WO1997043772A1

WO1997043772A1 - Gauge with variable resistor having cermet resistive element

Info

Publication number: WO1997043772A1
Application number: PCT/US1997/002391
Authority: WO
Inventors: Cecil M. Williamson
Original assignee: Rochester Gauges, Inc.
Priority date: 1996-05-13
Filing date: 1997-02-18
Publication date: 1997-11-20
Also published as: AU2126897A

Abstract

A liquid level gauge is provided having a variable resistor (104) with a cermet resistive element. A novel combination of wiper arm (74, 78) material, lubricant material and contact pressure are provided allowing the wiper arm (74, 78) to directly contact the exposed surface of the cermet resistive element (82), which has a uniform composition throughout its thickness in the axial direction (48), to yield a gauge for high powered applications having a very long service life.

Description

GAUGE WITH VARIABLE RESISTOR HAVING CERMET RESISTIVE ELEMENT

TECHNICAL FIELD OF THE INVENTION

This invention relates to a gauge for measuring a fluid level. In one aspect, it relates to a gauge incorporating a variable resistor having a cermet resistive element.

BACKGROUND OF THE INVENTION

This invention relates to fluid level sensing gauges. A common type of fluid level sensing gauge is the float gauge, which has a float that floats on the surface of the fluid being measured. The float is usually connected with other members which move with the float as the fluid level changes. Movement of the float and attached members is sensed by a gauge, typically through a magnetic coupling, to provide an indication, either electrical, visual or otherwise, of the fluid level. The indication may be used at the gauge site, transmitted to a remote location, or both.

It has been known for many years to incorporate a variable resistor in the float gauge to vary an electrical resistance with the change in the fluid level. The variable resistor typically comprises a conductive wiper arm that is moved along the contact surface (s) of a resistive element by the magnetic coupling as the float level changes. The resistance may be measured between one end of the resistance element and the wiper arm, providing a rheostat-type circuit whose resistance corresponds to the float position. Alternatively, a first resistance may be measured between one end of the resistance element and the wiper arm, and a second resistance may be measured between the wiper arm and the second end of the resistance element, to provide either two complementary rheostat-type circuits or a single voltage divider-type circuit having a voltage output which corresponds to the float position.

Although many configurations have been developed for variable resistors and many materials have been used for the resistive element, a variable resistor suitable for use in a magnetically coupled liquid level gauge must provide a specific combination of current capacity, power capacity, fixed resistance, and mechanical wiper drag to function properly in a given application. This is especially true when a new variable resistor or gauge is being designed to meet existing applications. For example, a popular application for magnetically coupled liquid level gauges in the fuel storage and automotive industry requires a gauge having a current capacity of 150 to 200 milliamps, a power capacity of 0.6 to 0.9 watts, a fixed resistance of less than 3 to 5 ohms for a nominal 0 to 90 ohm gauge, and wiper drag which allows the wiper to be fully responsive to a coupler transmitting one inch- ounce of torque or less.

Current capacity relates to the maximum amount of electrical current that a variable resistor or gauge can carry without failure, i.e., without experiencing a change which adversely affects the accuracy of the device or its structural integrity. Damage from excessive current usually takes the form of erosion or rapid wear on the contact surfaces of the wiper or resistive element. This erosion or wear changes the contact resistance of the device or the resistance of the resistive element itself such that its original accuracy is lost. It is theorized that microscopic electrical arcs occur between the contact surfaces during operation of a variable resistor. As current levels increase, these "micro-arcs" increase in power and welding or vaporization of the contact surfaces may occur. At excessive current levels, the loss or transfer of material from the contact surfaces due to such high power micro-arcs results in erosion, material transfer, or rapid wear which causes failure of the device. Increasing the contact area of a variable resistor generally increases the current capacity of the device. Where space is limited, however, or where larger contacts cannot be used for other design reasons, it has also been found that increasing the contact pressure, i.e., the normal pressure between the contact surfaces, increases the current capacity of the device, probably by suppressing the formation of destructive micro-arcs. When contact design is fixed, increased contact pressure is usually achieved through increased contact force, i.e., the normal force between the contact surfaces.

Power capacity relates to the maximum amount of electrical power that a variable resistor or gauge can dissipate without exceeding a safe operating temperature. Virtually all power dissipated in such a device takes the form of heat dissipated by the resistive element, thus excessive power typically causes undesired heating of the resistive element. In variable resistors having annular wound metal wire resistive elements, the element may become so hot that it melts surrounding components or the plastic case of the gauge. In variable resistors having thick film polymer resistive elements, it has been found that the resistance the element goes up sharply with increasing temperature, resulting in a "thermal run away" condition which may destroy the element itself. Forming the resistive element of heat-resistive material and mounting it on a substrate having high thermal conductivity has been found to allow greater power capacity.

Fixed resistance relates to the electrical resistance of a variable resistor or gauge when the wiper of the variable resistor is at the lowest resistance or "zero" end of its range of motion relative to the resistive element. For most applications, it is desirable that the fixed resistance be small relative to the nominal variable resistance of the device. For example, as previously described, in an existing application for a liquid level gauge having a nominal variable resistance of 0 to 90 ohms, it is preferred that the fixed resistance of the circuit not exceed 3 to 5 ohms. The fixed resistance of a variable resistor or gauge is typically comprised of contact resistance, terminal resistance, and miscellaneous circuit resistance.

Contact resistance relates to the resistance between the contact surfaces of the wiper and resistive element. Although the exact nature of contact resistance is very complex, it is known that contact resistance is affected by the conductivity of the materials used for the contact surfaces, the amount of resistive "tarnish" or corrosion present on the contact surfaces, the area of the contact surfaces, and the contact pressure, i.e., the contact force for a given contact area. Generally speaking, all other parameters being constant, increasing the contact pressure or contact force for a given contact area will decrease the contact resistance of a device.

