US2806928A - Adjustable precision potentiometers - Google Patents

Description

Sept 17, 1957 5. 0'0. WOODS ETAL 2,806,928

ADJUSTABLE PRECISION POTENTIbI AETERS Filed on. 15, 1954 4 Sheets-Sheet 1 INVENTORS a Q Whalep 1957 v IHI. 0'0. WOODS El'AL 2,806,928

ADJUSTABLE PRECISION POTENTIOMETERS Filed Oct. 13, 1954 4 Sheets-Sheet 2 INVENTORS a. m

Sept 17, 1957 E. OD. wooos ETAL 2,806,928

ADJUSTABLE PRECISION POTENTIOMETERS Filed Oct. 13, 1954 4 Sheets-Sheet 3 IN V EN TORS Sept 17, 1957 a 0'0. woops ETAL 2,806,928

ADJUSTABLE PRECISION POTENTIOIIETERS Filed Oct. 13,- 1954 4 Sheets-Sheet 4 7 J J 0' 1' 0% V I mz V G5 7/ 25. 8 N 7 G mo may:

United States Patent ADJUSTABLE PRECISION POTENTIOMETERS Elmer ODell Woods, Santa Monica, Wilbur John Mac- Cauley, Northridge, and Neil Anthony Kaufman, Venice, Calif., assignors to Helco Products Corporation, Santa Monica, Calif.

Application October 13, 1954, Serial No. 461,986 6 Claims. (Cl. 201-56) This invention relates to variable resistance devices such as potentiometers, and more particularly to improved adjustable precision otentiometers. The term potentiometer as used hereinafter will be understood as pertaining to variable resistance devices in general.

The requirements of precision potentiometers such as those of a rotary type variable resistance device or potentiometer used in various instruments, for example in control devices for aircraft and guided missiles, are very precise, forcing the technician to work with very small dimensional tolerances in all phases of their manufacture. The linearity, resolution, angle of rotation, total resistance, terminal resistances, and the electrical centers must in all cases be very precise and within very small tolerances. It can readily be seen that without adequate provision for adjustment and keeping a precise relationship between the above mentioned variables and the other variables that enter intosthe art of making precision potentiometers, the manufacturing costs may become excessive and prohibitive for many applications.

While the invention is broadly applicable to control devices, it has special utility in a multiple or ganged potentiometer of the rotary type for use in an instrument data computer system. i

The general object of the invention is to provide a rugged, efiicient, highly accurate potentiometer of relatively simple construction that may be manufactured in quantity with relatively low labor costfor fabrication and assembly.

More particularly, an object of this invention is to provide means for adjustment of the total resistance of the potentiometer readily and manually.

Another object is to provide a potentiometer compensating device adapted to compensate a potentiometer to have a precise predetermined total resistance.

Another object of this invention is to provide a potentiometer compensating device adapted to compensate a potentiometer to provide a predetermined resistance vs. a mechanical function.

Another object is to provide an etticient, trouble-free arrangement for shorting out a predetermined portion of a potentiometer resistor so that a-constant voltage is maintained over the corresponding portion of the potentiometer action.

Another object is to provide a means for compensating for variations in mechanical movement so that the electrical percent of potential may be corrected to equal the percent of mechanical movement.

Another object is to provide a means for accurate location of the zero percent and 100 percent of electrical potential of the potentiometer in relation to the mechanical length.

Another object is to provide means for accurate division of potentials.

Another object is to provide a means by which the desired voltage functions can be set with extreme precision from data presenting the function in a tabular form,

2,806,928 Patented Sept. 17, 1957 by shifting taps along the resistor or by measuring the amount of resistance between successive taps.

Still another important object is to provide an improved arrangement whereby the adjustment or synchronization of a plurality of precisiontype variable resistors or potentiometers may be obtained in a relatively simple manner.

Another object is the provision of means for accurately matching or aligning electrical centers, of ganged or multiple Potentiometers having a common actuating member.

Another object of this invention of primary importance is to provide a construction that is time saving with respect to the task of establishing accurate phase relationships in the course of assembly. The wiper on the rotor must be correctly phased relative to the resistor. The taps and terminals must be related with equal accuracy to the resistor.

