Specifically, it is directed toward a system that provides a durable, submersible sensor designed for installation in a fuel tank.

BACKGROUND OF THE INVENTION It is well-known that the fuel tank of a motor vehicle is a hostile environment for a sensor system. When electrical fuel level sensing systems were first developed, the transmitter units, which were intended to operate at least partially submerged in the fuel, were designed as potentiometric sensors of a wire-wound or metal foil type. A float arrangement was used to detect the liquid level, and by coupling the float to a sliding contact on the potentiometric sensor, a measurement system in the vehicle could track changes in resistance that occurred with variations in liquid level.

Unfortunately, the early potentiometric transmitter units were not durable enough to withstand the hostile environment in the fuel tank.

Breakage of wire in the wire-wound resistive sensor types, and peeling of foil in the metal-foil variety of sensor, often led to early failure of the sensing system. Of course, the deleterious effect of automotive fuel on the wire or metal foil tended to accelerate system wear.

Transmitter units manufactured using wire-wound or metal foil

techniques were eventually replaced by resistive film screen-printed on a durable substrate, such as a substrate of ceramic material. FIG. 1 illustrates a fuel tank mounted sensor of the prior art in which a dual wiper 3 moves along a printed resistive track 8 in response to fuel level changes conveyed to the wiper by an associated float (not shown). The printed resistive track 8 is deposited on a substrate 9 of ceramic material or porcelain coated steel for durability.

The printed resistive region 8, even though formed from glass frit in combination with precious metals, is still subject to wear due to friction with the wiper arm, and is prone to the adverse effects of gasoline (or other volatile fuel product) and its vapors. A variation on the early sensors of the prior art is illustrated in FIG. 2, in which the wiper contacts (3 in FIG. 1) are designed to slide across a network of metallized conductive traces 14 rather than the resistive film 12,13 itself. Designing the system so that the wiper makes contact with high metal content conductive regions provides a lower resistance path in operation.

The metallized traces 14 can also be designed for durability and long life in the presence of hostile solvents such as gasoline, but the materials for these traces 14 must generally be selected from among an expensive group of candidate materials. Suitable metals include palladium, platinum, nickel, gold, and silver, which can be combined into alloys that perform adequately in the intended environment.

Another feature often incorporated into level sensing units of the prior art is a non-linear resistance characteristic that tracks the actual shape of the fuel tank whose level is being measured. This non-linear characteristic is evident in FIG. 2 from the varying width profiles or steps that appear in the resistive film regions 12,13. As can be appreciated from the sensor layout of FIG. 2, the wider the resistive regions 12, 13 become, the longer the underlying conductive traces 14 must be in order to make proper electrical contact with the resistor areas. It has also

become common to introduce redundant conductive tracks (304a in FIG.

3) to minimize overall resistance and reduce the chance of sensor failure.

The use of both multiple conductive tracks and variable-width resistor areas to provide a non-linear characteristic has led to the use of larger quantities of expensive conductive material (such as palladium alloys) in the manufacture of sensor units. Consequently, a need arises for a measurement system implementation that is suitably rugged for fuel tank applications, relatively simple and economical to manufacture, and that provides reliability and repeatability in liquid level measurements.

SUMMARY OF THE INVENTION These needs and others are satisfied by the present invention, in which a variable resistor sensing unit for measuring liquid level in a container through interaction with a sliding electrical contact operating in conjunction with a float comprises a ceramic substrate, a first conductor pattern deposited on the substrate, a pattern of resistive ink making electrical contact with the first conductive pattern, a second pattern of conductive traces deposited over the pattern of resistive ink, and an electrically insulating material selectively deposited on exposed areas of the first conductor pattern, such that a portion of the second pattern of conductive traces defines a track of conductive traces over which the sliding electrical contact travels, and the electrical resistance of the variable resistor sensing unit indicates liquid level in the container.

Preferably, the ceramic substrate comprises 96% alumina.