Terminal resistance relates to the resistance of a device which results when portions of the resistive element are physically "inaccessible" to the wiper at the lowest resistivity end of the resistive element. For example, variable resistors having annular metal wire resistive elements are typically designed to stop the motion of the wiper at a point several turns of wire away from the end of the element to prevent the wiper from moving off the end of the element. However, these "inaccessible" turns add a resistance to the circuit even though the wiper is in its "zero" position. In some devices a conductive shunt is installed at the low resistivity end of the resistive element to bridge the inaccessible portion of the element and reduce the terminal resistance. Such shunts require additional manufacturing and assembly steps, however, adding to the cost of the device.

Miscellaneous circuit resistance relates to the resistance of the miscellaneous wiring and other components in a variable resistor or gauge.

Mechanical wiper drag relates to the frictional forces which must be overcome to move the contact surfaces of the wiper arm along the contact surfaces of the resistive element of a variable resistor or gauge. Wiper drag is a complex phenomenon, but it is known that drag is affected by the surface roughness of the contact surfaces and by the contact pressure. All other parameters being constant, increasing the surface roughness of the contact surfaces or increasing the contact pressure between the contact surfaces will increase the wiper drag. Wiper drag is especially significant in magnetically coupled liquid level gauges because of the modest coupling torques available from the magnetic couplers. For example, as previously described, one popular gauge application requires a gauge with a variable resistor which is fully responsive to a coupler producing one inch-ounce of torque or less.

Some variable resistor configurations currently used for magnetically coupled liquid level gauges may present problems to the cost-effective achievement of required performance. For example, magnetically coupled liquid level gauges are known using annular wound metal wire resistive elements in the variable resistor. Such elements typically have low contact resistance because of the direct metal to metal contact between wiper and resistive element. This allows relatively low contact pressure to be used, resulting in low wiper drag. Such wire wound elements have the following disadvantages, however: they are relatively expensive to produce; they have a resistive resolution which may be fixed by the number of turns of wire on the element; and their power capacity may be limited by their tendency to develop high temperature "hot spots" in the wire element at low resistance settings which can melt surrounding structures. In addition, the design of wire wound elements usually results in high terminal resistance due to inaccessible windings at the "zero" end of the element. A separate conductive shunt can be used to reduce the terminal resistance, but it further increases the cost of each unit.

Magnetically coupled liquid level gauges are also known using thick film conductive polymer on an insulative substrate for the resistive element of the variable resistor. Such elements may be less expensive to produce than wound metal wire elements. However, because the contact resistance between a metal wiper and such thick films is up to 100 times higher than metal to metal contact, it is generally not possible to exert enough contact pressure to produce acceptable fixed resistances without increasing wiper drag past the level where the magnetic coupler is affected. Existing magnetically coupled liquid level gauges using thick film resistors therefore employ an array of discrete conductive (usually metallic) members which are connected to different points along the resistive element and serve as contact surfaces for the resistive element, each conductive member producing a different resistance when contacted by the wiper. By having the wiper contact the discrete conductive members rather than the surface of the thick film resistive element itself, the contact resistance of the device may be lowered to acceptable levels without requiring excessive contact pressure and the associated high contact drag. However, forming the array of conductive elements adds to the complexity and cost of manufacturing a variable resistor using a thick film resistive element. The resolution of such a device may also be adversely affected by the use of discrete conductors, with such a device typically exhibiting a resistance that changes in "steps" as the wiper moves between conductive members in the array.

Magnetically coupled liquid level gauges are also known using an array of discrete surface mount resistors electrically connected to one another in series and having conductive (usually metallic) taps extending from the connection between each two resistors. These taps extend from the resistor array into an area where they can be contacted by the wiper. The wiper contacts one or more of these conductive taps in sequence to sense a range of resistivities as it moves. As with variable resistors using a thick film resistive element, variable resistors using an array of surface mount resistors achieves low contact resistance by having the wiper contact separately formed conductive elements rather than the surface of the resistive element itself, thus the need for high contact pressure and its associated high wiper drag is avoided. However, as with the thick film resistor device, forming the array of resistors and separate conductive taps adds to the complexity and cost of the manufacturing operation. Similarly, the resolution of such a device may be limited to the number of surface mount resistors in the array, and such a device will exhibit a resistance that changes in "steps" as the wiper moves between conductive taps.

In light of the limitations of existing devices, a need exists for a variable resistor suitable for use in a magnetically coupled liquid level gauge or the like, having infinite resistive resolution without "steps." A need also exists for a variable resistor suitable for use in a magnetically coupled liquid level gauge in which the wiper contacts the surface of the resistive element itself rather than separate conductive elements or taps. A ceramic-metallic material known as cermet has been used for the resistive element of variable resistors for numerous applications. A fluid ink or film of cermet is typically applied to an insulative substrate and then fired at high temperature to form a solid resistive element. This procedure allows the formation of solid cermet resistive elements having specific shapes in an economical operation. It is therefore desirable to utilize a cermet resistive element in the variable resistor of a magnetically coupled liquid level gauge if such a resistor can be formed which produces the desired current, power, fixed resistance, wiper drag, and resolution requirements.