Another object of this invention is to provide an improved, standardized structure that is simple, inexpensive and compact for joining together in a single housing assembly a plurality of electrical units adapted for operation in common, and to provide new and improved methods of mounting and adjusting potentiometer elements.

Another object of this invention is to provide improved standardized end-plate or partition structure for both joining together a plurality of easing units and for positioning and supporting electrical apparatus housed therein, and to provide a simple fastening means for retaining together in ganged relation a plurality of pote'ntiometers or adjustable resistors.

Another object of this invention is to provide a potentiometer in which the taps thereon may be adjusted after the potentiometer is assembled to obtain a precise value between such taps.

Another object of this invention is to eliminate the need for trimmer resistors for correcting the electrical value, and to provide more accurate linearity by the elimination of said trimmer resistors.

A further object of this invention is to avoid the necessity of external or additional parts, or the installation thereof, and to provide a potentiometer having the improved characteristics above noted without requiring more than a resistance element, the required adjustable spring taps, a traveling contact, and the proper housing.

As will be explained, one of the features of the invention with regard to the foregoing is the concept of combining a slip ring on the rotor with a wiper member that may be mounted on the rotor and staked in approximate position simply by a metal stake through the rotor. When so mounted the wiper member may be phase adjusted by simply rotating two tap terminals which will change the resistor position relative to the wiper member.

A further feature of the invention with respect to phase adjustment, and especially the saving of assembly costs in making phase adjustments, is the concept of making the various units of a ganged potentiometer freely adjustable relative to each other.

A further feature of the invention with respect to simplicity of structure and ease of assembly is the concept of employing a resilient brush structure for yielding engagement with a circumferential groove of the abovementioned slip ring for the purpose of energizing the slip ring and the wiper member carried thereby. 1n the preferred practice of the invention, the brush structure comprises one wire arm adapted for engagement with the slip ring groove. The brush structure is resilient radially so that it may be freely assembled after the rest of the potentiometer has been assembled as a unit, by simply pushing it through its particular hole in the case wall as it does not matter which side of the slip ring the resilient brush contacts.

The brush structure is then brought into register for permanent cooperation with the slip ring. Such resiliency is a safeguard against failure of the brush structure under severe service conditions.

The above and other objects and advantages of the invention will be apparent in the following detailed description of the presently preferred practices of the invention, considered with the accompanying drawings, which are to be regarded as merely illustrative.

In the drawings:

Fig. 1 is a longitudinal sectional view of one form of the invention, the rotor assembly being shown in side elevation;

Fig. 2 is a transverse section taken as indicated by the line 22 of Fig. 1;

Fig. 3 is a transverse section taken as indicated by the line33 'of Fig. '1.

Fig. -4 is a sectional view of a potentiometer embodying the invention, with a wiper element at forty-five degrees rotation in relation to the alignment holes.

Fig. 5 is a sectional view of the potentiometer embodying the invention, with the wiper element at ninety degrees rotation in relation to the alignment holes.

.Fig. 6 is a sectional view of apotentiometer embodying the invention, with one tap.

Fig. 7 is a sectional viewof a potentiometer embodying the invention, employing two taps.

Fig. 8 is a sectional view of a potentiometer embodying the invention, employing three taps.

Fig. 9 is a sectional view of a potentiometer embodying the invention, employing five taps.

Figs. 10-17 show schematically a few of the circuits i which this invention may be applied.

Referring now to Fig. l, in accordance with this invention in a specific embodiment thereof, a plurality of potentiometers 46 are mounted in casingiunits or cylindrical walls I39-41 having standardized rings 37 and shoulders 38 which mate with adjoining units and which also serve .to position and support potentiometer elements and associated electric equipment within the respective .casing units. The synchro-mount end cover 7 is connected to the cylindrical "Wall 39, and carries a suitable bearing 11 for one end of a rotor shaft 11. At the other end of the assembly, a cap 3 or end cover is connected with the end cylindrical wall 41 to complete the composite casing andto carry a second bearing 2 for the other end of the rotor shaft.

