BACKGROUND OF THE INVENTION Scales employing variable capacitors as weight responsive transducers are known in the art.

The following patents are believed to represent the state of the art: U. S. patents 4,585,082,4,273,204,4,381,040,4,243,114 as well as GB 2,223,849 and EP 18919.

SUMMARY OF THE INVENTION The present invention seeks to provide an improved low-cost pocket-size electronic scale employing a variable capacitor.

In a preferred embodiment of the present invention, the capacitor comprises a parallel plate capacitor, having a fixed bottom plate-conductor and a displaceable top conductor.

Preferably, the displaceable conductor comprises a plate with two fixed opposing sides.

Most preferably, one or more parallel leaf-springs are connected to the plate to form a displaceable member of prescribed stiffness. Such a member is capable of a predetermined deflection in response to, and preferably, co-directional with an applied load, resulting in a load induced change of the capacitance.

Repeatability and accuracy are two important characteristics of a weighing scale. These properties are achievable through a linear response of the capacitance over the scale's entire weighing range and an insensitivity of the capacitance response to off-center loading.

Both linearity and off-center loading insensitivity may be achieved by a suitable mechanical construction of the plate. The number of the leaf-springs, their individual design and mechanical properties as well as the connection therebetween are significant factors affecting the plate's deflection.

Preferably, the capacitor is designed to a prescribed steep response with respect to the applied load, so as to enable the manufacture of an ultra-compact, and in particular ultra-thin, accurate scale of pocket-size dimensions.

In preferred embodiments of the present invention, the electronic scale comprises a suitable display, such as a seven-segment LCD display, as known in the art, for displaying a determined weight of an object. Most preferably, the weight is displayed in user-selectable units.

The scale also comprises control circuitry, preferably comprising a microprocessor, capable of processing the data and controlling the various operational functions of the scale, such as display, reset and calibration.

In accordance with alternative embodiments of the present invention, the pocket-size electronic scale comprises enhanced processing capabilities, enabling the performance of various dietary related functions. For example, a user may receive such information as caloric and/or nutritional value of the weighed object.

A suitable power source, which supplies a predetermined voltage to the capacitor and to the other power consuming elements of the scale is also provided. Examples of such a power source is a disposable or rechargeable battery, or a light-activated power source.

In accordance with preferred embodiments of the present invention, the allowable weighing range of the pocket-size electronic scale spans between 0 to 500g. Weights ranging between 0 to 300 g may be determined with an accuracy better than 2 g. Weights between 300 to 500 g may be determined with an accuracy better than 15 g.

The scale components are preferably packaged within a pocket-size casing. The casing is preferably manufactured from resilient plastic materials or metals. The casing features a length of up to 100 mm, a width of up to 70 mm and a thickness which may vary between 4 to 8 mm. The total weight of the scale preferably does not exceed 100g.

Preferably, the casing is fitted with a tray on which an object to be weighed is placed.

The tray communicates with the displaceable conductor, so that the applied load is transmitted to the displaceable conductor causing a corresponding displacement thereof.

There is thus provided in accordance with a preferred embodiment of the present invention apparatus for determining a weight of an object including a fixed conductor, and a displaceable conductor including a plate with two fixed opposing sides arranged relative to the fixed conductor such that a capacitance may result between the two conductors, wherein the capacitance is changeable in response to a load applied to the displaceable conductor, wherein at least one leaf-spring is attached to the plate. Preferably the conductors include a parallel- plate variable capacitor.

In accordance with a preferred embodiment of the present invention the displaceable conductor displaces relative to the fixed conductor in response to the load applied thereon.

Further in accordance with a preferred embodiment of the present invention the conductors are arranged such that a change in capacitance therebetween is insensitive to off- center loading. Preferably the apparatus is pocket-size.

Still further in accordance with a preferred embodiment of the present invention a display is provided which displays the weight of the object being weighed. Preferably the display displays the weight in user-selectable units.

Additionally in accordance with a preferred embodiment of the present invention control circuitry is provided which controls operational functionality of the apparatus.

Preferably a power source which supplies power to the apparatus.