U.S. Patent No. 4,732,802 provides a discussion of use of cermet resistive elements for variable resistors and discloses numerous other patents involving the use of cermet resistive inks. Also discussed are a number of approaches taken toward minimizing the contact resistance of variable resistors using cermet elements. These approaches include the use of multi-fingered wipers to increase the number of contact points, use of noble metals in the wipers to reduce "tarnish" resistivity, increasing the force of the wiper against the resistive element, and the employment of chemical and mechanical (i.e., abrasive) means to remove surface irregularities and contaminants on the resistive film. The patent notes the drawbacks associated with each of these approaches. Another approach for lowering the contact resistivity of a cermet resistive element involves the application, through vapor deposition, screen printing or other methods, of very small "islands" of noble metals or other tarnish resistant high conductivity materials to the surface or near-surface layer of the cermet resistive element contacted by the wiper to form a matrix of discrete, high conductivity regions to be contacted by the wiper. Such techniques may also improve the resolution of the resistor by eliminating the "step" changes occurring when discrete conductor members are used. Such techniques have drawbacks, however, relating to the expense of using noble metals and the additional manufacturing operations required for the application of the metals to the cermet resistive element. In addition, such surface treatments may be susceptible to "encapsulation" by non-metallic components of their carriers during the high temperature operations of manufacture and by wear of the wiper during operation which may literally "wear through" the surface treatment. Such effects will change the contact resistance and shorten the useful life of the resistive element. It is therefore an object of this invention to provide a variable resistor having a cermet resistive element suitable for use in a magnetically coupled liquid level gauge. It is a further object of this invention to provide a liquid level gauge having a variable resistor with cermet resistive element which does not require separate non-cermet conductive members or the infusion of noble metals or other high conductivity materials into the surface of the resistive element.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a gauge is provided for measuring a fluid level. The gauge includes a base having a pivot pin extending therefrom having a pivot pin axis extending therethrough. A pointer assembly is mounted on the pivot pin for rotation about the pivot pin axis. An electrically conductive contact plate is included on pointer assembly, the contact plate having at least one primary wiper arm and at least one secondary wiper arm. A resistor assembly is positioned proximate the pointer assembly. The resistor assembly includes a substrate plate formed of an electrically insulative material and having a flat face disposed perpendicular to the pivot pin axis and confronting the pointer assembly. The resistor assembly also includes a resistive element adhering to the face of the plate, the resistive element being formed of an electrically resistive cermet material in a semi¬ circular arc at a predetermined radius about the pivot pin axis. The resistive element has first and second ends and an exposed surface extending circumferentially therebetween. The exposed surface of the resistive element lies in a plane generally perpendicular to the pivot pin axis. The resistive element may have either linear or nonlinear resistance along its circumferential length, however, at any given point the cermet material of the resistive element has a uniform composition throughout its thickness in the axial direction (i.e., parallel to the pivot pin axis) . The resistor assembly further includes a lubricant material applied to the exposed surface of the resistive element and a conductive element adhering to the face of the plate. The conductive element is formed of an electrically conductive material about the pivot pin axis and is spaced apart from the resistive element. The primary wiper arm of the contact plate is resiliently biased to contact with a contact force, the exposed surface of the resistive element at a contact point along the exposed surface determined by the angular position of the pointer assembly about the pivot pin axis. The contact force is within the range of about three grams to about nine grams when measured in a direction normal to the exposed surface. The secondary wiper arm of the contact plate contacts the conductive element on the face of the substrate plate. A coupler is provided attached to the pointer assembly for rotation of the pointer assembly about the pivot pin axis in response to changes in the fluid level, thus a first and second range of electrical resistances are provided between the conductive element and the first and second ends of the resistive element, respectively, when the fluid level changes through a range of positions. In a preferred embodiment of the current invention, the resistive element of the gauge can continuously dissipate at least 0.9 watts of power without failure. In a more preferred embodiment of the current invention, the primary wiper arm is formed of a material being selected from a group consisting of monel metal, nickel silver, and stainless steel. In still a more preferred embodiment of the current invention, the lubricant material is selected from a group consisting of specified greases, oils and other lubricants. In accordance with another aspect of the present invention, a variable resistor for a magnetically coupled gauge or the like is provided. The variable resistor includes a resistor assembly including a substrate plate formed of an electrically insulative material having a substantially flat face and having an axis formed perpendicular to the face. A resistive element adhering to the face of the plate is provided. The resistive element is formed of an electrically resistive cermet material in a semi-circular arc at a predetermined radius about the axis and has first and second ends and exposed surface extending circumferentially therebetween. The exposed surface of the resistive element lies on a plane generally perpendicular to the axis. The cermet material at any given point on the resistive element has a uniform composition throughout its thickness in the axial direction. A lubricant material is provided applied to the exposed surface of the resistive element. A conductive element adhering to the face of the plate is provided. The conductive element is formed of an electrically conductive material about the axis and is spaced apart from the resistive element. A pointer assembly is provided positioned proximate to the resistive element and adapted to pivot about the axis. The pointer assembly includes an electrically conductive contact plate. The contact plate includes at least one primary wiper arm and at least one secondary wiper arm. The primary wiper arm is resiliently biased to contact with a contact force the exposed surface of the resistive element at a contact point along the exposed surface of the resistive element. The contact point is determined by the angular position of the pointer assembly about the axis. The contact force is within the range of about three grams to about nine grams when measured in a direction normal to the exposed surface. The secondary wiper arm of the contact plate contacts the conductive element. Thus a first and second range of electrical resistances are provided between the conductive element and the first and second ends of the resistive element, respectively, when the pointer assembly is moved through a range of angular positions about the axis.

Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein several preferred embodiments of this invention are shown and described. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention will become more apparent from the following description and claims, and from the accompanying drawings, wherein: FIGURE 1 is a view of a float gauge assembly including a liquid level gauge according to the present invention and associated fluid sensing structure;

FIGURE 2 is a front elevation view of a gauge according to the current invention; FIGURE 3 is a vertical cross-sectional view of the gauge shown in FIGURE 2 taken along line 3-3 of FIGURE 2;

FIGURE 4 is an exploded perspective view of the components of a variable resistor according to the current invention; FIGURE 5a is a side view of the pointer assembly of FIGURE 3; and

FIGURE 5b is a plan view of the pointer assembly of FIGURE 3.