The potentiometer units 445, and the synchro-mount end cover 7, and the end cover 3, may be held together in assembled relationship in any suitable manner. In the preferred practice of the invention, however, two .screws r 10 are employed for this purpose and the holes through which they pass, 34 in Fig. 2, maybe used to relatively check and align the potentiometer units electrically to each other in an angular relation previous to their assemblyin a ganged relationship.

Each of the'cylindrical walls 39-41 is formed with an internal circumferential shoulder 18 against which is positioned a suitable resistor, having a configuration of a split ring, this resistor being resistor v of Fig. 1. In a preferred practice of the invention, each resistor 15 comprises a ring-shaped core 12 on which is wound a helical resistance wire. The core 12 may be made of metal coated with suitable insulating material toisolate the resistance wire.

The rotor shaft 1 is part of a rotor assembly which includes a sleeve 42 of suitable non-conducting material as shown in Fig. 2, which is slipped onto the rotor shaft. Slip rings 22, which are inthe form of coin silver sleeves, are assembled over this plastic non-conducting sleeve 42 which insulates them from the rotor shaft, there being one slip ring 22 for each of the potentiometer units.

Each of the slip rings 22 is suitably adapted to maintain constant contact with the corresponding brush means 19, and contacting each slip ring is a suitable resistor con- 1 tact or wiper element 26 with a coined tip 27 for contact with the corresponding resistor 15.

The space between the ends of core 12 in Fig. 3 is filled with a suitable material 13 forming a bridge for the Wiper.

The wipers and the slip ring of one unit are insulated from those of and adjacent unit by suitable insulators 23 which also provide insulation for the metal washers 25. The rotor assembly is then staked into a rigid assembly by tapered pin 20 driven into a tapered hole 21, which completes the rotor assembly.

In the preferred construction shown in the drawings, each slip ring 22 is provided with a shallow circumferential groove 24 for cooperation with the corresponding brush means 19. It is contemplated that the brush means 19 will be of resilient construction for the purpose of maintaining pressure contact with the slip ring in the groove 24 in the normal operation of the potentiometer. In the construction shown, each brush means 19 comprises a resilient wire arm, a portion of which is formed into a .helicalspring.

Each of the potentiometer units has two external terminals, in Fig. 2 designated 45 and 46 to energize the resistor 15 and has a terminal'8 for connection to the corresponding wiper arm .26. Terminal 8 carries a suitable brush means designated 19. The terminals have hexagon shanks 9 by which their adjustments are made.

In order to provide an accurate and precise termination for the unconnected ends of the resistance unit 15, variable or adjustable spring taps 43 and 44 in Fig. .2, are provided which permit the effective length of the resistance wire 15 to be accurately adjusted within very close limits. Engagement between the spring taps 43 and 44 on the casing 41 and the resistance wire 15 on the mandrel 12 is assured by the wire for each tap having a high tensile strength and springing against the potentiometer wires, so that the taps thereon firmly engage the wires on the resistance member. Terminal .slots 14 allow clearance for adjusting the taps. A suitable tool may be engaged with the feed throughs 9 in Fig. 2, so that either a clockwise or counter-clockwise rotation imparted to the tool will .cause the-.spring tap 43 or 44 to slide along the resistor and .either increase or decrease the resistance by altering the position of the termination.

It will be observed that the end termination of the resistance wire may be precisely adjusted to provide the desired resistance value, or angular resistance length.

The potentiometer may be provided with adjustable taps at its end and with additional taps between its ends to provide precise and accurate values of resistance, as will hereinafter be described in detail.

It is not necessary to use clamping means of special construction to interconnect the units. The units set to approximate alignment by suitable instruments are simply assembled with the two screws 10 through an alignment "hole or holes 34.

By means of measuring instruments, the positions of the resistance units in their individual casings may be adjusted relative to each other 'so as to provide the desired synchronization or adjustment of one resistor relative to the other. This adjustment is accomplished by rotating the feed throughs to the adjustable spring taps until the desired adjustment has been obtained. A simi larprocedure is followed for successive casings. By this arrangement the desired accuracy in the control may be obtained.

