Capacitive force load cell for weighing scale

A weight scale includes a capacitive force load cell for linearly transducing force into electrical capacity. A C-shaped spring supports two capacitor plates adjacent the distal ends of the arms with the surfaces of the plates being normally parallel. Weight is transmitted from the platform of the scale to a pin which acts to separate the arms against the spring force. The plates are shaped, preferably triangularly, so that the change in capacitance as they separate is linear. The load cell is connected in circuit with an oscillator so that the change in capacitance is directly proportional to change in frequency as a measure of the weight of the load on the scale's platform.

This invention relates to a capacitive force load cell for a weighing 
scale. More particularly, the present invention relates to a load cell 
which linearly transduces force into electrical capacity which can be used 
as a measure of the weight applied to the platform of the scale. 
An electronic scale distinguishes itself from a mechanical scale 
principally in the manner in which the measured weight is displayed. 
Typically, a mechanical scale transmits the weight of the load through a 
mechanical linkage to a movable display scale which is rotated or 
otherwise moved against a force (spring or counterbalance) until the 
weight number is brought into alignment with a pointer or hairline in a 
window. This usually occurs when the weight or force created by the load 
equals the scale counterforce. Sometimes a pointer is moved relative to a 
fixed display scale. In an electrical scale, the weight of the load is 
proportionately transmitted to a transducer which converts it into a 
measurable electrical parameter. The resultant electrical signal is 
processed and the weight is electronically displayed. Such electronic 
scales are sometimes referred to as digital scales although for the most 
part it is only the display that is digital. The scale otherwise remains 
basically analog in nature. 
Electronic scales have recently come into wide use for commercial purposes 
because they can also be used to perform calculating functions. Such 
scales can be used to display weight and also calculate sales price by 
multiplying measured weight times price per unit of weight. 
Although used commercially, electronic scales have not found their way onto 
the consumer and home market for use as bathroom and kitchen scales and 
the like. One reason why electronic scales have not been widely introduced 
into the consumer market is because they are expensive to manufacture. 
Although some bathroom scales have recently been introduced at prices well 
in excess of $100.00, such scales are not truly competitive with existing 
mechanical bathroom scales where the bulk of the market presently sells 
below $40.00 retail price. 
A major reason for the high sales price of electronic scales is the high 
cost of the force load cell which transduces the force created by the 
weight of the load into an electronic signal. In general, electronic 
scales use strain gauges which sense weight by varying resistance as a 
function of the amount of applied force. The problem with strain gauges is 
that they must be manufactured within narrow tolerances. Therefore, they 
tend to be priced relatively high even when manufactured in quantity. 
Moreover, high quality electronic amplifiers must be used to detect the 
signal generated by the strain gauges if a scale operable within 
acceptable tolerances, even for a bathroom scale, is to be produced. Thus, 
even with today's inexpensive microelectronic circuitry, bathroom scales 
are priced well above $100.00 at the retail level. 
The present invention is directed to providing an inexpensive force load 
cell which can be used to transduce force into a measurable electronic 
parameter that can be readily detected, processed and displayed. More 
particularly, the present invention is directed to a capacitive force load 
cell; that is, a force load cell which transduces the load force into a 
change in electrical capacitance which can be readily detected, processed 
and displayed in units of weight. 
In general, a force load cell should have certain characteristics. It 
should have a linear response over the entire range of force (weight) 
which is to be applied to it. Its electrical output as a function of the 
force should be repeatable over the useful life of the cell. Still 
further, the cell itself should be self-zeroing. In other words, it should 
have little or no hysteresis. If these parameters can be provided in a 
force load cell that is simple of construction and inexpensive to 
manufacture, then such a cell would be useful for electronic weighing 
scales. 
