US2538415A

US2538415A - Electric motor control system

Info

Publication number: US2538415A
Application number: US589224A
Authority: US
Inventors: William K Ergen
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1945-04-19
Filing date: 1945-04-19
Publication date: 1951-01-16
Anticipated expiration: 1968-01-16

Description

Jan. 16, 1951 w, ERGEN 2,538,415

' ELECTRIC MOTOR CONTROL SYSTEM Filed April 19, 1945 INVEN r0} WILL/HM Ir. mes/v A BY HTTOFNEY Patented Jan. 16, 1951 UNITED STATES PATENT OFFICE 2,538,415 ELECTRIC MOTOR CONTROL SYSTEM William K. Ergen, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application April 19, 1945, Serial No. 589,224

16 Claims. I

This invention relates to the held of electrical control devices, and more particularly to electrical telemetric systems in which the transmitter and receiver comprise endless resistance members similarly tapped and cooperating with contacting sliders.

It is an object of my invention to provide an improved telemetric system in which the contact friction in the transmitting member is reduced by reducing the number of sliding contacts.

It is another object of my invention to provide a telemetric system having improved sensitivity due to a novel interconnection of taps and a novel arrangement of sliders.

It is a further object of my invention to provide a telemetric system in which the transmitter and receiver are tapped endless resistance mem bers, the former having a single slider and the latter comprising three sliders unitarily mounted, the angles between the sliders being not less than the angles between the taps on the members.

Various other objects, advantages, and features of novelty which characterize my invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and objects attained .by its use, reference should be had to the subjoined drawing which forms a further part hereof, and to the-accompanying descriptive matter, in which I have illustrated and described certain preferred embodimentsofmyinvention. Innthe drawing,

for cooperation with winding 20 to make electrical connection therewith at any point around its circumference, and is rotated with respect to the winding in response to a condition by a mechanism I! which will now be described.

Purely by way of illustration I have shown my invention as embodied in a liquid level control system. Liquid is supplied to a tank 25 through Figure '-1 is a schematic representation 'of .a

' simplified embodimentof my invention, and

Figurer2 is a schematic showing of a moregeneralized embodiment of my invention.

Construction of Figure-1 a conduit 26 having a valve 21 arranged for motor operation. A float 30 is mounted by any suitable means (not shown) for vertical movement in the tank as the level of the liquid changes, and a cord 3| fastened to the float is arranged to pass around a drum 32 mounted for unitary rotation with slider 24, from which it is suitably electrically insulated. Thus, as the level of liquid in the tank 25 falls, slider 24 is caused to rotate in a clockwise direction with respect to winding 20: when the level of the liquid rises, slider 24 is moved in a counter-clockwise direction by any suitable spring or other conventional means (not shown). It will .be appreciated that any equivalent mechanism such as a rack and pinion drive may be substituted for the arrangement disclosed if desired.

Receiver II is shown to comprise an endless uniform resistance winding 33 tapped at three equidistant points 34, 35, and 36. As will later be pointed out, there is no necessity that only three taps be provided on each windingz" any desired number of taps greater than three-may also be, used. Moreover, although I have illustrated windings'wxandrfl as being circular in v outline and described them" as being uniform,

Referring flrstto'fi igure 1-, I' have shown-a ically actuated'by a conditionresponsive mechanism I2. The system is shown as energized from a source '13 of alternating current through a transformer l4, and the system supplies a signal r and although the taps are recitedas mutually equidistant, these features also are matters of choice only, since they are required only in an embodiment of the invention in which the angular displacement of the transmitter and the --receiver'from respective zero datu; points in pivotal relationship with winding 3.3 and to an amplifier l5 which is supplied with power from source l3. The amplifier energizes a motor [6, jointly with source l3, and operation of the motor is effective upon the receiver to reduce to zero the signal fed to the amplifier and also operate the controlling mechanism l1.