There is also provided in accordance with a preferred embodiment of the present invention a method for determining a weight of an object including providing a fixed conductor and a displaceable conductor, the displaceable conductor including a plate with two fixed opposing sides, arranged such that a capacitance may result between the two conductors, and wherein at least one leaf-spring is attached to the plate, applying a load to the displaceable conductor such as to change the capacitance, determining the capacitance, and relating the capacitance to a weight value.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a schematic, pictorial illustration of a pocket-size electronic scale, in accordance with preferred embodiments of the present invention; Fig. 2 is a cross-sectional view of the scale of Fig. 1, in accordance with a preferred embodiment of the present invention; Fig. 3A is a schematic illustration of a variable-capacitance capacitor under zero-load conditions, in accordance with a preferred embodiment of the present invention ; Fig. 3B is a schematic illustration of a variable-capacitance capacitor under loading, in accordance with a preferred embodiment of the present invention; Figs. 4A, 4B, 4C, 4D, 4E, 4F and 4G depict mechanical parts of the scale of Fig. 1, in accordance with a preferred embodiment of the present invention; and Fig. 5 is a schematic illustration showing control circuitry of the scale of Fig. 1, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Fig. 1 which illustrates a pocket-size electronic scale 20, constructed and operative in accordance with a preferred embodiment of the present invention.

Scale 20 comprises a casing 32, in which all parts of scale 20 are assembled, and a tray 22, fitted as a top-lid to casing 32. Casing 32 and tray 22 are preferably manufactured from

suitable resilient materials, such as PS or ABS plastics and appropriate metals. Preferably casing 32 is suitably electrically shielded, so as to minimize external electrical noise, which may affect the scale's operation.

A display 30, is assembled within a section 33 of casing 32 along with control circuitry 28 and a power source 26. A removable cover plate 34 is attached to the bottom of section 33, so as to enable easy replacement of power supply 26. Display 30, which displays a weight of an object 21 placed on tray 22, preferably comprises an appropriate seven-segment LCD display, as known in the art, such as a WK-T22027-R6 display manufactured by Wintek Corporation of Lafayette, Indiana, USA.

Power source 26, which supplies a prescribed voltage to control circuitry 28 and display 30, may comprise any suitable power source known in the art, such as disposable or rechargeable batteries or a light-activated power source.

A variable capacitor 24 is assembled within a second section 31 of casing 32. The capacitance of capacitor 24 varies in response to an applied load, such as the weight of object 21. Structure and operation of capacitor 24, in accordance with a preferred embodiment of the present invention, are shown in Figs. 2,3A and 3B and described with reference thereto.

Casing 32 of scale 20 preferably features a length of up to 100 mm, a width of up to 70 mm and a height that may vary between 4 to 8 mm. Preferably, scale 20 has a total weight of up to 100 g.

In accordance with preferred embodiments of the present invention, scale 20 is capable of determining weights of up to 500 g. Weights ranging between 0 to 300 g may be determined with an accuracy better than 2 g. Weights ranging between 300 to 500 g may be determined with an accuracy better than 15 g.

Reference is now made to Fig. 2, which is a cross-sectional view of scale 20, and to Figs. 3A and 3B, which are schematic illustrations showing capacitor 24 under zero-load and loaded conditions, respectively, in accordance with a preferred embodiment of the present invention.

In this embodiment, capacitor 24 comprises a parallel-plate capacitor. A fixed plate- conductor 48, shown in Fig. 4C below and described with reference thereto, is mechanically fastened to an electrically insulating block 50 by means of screws 44. Fixed conductor 48 is electrically connected to control circuitry 28.

A second, displaceable conductor 25 of capacitor 24, comprises a frame 46, shown in Fig. 4D and described with reference thereto, to which two parallel leaf-springs 52 are fastened

by means of flanged tubular rivets 42, so as to constitute a plate with two fixed opposing sides.

Leaf-springs 52, shown in Fig. 4E and described with reference thereto, are also fastened to insulator block 50 by means of screws 44, as is tray 22. Displaceable conductor 25 is electrically connected to control circuitry 28. More than two leaf-springs 52 may be used to effect a desired displacement in response to the applied load.

Under zero-load conditions, as shown in Fig. 3A, displaceable conductor 25 is separated from fixed conductor 48 by a zero-load spacing do corresponding to a zero-load capacitance. Spacing do is preferably smaller than 0.2 mm, so as to enable the manufacture of a 4 mm thick scale, as mentioned with reference to Fig. 1 above.

Upon placing object 21 on tray 22, as shown in Fig. 3B, a force F corresponding to the applied load is exerted on leaf-springs 52 resulting in a displacement xl thereof, relative to fixed conductor 48. A displacement xl of up to 0.5 mm corresponds to loads of up to 300 g, and a displacement xl of up to 0.75 mm corresponds to loads between 300 to 500 g.

Preferably, the displacement xl exhibits a generally linear dependency on the load F.

Mechanical non-linearity is preferably compensated for by means of a predetermined calibration look-up table programmed within control circuitry 28.

The stiffness of displaceable conductor 25 primarily derives from the stiffness of leaf- springs 52. Preferably, leaf-springs 52 and displaceable conductor 25 are arranged so that the displacement xl is insensitive to off-center location of object 21 on tray 22.

