US3636428A

US3636428A - Synchro servosystem in which a potentiometer with unique characteristics replaces the receiver

Info

Publication number: US3636428A
Application number: US714684A
Authority: US
Inventors: Joseph J Yanosik
Original assignee: Betatronix Inc
Current assignee: Betatronix Inc
Priority date: 1968-03-20
Filing date: 1968-03-20
Publication date: 1972-01-18
Anticipated expiration: 1989-01-18

Abstract

Potentiometer structures for synchro systems and synchro systems utilizing such structures. One or more potentiometers are employed in place of the usual rotor and stator inductive transformer arrangements employed in the control transmitter and control transformer of a synchro system. The potentiometer employed in the synchro control transformer has a resistivity which varies in accordance with a cotangent function of the displacement of the movable wiper of the potentiometer from a reference position. Essentially the control transformer potentiometer employs three similar resistive segments, each corresponding to 120 electrical degrees of rotation of the potentiometer. The equation expressing the resistance R at a location in a segment as a function of the angle of displacement is as follows: R ( 1/2 -( 3/6)ctn.(30*+X))Rt WHERE X equals the electrical angle of displacement which, for each segment, varies from 0* to 120*, and Rt is the open circuit resistance between the ends of each segment. The potentiometer employed as a synchro control transmitter to be used with the potentiometer-type synchro control transformer just described includes a square-shaped film of uniform resistivity against which bear three wipers of equal angular separation (120*). The wipers provide the output signal to the control transformer. The input to the square-shaped film is an AC or DC signal applied to opposing sides of the film. A midpoint of one or both of the two remaining sides is typically grounded. The signals from the three wipers are related to the DC input signal by a sine function, i.e., dependent upon the sine of the angle of rotation of the wipers from a reference position.

Description

United States Patent Yanosik [54] SYNCHRO SERVOSYSTEM IN WHICH A POTENTIOMETER WITH UNIQUE CHARACTERISTICS REPLACES THE RECEIVER [72] lnventor: Joseph J. Yanoslk, Huntington Station,

Long Island, NY.

[73] Assignee: Betatronlx, Inc., East Northport, NY. [22] Filed: Mar. 20, 1968 [21] App]. No.1 714,684

Primary Examiner-T. E. Lynch Attorney-Robert S. Dunham, P. E. Henninger, Lester W.

1451 Jan. 18, 1972 [57] ABSTRACT Potentiometer structures for synchro systems and synchro systems utilizing such structures. One or more otentiometers are employed in place of the usual rotor and stator inductive transformer arrangements employed in the control transmitter and control transformer of a synchro system. The potentiometer employed in the synchro control transformer has a resistivity which varies in accordance with a cotangent function of the displacement of the movable wiper of the potentiometer from a reference position. Essentially the control transformer potentiometer employs three similar resistive segments, each corresponding to I20 electrical degrees of rotation of the potentiometer. The equation expressing the resistance R at a location in a segment as a function of the angle of displacement is as follows:

R= 1/2- View. 30+ X)]R,

where X equals the electrical angle of displacement which, for each segment, varies from 0 to 120, and R, is the open circuit resistance between the ends of each segment.

The potentiometer employed as a synchro control transmitter to be used with the potentiometer-type synchro control transformer just described includes a squareshaped film of uniform resistivity against which bear three wipers of equal angular separation (120). The wipers provide the output signal to the control transformer. The input to the square-shaped film is an AC or DC signal applied to opposing sides of the film. A midpoint of one or both of the two remaining sides is typically grounded. The signals from the three wipers are related to the DC 'input signal by a sine function, i.e., dependent upon the sine of the angle of rotation of the wipers from a reference position.

Clark, Thomas F. Moran, Gerald W. Griffin, Howard J. 8Claims,9DrawingFigureS Churchill, R. Bradlee Boal, ChristopherC. Dunham and Robert Scobey AA/TE/V/t A .5? .24 J /-a4a f /0 S V/t/CHAO U 994 5 3 15 4 544 5 44..

i 20 5YA/C/7,0 2 1 GOA T201. OUTP T r/eAus/ o/wfz 1 H l I I 1 V 1 i ERR '95 5661 4 l 'T m Luca r 6 INPUT H VOLTAGE H PATENTED JAN 1 8 I972 SHEET 3 0F 3 UN/FOP/V FILM INVENTOR. JOSEPH J K /vos/x BY EM filly BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION This invention relates to potentiometer structures, andparticularly to potentiometer structures suitable for replacing the rotor and stator transformer coil arrangements found in synchro systems.