DETAILED DESCRIPTION

Referring now to the drawings, wherein like reference numerals designate like or corresponding parts through the several views, and in particular to FIGURE 1, there is illustrated a float gauge assembly 10 which provides an electrical indication, and possibly also a visual or other indication, of a fluid level when provided with a liquid level gauge 12 according to the present invention.

Float gauge assembly 10 includes gauge 12 resting in a recess 14 of mounting bracket 16 of a fluid level sensing structure 18. The gauge 12 and fluid level sensing structure 18 can be mounted to tank 20 by bolts 22 at a convenient position on the wall of tank 20 containing a fluid 24. As those skilled in this art will readily appreciate, mounting bracket 16 may also be separately bolted to the tank and the gauge attached to the mounting bracket by a separate set of screws or, alternatively, the portion of the mounting bracket extending through the tank can be supplied with screw threads to mate with a threaded opening in the tank. In this embodiment, the gauge 12 can be screwed into the face of the mounting bracket 16. The fluid contained in tank 20 may be a conventional liquid such as a fuel oil, gasoline, or water, or it may be a pressure-liquefied gas such as liquefied petroleum gas (LPG) or refrigerant such as Freon.

The fluid level sensing structure 18 includes a fixed arm 26 that extends from gauge 12 and into tank 20 to a fitting 28. A pivot arm 30 is mounted at a point along its length to fitting 28 for pivotal motion about an arm axis 32. A first gear 34 is mounted on pivot arm 30 for movement with the arm. A first end of pivot arm 30 mounts a float 36 which is so configured so as to float at surface 38 of fluid 24. A counterweight 40 is mounted at the opposite end of pivot arm 30. The first gear 34 meshes with a pivot gear 42 (not shown) on a shaft 44 extending within the fixed arm 26 to a position near gauge 12. The end of shaft 44 proximate to gauge 12 has a coupler 46 (not shown) mounted thereto.

Pivot arm 30 pivots about pivot axis 32 as the float 36 follows level 38 of the fluid. The pivotal motion of pivot arm 30 causes first gear 34 to rotate second gear 42 and shaft 44 to rotate coupler 46 about a gauge axis 48. In a preferred embodiment, coupler 46 is a magnet which allows transmission of forces to movements of float 36 across a short distance to gauge 12 without mechanical connection. This is especially advantageous for liquid level gauges since it does not require the use of dynamic fluid seals in the gauge which could allow the leakage of pressurized or flammable fluids from the tank.

As those skilled in this art will readily appreciate, FIGURE 1 illustrates but one possible embodiment of a fluid level sensing structure 18 which imparts movement to a coupler 46. Other fluid sensing structures can be used within the scope of the current invention.

Referring now generally to FIGURES 2-5b, the movement of coupler 46 about gauge axis 48 affects gauge 12 as described hereinafter. Referring specifically to FIGURES 3 and 4, gauge 12 has a base 50 including a pivot pin 52 extending from the base along pivot pin axis 48. In the embodiment shown in FIGURE 3, base 50 has side wall details including a recess 54 and an annular surface 56 forming a perimeter around recess 54, however, those skilled in this art will readily appreciate that bases having other side wall details would fall within the scope of the current invention. A pointer assembly 58 is provided with a recess 60 to fit over pivot pin 52 on base 50. The pointer assembly 58 thus pivots or rotates about pivot pin axis 48 with relatively little friction. On the other side of pointer assembly 58 opposite recess 60, there is formed a disk shaped recess 62 which receives an electrically conductive contact plate 64. Contact plate 64 includes at least one primary wiper arm 74 and at least one secondary wiper arm 78. In the embodiment illustrated in FIGURES 3 and 4, contact plate 64 is a "multi-finger" type comprising three primary wiper arms 74 and two secondary wiper arms 78, however those skilled in the art will readily appreciate that contact plates having other numbers of wiper arms are within the scope of this invention. Although in the preferred embodiment, contact plate 64 and primary and secondary wiper arms 74, 78 are formed from a single piece of electrically conductive material, those skilled in this art will readily appreciate that the contact plate and wiper arms could be formed separately and combined in a variety of configurations without departing from the scope of this invention provided each wiper arm is electrically connected to the contact plate. An alignment protrusion 66 on pointer assembly 58 and a mating alignment groove 68 on the contact plate 64 may be provided to orient plate 64 relative to pointer assembly 58 about pivot pin axis 48. A pointer 70 may be provided extending outwardly from one side of assembly 58 generally along a radius extending from pivot pin axis 48. Pointer 70 will allow gauge 12 to provide a visual indication of liquid level as well as an electrical indication. A coupler 72 is secured to pointer assembly 58 and interacts with coupler 46 of fluid level sensing structure 18 so that the position of pointer assembly 58 about axis 48 correlates directly with the position of float 36 and thus the surface level 38 of fluid 24. In a preferred embodiment, couplers 72 and 46 are magnets which allow the transmission of forces therebetween without requiring physical contact, however, those skilled in this art will readily appreciate that other coupling methods can be used without departing from the scope of the invention. Referring still to FIGURES 3 and 4, a resistor assembly 79 is provided positioned proximate to pointer assembly 58. Resistor assembly 79 includes a substrate plate 80 formed of an electrically insulative material and having a substantially flat face 81 disposed perpendicular to pivot pin axis 48 and confronting pointer assembly 58. A resistive element 82 is provided adhering to face 81 of plate 80. Resistive element 82 is formed of an electrically resistive cermet material in a semi-circular arc at a predetermined radius about pivot pin axis 48 and has first and second ends 84, 86 and an exposed surface 88 extending circumferentially therebetween. Exposed surface 88 of the resistive element lies in a plane generally perpendicular to pivot pin axis 48. In a preferred embodiment, resistive element 82 is formed by applying a track of cermet ink to substrate 80 and then firing it at high temperature. Resistive element 82 may have either linear or nonlinear resistance with respect to the circumferential direction about pivot pin axis 48. In a linear element, resistive element 82 has an end-to-wiper resistance that changes by a constant value for each degree of wiper rotation about pivot pin axis 48. In a nonlinear element, resistive element 82 has an end-to- . ,_{Λ Λ^},.,„

