This invention relates to precision non-linear and linear conductive plastic film potentiometers and methods of their manufacture.
Non-linear potentiometers are potentiometers which include an electrical resistance element of either plane or cylindrical configuration along which a wiper is moved to provide a voltage output value which varies non-linearly as a predetermined mathematical function of the distance from an end termination of the resistance element. In the case of certain functions, for example a logarithmic function, the rate of change of voltage output to distance causes the voltage output value near one end of the resistance element to be large compared to that near the other end of the resistance element. This is a serious problem in the design of resistance elements of the conductive film type, since it makes necessary the use of a longitudinal series of resistance elements of differing resistance values in order to keep the transverse dimensions of the resistance elements within practical limits while providing a minimum transverse dimension sufficient to be useful as a wiper path of travel.
In view of these problems, it is a major object of the present invention to provide a novel precision non-linear potentiometer of practical transverse dimension without the necessity of utilizing a longitudinally averaged series of resistance elements.
It is also an object of the invention to provide a novel precision linear potentiometer.
It is another object of the invention to provide a novel method of manufacturing such potentiometers.
According to the invention in its broadest aspect, there is provided a precision potentiometer having a predetermined function comprising a high resistance element, the slope of which approximates closely the maximum slope of the predetermined function, preferably a conductive plastic film on a suitable insulating base and having end terminations. The high resistance element provides a path of wiper travel between the end terminations for a wiper movable along the path. There is also provided a low resistance element, the slope of which approximates closely the minimum slope of the predetermined function, of varying transverse cross sectional dimension, preferably a film, in continuous electrical contact with the high resistance element, extending between the end terminations in continuous parallel electrical connection with the high resistance element, so that the high and low resistance elements together provide a voltage output value at the wiper varying as a predetermined function of distance from one end termination, either non-linearly or linearly. Slope ratios as high as 100,000:1 may be achieved by the invention.
Preferably, the low resistance element is spaced from the wiper path and has its edge remote from the wiper path at a varying distance from the adjacent edge to provide the varying transverse cross sectional dimension. Preferably, too, the edge of the low resistance element adjacent the path of wiper travel is diffused progressively transversely.
The novel method of the invention comprises the steps of providing a longitudinally extended high resistance element having end terminations, the high resistance element having a path of wiper travel between the end terminations. A longitudinally extended low resistance film is provided on the high resistance element in continuous parallel electrical connection therewith extending between the end terminations. The low resistance film is spaced from the wiper path. The transverse cross sectional dimension of the low resistance element is then progressively altered as a function of desired voltage output value and distance from the end termination by progressively sensing the voltage output value along the path of wiper travel and altering the transverse cross sectional dimension of the low resistance value responsively thereto. Preferably this is done by effectively varying the transverse dimension of the films on the side thereof remote from the path of wiper travel to provide a precision potentiometer element non-linearly varying as a function of distance from one end termination, or to provide a precision linear potentiometer.
For the purpose of more fully explaining the above and still further objects and features of the invention, reference is now made to the following detailed description of a preferred embodiment of the invention, together with the accompanying drawings, wherein:
FIG. 1 is a plan view of a precision potentiometer according to the invention;
FIG. 2 is a side view of the potentiometer of FIG. 1;
FIG. 3 is a cross sectional end view of the potentiometer of FIG. 1, taken on line 3--3 thereof and illustrating the method of the invention;
FIG. 4 is an equivalent circuit diagram of a potentiometer of the invention, and
FIG. 5 is a graph of a non-linear resistance function typical of a potentiometer of the invention.
Referring to the drawings, in which the thickness of the film layers has been greatly exaggerated for clarity, the potentiometer of the present invention comprises a suitable insulating base 12 having a longitudinally extended high resistance film 14 on it. Film 14 may be of any suitable material, such as a conductive plastic, for example, having a relatively high resistance value. Conventional end terminations 16 and 18 are provided on opposite ends of high resistance film 14.
A longitudinally extended low resistance film 20 is provided on high resistance film 14 for about half the transverse dimension thereof extending between end terminations 16, 18 in continuous parallel electrical connection with high resistance film 14. High resistance film 14 is thus exposed for about half its width to provide a path of travel between end terminations 16, 18 for a conventional wiper 22 movable along said path.
Low resistance film 20 may be of any suitable material, such as a conductive plastic, for example, having a relatively low resistance value, and preferably has its edge 24 adjacent the path of wiper 22 diffused progressively transversely to increase the resistance value thereof in a transverse direction toward said path and spaced from said wiper path.
In the preferred embodiment, base 12 is about 5 mils thick, high resistance film 14 is about 1 mil thick, and low resistance film 20 is about 1 mil thick.
In order to provide a voltage output value at wiper 22 varying as a predetermined function of distance between end terminations 16, 18, the edge 15 of high resistance film 14 and the edge 21 of low resistance film 20, that is, their opposite edges remote from the path of wiper 22, are at a varying distance from the low resistance film edge 24 adjacent the path of wiper 22 to provide a varying cross sectional dimension thereof. The equivalent circuit of the potentiometer of the invention is shown in FIG. 4 to consist of a network of two parallel series of resistors connected at indefinitely short intervals along their lengths.
In operation, the potentiometer of the invention, whether of the plane configuration as shown or of a conventional cylindrical configuration, has its wiper 22 moved along its path between terminations 16, 18 to provide the desired voltage output value, such as that of FIG. 5, for example.
The method of making the potentiometer element of the invention comprises the previously described steps of providing on base 12 a longitudinally extended high resistance film 14 having end terminations 16, 18 on opposite ends thereof and providing a path of travel of wiper 22 between said end terminations with a longitudinally extended low resistance film 20 on high resistance film 14 in continuous electrical contact therewith extending between said end terminations in continuous parallel electrical connection with said high resistance film and spaced from said wiper path.
In order to alter progressively the transverse cross sectional dimension of at least the low resistance film 20 as a function of desired voltage output value and distance from the end terminations 16, 18 by effectively varying the transverse dimension of said film on the edge 21 thereof remote from the path of travel of wiper 22, both low resistance film 20 and high resistance film 14 are cut by a suitable cutter 30, such as an etching tool, responsive to the voltage output value E at the wiper 22 as it is moved along the wiper path in synchronism with cutter 30. This may be accomplished by applying a constant voltage across end terminations 16, 18 and measuring the voltage value E at wiper 22 by a suitable voltmeter 32, and simultaneously advancing cutter 30 and wiper 22 at equal longitudinal distance D from end termination 16 while simultaneously moving cutter 30 transversely in accordance with the desired function of voltage and distance, as illustrated, for example in the graph of FIG. 5. As shown, the excess material of both the high and low resistance films is effectively removed from the potentiometer in an electrical sense, since cutter 30 extends into the insulating base 12. Of course, the cut could be made through the base for physical removal of excess film material if so desired.
The invention can be equally applied to linear elements in that the use of the double film permits extension of the resistance range in the lower direction, since the lower resistance film becomes the resistance and shunt that controls the total resistance of the entire element. In many cases, too, the lower resistance film has a high metallic content and thus has a difficult surface condition that may affect adversely the linear factors of the sliding contact.