Variable resistive element

Details Variable resistive element Variable resistive element

2001-09-13

2003-02-11

Easthom, Karl D. (Department: 2832)

Electrical resistors

Mechanically variable

Movable contact electrically adjustable over length of...

C338S162000, C338S185000

Reexamination Certificate

active

06518873

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
Not Applicable
FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
This invention relates generally to resistive elements for use in systems for measuring the level of liquid in a vessel and in particular for the measurement of fuel quantity, and is more particularly directed toward a resistive element that may be used in the construction of a submersible sensor designed for installation in a fuel tank.
It is well known that the fuel tank of an automobile 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. 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. In earlier versions of these screen printed sensors, a single or dual wiper moved along a printed resistive track in response to fuel level changes conveyed to the wiper by an associated float. The printed resistive track is typically deposited on a substrate of ceramic material or porcelain coated steel for durability. Of course, the printed resistive region, even though formed from glass frit in combination with precious metals, is still subject to wear due to friction with the wiper arm.
In a variation on the early sensors of the prior art, an example
100
of which is shown in
FIG. 1
, the wiper contacts are designed to slide across a network of conductors
101
rather than the resistive film
102
itself Designing the system so that the wiper makes contact with high metal content conductive regions provides a lower resistance path in operation.
The conductive regions
101
are also designed for durability and long life in the presence of hostile solvents such as gasoline, but the materials for these conductive areas must generally be selected from among an expensive group of candidate materials. Suitable metals include palladium, platinum, Gold, and silver, which can be combined into alloys that perform adequately in the intended environment.
A laser may be used to adjust, or trim, the thick film resistor to the required resistance value, by making a series of cuts, at appropriate points along the resistor.
The various processing steps, typically used in the manufacture of the prior art resistor element of
FIG. 1
, are illustrated in FIG.
2
. Conventionally, a number of resistor elements are typically fabricated on a single substrate. To facilitate their subsequent separation, the ceramic substrate is initially scribed
201
, for example by a laser scribing process. The conductive tracks are deposited
202
, using a conventional thick film screen printing process. The tracks are dried in an oven and then fired
203
in a furnace. Resistor material is then deposited, using a conventional thick film printing process
204
, over and between the conductive tracks. The resistor material is then dried in an oven and subsequently fired
205
in a furnace. The resistor is then laser trimmed
206
to the required resistance value, by making a series of cuts into the resistor, at appropriate points along the resistor. The location and size of cut is determined by reference to measurements made of the resistor value, facilitated by a series of test pads
108
formed with the conductive tracks.
The previously scribed ceramic is then broken
207
into individual resistor elements by breaking along the previously scribed lines and finally the individual elements are tested, packed and shipped to customers (shown as a single step
208
).
The conductive traces are arranged such that the wiper contact will only contact the conductive traces in one or more wiper contact areas (identified in dashed outline in
FIG. 1
as reference numerals
104
a
,
104
b
) over the working sweep of the wiper. In practice, two concentric wiper contact areas
104
a
,
104
b
may be used to reduce contact noise and increase sensor reliability. The wiper cannot directly connect with the thick film resistor
102
as the contact resistance would be excessive and the wiper contact would wear away before achieving the required number of life cycles demanded by system specifications.
The conductive traces
101
generally consist of precious metal alloys such as Palladium-Silver and to a lesser degree Gold-Platinum or Gold Platinum-Palladium. These alloys are resistant to the thick film manufacturing process and to 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 the hardness properties to either the gold or silver conductors, are selected from the Platinum Group of Metals (PGM) and in particular, Palladium. The addition of Palladium to Silver also mitigates against the tendency of silver to form ions in the presence of moisture, and 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. The Platinum Group Metals are expensive elements and significantly contribute to the overall material cost of the resistor element.
In recent years, increased environmental legislation has resulted in efforts to significantly reduce the sulphur content in automotive fuels. The process of removing the sulphur from fuels is known to leave residual traces of highly reactive sulphur compounds behind. These sulphur compounds have been found to react with the silver in palladium/silver traces on conventional fuel sensor elements forming non-conducting silver sulphide deposits, which can lead to sensor failure.
Consequently, a need arises for a resistor element that is suitably rugged for fuel tank applications, that minimizes the exposure of silver alloy traces, and has a reduced requirement for Platinum Group Metals.
SUMMARY OF THE INVENTION
These needs and others are satisfied by the present invention, in which a variable resistive element is provided for use with at least one associated sliding electrical contact. The variable resistive element comprises a substrate, a first conductor pattern deposited on the substrate, at least one resistive region making electrical contact with the first conductive pattern. The first conductor defines a contact area for the associated sliding electrical contact and in which portion the conductor pattern is plated with a first plating. The first plating may be nickel or a nickel alloy. The substrate is preferably ceramic.
In accordance with one aspect of the invention, the resistive element is incorporated in a sensor element, which further comprises a wiper arm having at least one electrical contact for contacting the resistive element. Preferably, the electrical contact is nickel or palladium nickel. The sensor element is particularly suited for use as a fuel card sensor element in a fuel sensor. The fuel level sensor may further include a wiper arm having at least one sliding electrical contact movable along a contact area of the first conductor pattern of the resistive element, and a float arrangement coupled to the wiper.
In ac

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