Using the adjustable taps as described herein, it is not difficult to terminate the resistor at accurately predetermined points. The adjustment of the potentiometer to fit a particular instruments errorsiso'as to render the correct signal output is therefore an easy task. The terminal feed through for each corresponding tap being of a close fit requires no additional tightening after it is once set in position. Although these feed throughs may be placed in position very simply with the proper tools, the high friction from their close fit will not allow them to be moved out of this position through accident. In the preferred embodiment the cylindrical walls 39 to 41 are made of linen base melamine which serves as an insulator as well as the resistor mount. The linen fibers aid in holding the feed throughs in their set positions. The cylindrical casings may, when required, be made of aluminum into which a melamine sleeve may be pressed for the feed through to pass through thus insulating it and giving it the desired frictional contact.

A problem is to afford a smooth wiper action throughout the rotary range of the potentiometer with absolutely constant voltage provided between accurately predetermined points of the resistor withoutthe generation of spurious signals or other troublesome phenomena.

Various attempts have been made to solve the problem by having the wiper traverse the resistor through the whole range of potentiometer operation with the successive turns of the resistor electrically interconnected over the portion of the range in which constant voltage is desired. Such attempts do succeed in providing a smooth wiperaction, thus eliminating any spurious signals that are due solely to the wiper action. Unfortunately, however, other difiiculties are introduced.

One expedient, for example, is to place a conductor .such as a thin wire along the core of the resistor in electrical contact with the successive turns of the resistance wire. It is exceedingly difficult to fabricate such a resistor with accurate placement of the transition points, especially so since the helically wound resistance wire tends to press the ends of the inserted conductor into the underlying core material.

Another expedient is to deposit a layer of metal along the resistor, the deposited metal interconnecting the successive turns of the resistor over the desired range of constant voltage. It has been found, however, that, notwithstanding great care and skill, such a deposit in combination with the turns does not provide uniform voltage. Consequently the traveling wiper in traversing the shorted portion of the resistor creates an undulating signal. which is imposed on a control system.

Broadly described, the present invention meets this problem by a shorting connection between one of the terminals and one of the taps, or if a portion of the resistance along the coil is to be shorted out soas to provide a portion of constant voltage, this may be accomplished by placing a shorting connection between two of the taps. The terminals and taps could then be adjusted to the length of that portion of the range of potentiometer operation over which constant voltage is der sired. Thus the area of equal voltage that the wiper may traverse can be varied to a very accurate length by the use ofthis invention.

This invention contemplates providing for compensating-a potentiometer-the compensation being applied in such a manner as to provide not only the exact total resistance of a potentiometer required, but also to provide the exact function of voltage output vs. shaft displacement requiredfwith a given fixed input voltage and a given load resistance connected to the wiper of the potentiometer.

If it is required that apotentiometer, having applied to one of its fixed terminals a given fixed voltage, must supply as the output of its wiper a voltage which is a given function of the potentiometer shaft rotation, it can be seen that the simple utilization of a potentiometer whose resistance vs. shaft rotation function required would not sufiice because of the presence of the load resistor. This insufiiciently follows because though the potentiometer is connected as a voltage divider, the voltage at the Wiper may not be computed simply by sealing down the supply voltage by the ratio of the resistance between the wiper and one fixed terminal to the total resistance of the potentiometer, but must be computed by reference to Kirchofis law of currents since of resistance wire as a resistance element.

the wiper does draw current. In addition, while potentiometers have heretobefore been available which would provide a given function of resistance vs. shaft rotation within limits of a few percent, since the tolerance could not be entirely eliminated, it has been necessary to adjust the resistance vs. shaft rotation characteristics of each potentiometer before it could be inserted in a circuit to attain the desired precise result. In addition, tolerances exist upon the total resistance of a potentiometer which therefore necessitated the adjustment of the total resistance of a potentiometer before it could be utilized in a precision computer circuit.

In resistance devices in which a resistance wire is wound in a plurality of turns on a support, it is sometimes found necessary in order to obtain predetermined precise resistance values to tap the resistancewire at precise points instead of simply connecting ends of the resistance wire to terminals and using the whole length The point of tapping may have to be so precise that a particular turn of the resistance wire must be tapped.

