US3223919A - Electrical potentiometers with a second set of conductors spaced differently from an integral multiple of spacings between a first set of conductors - Google Patents

Description

Dec. 14, 1965 E. M. LANGHAM 3,2 919 ELECTRICAL POTENTIOMETE WITH A SECOND SET 0 ONDUCT S SPACED DIFF NTLY OM INTEGRAL MULT E OF PAGINGS TWEEN A FI SET OF GONDUCTORS Filed June 12, 1 2 Sheets-Sheet 1 Dec. 14, 1965 E LANGHAM 3,223,919

ELECTRICAL POTENTIOME'fERS WITHA SECOND SET OF CONDUCTORS SPAGED DIFFERENTLY FROM AN INTEGRAL MULTIPLE OF SPAGINGS BETWEEN A FIRST SET OF CONDUCTORS Filed June 12, 1961 2 Sheets-Sheet 2 United States Patent 3,223,919 ELECTRICAL POTENTIOMETERS WITH A SEC- 0ND SET OF CONDUCTORS SPACE!) DIFFER- ENTLY FROM AN INTEGRAL MULTIPLE 0F SPACINGS BETWEEN A FIRST SET OF CON- DUCTQRS Eric Miles Langham, 14 The Close, Norwich, Norfolk, England Filed .Iune 12, 1961, Ser. No. 116,568 Claims priority, application Great Britain, .Iune 21, 1960, 21,753/60 8 Claims. (Cl. 323-435) This invention relates to electrical potentiometers, which term is used herein to denote a device enabling a desired and variable output voltage to be selected when a substantially steady input voltage is applied to the device.

One well-known form of variable voltage divider or potentiometer consists of a non-inductive former wound with resistance wire, the turns of which lie transversely of the former, and a movable contact which is arranged to travel along the length of the former and so select a desired turn of the winding to form a movable tapping, so that when an input voltage is applied between the ends of the winding, a portion of the input voltage is obtained as an output voltage between one end of the winding and the movable contact. The arrangement is such that the movable contact is always in contact with at least one turn of the winding, for example by using two contacts spaced apart a short distance axially of the former, and the output voltage then varies in a step-like manner as the movable contact moves along the former.

As a variation of that type of voltage divider, it is possible to use a winding of low ohmic resistance, wound on a former of highly magnetically permeable material arranged in a closed magnetic circuit, the input voltage in this case being an alternating voltage. Such a potentiometer is an inductive voltage divider, and the output voltage varies, as described above in connection with the resistive voltage divider, in a step-like manner.

It is possible to reduce the magnitude of the steps in such voltage dividers by providing several sliding contacts, arranged to move in unison along the former, but arranged in vernier fashion. Thus four such contacts could be arranged to span three turns of the winding, the contacts being connected each through an individual resistor to a common or star point. In such an arrangement, the output voltage is taken between the star point and one end of the winding. With such an arrangement, the output voltage will only satisfy accurately the desired output characteristic of the potentiometer at those instants when all the contacts are in contact with turns of the winding, and

as contacts move from one turn to the next, steps will occur in the actual output characteristic. Since the number of steps is increased by the use of the vernier arrangement, the size of the steps is reduced. A disadvantage of this type of potentiometer is that the star connection of the contacts causes shunting of the part of the winding bridged by the group of contacts, so introducing a further error in the potential dividing ratio.

An object of the present invention is the provision of improved potentiometer means in which the magnitude of the steps on the stepwise variation of the potential dividing ratio is reduced.

According to the present invention, electrical potentiometer means comprise a first set of conductors arranged in a line, a second set of conductors also arranged in a line and capable of movement in unison relative to the conductors of the first set in the direction of the line along the first set of conductors, the number of conductors in the second set being smaller than the number of conductors in the first set, and the pitch of the conductors in the 3,223,919 Patented Dec. 14, 1965 two sets being difierent, the conductors of the second set being coupled conductively, inductively or capacitatively to the facing conductors of the first set, means for applying in use electrical potentials to the conductors of the first set which increase in succession along the line in equal increments or steps, means for applying in use electrical potentials to the conductors of the second set which increase in succession along the line in equal increments or steps, the difference in electrical potential between the extreme conductors of the second set being substantially equal in magnitude and sense to the electrical potential between the extreme conductors of the part of the first set spanned by the second set, and two output connections between which the desired output voltage appears and which are coupled respectively to the conductors of the first set and the conductors of the second set.

