WEIGHING DEVICE AND METHOD

Abstract: A weighing device for measuring a load constructed of a loading platform (10) supported by a plurality of springs determines weight by measuring the displacement of the platform by a passive element (35) at an equivalent loading point (14) which is mathematically derived. At this point, the displacement is independent of the load location and is proportional only to the weight of the load.

Description: WEIGHING -DEVICE AND METHOD FIELD OF THE INVENTION The present invention relates to weighing devices, scales and the like, and more particularly, to a weighing device which measures the displacement between surfaces at a point invariant of load location, to provide a measured weight value. BACKGROUND OF THE INVENTION The prior art of weighing devices is based generally on the use of force-measuring devices and sensors which are calibrated to provide a weight measurement in relation to an applied force. Examples of such force- measuring devices operate based on the use volumetric displacement of a fluid within a deformable volume. US Patent 4,007,800 to Janach uses such a device as a spring element, and other examples of this approach are as follows: US Patent No. 5,234,065 to Schmidt; " 4,673,049 to Scheuter; " ' " 4,553,880 to Byrd et al; " 4,537,266 to Greenberg; " 4,498,550 to Menon; " 4,489,799 to Menon; " 4,248,046 to Fornell ; " 4,286,680 to Maltby; " " 4,084,651 to Lagneau; and " 4,056,156 to Dayton. Another type of weight measurement device is based on use of a hydraulic load cell, such as disclosed in US Patent No. 3,646,854 to Bradley et al, or a fluid-containing -load cell which changes Its volume, as per US Patent No. 3,791,375 to Pfeiffer. Still another type of weight measurement device is based on air pressure sensors, arranged to measure the pressure against suspended weight, as disclosed in US Patent 5, 119,895 to Gradert. Other variations of this type of measuring device include: US Patent No. 5,092,415 to Asano; " 4,306,629 to Powell; " 4,162,710 to Sjogren; " 4,103,752 to Schmidt; " 3,670,576 to Corry; " 3,648,791 to Van Raden; and " 2,087,494 to Annin. The patents which operate based on use of fluid are susceptible to liquid-gas equilibrium as a function of temperature, and the limitation to use of non-compressible fluids. The patents using air pressure in various cells have limited accuracy due to the fact that the total air pressure is not the sum of the partial pressures provided by each load cell. Another problem is the non-linear shape of the loaded surface which depends on the load distribution. The weight distribution between the sensors is a factor affecting measurement accuracy. Therefore, it would be desirable to provide a weighing scale which does not use fluids, and does not depend on a particular load distribution for accurate operation. SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to overcome the above-mentioned disadvantages of prior art weighing scales and provide a weighing device which measures the displacement between loaded surfaces at a point invariant of load location, to provide a measured weight value. In accordance with a preferred embodiment of the present invention, there is provided a weighing device comprising : a rigid frame for supporting a load positioned thereon; a plurality of spring elements for flexibly supporting said frame against a displacement thereof due to said load; means for measuring said frame displacement at a point invariant of said load distribution on said supporting frame , said frame displacement at said point being convertible to provide a weight measurement of said load. In a preferred embodiment, the inventive weighing device is constructed as a surface supporting a load, with a plurality of springs actively supporting the surface against displacement. Based on a mathematical derivation, an equivalent loading point is determined at which passive measurement of the surface displacement provides the weight value through a spring-force conversion. At this point, the surface displacement is independent of the load location and proportional only to the weight value, and is the most accurate point to measure weight. A passive weight measurement sensor can be provided which does not develop force against the load, such that the load supporting surface displacement provides the weight measurement. Advantages of the inventive method include the fact that it is not dependent on the use of fluids for measuring force and it is not affected by the load distribution. Thus, a weight measurement scale can be constructed which is accurate, small, light, strong and precise, and can be manufactured at low cost. Other advantages of the invention will become apparent from the following drawings and description. BRIEF-DESCRIPTION OF THE DRAWINGS For a better understanding of the invention with regard to the embodiments thereof, reference is made to the accompanying drawings, in which like numerals designate corresponding elements or sections throughout, and in which: Figs. 1a-c are schematic illustrations of a weight W arranged at various positions on a load supporting surface ; Fig. 2 is a perspective view of a load supporting surface arranged with a plurality of spring elements to support a load; Fig. 3 is a perspective view of the load supporting surface of Fig. 2 which defines an equivalent