A synchro system is a system for positioning a movable object in accordance with the position of a control mechanism, commonly termed a synchro control transmitter. In the past the synchro control transmitter has been a transformer arrangement formed from three fixed stator coils and one movable rotor coil. The position of the rotor coil induces varying voltages in the three stator coils. In the past these stator'coils have been coupled to three similar stator coils in a synchro control transformer which includes a movable rotor coupled to the movable object whose position is to be determined by the movement of the rotor of the synchrocontrol transmitter. The signal applied to the synchro control transmitter rotor and the signal induced in the rotor of the synchro control transformer are applied to a phase-sensitive amplifier. An output signal is developed which is used to drive a servomotor which moves the movable object. Because of the external mechanical coupling between this object and the rotor of the synchro control transformer the rotor is moved until the signal therefrom is zero indicating that this rotor is in the same relative position as the rotor of the synchro controltransmitter. The difference in phase between the signals applied to the amplifier determines the direction of movement of the servomotor. Y

As indicated, it has been common in the past to employ inductive"-type devices for the synchro control transmitter and synchro control transformer. There are many applications, however, because of space limitations, e.g., which render impractical the use of inductive-type devices. The present invention is directed to the use of potentiometer structures to replacethe conventional inductive-type devices used in the past.

A potentiometer structure is required which will simulate an inductive-type device, so that, e.g., a synchro control transformer may be a potentiometer whichreceives a signal from a conventional synchro control transmitter employing conventional-type coils. By the same token, it is desirable to provide a potentiometer for a synchro control transmitter which simulates the signals produced by the conventional coil type arrangement so that these signals may be applied to a synchro control transformer employing a potentiometer rather than coil-type arrangement.

It has been found in the present invention that, as far as the synchro control transformer is concerned, a potentiometer can be constructed to simulate the action of the conventional coil type arrangement. It has been discovered that the resistivity should vary in accordance with a cotangent function dependent upon the electrical angle of displacement with respect to a reference 'position. On the other hand, in a potentiometer for a synchro control transmitter, it has been discovered that a square-shaped film of uniform resistance should be used in conjunction with three wipers spaced 120 electrical degrees from each other. An AC or DC input signal is applied to the film to generate signals from the wipers varying in accordance with the sine of the angle of rotation of the wipers from a reference position.

The potentiometer structures proposed herein may be made relatively small so as to find application in instrument control systems and the like Where space requirements are important. Further, the potentiometers permit control by DC as well as AC signals (inductive-type transformers are limited to AC control signals), and calibration and adjustment is easily accomplished through DC measurement. The potentiometers thus provide a greater flexibility in use than transformers of the inductive type.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate representative embodiments of the invention.

FIG. I is a block diagram of a synchro system in accordance with the invention.

FIG. 2 is a sideview of a potentiometer structure useful in the system of FIG. 1 as a synchro control transformer.

FIGS. 3 and 4 are views of parts of the potentiometer structure of FIG, 2, looking in the direction of the arrows 33 and 4-4 of FIG. 2.

FIG. 4a is a curve plotting the output signal from a wiper of the potentiometer of FIG. 2 as a function of angular displacement of the wiper in degrees from a reference position.

FIG. 4b is a curve showing the thickness of a conductive plastic resistive film at any point in the film as a function of the angular displacement of that point from a reference position.

FIGS. 5 and 6 are views similar to FIGS. 3 and 4 showing an alternative potentiometer structure suitable as a synchro control transformer in FIG. 1 and designed to provide twice the degree of movement of the rotor with respect to movement of the synchro control transmitter rotor.

FIG. 7 is a view in schematic form of.a potentiometer structure suitable as a synchro control transmitter in the system of FIG. 1.