WO 97/4377

21

wiper resistance change per degree of wiper rotation about pivot pin axis 48 that is dependent upon the absolute angular position of pointer assembly 58. It is important that resistive element 82 can be produced in nonlinear form since that allows a gauge to provide an output that is proportional to the quantity of liquid in an oddly shaped tank. One method of producing nonlinear resistive elements is to vary the width (i.e., in the radial direction) of the element along its circumferential length. A second method is to print the resistive track forming the resistive element as a series of circumferential segments using different cermet inks having different resistivities for each segment. These methods can also be combined. Regardless of whether resistive element 82 is linear or nonlinear, however, the cermet material at any point on resistive element 82 has a uniform composition throughout its thickness in the axial direction (i.e., parallel to the pivot pin axis) . Thus, the resistive element does not have noble metals or other high conductivity materials infused, vapor deposited, plated, printed or otherwise incorporated into its exposed surface 88 or the region below the surface of resistive element 82. A lubricant material 90 (not shown) is applied to exposed surface 88 of resistive element 82. A conductive element 92 is also provided adhering to face 81 of plate 80. Conductive element 92 is formed of an electrically conductive material about axis 48 and is spaced apart from resistive element 82. In a preferred embodiment, a conductive ink is printed on face 81 of substrate plate 80 and then fired to form conductive element 92. This conductive ink may be silver based. After firing, the conductive areas may be plated with a second metal such as nickel or tin to improve durability. It is preferred that conductive element 92 provide low contact resistance with low contact force. Primary wiper arms 74 of pointer assembly 58 are resiliently biased to contact with a contact force exposed surface 88 of resistive element 82 at a contact point (or group of contact points, in the case of multiple contacts) along exposed surface 88 determined by an angular position 102 of pointer assembly 58 about gauge axis 48. The contact force between primary wiper arms 74 and the exposed surface 88 of resistive element 82 is within the range of about three grams to about nine grams when measured in a direction normal to exposed surface 88. In a wiper arm 74 having multiple "fingers" as shown in FIGS. 3 and 4, then the contact force is the total of the contact forces of all arms 74. The secondary wiper arms 78 do not contact the resistive element, thus the contact forces of the secondary wiper arms 78 are not added to this contact force. Those skilled in the art will recognize that while contact pressure is the parameter which actually affects many phenomena such as contact resistance, contact force is much more readily determined and thus is commonly specified in designs having total contact areas of less than about 0.001 square inches. Secondary wiper arms 78 contact conductive element 92 thereby completing a variable resistor circuit in which a first and second range of electrical resistances are provided between conductive element 92 and first and second ends 84, 86, respectively, of resistive element 82 as fluid level 38 is changed through a range of positions. In the preferred embodiment shown in FIGURE 4, resistor assembly 79 further includes electrical terminals 85, 87 and 93 which are electrically connected to resistive element ends 84, 86 and conductive element 92, respectively, by conductive traces 89, 91 and 95 to provide a convenient location for accessing the resistances produced by variable resistor assembly 104. Electrical terminals 85,87 and 93, like other conductive traces and pads, may be formed of silver based conductive inks which are fired and then plated with a second metal such as nickel or tin to improve durability. Those skilled in the art will readily appreciate, however, that other methods or materials can be used to form conductive traces and pads without departing from the scope of the current invention.

In a preferred embodiment of the current invention, variable resistor assembly 104 of gauge 12 can continuously dissipate at least 0.9 watts of power without failure, where failure is defined as the melting, burning, warping, or otherwise permanently changing the resistive element, the substrate plate, or any portion of the associated gauge structure so that the gauge will no longer function to provide an electrical resistance corresponding to measured fluid level, or where the resistance values for a given fluid level deviate from acceptable tolerances.

Referring again to FIGURE 3, in a more preferred embodiment of the current invention resistor assembly 79 is mounted in a cover 94 and cover 94 mates with base 50 to form a fluid tight seal enclosing the pointer assembly 58, contact plate 64 and resistor assembly 79. Two or more conductive electrical prongs 98 may be provided which pass through cover 94. If one such prong 98 is connected to conductive element terminal 93 and a second such prong 98 is connected to either first resistive element terminal 85 or second resistive element 87, then a rheostat-type circuit is provided, while if an electrical prong 98 is connected to each of the three electrical terminals 85, 87 and 93, then a voltage divider or two complementary rheostat-type circuits is provided. Cover 94 is preferably sealed with sealing material 99 at the point where electrical prongs 98 pass through the cover such that the fluid tight seal is maintained. Connecting wires 100 may be connected to the exterior portions of electrical prongs 98 such that resistive signals provided by gauge 12 can be transmitted to other equipment. An electrical connector cover 96 may be provided which mates with cover 94 to protect the junction between electrical prongs 98 and connecting wires 100. While not required, it is preferred that a potting material 104 be used to fill the cavity between cover 94 and electrical connector cover 96 so that any exposed metal portions of prongs 98 or connecting wire 100 are protected from moisture or corrosive fluids. The potting material may be an epoxy resin, a catalyzed or thermosetting liquid, or other potting materials known in the art.