loading point for measuring its displacement under a load; Fig. 4 is a schematic illustration of a modified load supporting surface arranged with a plurality of spring elements; and Fig. 5 shows a rigid frame supported by spring elements ; Figs. 6a-c show top and side views of a scale constructed in accordance with the present invention; Fig. 7 is a schematic diagram of a measurement sensor; Figs. 8a-b show a volume displacement sensor; and Figs. 9a-b show an alternative volume displacement sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figs. 1a-c, there are shown schematic illustrations of a load W arranged at various positions on a load supporting surface 10. The invention relates to measurement of the weight of load W by use of a scale in which load supporting surface 10 is depressed against a set of springs, each having a known spring constant. In the simplest case, as shown in Figs. 1a-c, load supporting surface 10 is flexibly supported by a pair of springs S1 , S2 having equal spring constants. In Fig. 1a, load W is placed at the center (X=0) of the distance between springs S1-S2, and surface 10 is displaced by an amount d . In Figs. 1b-c, if the load W is o placed directly above either of springs S1 or S2 , the displacement of surface 10 is d or d , each of which is 1 2 double the amount of d . However, the slope of surface 10 o intersects the point at which X=0, midway between points X1 and X2. Therefore, it can be said that the displacement d o at the center point of surface 10 is independent of the position at which load W is placed. The point at which the displacement of surface 10 is independent of the load W position is called the origin. For the arrangement shown in Fig. 2 having a multiplicity of springs Si, it can be proven that in general, with respect to the origin, the sum of the spring constants multiplied by the distance from the origin is equal to zero. The invention is based on the principle that, for a pair of surfaces separated by a multiplicity of springs or elastic elements, there exists a point at which the displacement between the surfaces is independent of the position of the load. Thus, at this point, defined as the origin, measurement of the displacement alone is constant with regard to a given load. The proof of the above principle is now presented in connection with the arrangement of Fig. 2. There are shown N different springs Si, each with a spring constant Ki for the i-th spring located at the point Ri (radius vector), where Ri is a function of coordinates Xi and Yi. The origin previously mentioned is defined as: N _£Ki Ri = 0, such that .Ki Xi = 0 and Ki Yi = 0 (1) i If, for a certain system of coordinates, equation (1) is not satisfied, the origin has to be shifted to the coordinates : £Ki Xi ' Ki Yi ' Xo = --- --- Yo = (2) ξKi :£Ki and in this transferred system the condition of equation (1) is satisfied. The lower ends of the springs Si are connected to the base 12 and the upper ends are connected to load supporting surface 10 such that the springs are vertical. If a load W is applied at a point Rw to load supporting surface 10, each displacement di of the upper spring ends will then satisfy the equations of equilibrium for compression forces and moments as follows: ΞKidl W (3) and £Kid? x Rf + W* x Rv = 0 (4) where the load multiplied by the radius vector Rw is equivalent to the bending moment. Without loss of generality, it can be seen that load supporting surface 10 is parallel to the lower one when unloaded. Thus, all the springs have the same initial length. When loaded, surface 10 shifts parallel to itself only in the case of a load applied at the origin. In other cases, the equilibrium position of the load supporting surface 10 is not parallel to the lower one. Thus, the vertical shift of load supporting surface 10 differs from point to point on its surface, but due to its rigidity it remains planar and this provides an additional constraint to the individual shifts of the upper spring ends, so that for each spring location the planar equation is satisfied: aX + bY + c = d (5) aXi + bYi + c = di , (6) such that a, b and c can be derived from the above force and moment equilibrium equations as follows, by substituting eq. (6) : i_ i (aXi + bYi + c) = W, (7a) representing the sum of forces in relation to their position ; *Ki (aXi + bYi + c) * Xi = W * Xw, (7b) representing the bending moment in relation to the Y axis; Ki (aXi + bYi . + c) * Yi = W * Yo, (7c) representing the bending moment in relation to the X axis. By expansion, these equations (7a-c) can be rewritten : a 2-KiXi + b £KiYi + c . i = W, 2 a i_KiXi + b £KiXiYi + c KiXi = W * Xw, 2 a s KiXiYi + b £KiYi + c ^ KiYi = W * Yw, (8) This system of three equations (8) defines coefficients a, b and c. Taking into account the equality expressed in equation (1), the equations (8) can be rewritten as follows: c SKi = W 2 a KiXi + b £KiXiYi = W * Xw, 2 a £ KiXi Yi + b ^ KiYi = W * Yw, (9) Vertical displacement d(X,Y) of the supporting surface at a point X,Y is d(X,Y) = aX + bY + c (10) Coefficients a and b depend on the load W and the point of application, while coefficient c depends on W only. If, for example, the coordinates at the origin a re Xo=0 and Yo=0, then in accordance with equation (10), the displacement d(X,Y) is a function of coefficient c only, since d = c. Therefore, it can