DETAILED DESCRIPTION Referring to FIG. 1, there is shown a synchro system in accordance with the invention. A conventional synchro control transmitter 10 is shown having a rotor 12 which is energized by a suitable varying signal applied to a pair of of conductors 14. The rotor 12 constitutes one coil of a transformer which is variable in position typically so as to rotate 360 about an axis. The angular position of the rotor corresponds to the position of some mechanism to be adjusted, for example, an antenna 16. To this end the synchro control transmitter 10 includes three

stator coils

18, 20 and 22 in which are induced electrical signals of a phase and magnitude in accordance with the an gular position of rotor coil 12.

Signals from the

stator coils

18, 20 and 22 are applied to a synchro control transformer 24. Conventionally the synchro control transformer is a rotor coil and stator coil structure as is the control transformer 10. However, in the present invention, the synchro control transformer 24 is a potentiometer structure such as that shown in FIGS. 2 to 4, to be described below. The synchro control transformer includes a rotor which produces an electrical signal on conductors 26 which is applied to an amplifier 28, typically a phase-sensitive amplifier. This amplifier also receives as a reference input signal the signal on the conductors 14 that energizes the synchro control transmitter rotor coil 12. Any potential difference between the two conductors 26 from the control transformer 24 causes an output signal to be generated on conductors 30 from the amplifier which are connected to a servo motor 32. The difference in phase between the signals on the conductors 26 and 14 determines the phase of the amplifier output signal and the direction of rotation of the servomotor. The servomotor includes a shaft 34 which drives the antenna 16. The shaft 34 is coupled to a shaft 34a, in turn coupled to the rotor of the synchro control transformer 24. As energized by a signal on the conductors 30, the servomotor 32 moves the antenna and hence the rotor of the synchro control transformer 24 until the signal on the conductors 26 (the potential difference between the conductors) is zero. When this occurs, the rotor of the synchro control transformer is in the same angular orientation with respect to a reference position as is the rotor coil 12 in the synchro control transmitter. The output signal on the conductors 30 from the amplifier 28 drops to zero, stopping movement of the servomotor. In this fashion, the antenna is positioned in accordance with the position of the rotor coil 12 of the synchro control transmitter.

As explained above, the synchro control transformer 24 is comprised of a potentiometer structure such as shown in FIGS. 2 to 4. A somewhat different potentiometer structure is shown in FIGS. 5 and 6, and a potehtiometer structure suitable as a replacement for the coil arrangement of the synchro control transmitter is shown in FIG. 7. These potentiometer structures will now be described in detail.

Referring to FIG. 2, the stator of the synchro control transformer 24 comprises a platelike structure 36 fixed in position by a bracket 38. The rotor of the synchro control transformer is formed from another platelike structure 40 having a hub 40a affixed to the shaft 34a that is connected to the antenna structure 16 in FIG. 1. The shaft 34a thus represents the movable mechanism whose position is to be adjusted in accordance with the position of the rotor coil 12 of the synchro control transmitter 10.

Referring to FIG. 4, the stator 36 of the potentiometer structure includes a resistive material 42 thereon. Typically this resistive material is in the form of a resistive film which is deposited on a plastic'base in accordance with conventional plastic film techniques. The resistive film 42 includes three segments 42a, 42b and 421:. Each segment constitutes I electrical degrees of rotation of the potentiometer. In this potentiometer structure the potentiometer rotor 40 is adapted to make one complete revolution for one complete revolution of the transmitter rotor 12in FIG. 1.

It will be noted that the shape of the resistive film 42 is not uniform. This is done so that the resistivity at any position in a film segment is a function of the angular displacement of that position with respect to a reference position, as will be explained in more detail below. The resistive film has three points of

connection

44, 46 and 48, spaced the equivalent of 120 electrical degrees from each other.