Referring now to FIGURE 4, in a preferred embodiment of the current invention, the electrically insulative material of substrate plate 80 is a ceramic. Most ceramic materials are electrically non-conductive, have high heat resistance and high thermal conductivity, all of these characteristics being desirable for substrate plate 80. In a more preferred embodiment, the electrically insulative material of substrate plate 80 is alumina. The useful life of a liquid level gauge may be measured in terms of the number of times the resistor assembly 104 may be cycled through its full range while maintaining a given resistance precision, i.e., while the maximum resistance of the gauge is within a given percentage of the original maximum resistance. For applications such as vehicle fuel tanks and long term storage facilities, it is desirable to have a liquid level gauge which can withstand five hundred thousand cycles and maintain a 5% resistive precision.

It has been determined that such a useful life may be provided by preferred embodiments of the current invention, such as a first preferred embodiment incorporating particular materials for the primary wiper arm 74, a more preferred embodiment incorporating a lubricant material 90 applied to exposed surface 88 of the resistive element 82, and a still more preferred embodiment specifying a range of contact pressures between tip 76 of primary wiper arm 74 and exposed surface 88 of resistive element 82.

In a preferred embodiment, primary wiper arms 74 are formed of a material selected from the group consisting of monel metal, nickel silver and stainless steel. If the material of primary wiper arms 74 is too soft, then the material will be removed from wiper arm tips 76 by the slightly abrasive exposed surface 88 of resistive element 82 and deposited onto exposed surface 88. The transferred materials will form a path of relatively low resistance along surface 88 that is in parallel with the inherent resistance of element 82. This low resistance path causes failure of the device by reducing the end-to-end resistance of element 82 below the acceptable range. On the other hand, if the material of primary wiper arms 74 is too hard, then the arm material will itself wear away the exposed surface 88 of the cermet resistive element 82, creating a "groove" or "trench" in the exposed surface 88. The groove reduces the current carrying volume of the cermet resistive element 82. This causes failure of the device by increasing the end-to-end resistance of element 82 above the acceptable range.

In a more preferred embodiment, a lubricant material 90 is applied to exposed surface 88 of cermet resistive element 82 to achieve a useful life for gauge 12 without the use of resistive elements having exposed surfaces infused or overlaid with conductive materials or having the wiper arm contact separate non-cermet conductor surfaces electrically connected to the resistive element rather than contacting the cermet resistive element itself. Without being bound by the following explanation, it is believed that the lubricant material applied to the exposed surface 88 of resistive element 82 must provide lubrication between the contact surfaces while resisting breakdown caused by the electrical micro-arcs present during current flow. Lubricant materials suitable for application to exposed surface 88 include a grease formed from halogenated silicone oil gelled with PTFE (polytetrafluoroethelyne) polymer (sold commercially as Nye Fluorocarbon Gel 813), an oil formed from the halogenation of phenol radicals attached to silicone oil (sold commercially as Nye NyoSil Oil) , a grease formed from completely fluorinated poly ether fluids and PTFE solids (sold commercially as Nye Fluoroether Grease 899- 1) , an oil formed from completely fluorinated poly ethers consisting of CF₂ groups interconnected by oxygen in a polymer with no side chains (sold commercially as Nye

Fluoroether Oil 499) , a lithium soap gelled grease using a low viscosity synthetic hydrocarbon as the base oil (sold commercially as Nye Rheolube 737B) , and a stabilant formed from a modified polyoxypropylene-polyoxyethylene block polymer of the polyglycol family (sold commercially as D.W. Electrochemical Stabilant 22) . In a still more preferred embodiment of the current invention, primary wiper arms 74 are resiliently biased to contact exposed surface 88 of resistive element 82 with a contact force within the range of about four grams to about eight grams when measured in a direction normal to exposed surface 88. If too little contact force is used, excessive contact wear or erosion will ultimately result because of breakdown of the lubricant caused by excessive micro-arcing. If too much contact force is used, the tendency for conductive material to transfer from arms 74 to surface 88 or for the abrasion of surface 88 by arms 74 will greatly increase. In addition, increasing force increases the drag experienced by the pointer assembly as it moves primary wiper arms 74 across the resistive element. High wiper drag will make a variable resistor unsuitable for use in a magnetically driven gauge. In a most preferred embodiment of the current invention, primary wiper arms 74 are formed of nickel silver material, lubricant material 90 comprises a lithium soap gelled grease using a low viscosity (i.e., viscosity at 100°F within the range of about 12 centistokes to about 25 centistokes) synthetic hydrocarbon as the base oil, and primary wiper arms 74 are resiliently biased to produce a total contact force against exposed cermet surface 88 within the range of about five grams to about seven grams when measured in a direction normal to exposed surface 88. A resistor assembly 104 using this combination of wiper arm material, contact pressure and lubricant may allow gauge 12 to have a useful life of over five hundred thousand cycles while maintaining a resistive precision within 5% of the original value.

Referring now to FIGURES 2, 3, and 4, In an even more preferred embodiment of the current invention, cover 94 is formed of a transparent material and pointer assembly 58 further defines a pointer 70 extending outwardly from one side of pointer assembly 58 generally along a radius extending from gauge axis 48. An annular face plate 106 having visible indicia 108 is mounted between base 50 and cover 94 within the fluid tight interior. The position of pointer 70 with respect to indicia 108 provides a visual indication of the fluid level being measured by gauge 12 in addition to the electrical indication provided by the resistor assembly 104.