It will be appreciated, therefore, in constructing potentiometers of the self-balancing type that it is highly desirable to construct the resistance unit in such a manner *that a precise over-all resistance value is assured and that some means should be provided for adjusting the value or effective length of the resistance unit after it has been assembled between terminations thereon.

The ideal resistance unit for a potentiometer of the self-balancing type should consist of a bare wire of resistance material so disposed that the sliding contact or arm may engage any point along its length and that the length thereof would be precise and of an exact value. However, due to the physical difficulties of obtaining suitable resistance values within the available dimensional limits, it is seldom that this ideal can be obtained without resorting to complicated mechanisms which, in themselves, tend to introduce errors that offset the precision attendant on the use of the continuous bare wires.

In potentiometers of the self-balancing type, it is usual, therefore, for the slide wire or resistance unit to be in the form of a continuous winding on a card or mandrel, with the turns of wire along one edge thereof exposed for contact with a sliding arm or contact member at successive turns. By the use of a very fine wire and a great many turns, the potential drop between successive convolutions may be made so small that a single step, that it, from one turn to the other or a momentary shortcircuit between turns is not perceptible in its effect on the apparatus in the circuit of which the potentiometer forms a part. But in the use of a very fine wire to obtain a great many turns the resistance would be increased to a quantity that is too high to be used in some circuits. This invention provides a means whereby a high resistance potentiometer may be reduced to a total relatively low resistance, and it may fit into a low resistance circuit and still maintain the advantage of a great number of turns.

Fig. 4 to 9 illustrate the relative positions of taps on ganged potentiometer uni-ts in accordance with circuits illustrated in Figs. 10 through 17. The application of this invention in various circuits are innumerable, and these circuits are given only to illustrate the versatility of this invention.

Fig. 10 shows the potentiometers in Figs. 4 and 5 ganged together, the distance A being the distance to the electrical center of resistor 54 from terminal 55 and the distance C being the distance from the electrical center of resistor 50 from terminal 53. Therefore the differences between the electrical centers in this ganged relationship is the distance B.

Fig. 11 shows a means of very accurately center tapping resistor 59. This is obtained by a combination of the potentiometer in Fig. 5 and the resistor unit of Fig. 6. A voltage is placed across resistor 59 and resistor 54, and tap 58 is adjusted for as near electrical center of I? resistor 59 as possible. Contact 61 is .not used in this combination; therefore, as Wiper '56 moves along resistor 54, a very fine adjustment of the electrical center of resistor '59 is obtained.

Fig. '12 shows resistor 64 of Fig. 7 in a circuit to render a portion of an end A at zero voltage and a portion of an end C at maximum voltage by the use of shorting c011- nections 69. The lengths A, B, and C may be adjusted very accurately.

Fig. 13 shows the resistor '59 from Fig. 6 with the center tap 58 placed far to one side. Two terminals 6-; and 58 may be adjusted to accurately coincide with the zero percent and 106 percent of mechanical movement. The value of resistance for distance A is very high, while the value of resistance for distance B is near the amount required for the given circuit. Terminal 62 may be adjusted to give the exact required resistance for the given circuit as the sections A and B are parallel with each other. The wiper 61 then travels between terminals 58 and 60 rendering very fine divisions of the voltage therein.

Fig. 14 shows resistor 72 of Fig. 8 in a circuit to render a non-linear voltage output in relation to the wiper movement. The load resistor 87 impresses voltages on resistor 72 differing from the percent of mechanical movement. curately.

Fig. 15 shows the resistor 79 of Fig. 9 and resistor of Fig. 4 ganged to give a varied function output signal. The wipers 82 and 52 are on a common shaft. As the wipers move along their respective resistors 79 and 5t), wiper 52 produces a linear voltage while wiper 82 produces a cycling voltage from zero to 100 percent and back to zero returning to 100 percent etc.