The invention will now be described, by way of example, with reference to the accompanying partly diagrammatic drawings, in which:

FIGURE 1 is a diagram of a potentiometer circuit in which the input voltage is an alternating voltage;

FIGURE 2 is perspective view of two relatively movable parts of a second form of potentiometer circuit;

FIGURE 3 is a sectional side elevation of part only of two relatively movable parts shown in FIGURE 2;

FIGURE 4 is a circuit diagram in which the parts shown in'FIGURES 2 and 3 are incorporated;

FIGURE 5 is a circuit diagram of a third form of potentiometer circuit;

FIGURE 6 is a perspective view of part of a potentiometer member incorporated in the circuit of FIGURE 5; and

FIGURE 7 is a perspective view of the main paltsof the circuit diagram shown in FIGURE 5.

Referring first to the embodiment of the invention shown in FIGURE 1, a cylindrical former 1 of insulating material is wound with an evenly distributed winding 2 formed of resistance wire, the spacing between the turns of the Winding 2 being exaggerated for clarity. The primary winding 3P of a transformer 3 is connected in parallel with the winding 2, the input voltage VI thus being applied to both these two windings, and the secondary winding 38 of this transformer is provided with four tapping points 5. Each end of the secondary winding 38, and each of the tapping points 5, is connected through an associated resistor 6 to one of six contacts 7. All six contacts 7 are mounted on a slider 8 so that they move in unison along the former 1 with the contacts 7 engaging exposed parts of the wire of winding 2, and contacts 7 are evenly spaced apart. The spacing between pairs of adjacent contacts 7 is made unequal to, and unequal to any constant multiple of, the spacing between the turns of the winding 2, so that a vernier effect is obtained. Thus the six contacts are shown as spanning eight turns. The turns ratio of the transformer 3 is so selected that the across the outer two contacts 7A and 7B is equal to, and of the same phase as, the voltage drop across the part of the winding 2 spanned by these two contacts. Thus the E.M.F. between successive taps on the secondary winding 38 is equal to the voltage drop across the part of the Winding engaged by the contacts 7 connected to those two taps. The output voltage V0 of the potentiometer circuit is taken from between a centre tap 9 of the secondary winding 35 and a connection made with one end of the winding 2.

During use of the potentiometer circuit, as the contacts 7 move in unison along the former each contact 7 will periodically pass from one turn of the winding to an adjacent turn, so that that contact experiences a stepwise change in potential. This will modify the currents flowing through the various resistors 6 and the sections of the secondary winding 38, but the variation caused in the output voltage extracted from the centre tap 9 will vary less than the change in voltage experienced by the individual contact 7. At the same time, since the E.M.F in the transformer winding 33 is equal to and in opposition to the voltage drop in the winding 2 between contacts 7A and 7B, substantially no shunting of this part of winding 2 takes place.

Thus, it will be apparent that with a known form of resistive potentiometer having a wound former and single wiping contact, the steps in voltage experienced by the contact as, during movement thereof, it connects with successive turns on the former, are greater than those experienced at the output voltage centre tapping of the secondary winding 38 in the potentiometer circuit of the present invention. Thus improved accuracy in the variations of output voltage as a function of the positions of the slider is obtained.

Referring now to the embodiment of the invention shown in FIGURES 2 to 4, the inductive potentiometer circuit shown therein comprises a rod former 21 of highly magnetically permeable material wound uniformly with a sectionalised winding 22 of a wire which is a relatively good conductor, for example of copper. The sections of the winding 22 are insulated from one another and they consist of equal numbers of turns. A transformer 23 (see FIGURE 4) includes a primary winding 23F to which the input voltage VI is applied and a secondary winding 235, the winding 23S being provided with tappings 24 one less in number than the number of sections in the winding 22. One end of the winding 23S and each of its tappings 24 are connected severally to the start ends (marked S) of the difierent sections of the winding 22. The second end of winding 238 is connected to one of a pair of output terminals OT. The finish ends of the different sections of the winding 22 (marked F) are left unconnected.