Transmitter Ill is shown to comprise an endless uniform resistance winding 20 which is tapped at three equidistant points H, 22, and 23. A contacting slider 24 is pivotally mounted carries aplurality of mutually insulated contactingslider's .40,--"4-l-,-'and" 42 arrange'dfto make electrical contact-with the winding at any-point on-its circumference. Member 31 is arranged for rotation with respect to winding 33 by operation of motor It as will presently be set-forth in greater detail. In this form of my invention, sliders 40, 4|, and 42 are mutually equidistant, but it must be understood that, so long as the angle between sliders 40 and 4| and that between

sliders

40 and 42 are not less than the greatest angular displacement between any two adjacent taps on winding 33, it is not necessary that the angles between the sliders be equal or equal to the angles between the taps.

Taps 2|, 22, and 23 of transmitter H) are connected respectively with taps 34, 35, and 36 of receiver II by

conductors

39, 44, and 45. This also is a matter of preference, as it is only necessary that the progression of taps around the windings be sequential: the sequence may progress around the two windings in the same or in opposite directions at the choice of the designer.

The telemetric system made up of transmitter l and receiver II is energized from a source |3 of alternating voltage of a desired frequency through transformer 14 which has a primary winding 46 and a secondary winding 41. Primary winding 46 is connected to source l3 through conductors 56, and 52 and

conductors

53, 54, and 55. The system is energized from secondary winding through a transmitter ground connection 56 connected to one end of secondary winding 41, a receiver ground connection 51, resistor 49, conductor 48, slider 40, a portion of winding 33, one or more of

conductors

39, 44, and 45, depending upon the position of member 24, a portion of winding 20, slider 24, and conductor 56.

Amplifier l5 may be of any desired type in which the voltage of the output is substantially either in phase with or 180 degrees out of phase with the input voltage: amplifiers introducing such low phase shift are known, and details of the structure of amplifier l5 are therefore not included in this application. The amplifier includes its own power supply, energized through terminals 59 and 60, and also has signal input terminals 6| and 62 and output terminals 63 and 64. Sliders 4| and 42 of receiver H are respectively connected to input terminals 6| and 62 of amplifier, l5 by

conductors

65 and 66..

Power is delivered to the amplifier from source I: through conductors 60 and 61 and conductors 53 and 68.

Motor I6 is shown to comprise a rotor 16 mounted on a shaft 13, a line phase" stator winding 1|, and an "amplifier phase stator winding 12. The motor is arranged in driving relation with valve 21 and member 31 through such gear reduction means as may be desirable: this drive means is suggested by shaft 43. While the motor may comprise any suitable number of electrical poles, I have shown it to comprise a pair of such poles, which conventionally comprise four mechanical poles spaced about the stator by 90 mechanical degrees.

As is well known to those skilled in the art. if one electrical pole of such a motor is energized by a first alternating voltage and the second electrical pole is energized by a second alternating voltage in quadrature therewith, the motor will operate, the direction of operation of the motor being determined by which of the voltages leads and which lags.

Associated with motor 26 are a pair of capacitors 13 and 14. Capacitor 13 is connected in a series circuit 15 with winding 1| by conductor 16, and the circuit is continuously energized from source 3 by conductors 50, 5|, and 11 and

conductors

53, 54, and 80. The capacitance of capacitor 13 is so chosen, with respect to the inductance of winding 1|, that at the frequency of source |3 the series circuit is resonant, thus promoting efficient operation of the motor. The voltage drop across windin 1| leads that of the source by approximately 90 degrees, the

exact angle depending on the ohmic resistance of winding 1|.

Capacitor 14 is connected in a parallel circuit 8| with winding 12 by conductors 32 and 63, and the parallel circuit is connected to output terminals 63 and 64 of amplifier l5 by conductors 84 and 85. The capacitance of capaci tor 14 is so chosen, with respect to the inductance of winding 12, that at the frequency of source I3 the parallel circuit is resonant, again promoting efiicient operation of the system. The voltage drop across winding 12 is in phase with the output of the amplifier.

It should be understood that substitution of motors of other types together with known control systems therefor is an obvious expedient to those skilled in the art, when such other types of motors are considered more advantageous.