Desired weight-determination repeatability and accuracy of the scale are ensured by the linear response and off-center location insensitivity.

Reference is now made to Figs. 4A, 4B, 4C, 4D, 4E, 4F, 4G and 4H, which depict casing 32, fixed capacitor plate 48, frame 46, leaf-spring 52 and tray 22, respectively, in accordance with a preferred embodiment of the present invention.

Fig. 4A is a bottom view of casing 32 and Fig. 4B a cross-sectional view IVB-IVB thereof. Casing 32 is manufactured from suitable resilient materials, such as PS and ABS plastic materials or appropriate metals. Casing 32 is preferably partitioned into at least two sections 31 and 33 designed to accommodate mechanical and electrical components, respectively, of scale 20. Removable cover plate 34 screwed to the bottom of section 33 enables easy access to the power source or any of the electrical components within section 33.

Typically, but not necessarily, section 31 has a length of 73 mm, section 33 is 27 mm long, and the resultant total length of casing 32 is 100 mm.

Fig. 4C illustrates fixed conductor 48 comprising a plate having a length of 50 mm, a width of 64 mm and a thickness of 0.5 mm. Conductor 48 is manufactured of a suitable electrically conducting material, such as steel or copper and fastened to insulating material block 50, as described above.

Fig. 4D depicts frame 46 having a length of 70 mm and a width of 66 mm and featuring two M2 threaded holes 47 and four additional bores 49 for fastening leaf-springs 52 by means of rivets 42. A rectangular opening (typically 54 by 25 mm) is cut within frame 46 to allow the displacement of leaf-springs 52, as described hereinabove with reference to Fig. 3B.

Leaf-spring 52, shown in Fig. 4E in accordance with a preferred embodiment of the present invention, has a length of 70 mm, a width of 20 mm and a thickness of 0.18 mm Alternatively, leaf-spring 52 may have different dimensions, so as to comply with prescribed stiffness values. Leaf-spring 52 comprises four bores 51 through which rivets 42 are inserted to fasten the leaf-springs to frame 46. Four additional bores 53 are used to fasten the leaf- springs to insulating material block 50 using screws 44. Leaf-spring 52 is preferably manufactured to a prescribed stiffness from suitable materials, such as stainless steel SS 302 having a Rockwell hardness RC 40-45.

Frame 46 and leaf-springs 52 constitute a plate with two fixed opposing sides, as mentioned above. Such a plate is capable of a controllable and determinable deflection, generally determined by the dimensions and mechanical properties, as is well known in the art..

Attaching to the plate the plurality of leaf-springs, further enhances the controllability of its deflection responsive to an applied load. A suitable choice of the number, properties and structure of the leaf-springs enables the design of a capacitor having any desirable response characteristic, at a reasonable manufacturing cost.

Fig. 4F is a top view of tray 22 and Figs. 4G and 4H depict cross-sectional views IVG- IVG and IVH-IVH, respectively. Tray 22 comprises a central section having a tapered cross- section IVG-IVG with a maximal thickness of 2.5 mm and 1 mm thick edges. A cross-section IVH-IVH having a constant thickness of 1 mm is typical of a peripheral area of tray 22. Four threaded holes 55 are used to fasten tray 22 to insulating material block 50 using screws 44.

Tray 22 is free to move in a vertical direction, thereby displacing leaf-springs 52 in response to the load applied by object 21 placed thereon for weighing.

Fig. 5 is a schematic illustration showing control circuitry 28, in accordance with a preferred embodiment of the present invention. Circuitry 28 is operative inter alia to process

raw load data, generated as capacitance values of capacitor 24, and to display the data on display30.

In accordance with this embodiment control circuitry 28 comprises a board 40 on which are mounted various components of the circuitry, including a variety of suitable capacitors c and resistors R and a switch 54 operative to switch scale 20 between"on"and"off'modes.

Circuitry 28 preferably comprises a microprocessor 36 capable of performing a set of control functions and processing raw weight data. The set of control functions may comprise such functions as reset, calibration, display and weight-units transformations, as determined by a predefined operational functionality of scale 20. A single-chip 4-bit KS57C2102 microprocessor manufactured by Samsung Electronics, Suwon City, Kyungki-Do, Korea, is an example of such a microprocessor. Control circuitry 28 further comprises a binary counter and oscillator 39, as known in the art, such as a 14-bit binary counter and oscillator MC140806P manufactured by Motorola, Schaumberg, Illinois, USA. Binary counter 39 is connected to microprocessor 36 and to the two conductors 48 and 25 of capacitor 24. A clock 38 is connected to microprocessor 36.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of features described hereinabove as well as variations and developments thereof which would occur to a person of skill in the art upon reading the foregoing description, and which are not in the prior art.