Dual Track Resistor Element Having Nonlinear Output

1. Technical Field

This invention relates to the use of resistance compositions to produce resistive potentiometers having nonlinear output characteristics.

2. Background Art

Prior art proposals include a variety of approaches for effecting a nonlinear resistance output. These proposals include the screen printing of one layer of resistive paint over the top of another layer of resistive paint, the upper layer being configured differently to expose portions of the lower layer. The problems inherent with this method are that the ratio of the resistivities of the paints is critical, a high resistance interface may develop between the two adjacent resistive paint layers, and the paints may have different wear characteristics thereby causing the output curve to shift during the useful life of the product. Another approach is the use of a variably shaped resistance element to produce a nonlinear potentiometer, such as described in H. B. Casey U.S. Pat. No. 3,325,763 issued June 13, 1967, Fujii et al. U.S. Pat. No. 3,564,475 issued Feb. 16, 1971, and Katz U.S. Pat. No. 2,833,901 issued May 6, 1958. Another approach comprises the firing of different cermet resistive materials adjacent one another on a substrate, as disclosed in Wright U.S. Pat. No. 3,379,567 issued Apr. 23, 1968. Prior art nonlinear resistors and potentiometers utilizing techniques such as varying the width or configuration of the resistive track, using different paints to produce different sections of the nonlinear output curve, voltage clamping, multiple layers of paints, or combinations of the above, each have drawbacks to their respective technique.

Another prior art proposal comprises the use of wirewound resistive elements having different widths, a different number of turns per length, and variance of the taper or diameter of the wire itself, as disclosed in Van Alen U.S. Pat. Nos. 2,468,144 issued Apr. 26, 1949 and 2,487,839 issued Nov. 15, 1949. This method also includes the use of multiple contactor elements. Other prior art proposals for producing nonlinear resistance outputs are described in Leahy U.S. Pat. No. 3,544,945 issued Dec. 1, 1970 and Kogo et al. U.S. Pat. No. 3,890,589 issued June 17, 1975, and Carter U.S. Pat. No. 4,237,442 issued Dec. 2, 1980. Many of the prior art techniques are outlined in the Bourns, Inc. publication entitled The Potentiometer Handbook by McGraw Hill, published 1975. A more recent technique consists of the use of laser trimming, sand abrading, electron beams, mechanical scribing, or chemical etching to remove portions of a variable width resistive element as described in Steigerwald et al. U.S. Pat. No. 4,243,969 issued Jan. 6, 1981.

It is the purpose of the present invention to overcome the deficiencies of the prior art proposals for resistors and potentiometers having a nonlinear output characteristic, and to produce a compact resistor element having a nonlinear voltage output such as a logarithmic curve, that may be utilized in a potentiometer, having a wide range of industrial uses, and requiring a minimum number of parts and manufacturing steps.

The dual track variable resistor element of the present invention comprises two thick film resistance layers electrically insulated one from another on a substrate, each track comprising a rheostat having a different rate of change of resistance per length of travel. The combined rates of change of resistance produce collectively a nonlinear output characteristic. The respective resistance layers have termination means, and metallic contactor having resilient fingers is disposed for wipable engagement of one set of resilient fingers with the surface of each of the resistance layers. As the contactor moves along the resistance layers, the contactor utilizes each of the two resistance layers to complete a circuit across the layers. The voltage present at the contactor is used as the output of the potentiometer. The different rates of change of resistance of the dual track variable resistor element may be effected by two different constructions. The resistance layers may consist of the same resistance composition, i.e. each track has the same resistivity, but the width dimensions of the respective tracks are different, thereby producing a different resistance for each track. Another embodiment comprises each resistance track having the same width and length dimensions, but each track consisting of a composition having a different resistivity, thereby producing different rates of change of resistance for the respective resistance layers. The dual track resistor element of the present invention effects a nonlinear voltage output characteristic that accurately tracks a logarithmic curve.

FIG. 1 illustrates schematically an embodiment of the dual track resistor element wherein the tracks have the same width and length dimensions but have different resistivities;

FIG. 2 illustrates an embodiment of the dual track resistor element wherein the tracks are comprised of the same resistive composition but the tracks have different but consistent widths;

FIG. 3 is a schematic of the equivalent electrical circuit of the dual track resistor element;

FIG. 4 is a graph of the percentage of voltage out/voltage in versus the degrees of actuation of a rotary potentiometer;

FIG. 5 illustrates a dual track resistor element utilizing a single resistive paint for both tracks having different widths, and the narrower track having a variable width in order to produce exactly the desired nonlinear output characteristic.

Referring now to the drawings, and in particular FIGS. 1 and 2, the dual track variable resistor element is designated generally by reference numeral 10. FIG. 1 illustrates an embodiment wherein the tracks or resistance layers of the resistor element have the same width dimension, but the tracks consist of resistance compositions having different resistivities. The tracks of the dual track resistor element are produced by printing the resistance materials on an insulating substrate 11, such as a polyimide film. The polyimide film provides a flexible insulating substrate 11 easily mounted in the housing of a rotary of linear travel potentiometer. Resistor track or resistance layer 12 consists of a resistive material having a resistance of 10 ohms per square, while resistor track 14 consists of a resistive material having a resistivity of approximately 231/2 ohms per square. Each resistance layer or track has at least one conductive termination 16, 17, 18. A metallic contactor 20 having resilient wiper fingers 22 is positioned for wipable engagement with the tracks 12, 14. As the contactor 20 moves laterally to the left or to the right, the contactor completes a circuit across the resistor tracks 12, 14, and there is a rate of change of resistance along each track as the wiper moves laterally, the combination of the rates of change of resistance of the respective tracks determining the nonlinear output characteristic.

