 134, 136, 
and 138 between the arms of mounting structure 120 helps to stiffen 
mounting structure 120 with respect to deformations out of the plane of 
the paper in FIG. 3. (The "breadth" of the pivotal connections thus 
referred to is measured perpendicular to the plane of the paper in FIG. 
3.) Mounting structure 120 therefore strongly resists any twisting 
deformations which would subject crystal resonator 150 to non-axial 
forces. As mentioned above, such non-axial forces reduce the accuracy of 
the transducer output and may also cause the relatively fragile crystal 
resonator to break. The above-described stiffness of mounting structure 
120 is further enhanced by the use of a configuration including four 
hinges 132, 134, 136, and 138, all of which tend to reinforce one another 
to resist twisting deformation of the structure. The use of an integral 
mounting structure with flexural hinges further contributes to the ability 
of the structure to resist twisting deformation. 
It should also be noted that coupling assembly 90 applies force to mounting 
structure 120 at a point which is longitudinally spaced from both hinge 
132 and crystal resonator 150. This allows mounting structure 120 to be 
used as a lever arm assembly to reduce or amplify the force applied to 
crystal 150 as desired. 
Although the arms in the mounting structure 120 described above are all 
substantially rigid, one or more arms in that structure may be made 
flexible in bending to a small degree if desired. In that event, resilient 
diaphragm 106 may be eliminated because the flexible mounting structure 
arm or arms will provide the function of allowing sufficiently large 
vertical motion of vertical post 32 to permit use of mechanical stops 82 
and 86 to limit that motion. 
While the mounting structure configuration 120 shown in FIGS. 3 and 4 is 
especially preferred, other mounting structures can be alternatively 
employed in accordance with the principles of the invention. FIG. 8, for 
example, shows a transducer system similar to the one shown in FIGS. 2-7 
but with an alternative mounting structure 160. This mounting structure 
includes two vertically spaced, substantially parallel arms 162 and 164 
connected at one end by flexural hinge 166. As in the previously described 
embodiment, mounting structure 160 is preferably integral. Flexure mode 
crystal resonator 150 is mounted between the ends of arms 162 and 164 
remote from hinge 166. Arm 162 is rigidly mounted on scale frame 60. The 
lower end of push rod element 110 bears on arm 164 at a point intermediate 
hinge 166 and transducer crystal 150. Accordingly, in this embodiment a 
load on platform 12 places crystal resonator 150 in compression. The load 
is measured by the resulting change in the electrical characteristics of 
crystal resonator 150. 
The alternative embodiment shown in FIG. 9 is similar to the embodiment of 
FIG. 8, except that mounting structure arm 174 extends beyond flexural 
hinge 176 in the direction away from flexure mode crystal resonator 150. 
The lower end of push rod element 110 bears on this extension of arm 174 
so that a load on the scale places crystal resonator 150 in tension. 
The alternative embodiment shown in FIG. 10 is again generally similar to 
the previously discussed embodiments, especially the embodiment of FIG. 9. 
In FIG. 10, however, mounting structure 180 is a four bar linkage 
including arms 182, 184, 186, and 188 connected in a circuit by flexural 
hinges 192, 194, 196, and 198. Arm 182 is the fixed arm attached to scale 
base 60 by bolt 142 and is cantilevered above base 60 to the right of bolt 
142 as viewed in FIG. 10. As in the embodiment of FIGS. 2-7, cantilevering 
the major portion of mounting structure 180 above base 60 helps to 
insulate the mounting structure from spurious forces due to distortion of 
base 60. The mounting structure is loaded by the lower end of push rod 
element 110 which bears on an interior point near the middle of the length 
of bar 186. As in the mounting structure 120 shown in FIGS. 3 and 4, the 
use of four flexural hinges as shown in FIG. 10 helps increase the 
resistance of mounting structure 180 to twisting deformations (i.e., 
deformations out of the plane of the paper in FIG. 10). 
It will be understand that the foregoing embodiments are only illustrative 
of the principles of the invention, and that various modifications can be 
made by those skilled in the art without departing from the scope and 
spirit of the invention. For example, parallelogram linkage 30, 40, 50, 
and 60 can include other types of flexure arm assemblies 30 and 40 as 
mentioned above.