Such a force load cell is provided in accordance with the present 
invention. The force load cell of the present invention comprises a pair 
of metallic capacitor plates which are insulatively mounted adjacent the 
distal end of a pair of support arms. The support arms normally hold the 
plates in spaced substantially parallel relation to each other to form a 
capacitor with an air dielectric. The support arms are joined at their 
opposite ends to a spring. The force created by the weight of the load on 
the platform of the scale is transmitted to the arms which move against 
the force of the spring means. This results in a change in the spacing 
between the capacitor plates which thus effects a change in electrical 
capacity. The plates are physically shaped so that the change in 
capacitance is linear even though the plates move out of parallelism. When 
the force separating the arms is relieved, the spring returns them and 
hence the capacitor plates to their original position. 
Such a cell meets all of the characteristics described above. The arms and 
spring can be a single piece of C-shaped steel. The capacitive plates 
themselves can be made of practically any metal. The capacitor can be 
connected as part of the circuit of an oscillator whose frequency changes 
in accordance with the change in capacitance. Oscillators are inexpensive 
electronic devices and the need for expensive electronic amplifiers is 
eliminated because changes in frequency can be measured with inexpensive 
electronic components. Another advantage of the load cell is that it is 
readily used with common existing linkages for mechanical scales which 
ordinarily transmit the load force from the platform of the scale to the 
movable scale face or pointer. 
For the purpose of illustrating the invention, there is shown in the 
drawings a form which is presently preferred; it being understood, 
however, that this invention is not limited to the precise arrangements 
and instrumentalities shown.

Referring to the drawings in detail, wherein like numerals indicate like 
elements, there is shown in FIG. 1 a weighing scale designated generally 
by the numeral 10. The scale is shown in the configuration of a bathroom 
scale, although it should be understood that the capacitive force load 
cell described herein can be used within scales having other 
configurations and other uses, such as kitchen scales for weighing food. 
As shown, the scale 10 includes a platform 12 upon which the load to be 
weighed may be supported. In the case of a bathroom scale, the load would 
normally be a person standing on the platform. The platform is supported 
on a mechanism 20 for transmitting the force developed by the load to the 
load cell in a manner to be described hereinafter. At the top of the 
platform, there is provided a window 14 in which the weight of the load 
may be displayed. Such display is preferably made by the use of light 
emitting diodes or liquid crystal display for representing the digits. 
As shown in FIG. 2, the weighing scale 10 also includes a base 16 upon 
which is supported the load force transmitting means as well as the 
capacitive force load cell 18. 
The means for transmitting the force developed by the load to the 
capacitive force load cell is shown through the cut-away portion of the 
platform 12 in FIG. 1 and in elevation in FIG. 2. In both figures, it is 
indicated generally by the numeral 20. The force transmitting means 20 is 
conventional and has previously been used in scales for the purpose of 
transmitting the load force to the mechanism for operating the mechanical 
scale or pointer. Its advantage is that it reasonably accurately transmits 
the force even though the load is not evenly distributed on the platform 
scale. The force transmitting means 20 has been modified somewhat for use 
with the capacitive force load cell. In particular, the force developed by 
the weight of the load is brought to a single point for application to the 
capacitive force load cell. Other means could be used. 
Referring to FIGS. 1, 2, 5 and 6, the mechanism of the force transmitting 
means 20 supports the platform 12 in each of the corners of the weighing 
scale 10. Since the support is the same in each of the corners, only one 
is shown in each of FIGS. 5 and 6. A U-shaped member 22 is fixed to the 
bottom of the platform 12 by a weld (as shown), rivet or other 
conventional fastener. The U-shaped member provides two arms supporting a 
cross arm 24 which has an inverted and truncated V relief section 26 
formed in its lower edge. The apex of the V section 26 rests in a V 
section 28 formed in the beam 30. A notch 32 in the lower edge of the beam 
30 is positioned outwardly of the point of engagement between the cross 
arm 24 and the V section 28. The notch 32 engages the beam 30 within a 
notch 34 in the upper edge of the upright 36. This forms the fulcrum of 
the beam 30. The free end of the beam 30 is provided with a pair of tabs 
38 and 40 which permit it to be engaged with the plate 42 by inserting the 
tabs into holes formed within the surface of the plate. This form of 
engagement permits the beam to be displaced by the load force while still 
remaining engaged with the plate 42. 