Another aspect of the current invention provides a high power variable resistor assembly suitable for a magnetically driven gauge or other low rotational drag application. Referring to FIGURE 4, variable resistor assembly 104 incorporates many components which were previously described in the liquid level gauge 12 of the previous aspect of this invention. Variable resistor 104 comprises a resistor assembly 79 and a pointer assembly 58. Resistor assembly 79 includes a substrate plate 80 formed of an electrically insulative material having a substantially flat face 81 and having an axis 48 formed perpendicular to face 81. Resistive element 82 adheres to face 81 of plate 80. Resistive element 82 is formed of an electrically resistive cermet material in a semi-circular arc at a predetermined radius about axis 48 and has first and second ends 84, 86 and an exposed surface 88 extending circumferentially therebetween. Exposed surface 88 lies in a plane generally perpendicular to axis 48. Resistive element 82 may have either linear or nonlinear resistance characteristics along its circumferential length, however, at any given point the cermet material of resistive element 82 has a uniform composition throughout its thickness in the axial direction. A lubricant material 90 (not shown) is applied to exposed surface 88 of resistive element 82. A conductive element 92 adheres to face 81 of plate 80. Conductive element 92 is formed of an electrically conductive material about axis 48 and is spaced apart from resistive element 82. Pointer assembly 58 is positioned proximate to resistive element 82 confronting face 81 and is adapted to pivot about axis 48 under the influence of a torque of one inch-ounce or less. In a more preferred embodiment, pointer assembly 58 is adapted to pivot about axis 48 under the influence of a torque of 0.5 inch-ounce or less. Pointer assembly 58 includes an electrically conductive contact plate 64. Plate 64 is affixed to pointer assembly 58 so that it will move with pointer assembly 58 as the assembly pivots about axis 48. Contact plate 64 includes at least one electrically conductive primary wiper arm 74 and at least one electrically conductive secondary wiper arm 78. In a preferred embodiment, contact plate 64, three primary wiper arms 74, and two secondary wiper arms 78 are formed from a single piece of electrically conductive material, however, those skilled in this art will readily appreciate that one or both of the wiper arms may be formed separately from contact plate 64 provided there is an electrical connection between the wiper arms 74, 78 and contact plate 64. Primary wiper arms 74 are resiliently biased to contact with a combined contact force exposed surface 88 of resistive element 82 at a contact point (or group of contact points in the case of multiple arms 74) along exposed surface 88 determined by an angular position 102 of pointer assembly 58 relative to axis 48, the contact force being within the range of about three gram to about nine grams when measured in a direction normal to exposed surface 88. Secondary wiper arms 78 contact conductive element 92 thereby establishing a variable resistor circuit providing a first and second range of electrical resistances between conductive element 92 and first and second ends 84, 86 of resistive element 88, respectively, when pointer assembly 58 is moved through a range of angular positions 102 about axis 48.

In a more preferred embodiment of the current invention, resistive element 88 can continuously dissipate 0.9 watts of power without failure. In another embodiment of the current invention, the electrically insulative material of substrate plate 80 is a ceramic, since ceramics have desirable characteristics with respects to mechanical strength, heat resistance, thermal conductivity and electrical insulation. In a more preferred embodiment of the current invention, the electrically insulative material on substrate plate 80 is alumina.

As discussed in the description of the variable resistor assembly for liquid level gauge 12 of the first aspect of this invention, the selection of a material for the primary wiper arm, the contact force between primary wiper arms and the exposed surface of the resistive element, and the composition of a lubricant material applied to the exposed surface of the resistive element may affect the useful service life of a variable resistor having a continuous power rating of one watt or more. In one aspect of the current invention, variable resistor 104 has primary wiper arms 74 formed of a material being selected from a group consisting of monel metal, nickel silver and stainless steel. In another embodiment of the current invention, the combined contact force between primary wiper arms 74 and exposed surface 88 of the resistive element 82 is within the range of about four grams to about eight grams when measured in a direction normal to exposed surface 88. In yet another preferred embodiment, the lubricant material applied to the exposed surface 88 of resistive element 82 is selected from the group of greases, oils and stabilants previously described for the first aspect of the invention. In a most preferred embodiment of this invention, variable resistor 104 incorporates primary wiper arms 74 formed of nickel silver material, wiper arms 74 are resiliently biased to press against exposed surface 88 of resistive element 82 with- a combined contact force within the range of about five grams to about seven grams when measured in a direction normal to exposed surface 88, and lubricant material 90 applied to exposed surface 88 is a lithium soap gelled grease using a low viscosity synthetic hydrocarbon as a base oil, such as that commercially sold under the name Nye rheolube 737B. While presently preferred embodiments of the invention have been illustrated and described, it will be understood that the invention is not limited thereto, but may be otherwise variously embodied within the scope of the following claims.

Claims

I CLAIM :

1. A gauge for measuring a fluid level, comprising:

(a) a base having a pivot pin extending therefrom having a pivot pin axis therethrough;

(b) a pointer assembly mounted on the pivot pin for rotation about said pivot pin axis; said pointer assembly including an electrically conductive contact plate thereon; said contact plate including at least one primary wiper arm and at least one secondary wiper arm;

(c) a resistor assembly positioned proximate said pointer assembly, said resistor assembly comprising:

(i) a substrate plate formed of an electrically insulative material and having a substantially flat face disposed perpendicular to said pivot pin axis and confronting said pointer assembly; (ii) a resistive element adhering to said face of said substrate plate; said resistive element formed of an electrically resistive cermet material in a semi-circular arc at a predetermined radius about said pivot pin axis and having first and second ends and an exposed surface extending circumferentially therebetween; said exposed surface lying in a plane generally perpendicular to said pivot pin axis; said cermet material at any given point of said resistive element having a uniform composition throughout its thickness in the axial direction; (iii) a lubricant material applied to said exposed surface of said resistive element;

(iv) a conductive element adhering to said face of said substrate plate; said conductive element formed of an electrically conductive material about said pivot pin axis and being spaced apart from said resistive element;

(d) said primary wiper arm being resiliently biased to contact with a contact force said exposed surface of said resistive element at a contact point along said exposed surface determined by an angular position of the pointer assembly about said pivot pin axis said contact force being within the range of about three grams to about nine grams when measured in a direction normal to said exposed surface;

(e) said secondary wiper arm contacting said conductive element; and

(f) a coupler attached to said pointer assembly for changing said angular position of said pointer assembly about said pivot pin axis in response to changes in a fluid level; whereby a first and second range of electrical resistances are provided between said conductive element and said first and second ends of said resistive element, respectively, when said fluid level changes through a range of positions.