In accordance with one aspect of the invention, the first conductor pattern includes at least two segments spaced apart from one another, and the pattern of resistive ink includes first and second lateral end points, the pattern of resistive ink making electrical contact with one segment of the first conductor pattern at the first lateral end point, and making electrical contact with the other segment of the first conductor pattern at the second

lateral end point. The first conductor pattern may be formed by depositing low cost, silver alloy conductive traces onto the ceramic substrate. The silver alloy conductive traces may comprise platinum- silver alloy having a relatively low platinum content. In one form of the invention, the pattern of resistive ink is formed from a ruthenium oxide resistive ink including at least trace amounts of palladium and silver.

In accordance with another aspect of the invention, the second pattern of conductive traces is formed by depositing palladium silver alloy conductive traces such that the second pattern of conductive traces substantially overlies at least the pattern of resistive ink. The second pattern of conductive traces may comprise a palladium-silver alloy composition having a Ag: Pd ratio of at most 3: 1 by weight. In one form of the invention, the electrically insulating material comprises a thermoset polymeric material. The thermoset polymeric material may comprise, at least in part, an epoxy resin.

In accordance with yet another aspect of the invention, a variable resistor sensing unit for measuring liquid level in a container through interaction with a sliding electrical contact operating in conjunction with a float comprises a ceramic substrate; a first conductor pattern deposited on the substrate, the first conductor pattern including at least two segments spaced apart from one another; a pattern of resistive ink including first and second lateral end points, the pattern of resistive ink making electrical contact with one segment of the first conductor pattern at the first lateral end point and making electrical contact with the other segment of the first conductor pattern at the second lateral end point; and a second pattern of conductive traces deposited over the pattern of resistive ink, wherein the second pattern of conductive traces is formed by depositing palladium silver alloy conductive traces such that the second pattern of conductive traces substantially overlies at least the pattern of resistive ink. An electrically insulating material is then

selectively deposited on exposed areas of the first conductor pattern. A portion of the second pattern of conductive traces defines a track of conductive traces over which the sliding electrical contact travels, and electrical resistance of the variable resistor sensing unit indicates liquid level in the container.

In still a further aspect of the invention, a variable resistor sensing unit for measuring liquid level in a container through interaction with a sliding electrical contact operating in conjunction with a float is produced by a process comprising the steps of providing a ceramic substrate; depositing a first conductor pattern on the substrate; depositing a pattern of resistive ink making electrical contact with the first conductive pattern; depositing a second pattern of conductive traces over the pattern of resistive ink; and selectively depositing an electrically insulating material on exposed areas of the first conductor pattern. A portion of the second pattern of conductive traces defines a track of conductive traces over which the sliding electrical contact travels, and electrical resistance of the variable resistor sensing unit indicates liquid level in the container.

In one form of the invention, the step of providing a ceramic substrate comprises providing a ceramic substrate of approximately 96% alumina. The step of providing a first conductor pattern preferably comprises providing a first conductor pattern including at least two segments spaced apart from one another, and the step of depositing a pattern of resistive ink comprises depositing a pattern of resistive ink that includes first and second lateral end points, the pattern of resistive ink making electrical contact with one segment of the first conductor pattern at the first lateral end point, and making electrical contact with the other segment of the first conductor pattern at the second lateral end point.

In another form of the invention, the first conductor pattern is formed by depositing low cost, silver alloy conductive traces onto the ceramic substrate. The silver alloy conductive traces may comprise

platinum-silver alloy having a relatively low platinum content.

In yet another form of the invention, the pattern of resistive ink is formed from a ruthenium oxide resistive ink including at least trace amounts of palladium and silver. The step of depositing a second pattern of conductive traces may further comprise depositing palladium silver alloy conductive traces such that the second pattern of conductive traces substantially overlies at least the pattern of resistive ink. The second pattern of conductive traces preferably comprises a palladium-silver alloy composition having a Ag: Pd ratio of at most 3: 1 by weight. The step of selectively depositing an electrically insulating material may comprise selectively depositing a thermoset polymeric material. The thermoset polymeric material may comprise, at least in part, an epoxy resin.