As best shown in FIG. 1, the scale includes three additional beams 46, 48 
and 50, all mechanically interconnected with the platform by the same 
mechanism shown in FIGS. 5 and 6. The beam 50 is connected to the plate 
42. However, the beams 46 and 48 are connected to the plate 44 as shown in 
FIG. 1. The manner of connection is the same as described in respect to 
beam 30. 
The forces applied by the load to the platform 12 are transmitted by the U 
members 22 to each of their respective cross arms 24. These forces now 
press downwardly against the beams 30, 44, 46 and 50 which tend to pivot 
about their respective fulcrum, as provided by the upright 36 and notch 
32. Thus, the force is transmitted along the beams and applied to the 
plates 42 and 44. 
The plate 42 includes an integral arm 52 which extends outwardly and rests 
upon the knife edge formed at the top of the post 54 fixed to the base 16. 
The knife edge of the post 54 is positioned closely adjacent to the distal 
end of the extension arm 52. The opposite edge of the plate 42 rests upon 
the apex of a pin 56 which is fixed to the lowermost arm of the capacitive 
load cell 18 as explained hereinafter. In this manner, the post 54 and pin 
56 support the entire central portion of the force transmitting means 
including the plate 44 which is connected to the beams 46 and 48. The 
plate 44 is loosely connected below the plate 42 by a headed pin 58 which 
extends through an opening in plate 42 and is held in position by the nut 
60 threaded to its uppermost end. The plates 42 and 44 are maintained in 
spaced apart relation by the spring 62. Since the plate 44 is below the 
plate 42, the downward forces transmitted to it by its respective beams 46 
and 48 are transmitted through the headed pin 58 to the plate 42. Thus, 
all forces developed by a load applied to the platform 12 are ultimately 
transmitted to the plate 42 which rests upon the post 54 and pin 56. 
The plate 42 is held in position on the post 54 and pin 56 by the spring 64 
connected to it and to the bracket 66 which is fixed to the base 16. The 
bracket 66 supports the capacitive force load cell 18 above the base 16. 
In particular, it is connected by a weld or similar attachment means to 
the support 68 for the capacitor plates 70 and 72. The plates 70 and 72 
are insulatively fixed to the support 68. By way of example, the capacitor 
plates 70 and 72 may be connected to the support 68 by an epoxy resin and 
glass insulators 74 and 76. 
As shown in FIG. 3, the support 68 includes a pair of arms 78 and 80 which 
are integrally connected by a portion 82 which forms the bight of a 
C-shaped support. The arms 78 and 80 support the capacitor plates 70 and 
72 adjacent their distal end. The support 68 is preferably made of heat 
tempered steel or spring steel. Heat tempered cold rolled steel may be 
chosen because it is relatively inexpensive and easy to use in 
manufacturing processes. If desired, the support 68 need not be 
manufactured as a single C-shaped piece. For example, the bight shaped 
portion 82 could be separate spring individually connected to the arms 78 
and 80. Other configurations could be used so that capacitor plates 70 and 
72 are mounted to separate when a force is applied against a spring member 
which will draw them back into their normal position when the force is 
relieved. 
The plates 70 and 72 are mounted so as to be parallel to each other when no 
load is on the platform 12 of the scale 10. The air between the plates 70 
and 72 provides the dielectric, and the spacing between the plates may, by 
way of example, be 0.005 inch. For ease of illustration, the spacing 
between the plates 70 and 72 has been greatly exaggerated in the drawings. 
The pin 56 is fixed only to the arm 80. It extends upwardly through a hole 
in the arm 78 but is not fixed to it. Arm 78 is, however, fixed to bracket 
66 as shown. When a force is applied to the pin 56 by plate 42, it forces 
arm 80 to move away from arm 78 which is fixed in position by the bracket 
66. This results in a separation of the plates 70 and 72 thereby 
increasing the spacing between them. The capacitance of an electrical 
capacitor is directly proportional to the spacing between its plates. 