2. The gauge of claim 1 wherein said resistive element can continuously dissipate at least 0.9 watts of power without failure.

3. The gauge of claim 2 further defining a cover, said resistor assembly mounted in said cover, said cover secured to said base to form a fluid tight seal enclosing the pointer assembly, contact plate and resistor assembly.

4. The gauge of claim 3, wherein said coupler is a magnet.

5. The gauge of claim 3, wherein said electrically insulative material of said substrate plate is a ceramic.

6. The gauge of claim 5, wherein said electrically insulative material of said substrate plate is alumina.

7. The gauge of claim 3, wherein said primary wiper arm is formed of a material being selected from a group consisting of monel metal, nickel silver, and stainless steel.

8. The gauge of claim 3, wherein said contact force is within the range of about four grams to about eight grams when measured in a direction normal to said exposed surface.

9. The gauge of claim 3, wherein said lubricant material is selected from a group consisting of: a grease formed from halogenated silicone oil gelled with a PTFE (polytetrafluoroethylene)polymer; an oil formed from the halogenation of phenyl radicals attached to silicone oil; a grease formed from completely fluorinated poly ether fluids and PTFE (polytetrafluoroethylene) solids; ,„„„„ O 97/43772

35

an oil formed from completely fluorinated poly ethers consisting of CF₂ groups interconnected by oxygen in a polymer with no side chains; a lithium soap gelled grease using as a base oil a synthetic oil having a viscosity at 100^CF within the range of about 12 centistokes to about 25 centistokes; and a stabilant formed from a modified polyoxypropylene- polyoxyethylene block polymer of the polyglycol family.

10. The gauge of claim 3 wherein said primary wiper arm is formed of nickel silver, said contact force is within the range of about five grams to about seven grams when measured in a direction normal to said exposed surface, and said lubricant material is a lithium soap gelled grease using as a base oil a synthetic oil having a viscosity at 100°F within the range of about 12 centistokes to about 25 centistokes.

11. The gauge of claim 10 wherein said cover is formed of a transparent material, and wherein said pointer assembly further defines a pointer extending outwardly from one side of said pointer assembly generally along a radius extending from said axis, whereby a visual indication is provided of said liquid fluid level.

12. A variable resistor for a magnetically driven gauge or the like, said variable resistor comprising: a resistor assembly including: a substrate plate formed of an electrically insulative material having a substantially flat face and having an axis formed perpendicular to said face; a resistive element adhering to said face of said plate; said resistive element formed of an electrically resistive cermet material in a semi-circular arc at a predetermined radius about said axis and having first and second ends and an exposed surface extending circumferentially therebetween; said exposed surface lying in a plane generally perpendicular to said axis; said cermet material having, at any given point of said resistive element, a uniform composition throughout its thickness in the axial direction; a lubricant material applied to said exposed surface of said resistive element; a conductive element adhering to said face of said plate; said conductive element formed of an electrically conductive material about said axis and being spaced apart from said resistive element; and a pointer assembly positioned proximate to said resistor assembly and confronting said resistive element and adapted to pivot about said axis under the influence of a torque of one inch- ounce or less; said pointer assembly including an electrically conductive contact plate thereon, said contact plate including at least one primary wiper arm and at least one secondary wiper arm- said primary wiper arm being resiliently biased to contact with a contact force said exposed surface of said resistive element at a contact point along said exposed surface determined by an angular position of the pointer assembly about said axis, said contact force being within the range of about three grams to about nine grams when measured in a direction normal to said exposed surface; and said secondary wiper arm contacting said conductive element; whereby a first and second range of electrical resistances are provided between said conductive element and said first and second ends of said resistive element, respectively, when said pointer assembly is moved through a range of angular positions about said axis.

13. The variable resistor of claim 12, wherein said resistive element can continuously dissipate 0.9 watts of power without failure.

14. The variable resistor of claim 13, wherein said electrically insulative material of said substrate plate is a ceramic.

15. The variable resistor of claim 14, wherein said electrically insulative material of said substrate plate is alumina.

16. The variable resistor of claim 13, wherein said primary wiper arm is formed of a material being selected from a group consisting of monel metal, nickel silver, and stainless steel.

17. The variable resistor of claim 13, wherein said contact force is within the range of about four grams to about eight grams when measured in a direction normal to said exposed surface.

18. The variable resistor of claim 13, wherein said lubricant material is selected from a group consisting of: a grease formed from halogenated silicone oil gelled with a PTFE; an oil formed from the halogenation of phenyl radicals attached to silicone oil; a grease formed from completely fluorinated poly ether fluids and PTFE (polytetrafluoroethylene) solids; an oil formed from completely fluorinated poly ethers consisting of CF₂ groups interconnected by oxygen in a polymer with no side chains; a lithium soap gelled grease using as a base oil a synthetic oil having a viscosity at 100°F within the range of about 12 centistokes to about 25 centistokes; and a stabilant formed from a modified polyoxypropylene- polyoxyethylene block polymer of the polyglycol family.

19. The variable resistor of claim 13 wherein said primary wiper arm is formed of nickel silver, said contact force is within the range of about 5 grams to about 8 grams when measured in a direction normal to said exposed surface, and said lubricant material is a lithium soap gelled grease using as a base oil a synthetic oil having a viscosity at 100°F within the range of about 12 centistokes to about 25 centistokes.