The sectionalised winding 22 is embraced by a sleeve 25 which is shorter in length, axially of the former 21, than the winding 22, and so embraces part only of the length of that Winding. The sleeve 25 is channel shaped in longitudinal cross section, as seen most clearly in FIG- URE 3, and Within the gap in the channel section are disposed two co-axial windings 27 and 28 of which the winding 28 is within the winding 27. These windings span more than one section of the sectionalised winding 22. The outer winding 27 has its ends connected respectively to the ends of a further secondary winding 2ST of the transformer 23. The winding 28 has its ends connected respectively through resistors 30 and 31 to the second output terminal OT.

In use of the potentiometer circuit of FIGURES 2 to 4 a magnetic circuit comprising the permeable sleeve 25, the permeable rod 21 and the air gaps between the flanges of the channel section of the sleeve 25 and the rod, couples by transformer action the outer winding 27 with the inner winding 28, so that the energised outer winding 27 energises the inner winding 28.

The number of turns in the winding 28 diifers slightly from the number of turns in the sectionalised winding 22 which they span, preferably being greater in number. The resistors 30 and 31 have values so selected that the volts per inch along the windings 28 and 22 are equal to one another.

In the example shown, a resistive connection is provided between the winding 28 and the winding 22. Thus facing axlfllly extending areas of the windings 28 and 22 are bared and the winding 28 is a close sliding fit on th i ing 22. In this manner, a large number of high impedance contacts are established between the spanned turns of the winding 22 and the facing turns of Winding 28, these being indicated diagrammatically by resistors 35 in FIGURE 4.

The voltage appearing between the output terminals OT will depend upon the effective turns ratio of the transformer formed by winding 23F and the effective part of the winding 238. It will be appreciated, upon tracing the circuit from one terminal OT through winding 235, the effective sections of winding 22, the high impedance paths indicated as resistors 35, the winding 28 and the res1stors 3t and 31, that the potentiometer circuit of FIGURE 4 is basically a transformer having a plurality of adjustable tapping points which move in unison as the winding 28 slides over the winding 22.

Since an is generated in the winding 28, by transformer action from the winding 27, with a volts/ inch potential gradient along the winding 28- equal or substantially equal to the volts/inch potential gradient along the winding 22, the winding 28 does not substantially shunt the part of the winding 22 which it spans.

During use of the potentiometer circuit, as the sleeve 25 moves along the rod 21 each of the contacts represented by the resistors 35 will transfer from one turn of the winding 22 to an adjacent turn, so that that contact experiences a step-wise change in potential. This will modify the currents flowing through the various resistors 35 and the sections of the winding 22, but the variation caused in the output voltage at the output terminals OT will vary less than the change in voltage experienced by the individual contact represented by a resistor 35.

In the embodiment of the invention described above in connection with FIGURES 2 to 4, the high impedance connections between windings 28 and 22 are resistive in nature, and are represented by the resistors 35. Alternatively, a capacitative connection can be effected between the windings 28 and 22. Thus facing areas of the windings 28 and 22 can be bared as described above in connection with FIGURES 2 to 4, and the exposed areas coated with a suitable dielectric material, e.g. varnish or plastics material. If desired aluminium wire can be used for the windings 28 and 22, and the dielectric provided by anodizing the surfaces of the windings.

It will be apparent that whether resistive or capacitative connections are employed, the inner winding 28 in effect provides a large number of high impedance contacts with the spanned turns of the sectionalised winding 22 on the rod 21.

As a further modification of the arrangement shown in FIGURES 2 to 4, the inner winding 28 can be dispensed with and the plurality of equally spaced contacts between the winding 28 and the sectionalised winding 22 replaced with a plurality of equally spaced high impedance connections between selected turns on the winding 27 and the turns on the winding 22. As in the example of FIGURES 2 to 4, the volts/inch gradient along the contacts with the winding 22 are made substantially equal to the volts/inch gradient along the winding 22. The second of the output terminals OT would, in this modification, be connected to a centre tap on the winding 28.