Operation of Figure 1 In order to understand the operation of my invention, the points of contact of sliders 24, 40, 4|, and 42 with the associated resistance windings, at any instant, are identified by reference numerals 36,31, 36, and 9|, respectively. It will be observed that a circuit mesh constituting a Wheatstone bridge may be traced as follows: tap 2|, conductor 33, tap 34, that portion of resistance windin 33 including point 81, tap 36, conductor 45, tap 23, and that portion of resistance winding 26 including point 36.

Points

66 and 31 can be considered the input terminals of the bridge, which is energized from transformer I4, and

conductors

39 and 46 may be considered the output conductors of the bridge.

when sliders 24 and 46 are in such a position that the angle K 15 equal to the angle M as shown in the ngure, the bridge is balanced and there is no potential difference appearing between

conductors

39 and 45, that is, between taps 34 and 36. For any other relative position of sliders 24 and 40, a potential difference appears between taps 34 and 36, tending to cause flow of current from tap 34 to tap 36 through tap 35. These currents cause IR. drops in the resistance winding, and these in turn are picked oil! by sliders 4| and 42, so that a voltage is impressed on amplifier I5 whose magnitude and phase depend on the relationship between the positions of slider 24 and slider 43. As a result of this, the amplifier supplies an output voltage to stator winding 63 having the phase characteristics indicated above.

Suppose the level of liquid in tank 25 rises, resulting in counterclockwise movement of slider 24. Angie K becomes greater than angle M. Then in the bridge referred to the voltage between

conductors

39 and 45, and hence that between sliders 4| and 42, is substantially 180 degrees out of phase with the voltage between the right and left secondary te'rminals of transformer i4, and therefore in phase with source l3: this voltage is transmitted without apprecibale phase shift through amplifier I5 and impressed upon winding 12 of motor l6. Since winding 1| is continuously energized with a voltage degrees out of phase with that of the source, operation of motor l6 takes place in a first direction.

The connection to winding 12, and such gear reductions as are included in means 43, are so chosen that when thus energized motor l6 operates in a direction to increase angle M, and this operation continues until angle M is made equal to angle K. When this happens the bridge is ba.anced, no signal is transmitted through the amplifier, and energization of winding 12 is interrupted. Although winding 1I continues to be energized, this is not suflicient to maintain rotation of rotor of motor I6.

If the level of liquid in tank 25 falls instead of rising, exactly the opposite effect takes place in each step of the operation thus far traced, and these details need not be repeated. If, however, due to interrupted energization of the system or to some other cause, slider 24 passes tap 2I into the portion of winding 20 between taps 2I and 22,

while slider 40 is still between taps 34 and 36, a

different condition arises. For this special and transitory case, the bridge explanation is not valid, 9. much more complicated network taking the place of the bridge. However, it is found that movement of slider 40 follows that of slider 24 through this critical portion of its range just as it does when the sliders are in similar portions of their resistance windings.

Since both slider 24 and slider 40 are rotatable through 360 degrees, it sometimes occurs that the former comes to rest exactly on one of the taps on winding 20. Where the output to the amplifier should be zero, slider 40 must come to rest on the corresponding tap of winding 33, and this will result in a direct short circuit across the secondary winding. To prevent this from having an injurious effect on the transformer, the slider may be especially wound, but I prefer to insert resistance 49 in series with the sliders so that a direct short circuit can at no time take place.

The foregoing is a description of a specific embodiment of my invention, and it will be seen that a maximum signal is provided to the amplifier when slider 24 is displaced from the related position of slider 40 by one-third of a circle, with slider 40 at or near tap 36 and with slider 24 at or near tap 2| However, a much greater sensitivity is attained by the practice of my invention if the angle between taps 2| and 22 is reduced: a movement of the responsive slider through a given angle is effective to cause a much greater voltage unbalance of the bridge if the angle between the taps is decreased. For a disclosure and description of this embodiment of my invention, attention is now directed to Figure 2.

Construction and operation of Figure 2 Since the structure of Figure 2 is very similar to that of Figure 1, I have used in Figure 2 reference numerals of a series from IN to 200 which are the same as reference numerals from I In I00 for members of Figure 1 having analogous functions. Thus, Figure 2 shows a telemetric system comprising a transmitter H0 and a receiver III. -The system is shown as energized from a source II3 of alternating current through a transformer I I4 and the system supplies a signal to an amplifier I I5 supplied with power from source I I3. The amplifier energizes a motor I I6 jointly with source H3, and operation of the motor is effective upon the receiver to reduce to zero the signal fed to the amplifier, and also operates an indicator II1.