Fig. 16 shows a complex circuit using the resistors of Fig. 7 and Fig. 9. The wipers 66 and 82 are assembled on a single rotor and the two potentiometer units are in a ganged relationship. Now consider only the voltage output of wiper 32 from resistor 79. As the wiper 82 traverses from terminal 81 to terminal 33 there is a rapid decrease in voltage until terminal is reached. The voltage then remains constant until terminal 78 is reached and again a rapid decrease in voltage to zero as terminal 77 is reached. Then there is a rapid increase in voltage until tap is reached. Then there is a slower increase in voltage until tap 84 is reached and then a rapid increase in voltage to percent as terminal 33 is reached. Simultaneously wiper 66 traveling from terminal 65 has a rapid decrease in voltage until terminal 63 is reached. Then a very slow increase in voltage until terminal 67 is reached. This arrangement of taps and terminals provides the means by which the desired voltage function can These differences may be adjusted very ac be set with extreme precision from data presenting the function in a tabular form, by shifting the taps along the resistor until the desired variations are obtained in a very accurate relationship.

Fig. 17 shows the potentiometer as described in Fig. 7 with the resistor 64 Wound of very fine wire and a great many turns. In normal practice this winding arrangement would increase the resistance to a value too large to be utilized some circuits. But to maintain the fine resolution feature of this arrangement, the portions of resistor 64 represented by A and C are placed in parallel with that portion B thus giving a total decrease in resistance. Therefore, as wiper 66 moves between terminal 63 and 68 along resistor 64 we obtain a very fine gradient of voltage increase. Taps 63 and 63 may be set for very accurate mechanical locations equal to the mechanical movement of wiper 66 while terminals 67 and 65 may be adjusted to render a very accurate total resistance.

Cir

While we have elected to describe our invention as embodied 'in 'a "rotary type potentiometer, it is obvious that the same principles are applicable to helical potentiometers as well as potentiometers that operate with straight line relative motion. It is also apparent that our specific disclosure of'the presently preferred practices of the invention will suggest to those skilled in the art various changes, substitutions and other departures that properly lie within the spirit and scope of our appended claims.

We claim:

1. In a variable resistance device comprising a mandrel of insulating material and a resistance wire wound in a plurality of convolutions on said mandrel, the combination comprising: a casing; means in said casing for supporting said mandrel; an adjustable Wiper in engagement with said resistance wire; at least one .adjustable terminal including a feed through passing through said casing and provided with means by which it may be rotated clockwise or counter clockwise; a spring tap connected to said feed through for making pressure contact with the resistance wire, said spring tap being adapted to be moved along said resistance wire upon rotation of said feed through to vary the point of contact of said tap with said resistance wire.

2. A variable resistance device according to claim 1, including an additional adjustable terminal similar to said one adjustable terminal for tapping said resistance wire between its terminals.

3. In .a rotary variable resistance device, the combination of: a casing; afixed curved resistor secured within said casing; a rotary slip ring having a circumferential groove; a wiper associated with said slip ring to electrically connect the slip ring with said resistor; a brush structure in contact with said slip ring, said brush structure having a resilient 'arm on a feed through passing through said casing tomake contact withsaid groove, said resilient arm being smaller in diameter than said feed through, whereby said brush structure may be inserted through said casing after said resistance device has been assembled.

4. A rotary variable resistance device according to claim 3, in which part of said resilient arm is formed like a helical spring, smaller in diameter than the feed through, over a portion of its length to increase its resiliency.

5. In a rotary variable resistance, the combination of: a curved resistor; a wiper in contact with said resistor; actuating means to cause relative rotation between said resistor and wiper about a given axis; means for adjustably gauging together a plurality of such resistors in adjusted angular relation to each other, comprising a casing for each resistor, each casing having adjustable terminals and alignment holes to which the electrical positions of .the resistorshave been adjusted; and alignment screws to pass through these alignment holes to lock the resistors relative to each other.

6. A rotary variable resistance device according to claim 5, including a common control shaft for said resisters.

References Cited in the file of this patent UNITED STATES PATENTS 1,026,798 Gill May 21, 1912 2,592,392 Canziani et a1 Apr. 8, 1952 2,657,295 Barclay Oct. 27, 1953 2,662,146 Wilentchik Dec. 8, 1953

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