In the specific embodiments described above, the windings have been in the form of helical coils wound on formers of cylindrical cross section, but the invention is also applicable to other forms of windings and other shapes of former. Further, the requisite relative movement when the output voltage of the potentiometer circuit is to be varied can be eifected between relatively movable contact or coupling members, while the actual resistive or inductive windings remain stationary.

In the potentiometer circuit illustrated in FIGURES 5 to 7, no relative movement is eifected between the windings, and the variable couplings are capacitative couplings, as described above in connection with one of the modifications of FIGURES 2 to 4. Thus the alternating input voltage VI is applied to terminals 51, 52, which are connected to the ends of the winding 53 of a toroidal voltage transformer 54. This transformer 54 is provided with a large number of equally spaced taps 55, and is equivalent to the winding 2 shown in FIGURE 1. Also connected across the terminals 51, 52 is a primary winding 57 P of a second toroidal voltage transformer 57, this transformer 57 having an isolated secondary winding 578 provided with a large number of equally spaced taps 58. It will be seen that this transformer 57 is equivalent to the transformer 3 of FIGURE 1. The desired capacitative couplings between the taps 55 and 58 is effected by the arrangement shown in FIGURES 6 and 7 A number of slabs 62 are provided, each slab consisting (as shown most clearly in FIGURE 6) of an elongated flat plate of insulating material such as glass or Bakelite on one face of which are affixed metal strips 63, the strips extending transversely of plate 62, being set close together and equally spaced apart along the length of the plate. Conveniently the strips are 0.490 inch wide and are spaced at a pitch of half an inch, so that there is a gap 64 which is 0.010 inch wide between adjacent strips. The strips extend at one end beyond the edge of the plate 62, being bent over as indicated to provide terminal tabs 63A. The strips 63 can be stuck to the plate by the use of a suitable adhesive, they can be held in place by screws or bolts, or they can be provided by photo-etching of a metallic film provided on the plate.

The arrangement shown in FIGURE 7 comprises several such plates 62 arranged end to end and suitably mounted on a backing member (not shown). A movable plate 72 is provided with strips 73 (see FIGURE 5) in a manner similar to that used for the plate 62, but the strips 73 are narrower than the strips 63. Thus, over a ten inch length of the plate 62 twenty strips 63 are provided, but over a ten inch length of the plate 72 twentytwo strips 73 are provided. The object of this difference in the pitch of the strips is to provide a Vernier effect, as will be explained below. Strips 73 have terminal tabs 73A. The-movable plate 72 is carried by suitable guides, not shown, so that it can be moved along the assembly of plates 62 with the strips 63 separated from the strips 73 by an air gap of 0.0005 inch.

The strips 63 are connected respectively to the taps 55 and the ends ofthe winding 53 of transformer 54, so that a step-,by-step potential gradient is set up along the assembly of plates 62. It will be appreciated that, for any practical number of taps 55 and strips 63, this assembly of plates 62 forms a potentiometer of very coarse discrimination, since the discrimination is limited by the widths of the strips.

The strips 73 of the movable plate 72 are connected respectively to the taps 58 and the ends of the winding 578 of the transformer 57.

The turns ratio of the transformer 57 is so selected that the mean potential gradient along the plate 72 is substantially equal to that along the facing part of plates 62. Two output terminals OT are connected respectively to the input terminal 52 and to a selected one of the taps 58 on transformer secondary winding 578. Which tap 58 is selected will depend upon the setting of the plate 72 relative to the assembly of plates 62 at which it is desired to obtain Zero output voltage, and in FIGURE 5 the selected tap 58 is that connected to the end strip 73, so that when the plates 72 and v62 have the relative positions shown in FIGURE 7 the output voltage is zero. If desired, an alternative output terminal 75 connected to a centre tap on winding 578 could be utilised.

It is convenient, at this point, to consider how the potentiometer circuit of FIGURES 5 to 7 would operate if only the end strip 73 connected to an output terminal OT were provided. If this strip were narrow compared with the strips 63, the output voltage, as a function of the distance moved by the plate 72, would be a staircase and would be a very inaccurate representation of a linear function. If this strip was made comparable in width with the width of the strips 63, the output voltage, as a function of the distance moved, would be smoother because the strip 73 would act as a variable capacitor between adjacent strips 63. The variation of the output voltage would not be linear, however, because of the effects of the gaps between adjacent strips 63 and because of variations of the effective width of the gap between the single strip 73 and strips 63. Normally the errors would repeat as the strip 73 passed over each strip 63.