be seen from equation (9) that the vertical displacement d(Xo,Yo) of supporting surface 10 at the origin is equal to W d(Xo,Yo) = c = -— (11) *Ki, which is independent of the load location on supporting surface 10, and is proportional to the load value only, so that it is the best point at which to make weight measurements of load W. In Fig. 3, there is illustrated a perspective view of load supporting surface 10 of Fig. 2, with a plurality of springs, in which the origin defines an equivalent loading point 14 for measuring displacement under a load. In this arrangement, load W is not placed directly over the origin, and the displacement of load supporting surface 10 is measured at point 14. As defined above, the displacement is used to provide the weight measurement in accordance with equation (11), such that if the displacement at the origin is d , the weight is: W = d 0 Ki (12) As can be appreciated from the above, in accordance with the principles of the invention, measurement of the displacement di of load supporting surface 10 is a simple, easily achievable measurement which does not involve the use of complicated fluid or air-based force measurement devices. Therefore, a measurement scale can be constructed which is accurate, small, light, strong and precise. In the schematic illustration of Fig. 4, there is shown a modified load supporting surface 16 arranged with a plurality of spring elements Si. The modified load supporting surface is formed with projections 18 forming an irregular, non-smooth underside. The spring elements are connected at their upper ends to each of projections 18, thus, each has a different height. Since surface 16 is rigid, when it is displaced under a load W, projections 18 are also displaced, and the upper ends of spring elements are displaced by dn, d1 and d2. As before with equation (6), the rigidity of surface 16 requires that it maintains a planar shape, and this allows the displacement to be defined in an equation relating the spring location and displacement as follows: di = d(Xi ,Yi ) = aXi + bYi + c (13) Thus, the solution provided by equation (12) above also applies to the modified load supporting surface 16 of Fig. 4. The rigidity of surface 16 may also be achieved using a rigid frame connecting a number of points and forming any shape. For example, as shown in Fig. 5, a star- shaped rigid surface 19 can be supported by a plurality of spring elements Si, one at each point. Once again, the equations describing the spring location and displacement can be applied, as in equation (13). In Figs. 6a-c, there are shown top and cross- sectional side views of a scale 20 constructed in accordance with the present invention. As shown in the cross-sectional view of Fig. 6b taken along section lines A-A of Fig. 6a, scale 20 is constructed with a base 12, and a load supporting surface 10. A cover 26 of scale 20 is provided with spacers 27 between it and load supporting surface 10, thus preventing deformation of load supporting surface 10 when loads are placed on cover 26, to insure accuracy of weight measurements. A set of spring elements Si flexibly supports surface 10 over base 12, and as described in the equations above, an origin is defined as the equivalent loading point 14, at which the displacement of surface 10 is measured, between points 30 and 32. Ball supports 33 between base 12 and feet 34 define points of load application in the same plane, and these points define the system of coordinates upon which derivation of the origin is based in accordance with Eq. 11 above. A passive measurement of the displacement between points 30,32 is performed by a sensor 35 which does not present any resistance since it does not measure force, but measures only the spacing between surface 10 and base 22. In Fig. 6c, an enlarged view of sensor 35 is shown. Sensor 35 comprises a conventional LVDT 37 (low voltage differential transformer) having a transformer core 36 rigidly attached to arm 38. Movement of arm 38 is responsive to movements of load supporting surface 10, such that core 36 responds to the displacement between points 30,32. Using a known electrical circuit, movements of core 36 are converted into a voltage representing the displacement value, and this can be provided as. the weight value in a digital readout. A schematic diagram of an alternative displacement sensor arrangement is illustrated in Fig. 7. Load supporting surface 10 is supported by three springs, S1 , S2 and S3, mounted on legs 40. A set of pulleys 42 is arranged on load supporting surface 10 and legs 40 as shown, and a string 44 is threaded over pulleys 42.between an end wall and leg 40, under tension of spring 46. Springs S1-S3 have spring constants much larger than spring 46, such that the latter develops negligible force. An LVDT is arranged such that when a load is placed on load supporting surface 10, core 36 moves with string 44 movement. When converted to voltage, core 36 movement represents the sum of the deformation of springs S1-S3, regardless of load distribution. As above, this is a passive measurement, without developing force. Referring now to Figs. 8a-b, there is shown a volume displacement sensor 50 arrangement featuring a bellows 52 having a spring Si disposed therein to provide an initial height to the upper end 53 of the bellows. An open- ended tube 54 is connected in fluid