In accordance with yet a further aspect of the invention, a variable resistor sensing unit for measuring liquid level in a container through interaction with a sliding electrical contact operating in conjunction with a float is produced by a process comprising the steps of providing a ceramic substrate ; depositing a first conductor pattern on the substrate, the first conductor pattern including at least two segments spaced apart from one another; depositing a pattern of resistive ink including first and second lateral end points, the pattern of resistive ink making electrical contact with one segment of the first conductor pattern at the first lateral end point, and making electrical contact with the other segment of the first conductor pattern at the second lateral end point; depositing a second pattern of conductive traces over the pattern of resistive ink, wherein the second pattern of conductive traces is formed by depositing palladium silver alloy conductive traces such that the second pattern of conductive traces substantially overlies at least the pattern of resistive ink; and selectively depositing an electrically insulating material on exposed areas of the first conductor pattern. A portion of the second pattern of conductive traces defines a track of conductive traces over which the

sliding electrical contact travels, and electrical resistance of the variable resistor sensing unit indicates liquid level in the container.

Further objects, features, and advantages of the present invention will become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a first type of fuel level sensing arrangement of the prior art; FIG. 2 depicts a second type of of fuel level sensing resistor element of the prior art; FIG. 3 shows a third type of prior art level sensing resistor element; FIG. 4 illustrates a substrate after deposition of a first conductive pattern, in accordance with a preferred embodiment of the present invention; FIG. 5 shows resistor material added to the substrate of FIG. 4; FIG. 6 shows palladium silver conductive traces deposited on the substrate of FIG. 5; FIG. 7 depicts a dielectric material deposited on the substrate of FIG. 6; FIG. 8 illustrates electrical connection points on the substrate of FIG. 7 to form a sensor in accordance with a preferred embodiment of the present invention; FIG. 9 shows a contact assembly and electrical wiring connections in conjunction with the sensor of FIG. 8; FIG. 10 illustrates a sensing resistor element in accordance with a preferred embodiment of the present invention, including a collector track; FIG. 11 is a flow chart of process steps in accordance with the present invention; and FIG. 12 depicts a fuel level measurement circuit that may use a

sensor in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION There is described herein a fuel level sensing system that offers distinct advantages when compared to the prior art. FIG. 3 depicts a portion of another existing liquid fuel sensor system that includes a variable resistor element 300 coupled to a float that rides along the surface of the fuel in a storage tank. As the fuel level varies, the float position is displaced, moving a wiper in contact with tracks 304,305, and in particular making contact within the working wiper contact areas 304a, 305a on the variable resistor element 300. Movement of the wiper changes the resistance between the wiper and a reference point 306 on the resistor element 300.

Conventionally, the resistor element 300 comprises a ceramic substrate 301 and a series of thick film conductive traces 302, over which is printed a thick film resistor. A laser may be used to adjust or trim the thick film resistor to the required resistor value by making a series of cuts at appropriate points along the resistor 303.

The process of manufacturing a resistor element 300 of the prior art begins by printing the conductive tracks 302 on the ceramic substrate 301.

The resistor material 303 is then printed over and between the conductive tracks 302. The resistor area 303 may then be trimmed to the proper value, if required, by making a series of cuts (by laser, for example) at appropriate points along the resistor 303.

The conductive tracks 304,305 are arranged such that wiper contacts (not shown) only make electrical contact with the conductive tracks 304,305 over the working sweep of the wiper. Because of the geometry of the resistor element layout, the wiper cannot directly contact the thick film resistor material 303, or the underlying abrasive ceramic substrate 301. As noted previously, direct contact with the resistor

material 303 would lead to excessive contact resistance and an eventual wearing away of the resistor material 303 before the desired number of life cycles were achieved by the system.

The conductive traces generally comprise precious metal alloys such as palladium-silver, and, to a lesser degree, gold-platinum, or gold- platinum-palladium. These alloys are compatible with the thick film manufacturing process, and can survive subsequent long-term exposure to various fuels. They are also formulated to be sufficiently hard to withstand the wear associated with hundreds of thousands of cycles of the wiper contact. The alloying elements, which are used to impart desired hardness properties to either gold or silver conductors, in an air-fireable thick-film process, are generally selected from the Platinum Group Metals (PGM), and, in particular, palladium.