This action is modified by the vernier arrangement of the strips 63 and 73 and the manner in which they are connected. Each strip 73 acts as an individual tap to the potentiometer formed by the strips 63. Interpolation is provided to each strip by virtue of the variable capacitance between the strips 63 and 73.

If the voltage per ten inches along the strips 63 were ten volts, then in the embodiment described above the voltage occupied by the strips 73 would ideally be l/2.2 volts per strip. Actually, some strips 73 will have higher voltages and others lower voltages because of the errors from linearity in the effective variable condenser formed between one strip 73 and the two nearest strips 63. The transformer winding 578 is arranged to provide an of ten volts, and this winding has taps 58 at 1/22 volts from each other. As described above, ,these taps 58 are connected successively to the strips 73. There is now a direct connection through the Winding 57S between all of the strips 73 and yet this direct connection also holds the strips to their nominal voltage separation. There is a low impedance path between strips 73 (by the transformer action) so that the strips 73 which have a high positive error from their ideal voltage pass through the transformer winding 573 current into the strips 73 which have a high negative error. Since the effective output impedance of the variable capacitors formed between strips 63 and 73 is high, this causes the effect of the high positive error strips to be cancelled by the high negative error strips.

Thus, the effect of the errors in the interpolation of voltage between strips on the fixed member is very much reduced.

In the example shown in FIGURE 5, one of the output terminals OT is connected to the lowermost strip 63. If this connection is removed and applied to another of the strips 63, that output terminal will be at the voltage of that strip 63, so that the output voltage across terminals OT can be made zero at a different, predetermined point in the travel of the plate 72.

In the embodiments of FIGURE 1 and FIGURES 2 and 4, a potentiometer of a type normally designed for high resolution was described and used. However, it has been found that the effect of the interpolation provided by the potentiometer circuits of the present invention is so good that the construction of the potentiometer circui can be simplified by using a potentiometer arrangement, i.e. the arrangement of strips 63, which would ordinarily have extremely coarse resolution.

In the embodiments of the invention described above, linear relative movements between relatively moving parts of the potentiometer circuit have been used, but it will be clear to those skilled in the art that the same principles may be applied to potentiometer circuits in which the relative movement is a rotary motion. For example, the construction of FIGURES 5 to 7 can be modified by the use of sector shaped strips arranged in a circle instead of the strips 63 and 73.

I claim:

1. Electrical potentiometer means comprising:

a first mounting;

a first set of several uniformly spaced conducting elements supported on the first mounting and arranged transversely to a line intersecting each of said elements;

ship with elements of the first set, the spacing between the conducting elements of one of the sets differing from that and from an integral multiple of that obtaining between the elements of thet other set of conducting elements; I

means for applying in use electrical potentials to the respective elements of the first set which increase in equal increments successively from element to element of the first set;

means for applying in use electrical potentials to the respective elements of the second set which increase in equal increments successively from element to element of the second set, the difference in electrical potential between end elements of the second set being substantially equal in magnitude and sense to the electrical potential between end elements of that part of the first set spanned by the second set; and

two output voltage connections respectively so coupled to the first and second sets of conducting elements as to provide in use a desired output voltage dependent upon the relative position of the second to the first set of conducting elements.

2. Electrical potentiometer means as claimed in claim 1, wherein the conducting elements of the second set are coupled respectively to tappings of an energised inductance.

3. Electrical potentiometer means as claimed in claim 2, wherein the energised inductance is a secondary winding of a transformer.

4. Electrical potentiometer means as claimed in claim 3, wherein the coupling between the conducting elements of the first and second sets is a conductive coupling and of relatively high impedance compared with the impedance between successive elements of each set.

5. Electrical potentiometer means as claimed in claim 1, wherein the first set of conducting elements are turns of a continuous resistance winding, the second set of conducting elements are contacts arranged to engage bared portions of the turns of the resistance winding, means are arranged for applying an input voltage to the ends of the resistance winding, the means for applying the electrical potentials to the conducting elements of the second set comprise a transformer the primary winding of which is connected in parallel with the resistance winding, a secondary winding of which is provided with taps, and the taps of the winding are connected to the conductors of the second set through resistors of relatively high impedance compared with the impedance between successive elements of each set.