FIG. 2 illustrates another embodiment of the dual track resistor element 10. In this embodiment, the resistor tracks 40 and 50 consist of the same resistive paint material and therefore have the same resistivities. However, the width dimensions of the respective tracks and different. In this case, the width of track 40 is greater than the width of the track 50, and thus the resistance of track 50 is higher than the resistance of track 40. The contactor 20, as it is moved laterally, completes a circuit across resistor tracks 40 and 50, each track having a different rate of change of resistance. Again, it is the combination of the rates of change of resistance which determines the nonlinear output characteristic. Resistor tracks 40 and 50 each have terminations 66, the dual track resistor element 10 comprising a potentiometer.

The tracks of the dual track resistor elements 10 each comprise a rheostat having a respective rate of change of resistance. The schematic of the dual track resistor element 10 is illustrated in FIG. 3, wherein there is an electrical connection 70 for the first resistor track 80, an electrical input 72 for the second resistor track 90, an output termination 74 connected to the other end of the track 80, and a single contactor 100 completing the circuit across the resistor tracks 80 and 90 of the dual track resistor element designated generally by reference numeral 110. As the contactor 100 moves along the resistor tracks 80 and 90, the combination of the respective rates of change of resistance determines the nonlinear output of the element 110.

FIG. 4 is an illustration of the nonlinear output characteristic of a rotary potentiometer utilizing the dual track resistor element of the present invention. The ordinate is scaled for the percent of Voltage Out/Voltage In and the abscissa illustrates the Degrees of Shaft Turn of a rotary potentiometer wherein the contactor is connected to the shaft and wipably engages the dual track resistor element. As illustrated by FIG. 4, the dual track resistor element produces an accurate nonlinear output characteristic, in this case a logarithmic curve. The nonlinear output characteristic of the dual track resistor element of the present invention can match a logarithmic output curve with a variance of only one percent. To attain an exact match with a logarithmic curve, the width of one of the tracks can be varied or modulated slightly by adding a small amount of resistance material to the track, this being done with the aid of a computer in order to produce an exact logarithmic output curve. This is illustrated in FIG. 5 wherein the resistance layers of tracks 210 and 220 of the dual track resistor element designated generally by reference numeral 200, having different widths in accordance with the embodiment illustrated in FIG. 2. In FIG. 5, the track 220 has a variable width along its length. By varying slightly the width of one, or both, of the resistor tracks, the output characteristics of the potentiometer can match exactly a logarithmic output curve.

The dual track resistor element of the present invention may also comprise a pair of resistor tracks of layers of different resistance compositions and thereby having different resistivities, and the tracks may also have different widths. In order words, there can be a combination of the embodiments shown in FIGS. 1 and 2, in order to produce a nonlinear output characteristic. Of course, to refine further the nonlinear output characteristic, the widths of the tracks can be slightly modulated as described above. It is also possible to take the output of the resistor element from a third conductive track disposed under the conductive wiper. However, in this potentiometer application it is desirable to take the output of the resistor element from an end of one of the resistance tracks as illustrated in FIGS. 1, 2, and 5.

The voltage outputs can be calculated in a manner applicable to the respective embodiments.

Utilizing FIG. 1 as an example:

V.sub.in =voltage applied between terminations 17 and 18;

V.sub.out =the resulting voltage between wiper 20 and terminal 18;

d=the variable distance from termination 18;

r.sub.1(d) =the resistance between termination 18 and the wiper 20 along track 14 where r.sub.1 is a function of the distance d.

r.sub.2(d) =the resistance between termination 17 and wiper 20 along track 12 as a function of distance d.

Thus the voltage output may be expressed by the equation: ##EQU1##

In the case where the widths of the respective resistance elements vary (FIG. 5), r.sub.1(d) and r.sub.2(d) will be nonlinear and a computer procedure can be utilized in order to match a desired voltage output (V.sub.out /V.sub.in).

In the embodiment wherein the tracks are uniform films of constant widths (FIG. 1), the relationship may be expressed as follows: ##EQU2## .rho..sub.1 and .rho..sub.2 equal the respective resistivities of the compositions and the ratio of the voltage output to voltage input may be expressed by the equation: ##EQU3## 100% (.rho..sub.2) which equals R.sub.2 is the total resistance of track 12 of FIG. 1 and has been factored out of the equation.

The characteristic curve of this equation is similar to a logarithmic curve. Many logarthmic curves can be approximated to within a few percent by choosing easily obtainable values for .rho..sub.1 and .rho..sub.2.

The dual track resistor element in the present invention may be utilized in electrical applications requiring a nonlinear output characteristic.

Although the present invention has been illustrated and described in connection with example embodiments, it will be understood that this is illustrative of the invention, and it is by no means restrictive thereof. It is reasonably to be expected that those skilled in the art can make numerous revisions and additions to the invention and it is intended that such revisions and additions will be included within the scope of the following claims as equivalents of the invention.