communication with the bellows interior and a mercury bead 56 is situated in tube 54. Since the tube diameter is small, mercury bead 56 develops a minimum amount of friction with tube 54, and thus no resistance. Tube 54 is calibrated with markings 55 indicating the bellows upper end 53 displacement as a function of the volume displacement of bellows depression. Thus, when placed at origin 14 under load supporting surface 10 of scale 20, volume displacement sensor 50 provides a passive weight measurement (W). In Figs. 9a-b, there is shown an alternative embodiment of a volume displacement sensor 60, constructed as a pair of flexible rubber mats 62 having formed therebetween a plurality of air pockets 64 which are connected together and communicate with open-ended tube 54, as in the sensor 50 of Figs. 8a-b. As before, tube 54 contains a mercury bead 56 and is calibrated with markings indicating the volume displacement as a function of the depression of mats 62. Thus, a distributed weight can be measured via the distributed depression of mats 62, and when placed under load supporting surface 10 of scale 20, volume displacement sensor 60 provides an integrated, passive weight measurement. In summary, the inventive weighing device is constructed such that an equivalent loading point is provided at which the surface displacement is independent of the load location and by passive measurement of the surface displacement, the weight value is provided. Thus, by use of passive weight measurement sensors which do not develop force against the load, the load supporting surface displacement provides the weight measurement. Advantages of the inventive method include the fact that it is not dependent on the use of fluids for measuring force and it is not affected by the load distribution. Thus, a weight measurement scale can be constructed which is accurate, small, light, strong and precise, and can be manufactured at low cost. Having described the invention with regard to • certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications , may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims .

Claims: C LA I MS : 1. A weighing device comprising: a rigid frame for supporting a load distributed thereon ; a plurality of spring elements for flexibly supporting said frame against a displacement thereof due to said load; means for measuring said frame displacement at a point invariant of said load distribution on said supporting frame, said frame displacement at said point being convertible to provide a weight measurement of said load. 2. The device of claim 1 wherein said frame displacement (di) at said point against said spring elements having spring constants (Ki) is convertible to said weight measurement (W) in accordance with the relation: W = di^Ki. 3. The device of claim 1 wherein said measuring means comprises a measurement sensor providing passive measurement of said frame displacement. 4. The device of claim 3 wherein said measurement sensor comprises an arm resiliently flexible against said frame, said arm having attached thereto a core of an LVDT providing conversion of said frame displacement to a voltage measurement. 5. The device of claim 3 wherein said measurement sensor comprises a volume displacement sensor, having a compressible volume in fluid communication with a tube containing a mercury bead, changes in said compressible volume causing bead movement. 6. The device of claim 5 wherein said tube is calibrated to indicate said frame displacement as a function of said changes. 7. The device of claim 5 wherein said compressible volume comprises a bellows. 8. The device of claim 5 wherein said compressible volume comprises a pair of flexible mats having air pockets between them. 9. A method of measuring weight comprising the steps of providing a rigid frame for supporting a load distributed thereon; providing a plurality of spring elements for flexibly supporting said frame against a displacement thereof due to said load; and measuring said frame displacement at a point invariant of said load distribution on said supporting frame, said frame displacement at said point being convertible to provide a weight measurement of said load. 10. The method of claim 9 wherein said frame displacement (di) at said point against said spring elements having spring constants (Ki) is convertible to said weight measurement (W) in accordance with the relation: W = di_ Ki. 11. The method of claim 9 wherein said frame displacment measurement is performed by a passive measurement sensor. 12. The method of claim 11 wherein said passive measurement sensor converts said frame displacement to a voltage measurement. 13. The method of claim 11 wherein said passive measurement sensor converts said frame displacement to a volume displacement measurement. 14. A weighing device providing passive measurement of a frame displacement under load, substantially as described herein by way of example and with reference to the drawings. 15. A method of measuring weight by passive measurement of a frame displacement under load, substantially as described herein by way of example and with reference to the drawings.

Inventors: INGMAN DOV, INGMAN, DOV
Applicants: INGMAN DOV, TECHNION RES & DEV FOUNDATION, INGMAN, DOV, TECHNION RESEARCH & DEVELOPMENT FOUNDATION LIMITED
Application year: 1997
Main classification: G01G3
IPC codes: G01G3