6. Electrical potentiometer means as claimed in claim 5, wherein the two output connections are connected respectively to one end of the resistance winding and to a centre tap of the secondary winding,

7. Electrical potentiometer means as claimed in claim 1, wherein the first set of conducting elements are turns of a sectionalised winding wound on a former of highly magnetically permeable material, and the means for applying electrical potentials to the conducting elements of the first set comprise an energised inductance.

8. Electrical potentiometer means as claimed in claim 7, wherein the energised inductance is the secondary winding of a transformer.

References Cited by the Examiner UNITED STATES PAT ENTS 2,109,214 2/1938 Harder 338-424 X 2,342,084 2/ 1944 Lennox 323--43.5 2,572,545 10/1951 Walker 32379 2,843,822 7/1958 Scott 32379 2,886,677 5/1959 Bourns 32379 X 2,888,636 5/1959 McManis 323*79 X 2,889,504 6/1959 Spencer 323-43.5 2,976,476 3/ 1961 Snowdon 323-79 3,032,702 5/1962 Scott 323-79 3,065,418 11/1962 Dauphinee 32379 3,113,262 12/1963 Green 323-79 LLOYD MCCOLLUM, Primary Examiner.

MILTON O. HIRSHFIELD, RALPH D. BLAKESLEE,

Examiners.

Claims (1)

1. ELECTRICAL POTENTIOMETER MEANS COMPRISING: A FIRST MOUNTING; A FIRST SET OF SEVERAL UNIFORMLY SPACED CONDUCING ELEMENTS SUPPORTED ON THE FIRST MOUNTING AND ARRANGED TRANSVERSELY TO A LINE INTERSECTING EACH OF SID ELEMENTS; A SECOND MOUNTING; A SECOND SET OF SEVERAL CONDUCTING ELEMENTS SUPPORTED ON THE SECOND MOUNTING, SAID SECOND SET OF CONDUCTING ELEMENTS BEING FEWER IN NUMBER THAN THE ELEMENTS OF THE FIRST SET, UNIFORMLY SPACED, ARRANGED TRANSVERSELY TO A LINE INTERSECTING ECH OF SAID ELEMENTS OF SAID SECOND SET AND IN FACE TO FACE RELATIONSHIP WITH ELEMENTS OF THE FIRST SET, THE SPACING BETWEEN THE CONDUCTING ELEMENTS OF ONE OF THE SETS DIFFERING FROM THAT AND FROM AN INTERGRAL MULTIPLE OF THAT OBTAINING BETWEEN THE ELEMENTS OF THET OTHER SET OF CONDUCTING ELEMENTS; MEANS FOR APPLYING IN USE ELECTRICAL POTENTIALS TO THE RESPECTIVE ELEMENTS OF THE FIRST SET WHICH INCREASE IN EDQUAL INCREMENTS SUCCESSIVELY FROM ELEMENT TO ELEMENT OF THE FIRST SET; MEANS FOR APPLYING IN USE ELECTRICAL POTENTIALS TO THE RESPECTIVE ELEMENT OF THE SECOND SET WHICH INCREASE IN EQUAL INCREMENTS SUCCESSIVELY FROM ELEMENT TO ELEMENT OF THE SECOND SET, THE DIFFERENCE IN ELECTRICAL POTENTIAL BETWEEN END ELEMENTS OF THE SECOND SET BEING SUBSTANTIALLY EQUAL IN MAGNITUDE AND SENSE TO THE ELECTRICAL POTENTIAL BETWEEN END ELEMENTS OF THAT PART OF THE FIRST SET SPANNED BY THE SECOND SET; AND TWO OUTUT VOLTAGE CONNECTIONS RESPECTIVELY SO COUPLED TO THE FIRST AND SECOND SETS OF CONDUCTING ELEMENTS AS TO PROVIDE IN USE A DESIRED OUTPUT VOLTAGE DEPENDENT UPON THE RELATIVE POSITION OF THE SECOND TO THE FIRST SET OF CONDUCTING ELEMENTS.

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