As noted above, these are expensive elements, and they contribute significantly to the overall material cost of a resistor element. The addition of palladium to silver also mitigates against the tendency of silver to form ions in the presence of moisture, and to physically migrate between conductive tracks, under the influence of an electrical potential.

This phenomenon, known as metal migration, is minimized as the proportion of palladium in the alloy is increased.

In the present invention, the PGM content of the resistor element is minimized through the use of alternative materials, and by modification of the resistor element manufacturing process. In the process contemplated by the invention, a low cost silver alloy conductive track 401, and first and second termination pads A, B, respectively, are printed onto a ceramic substrate 301, the latter preferably being formed from 96% alumina (A1203), as illustrated in FIG. 4. Preferably, the silver alloy conductive track 401 and the pads A, B are formed from a platinum- silver alloy having a low platinum content (about 0.5%). Of course, other materials may also be suitable. The first termination pad A is remote

from one end of the track 401, while the second termination pad B is contiguous with the opposite end of the track 401.

This process of screen printing the conductive track 401 and the pads A, B is also indicated as step 1103 in the process flow chart of FIG.

11. It should be noted that multiple variable resistor elements are generally fabricated from a single sheet of ceramic material. Thus, for ease of separation of the individual units, the ceramic sheet is scribed (step 1101) prior to the first screen printing operation. Of course, individual screens for each screen printing operation are manufactured offline (step 1102). In the next operation 1104, the screen printed conductors are preferably dried for about ten minutes at approximately 150C. Next (step 1105), the ceramic is fired in a conventional furnace for about ten minutes at a temperature of about 850_C.

Next, as FIG. 5 indicates, resistor material 501 is printed on the substrate 301 such that the resistor material 501 makes electrical contact between the first conductive termination pad A and the conductive track 401. This resistor printing process is step 1106 in FIG. 11. The resistor material is typically a resistance ink that is preferable made from ruthenium oxide (Ru02), with traces of palladium and silver to aid the bonding to the conductive tracks and to modify the resistance of the ink.

A variety of suitable products are available from different suppliers, but the various inks are based upon generally the same technology.

The composition of the resistor material is also dependent on the resistance range required for a particular application. For low ohmic values, palladium silver (PdAg) is an example of a suitable material. For high ohmic values, a compound containing lead, ruthenium, and oxygen (PbRu03) may be suitable, among others.

The conductive pads A, B (FIG. 4) are printed so that the resistor blend can be verified before the complete order is printed. This process (known as"targeting") involves the measurement of the resistance

between the conductive pads A, B and a comparison of this measured value against the mathematically predicted resistance for the desired resist blend. If a discrepancy is evident, the resistor blend may be modified and further evaluations undertaken until the desired resistance is achieved.

Subsequent to resistor printing, the ceramic substrate is preferably dried once again at 150C for ten minutes (step 1107 in FIG. 11), then fired at 850C for a similar time period (step 1108). It is worth noting here that the temperature should not be raised too high during the firing process, as the resistance value can shift unpredictably under those conditions. If, on the other hand, the firing temperature is too low, proper sintering of the ink powders and good adhesion to the substrate will not be achieved.

A pattern of palladium silver alloy conductive traces 601 is then printed in the wiper area of the resistor element, as shown in FIG. 6.

Preferably, the palladium silver alloy material is formed from palladium silver having a Ag: Pd ratio of at most about 3: 1 by weight. The higher the palladium content, the more fuel and chemical resistant the conductive traces will be.

In the process flow chart of FIG. 11, the screen printing operation for these palladium silver conductive traces is indicated as step 1109.

Subsequent to screen printing, the ceramic substrate is dried once again (step 1110), then fired (step 1111). In order to minimize the usage of this palladium silver alloy material, the resistor material 501 is printed in the working wiper area (802 in FIG. 8), and the palladium silver conductive traces 601 are printed directly on top of the resistor material 501.