Conductors

44a, 46a and 48a are connected to these points of connection to the resistive film. These conductors are designated by the same reference numerals in FIG. 1 and are shown connected to the synchro control transmitter coils 18, 20 and 22. Two

wipers

50 and 52 are positioned on the stator plate 36, as shown in FIGS. 2 and 4, and bear respectively against

conductive tracks

50a and 52a on the rotor plate 40 shown in FIG. 3. A wiper 52b is electrically connected to the track 52a, and another wiper 50b is connected to the track 50a. The wipers 50b and 52b are positioned I80 electrical degrees from each other and bear against the resistive film 42. Thus the wipers 50b and 52b pick off signals from the resistive film 42 and transfer these signals to the

conductive tracks

50a and 52a which in turn transfer the signals to the

wipers

50 and 52. The

wipers

50 and 52 are connected to conductors 26 which, as shown in FIG. 1, constitute the synchro control transformer rotor conductors applying an error signal to the phase-sensitive amplifier 28. The

wipers

50 and 52 are spaced so that the wipers 50b and 5211 may pass freely therebetween as the potentiometer rotor 40 rotates.

As noted above, the configuration or shape of the resistive film 42 varies in accordance with position at any point in the film from a reference position. Specifically, each of the segments 42a, 42b and 420 has the same resistivity characteristic as a function of position. Taking the potentiometer segment 42a as an example, and considering a reference position of 20 degrees as passing through the point of connection 44, the resistivity R between the point of connection and one of the wipers 50b and 52b at an angular displacement of X, degrees from the reference or zero position is given by the following equation:

R=[%-( 3/6) ctn. (30+X)]R,, (l) in which R, is the open circuit resistance between the points of

connection

44 and 46 at the ends of the potentiometer segment 42a.

It has been found that if the resistivity of each segment of the resistive film 42 varies in accordance with the above equation, signals will be developed on the conductors 26 for varying positions of the rotor the same as signals developed from a conventional rotor coil of a conventional coil-type synchro control transformer. In particular, the

wipers

50b and 52!) pick off equal potentials (a zero signal from the resistive film 42 when the rotor 40 is displaced angularly from a reference posi' tion corresponding to'the angular displacement of the transmitter rotor from a reference position). Such a zero signal is developed by the rotor coil of a conventional synchro control transformer when in this same angular orientation, and hence the potentiometer-type control transformer of the present invention may directly replace the conventional coil-type control transformer in a synchro system.

In practice, the variation of resistivity as a function of an gular displacement may be achieved by a number of different techniques. A first technique is to employ a resistive film 42 which is constant in thickness and uniform throughout the entire extent thereof except for the shaping of the film by etching or otherwise so that it assumes the shape shown in FIG. 4. In this case the width of the film (in a radial direction) varies in accordance with the angular displacement X. It is possible to maintain the width of the film constant and to vary the thickness, for example, in order to achieve the appropriate resistivity variation as a function of angular displacement. Other techniques are by varying the width and thickness of the resistive film simultaneously, or by shunting various portions of the resistive film either with resistors mounted external to the stator plate 36 or by the use of an overlay of resistive material deposited selectively on various portions of the film. Another technique is to change the composition of the material for various positions in the film segments.

No matter which technique of resistance variation is employed, the fabrication of the potentiometer film 42 is easily accomplished by applying a signal, for example, between the connection points 44 and 46 and measuring the potential difference between the connection point 44 and one of the wipers 50b and 52b (as measured at one of the conductors 26) as that wiper is moved from adjacent connection point 44 on the segment 42a to adjacent the connection point 46. The output signal that should be obtained (expressed as a fraction of the input signal applied to the connection points 44 and 46) is shown in FIG. 4a, by the curve designated 60. In this regard, it is assumed that one of the wipers is moving from adjacent the connection point 44 to adjacent the connection point 46 and that the connection point 44 is grounded, for example, and that a full input signal of L00 is applied to the connection point 46. The curve 60 is in the form of a cotangent function. As the wiper is moved through the of displacement from adjacent the connection point 44 to adjacent the connection point 46, the potentials indicated by the curve 60 should be obtained. As shown in FIG. 4, the width of the film may be made to vary to achieve the appropriate relationship between resistivity and angular displacement. Depending upon the signal potentials detected at the various angular positions, the film segment 42a is appropriately changed in width to achieve the desired relationship. Conductive bits of material, for example, may also be applied to the film to shunt various portions of the film to change the resistivity, if desired, so that the signal detected from the wiper follows a curve such as curve 60 shown in FIG. 4.