Thus, this change in spacing changes the electrical capacitance of the 
capacitor formed by the plates 70 and 72 and it is directly proportional 
to the force applied by the pin 56. From what has been described 
hereinbefore, the force applied by the pin 56 is directly proportional to 
the weight of the load on the platform 12. 
A unique advantage of the present invention is that only a single force is 
transmitted to the capacitive force load cell 18 through the pin 56. The 
spacing between the plates 70 and 72 is allowed to become non-parallel 
when forced apart. This is advantageous because mechanical mechanisms for 
separating the plates 70 and 72 while maintaining their surfaces parallel 
to each other require two force applying mechanisms which are complex and 
therefore make the weighing scale 10 more expensive to manufacture. 
The disadvantage of allowing the plates to become non-parallel is the 
resultant non-linearity of the change in capacitance. By way of example, 
at maximum load, the spacing may vary from 0.005" to 0.050". This, 
however, can be compensated for over the range of the scale of properly 
shaping the plates. As shown in FIG. 4, each of the plates is triangular 
in shape; more particularly a truncated triangle with the apex of the 
triangle being positioned inwardly from the distal end of the arms 78 and 
80 adjacent to which the plates 70 and 72 are mounted. As the arms 78 and 
80 separate by the application of a force through the pin 56, the apices 
of the triangular plates 70 and 72 remain closely spaced together at their 
initial spacing while the bases of the triangle are at the widest spacing. 
However, the triangular shape compensates for the variation in spacing 
along the height of the triangle since the area of the plates also affects 
its capacitance. Thus, by positioning maximum area adjacent maximum 
spacing between the plates, the change in capacitance is linearly 
proportional to the force applied to the pin 56 even though the plates are 
displaced out of parallel with each other. 
It should be understood that a reversal of parts is entirely possible. 
Thus, the arms 78 and 80 could be forced toward each other. In this case, 
the plates 70 and 72 should be reversed with the apices at the outermost 
end. 
The spring force created by the support 68 returns the plates to their 
normal parallel position when the force applied to the pin 56 is relieved. 
This provides automatic and simple self-zeroing of the scale. The 
hysteresis of the spring is negligible. 
Referring now to FIG. 7, there is shown an electronic circuit which may be 
used in conjunction with the capacitive force load cell 18. The capacitor 
formed by the plates 70 and 72 is connected by electrical wires (see FIG. 
3) to an oscillator 84. More particularly, the capacitor may be connected 
in the tank circuit of the oscillator so that the changes in capacitance 
result in a change in the output frequency of the oscillator. The output 
of the oscillator 84 is connected to a phase lock loop filter 86 which in 
turn is connected to the frequency difference circuit 88. A reference 
oscillator 90 generates a fixed frequency which also is connected to the 
frequency difference circuit 88. In a conventional manner, the output of 
the frequency difference circuit 88 is a frequency which is either the sum 
or the difference of the output of the oscillators 84 and 90 (F.sub.0 
-F.sub.(w)). The number of cycles in the frequency received from the 
frequency difference circuit 88 is counted in the counter 92 which is also 
provided with a timing base by the clock 94. The output of the counter 92 
may be displayed by an LED or LCD display 96. It is understood that 
appropriate BCD circuits and drivers for the display are also provided. 
Since such circuitry is conventional, it has not been described. The 
function of the clock 94 is to provide a time base so that the frequency 
counted by the counter 92 is in the correct unit of measurement (e.g., 
pounds, ounces, kilograms). 
Since it is desirable that the power for the electronic circuit be provided 
by batteries, it is also necessary to minimize the drain on the batteries 
by the display. The provision of an on/off switch for a bathroom scale is 
not a desirable feature. To eliminate the necessity for such a switch, the 
circuit shown in FIG. 7 provides the frequency detector 98. Detector 98 
detects the presence of a frequency at the output of the frequency 
difference circuit 88. The circuit generates a signal which can be used to 
turn on the power to the display. Power to the display is controlled 
because that is the major drain upon the batteries. The remaining 
electronic circuitry has negligible power drain upon the batteries. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification as indicating the scope of the invention.