Transmitter III] is shown to comprise an endless uniform resistance winding I20 having a plurality of equally spaced taps, in this case seven in number, of which two are indicated by the reference numerals I2I and I23. A contacting slider I24 is pivotally mounted for cooperation with winding I20 to make electrical contact therewith at any point around its circumference, and is rotated with respect to the winding in response to a condition. In order to avoid unnecessary complication of the drawing, the means by which slider I24 is actuated in response to the condition is not specifically illustrated in this figure.

A member I31 of insulating material is schematically shown as mounted in pivotal relationship with winding I33 and carries a plurality of mutually electrically insulated contacting sliders I40, I, and I42, arranged to make electrical contact with the winding at any point on its circumference. Member I31 is arranged for rotation with respectto winding I33 by operation of motor II6 which will presently be set forth. In this embodiment of my invention, sliders HI and I42 are equally spaced from slider I40, the spacing being the same as that between the taps on winding I33.

Taps I2I and I34 are joined by conductor I39, taps I23'and I36 are joined by conductor I45, and the remaining taps are similarly joined by suitable conductors in the proper sequence.

The system is energized from source II3 through transformer II4, which has a primary winding I46 and a secondary winding'I41. The primary winding is energized from source II3 through conductors I52 and I53. The system is energizedfrom secondary winding I41 through transmitter ground connection I56 connected to one end of secondary'winding I41, receiver ground connection I51, conductor I48, resistor I49, slider I40, a portion of winding I33, one or more of conductors I39, I45, and so forth, depending on the position of s'ider I40, a portion of winding I20, and slider I24.

Amplifier I I5 is shown to have power input terminals I59 and I60, signal input terminals I6I and I62, and output terminals I63 and I64. Sliders MI and I42 of receiver III are respectively connected to input terminals I6I and I62 of amplifier II5 by conductors I65 and I66. Power is delivered to amplifier II5 from source II3 through conductors I61 and I68. The amplifier has the same structure and function as that described in Figure 1, and details of the structure are omitted here for the same reason that they were omitted from the first figure.

Motor I I6 is in all respects identical with motor I6, but is more schematically illustrated. The capacitors explicitly disclosed in Figure 1 are comprised within the motor housing in Figure 2, and the motor is energized from the amplifier through conductors I84 and I85 and from source II3 through conductors I11 and I80.

The detailed mechanical connection between motor H6 and member I31 carrying sliders I40, I, and I42 is suggested schematically in Figure 2, and shaft I19 of motor H6 is also shown as being extended through suitable means I43 to operate the index of indicator I I1 over a scale fixed with respect thereto.

In Figure 2 the bridge relationship between corresponding portions of winding I20 and I33 is more clearly evident than in Figure 1, and the method of operation in this case is exactly the same as that recited in detail in connection with the previous figure. From a comparison of the two figures it is evident that a displacement of five degrees, for example, in slider I24 of Figure 2 is effective to cause the signal to amplifier II5 which is seven-thirds the corresponding signal supplied to amplifier I5 due to a displacement of slider 24 throughthe same angle with respect to winding 20.

Numerous objects and advantages of my invention have been set forth in the foregoing description, together with details of the structure and function of the invention, and the novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrative only, and I may make changes in detail, especially in matters of shape, size and arrangement of parts, within the principle of the invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expresses;

I claim as my invention:

1. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the taps of each resistance member dividing the member into at least three sections; means connecting corresponding taps of the two devices together, said sliders and the taps at the opposite'ends of the section of the receiving resistance member engaged by the slider thereof defining repectively first and second pairs of electrically spaced pointsp a source of electrical energy adapted for transmission along a single pair of conductors; means energizing one only of said pairs of electrically spaced points from said source; and means responsive to the voltage across the other of said pairs of electrically spaced points.

2. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the taps of each resistance member dividing the member into at least three sections, corresponding sections of the resistance members being of the same angular length; means connecting corresponding taps of the two devices together, said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points; a source of electrical energy adapted for transmission along a single, pair of conductors; means energizing one only of said pairs of electrically spaced points from said source; and means responsive to the voltage across the other of said pairs of electrically spaced points.

3. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising a uniform endless resistance member, at least three taps, and a slider, the taps of each resistance member dividing the member into at least three sections, corresponding sections of the resistance members being of the same angular length; means connecting corresponding taps of the two devices together, said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs oi electrically spaced points; a source of electrical energy adapted for transmission along a single pair of conductors; means energizing one only of said pairs of electrically spaced points from said source; and means responsive to the voltage across the other of said pairs of electrically spaced points.

4. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider,

8 the taps of each resistance member dividing the member into at least three sections; means connecting corresponding taps of the two devices together, said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points; a source or electrical energy adapted for transmission along a single pair of conductors; means energizing one only of said pairs of electrically spaced points from said source; and means responsive to the voltage across the other of said pairs of electrically spaced points, said last named means including a pair of further sliders oppositely spaced from said slider of said receiver and contacting said resistance member of said receiver, the angular spacing between said further sliders being no less than the greatest spacing between said taps of said resistance member of said receiver.

5. In a device of the class-described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the taps of each resistance member dividing the member into at least three equal sections; means connecting corresponding taps of the two devices together, said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points; means for impressing a potential across one of said pairs of electrically spaced points only; and means responsive to the voltage across the other of said pairs of electrically spaced points, said last named means including a pair of further sliders equally and oppositely spaced from said slider of said receiver and contacting said resistance member of said receiver, the angular spacing between said further sliders being equal to the spacing between said taps of said resistance members.

6. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the taps of each resistance member dividing the member into at least three sections; means connecting corresponding taps of the two devices together, said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points; a source of electrical energy adapted for transmission along a single pair of conductors; means energizing one only of said pairs of electrically spaced points from said source; means responsive to the voltage across the other of said pairs of electrically spaced points; means actuating said slider of said transmitting device in response to a condition; and motor means actuated by said voltage responsive means. i

'7. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the taps of each resistance member dividing the member into at least three sections; means connecting corresponding taps oi the two devices together; said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points; a source of electrical energy adapted for transmission along a single pair of conductors; means energizing one only of said pairs of electrically spaced points from said source; means responsive to the voltage across the other of said pairs of electrically spaced points, said means including a pair of further sliders oppositely spaced from said slider of said receiver and contacting said resistance member of said receiver, the angular spacing of said sliders being no less than the greatest spacing between said taps of said resistance member of said receiver; means actuating said slider of said transmittng device in response to a condition; and motor means actuated by said voltage responsive means.

8. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the taps of each resistance member dividing the member into at least three equal sections; means connecting corresponding taps of the two devices together; said sliders and the taps at the opposite ends of the section of the receiving potentiometer engaged by the slider thereof constituting two pairs of electrically spaced points; means for impressing a potential across one of said pairs of electrically spaced points only; means responsive to the voltage across the other of said pairs of electrically spaced points, said means including a pair of further sliders equally and oppositely spaced from said slider of said receiver and contacting said resistance winding of said receiver, the angular spacing of said sliders being equal to the spacing between said taps of said resistance member of said receiver; means actuating said slider of said transmitting device in response to a condition; and motor means actuated by said voltage responsive means.

9. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the

taps of each resistance member dividing the member into at least three'sections; means connecting corresponding taps of the two devices together; said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points; means for impressing a potential across one of said pairs of electrically spaced points only; means responsive to the voltage across the other of said pairs of electrically spaced points; means actuating said slider of said transmitting device in response to a condition; motor means actuated by said voltage responsive means; and means connecting said sliders of said receiving device with said motor means for unitary actuation thereby.

10. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider,

the taps of each resistance member dividing the member into at least three sections; means connecting corres onding taps of thetwo devices together; said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points: means for impressing a potential across one of said pairs of electrically spaced points only; means responsive to the voltage across the other of said pairs 01' electrically spaced points, said last named means including a pair of 10 p ,l further sliders oppositely spaced from said slider of said receiver and contacting said resistance member of said receiver, the angular spacing of said sliders being no less than the least spacing between said taps of said resistance member of said receiver; means actuating said slider of said transmitting device in response to a condition; motor means actuated by said voltage'responsive means; and means connecting said sliders of said receiving device with said motor means for unitary actuation thereby.

11. In a device of the class described, in combination, a transmitting device and a receiving device, each device comprising a uniform endless resistance member, at least three taps, and a slider, the taps of each, resistance member dividing the member into at least three equal sections; means connecting corresponding taps of the two devices together; said sliders and the taps at the opposite ends of the section of the receiving resistance member engaged by the slider thereof defining respectively first and second pairs of electrically spaced points; means for impressing a potential across one of said pairs of electrically spaced points only, and means responsive to the voltage across the other of said pairs of electrically spaced points; said last named means including a pair of further sliders equally and oppositely spaced from said slider of said receiver and contacting said resistance winding of said receiver, the angular spacing of said sliders being equal to the spacing between said taps of said resistance member of said receiver; means actuating said slider of said transmitting device in response to a condition; and motor means actuated by said voltage responsive means; and means connecting said sliders of said receiving device with said motor means for unitary actuation thereby.

12. In a device of the class described in combination: first and second endless im edance members, said members being provided with like numbers of equally s aced taps; a movable slider engaging one of said members for continuously potential difierence between said slider of said first member and said first slider of said second member; and electrically responsive means connected between said further sliders of said second member. 13. In a device of the class described in combination: first and second endless resistance members, said members being provided with like numbers of equally spaced taps; a movable slider engaging one of said members for continuously making electrical connection therewith; a first movable slider similarly engaging the second of said members and further sliders similarly-enaging said second member and oppositely spaced from and movable unitarily with said first slider, the spacing between said first slider and said further sliders being the same as the spacing between said taps; electrically conducting means joining each tap on one of said members with a tap on the other member in the same sequence;

7 means maintaining an alternating potential difor said members for continuously making electrical connection therewith; a first movable slider similarly engaging the second of said members and further sliders similarly engaging said second member and oppositely spaced from and movable unitarily with said first slider, the spacing between said first slider and said further sliders being no less than the greatest spacing between said taps; electrically conducting means joining each tap on one 01' said members with a tap on the other member in the same sequence; means maintaining an alternating potential diflerence between said slider of said first member and said first slider or said second member; and electrically responsive means connected between said further sliders of said second member.

15. In a device of the class described, in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and movable means for making electrical connection to said resistance member at any point therealong, the taps of each resistance member dividing the member into at least three sections; means connecting corresponding taps oi. the two devices together: said movable means of said receiving device com rising a pair of angularly spaced sliders contacting said resistance member of said device, the an ular spacing between said s iders being no less than the greatest spacing between said tape oi said receiver; said pair 01' sliders defining a first pair of electrically spaced points, and said movable means of said transmitting device and a point at a potential midway between the potentials of said pair of sliders constituting a second pair of electrically spaced points; a. source of electrical energy adapted for transmis- 12 sion along a single pair of conductors; means energizing one only of said pairs of electrically spaced points from said source; and means responsive to the voltage across the other of said pairs 01' electrically spaced points.

16. In a device 0! the class described. in combination: a transmitting device and a receiving device, each device comprising an endless resistance member, at least three taps, and a slider, the taps 01 each resistance member dividing the member into at least three sections means connecting corresponding tape of the two devices together; a pair of further sliders oppositely spaced from said slider of said receiving device and contacting said resistance member of said device, the angular spacing between said further sliders being no less than the greatest spacing between said taps of said receiver; said pair 0! further sliders defining a first pair of electrically spaced points, and said slider of said transmitting device and a point at a potential mid-way between the potentials of said further sliders constituting a second pair of electrically spaced points; a source of electrical energy adapted for transmission along a single pair of conductors; means energizing one only 01 said pairs of electrically spaced points from said source; and

means responsive to the voltage across the other of said pairs of electrically spaced points.-

WILLIAM K. ERGEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS (Add. No. 25,758)