In the same fashion, conductive segments 42b and 420 are shaped to provide the appropriate resistivity variation as a function of angular displacement. The curve 62 in FIG. 4a shows the variation in signal detected at one of the wipers that should be obtained as the wiper is moved from adjacent the connection point 46 to adjacent the connection point 48, with the point 46 having full input potential applied thereto and point 48 grounded, for example. Finally, the curve 64 in FIG. 4a shows the variation in signal from one of the wipers as the wiper is moved over the resistive segment 42c from adjacent the connection point 48 to adjacent the connection point 44, and with connection point 48 being grounded and the full input signal being applied to connection point 44.

FIG. 4b shows the variation of film thickness as a function of angular displacement in degrees to obtain the desired resistivity variation by varying solely the thickness of the film and maintaining the width constant. It is indicated in FIG. 4b that the three taps or connection points (corresponding to connection points 44, 46 and 48) are at 60, 180 and 300 corresponding to points of minimum thickness.

As noted above, no matter how the resistivity is varied, the variation is in accordance with a cotangent function as noted above in equation (1). Because the synchro control transformer constitutes a potentiometer, the potentiometer may be fabricated by monitoring a signal, as explained above in connection with FIG. 4a, and varying the resistivity so that the proper signal is obtained as the potentiometer rotor is moved. It should be noted that when shape or configuration is varied, such as by varying width or thickness or both, the variation at any point is determined from the derivative'of the resistivity equation l above.

Equation (1) above gives the relationship between resistivity and angular displacement in which a single complete rotation of the rotor 12 of the synchro transmitter produces a single complete rotation of the rotor 40 of the synchro receiver 24. With respect to equation (1), the electrical zero position of the control transformer rotor 40 will be achieved when the following relationship exists between the angular displacement 8 of the control transmitter rotor 12 from a reference position and the angular displacement X of the control transformer rotor 40 from a reference position.

kElO -X. (2) Equation (1) above may be generalized for the case in which the synchro transmitter rotor 12 rotates one complete revolution and the receiver rotor 40 rotates a fraction of a revolution or some other number of revolutions. Equation (1 expressing resistivity R in each segment as a function of angular displacement X, is written for the general case, as follows:

in which A is the ratio of the rotation of rotor to the rotation of therotor 12. Specifically, equation (3) is as follows, when the control transmitter rotor 12 makes one complete revolution and produces two complete revolutions of the control transformer rotor 40:

FIGS. 5 and 6 show potentiometer structures similar to that of FIGS. 2 to 4, specifically designed to provide twice the degree of movement of the control transformer rotor with respect to movement of the control transmitter rotor. FIG. 5 shows the rotor structure comprising a plate 70 and

conductive tracks

72a and 74a. Wipers 72b and 74b are connected to the tracks directly adjacent each other, and these wipers in turn make contact with resistivefilm sections or tracks on the stator structure 80. Specifically, the stator structure includes an outside section or track 82 and an inside section or track 84 contacted respectively by the wipers 72b and 74b. The

resistive tracks

82 and 84 vary in resistivity dependent upon displacement from a reference position in accordance with equation (4) above. The outside track includes spaced ends 82a and 82b, while the inside track includes spaced ends 84a and 84b. The ends 82a and 84b are joined together by a conductor 88 and constitute one point of connection of the potentiometer structure connected to a conductor 90. The ends 82b and 84a are also connected together by a conductor 92. Other connection points are taken from the resistive film tracks as at a point 94 on the inner track 84 (connected to a conductor 96) and at a point 98 connected to the outer track 82 (in turn connected to a conductor 100).

Wipers

102 and 104 connected to

conductors

106 and 108 make contact with the

conductive tracks

72a and 74a of the rotor to transfer the signal picked off the resistive tracks by the wipers 72b and 74b as an output signal. In this regard, the

conductors

90, 96 and 100 correspond to

conductors

44a, 46a and 48a in the system of FIG. 1, while the

conductors

106 and 108 correspond to the conductors 26 in the system of FIG. 1.