As shown in FIG. 7, a suitable dielectric material 701 is then printed over the exposed, low cost, silver alloy track 401 to form a protective barrier against metal migration, and to reduce susceptibility to chemical attack by the fuel to which the sensor is exposed (step 1112 in FIG. 11). The dielectric material 701 is preferably a thermoset

polymeric material that is cured at 150gC to 200°C (for about 30 minutes, as indicated in process step 1113 of FIG. 11). There are a number of such materials available, some of which are based upon epoxy resins.

Alternatively, a glass dielectric paste could be used that is fired at 500°C to 850°C. The higher temperature glass materials are less desirable, as each subsequent re-firing of the resist material can lead to a resistance shift on the product.

The resistor area 501 may then be trimmed (step 1114), by laser or through an abrading process, to the required resistance value by making a series of cuts into the resistor at appropriate points. The previously scribed ceramic is then broken into individual resistor elements by breaking along the scribe lines (step 1115). The individual elements are then tested, packed, and shipped to customers (steps 1116 through 1118).

In some designs, the second termination pad B may be adjacent to the wiper area, and in these cases the first conductive print may utilize the high-cost palladium silver alloy ink. In these cases, the need for the dielectric layer is eliminated. Naturally, the additional material cost will be offset by the elimination of the extra print operation.

FIG. 9 illustrates the interconnection of a wiper assembly 901 with a variable resistor sensing element in accordance with the present invention. As shown, the wiper assembly 901 includes wiper contacts 902 that are positioned such that the wiper contacts 902 make electrical contact with the palladium silver conductive traces 601 disposed within the working wiper area 802. Electrical leads 903,904 make electrical contact between the substrate and the wiper assembly 901 for connection to a measurement circuit.

The wiper contacts 902 themselves are preferably precious metal buttons, but there are also multi-finger wiper assemblies known in the art that are stamped from a metal strip. It is also known to use multi-wire wipers using a plurality of formed wires bonded or welded together.

Typical wiper contact materials include silver nickel (a sintered composite<BR> containing silver and nickel), pure nickel, nickel silver (also called German silver, but actually an alloy of copper, nickel, and zinc that contains no silver). There are also precious metal alloys known as Paliney 6 and Paliney 7, produced by J. M. Ney, that are known to be suitable, and hard gold (a gold/nickel alloy) is also a workable alternative.

FIG. 12 illustrates a typical measurement circuit. The electrical leads 903,904 (FIG. 9) extend from the sensing element/float assembly 1201 to form a series circuit with a regulated power supply 1202, a series resistor 1203, and a fuel level indicator 1204. The regulated power supply 1202 may be the output of the automotive voltage regulator, with a nominal output voltage of 12 volts DC. A relatively small fixed resistor 1203 is generally incorporated in series with the indicator 1204. The resistance range that can be expected from the sensing element/float assembly 1201 is from about zero ohms to 400 ohms, depending upon the level of fuel. The fuel level indicator 1204 can be, for example, a conventional moving-coil meter movement, a well-known digital display based upon current or voltage measured in the measurement circuit, or even an analog indicator whose position is set by an associated stepper motor assembly.

There are, of course, other possible configurations for a variable resistor sensor element. FIG. 10 illustrates an arrangement with a collector track 1001 disposed along a circular arc that is concentric with a working wiper area 1004. In this configuration, a wiper assembly (not shown) includes a pair of electrical contacts that, unlike the wiper 901 of FIG. 9, are not electrically connected together. In the structure depicted in FIG. 10, the wiper forms a bridge between the collector track 1001 and the conductive track within the working wiper area 1004. Electrical leads may be attached to the individual wiper contacts or to solder pads 1002,

1003 provided on the sensor element itself. Although this is convenient in terms of ease of assembly, it is not particularly economical in terms of precious metal ink usage. In addition, because of the disparate contacts, this configuration is more prone to contact electrical noise. However, using the techniques of the present invention does allow optimization of ink usage for the non-collector track side of the circuit.

There has been described herein a fuel level sensing system that offers distinct advantages when compared with the prior art. It will be apparent to those skilled in the art that modifications may be made without departing from the spirit and scope of the invention.

Accordingly, it is not intended that the invention be limited except as maybe necessary in view of the appended claims.