The connection points 88, 94 and 98 are spaced 120 electrical degrees form each other, although actually they are spaced 240 in rotation as will be explained. Commencing from the connection point 98, traveling clockwise, the wiper 72b travels from adjacent this connection point on the outer track 82 for 240 of rotation until the connection 88 is reached. For further clockwise rotation of the rotor 70, the inner wiper 74b engages the inner track 84 for 240 of rotation until connection point 94 is reached. Finally, continued clockwise rotation of the rotor provides travel of the inner wiper 74b to the conductor 92 linking the ends 84a and 82b of the inner and outer tracks, the outer wiper 72b moves along the outer track 82 until 240 of rotation have been achieved, again back to the connection 98. Two complete revolutions of the wipers constitute 360 electrical degrees. Although the connection points are spaced 240 physically, they are spaced 120 electrically. For this reason the two wipers 72b and 74b are spaced 180 electrically from each other, although physically the travel from one wiper to another constitutes a full 360 of rotor rotation.

It will be noted that at the region of the connection 88, there is a discrete spacing between the ends of the inner and outer

resistive tracks

82 and 84. As the outer wiper 72b passes from the end 82a to the end 82b, there is an abrupt change in resistivity. Similarly, as the inner wiper 74b passes from the end 84a to the end 84b, there is another abrupt change in resistivity. However, since the ends 84a and 82b are linked together, as are the ends 82a and 84b, and since these linked ends have the same resistivity, there is essentially no abrupt change in resistivity between the wipers 72b and 74b or in the signal developed between the wipers. It is recognized that if the wipers fall inside the region between the resistive tracks, there will be no engagement with the tracks. This cannot be tolerated, and hence the potentiometer of FIGS. 5 and 6 is used only in connection with mechanical stops (not shown) preventing travel of the wipers through the region between the resistive tracks. This limits the use of the potentiometer to a system in which it rotates just short of 360, 'e.g., 350, corresponding to 175 of rotation, e.g., of the control transmitter rotor.

The embodiments shown in FIGS. 2 to 6 have all involved rotational movement of the potentiometer. The invention is applicable to linear movement or any movement other than rotational. Equation (1) above may be further expanded for the general case involving any type of motion, as follows:

R==[ 3/6) ctn. (30+(AY/Y)l20)]R,, (5) where y,is the distance between adjacent connection points, and AY is the distance between a point at which the resistivity R is given and a reference point at an end of the resistive segment, and R, is the open circuit resistance between these two connection points. It should be noted that two adjacent connection points constitute 120 electrical degrees of potentiometer movement, and hence for a full 360 electrical degrees there should be a total of three resistive segments, each segment the same as another segment, with the resistance of each segment varying in accordance with a reference position as in the above equation.

FIG. 7

FIG. 7 shows a potentiometer arrangement suitable for replacing the transformer-type coils in the synchro transmitter 10. FIG. 7 has been drawn schematically and involves a uniform film of resistive material 101 against which bear three wipers 103, and 107. The wipers are driven by a shaft 109 whose position with respect to a reference position determines the movement of the antenna 16 in FIG. 1. Film 101 is square shaped, and the wipers are equidistantly spaced 120 apart and are of equal length. An AC or DC input potential is supplied between

terminals

110 and 112 connected respectively to

conductive strips

114 and 116, which supply a potential gradient across the film. Typically, a midpoint 117 of one or both of the other sides is grounded to provide a sine function signal from the potentiometer varying above and below ground. For an angle X of displacement with respect to a reference position, the potential developed by the wiper 103 may be expressed as:

V =EsinX, (6) in which Y is the potential of wiper 103, Eis the input signal across the

strips

114 and 116, and X is the angle of displacement.

Similarly, for the same angular displacement, the potential of the wiper 105 may be expressed as:

, V =E sin (X-HZO), 7 and the potential of the wiper 107 may be expressed as:

V =E sin (X+240). (8) If these wipers are connected to the stator connections of the synchro control transformer 24 in FIG. 1, an operative synchro system will result. In this case the system must employ a potentiometer-type control transformer. A potentiometer for the synchro control transmitter is not compatible with an inductive coil-type control transformer.

SUMMARY A synchro system has been disclosed involving potentiometers to replace the coil arrangements in the synchro control transmitter and control transformer. While specific embodiments have been disclosed, it should be understood that the invention is not limited to the specific embodiments, which are simply representative. The invention, therefore, should be taken to be defined by the following claims.

I claim:

1, In a synchro system that includes a synchro control transmitter having a stator with three stator outputs, the improvement comprising a three-region resistive material serving as and directly interchangeable with a synchro control transformer and which is coupled to and energized by the stator outputs and of a resistivity in each region which varies as a function of position within the region with respect to a reference position in accordance with a cotangent function.

2. A synchro system as defined in claim 1, wherein the stator comprises a square-shaped resistive material, three wipers of equal lengths connected to and spaced 120 about an axis of rotation, the wipers constituting the outputs of the stator, and means applying an input signal to two opposing sides of the square-shaped material.

3. A synchro system as defined in claim 2, wherein at least one of the remaining two sides of the square-shaped material is electrically grounded.

4. A synchro system, comprising a synchro control transmitter having a movable rotor and three stator outputs, a potentiometer serving as and directly interchangeable with a synchro control transformer and which comprises a three-region resistive material, points of connection to the resistive material being coupled to the stator outputs of the synchro control transmitter, a movable rotor forming part of the potentiometer and having two contacting means for engaging the resistive material at spaced points thereon so as to pick off the potentials of the engaged points of resistive material, the resistivity of the material varying in each region as a function of position of the potentiometer rotor with respect to a reference position and in accordance with a cotangent function so that the output potentials picked off by the contacting means are equal when the potentiometer rotor is in a position with respect to a reference position corresponding to a similar position of the rotor of the synchro control transmitter with respect to a reference position in the control transmitter.

5. A synchro system as defined in claim 1, wherein the resistive material comprises a plurality of segments, each of which has the same resistivity in accordance with a cotangent function as another segment.

6. A synchro system as defined in claim 1, wherein the re-I sistivity of the material is expressed substantially by the following formula:

=t cm. 9+X l wherein R IS the resistivity at a position in the material, X Is the displacement of this position in electrical degrees from a reference position at an end of a segment of the material, which segment represents electrical degrees, and R, is the open circuit resistance between the ends of the resistive segment.

7. A synchro system as defined in claim 6, wherein the resistive material is formed from three l20-electric'al-degree segments as defined in claim 22, the resistive material has three points of connection thereto which are spaced 120 electrical degrees from each other, and these three points of connection are coupled to the stator outputs of the synchro control transmitter.

8. A synchro system as defined in claim 7, including a rotor having two wipers contacting the resistive segments and which are spaced electrical degrees from each other.

Claims

1. In a synchro system that includes a synchro control transmitter having a stator with three stator outputs, the improvement comprising a three-region resistive material serving as and directly interchangeable with a synchro control transformer and which is coupled to and energized by the stator outputs and of a resistivity in each region which varies as a function of position within the region with respect to a reference position in accordance with a cotangent function.

2. A synchro system as defined in claim 1, wherein the stator comprises a square-shaped resistive material, three wipers of equal lengths connected to and spaced 120* about an axis of rotation, the wipers constituting the outputs of the stator, and means applying an input signal to two opposing sides of the square-shaped material.

6. A synchro system as defined in claim 1, wherein the resistivity of the material is expressed substantially by the following formula: R ( 1/2 -( 3/6) ctn. (30*+ X))Rt wherein R is the resistivity at a position in the material, X is the displacement of this position in electrical degrees from a reference position at an end of a segment of the material, which segment represents 120 electrical degrees, and Rt is the open circuit resistance between the ends of the resistive segment.

7. A synchro system as defined in claim 6, wherein the resistive material is formed from three 120-electrical-degree segments as defined in claim 22, the resistive material has three points of connection thereto which are spaced 120 electrical degrees from each other, and these three points of connection are coupled to the stator outputs of the synchro control transmitter.

8. A synchro system as defined in claim 7, including a rotor having two wipers contacting the resistive segments and which are spaced 180 electrical degrees from each other.