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A resolver , sometimes referred to as a resolution transformer , rotary field encoder or sometimes with the English name Selsyn (a case word from self and synchronizing ) or synchro , is an electrical machine whose output voltage depends on the applied backup voltage and the angle of rotation between rotor and stator . The combination of two or more spatially separated and only electrically connected resolvers is used in downstream systems for electrical angle transmission and as an inductive measuring instrument for the angle of rotation, for a torque or an angle difference.

A resolver transducer converts the rotation of the rotor into an electrically measurable and transferable quantity; a response receiver converts this electrical quantity back into a mechanical quantity. A speed signal can be derived from the output signal. The rotary movement on the resolver receiver is synchronous with the rotation on the resolver encoder, as if the axes of both resolvers were mechanically connected to one another (for example with a flexible shaft ).

Resolvers look like small cylindrical motors 1.5 cm to 15 cm in diameter. The connection with electrical lines is more flexible than a mechanical connection. However, only relatively low torques can be transmitted, which can be improved by an external circuit with amplifiers and servo motors .


Resolver with pin assignment (here for 400 Hz operation)


Principle of a direct current resolver, forerunner of the resolver based on the induction principle: The black circle consists of a resistance layer.

The first documented use of resolver systems with resolver encoders and receivers was in 1914/1915 at the locks of the Panama Canal for the transmission of gate locks, valve positions and water levels. These first systems worked with direct current and used a special, precise potentiometer whose resistance layer or winding made of resistance wire was built in a continuous circular shape with several taps. The slider had two diametrically isolated contacts and could be rotated on the resistance layer without stopping. The grinder applied a DC voltage to the resistance layer. Depending on the angular position of the grinder, a direct voltage with a specific value for this angular position could be tapped at the taps. On the resolver receiver side, either a permanent magnet was moved or an identical potentiometer was moved as a control loop by a DC motor until the voltage differences between the resolver encoder and the resolver receiver were zero. These DC current indicators could only be used for simple applications.

Lieutenant William R. Furlong had seen control loops with resolvers on his study trips to Europe and wanted to use the principle to control weapon systems. From 1918 to 1920, together with the engineer Edward Hewlett, he developed alternating current rotary indicators for the General Electric company that work on the induction principle. For the intended purpose of weapon control, they were much more accurate than the DC systems known until then. These new systems were used in the battleships Colorado and Maryland . The DC resolvers then lost their importance as angle sensors.

The resolution systems developed by General Electric for remote control purposes were named with the trademark Selsyn® . This name was unfortunate because it was also given for the remote control principle with the motor-controlled potentiometers. This system was called DC-Selsyn since it works with direct current; To distinguish it, the inductive resolver system was then called AC-Selsyn or inductive resolver . Kollsman, a company engaged in avionics , used the name Teletorque for its resolver system , and the Pioneer Instrument Division of Bendix Corporation called its devices Autosyn . It was not until 1944 that the name Synchro, used by the Sperry Gyroscope company, became uniform for all resolvers (except for the resolver ), as this designation was used in the American Navy, which at that time was a main user of the system. Other historical names were Telesyn, Synchrotel and Synchrotie.

The General Electric Company used their resolver system in many remote-controlled applications, such as lighting control in the Rockefeller Center theater in New York. The system installed there in 1922 also made it possible to call up a number of preset sequences . The company Western Electric used this system in 1922 to synchronize the recordings from the camera and record player for the first sound films. For the film recordings for the film "I, the Jury" produced in 3D technology in 1953, Hannah Lee used a resolver system that controlled two resolver receivers with one encoder in order to synchronize the pans of both cameras.

In 1942, a two-channel rotary reporting system was developed and patented by Robert F. Hays of the Sperry Gyroscope company . As a threshold switch for the coarse channel, he used two anti-parallel connected diodes . In Germany, the "Transmitter 37" equipped with resolvers was used in the Würzburg radar device to control anti-aircraft cannons and has been in series production for the Würzburg D and Würzburg-Riese devices since 1940 . The unusually precise accuracy of the angle measurement at the Würzburg giant was given as ± 0.2 degrees.

Construction of inductive resolvers

Internal circuit of various types of resolvers:
Shown above: the rotors of the resolver; Shown below: their stators;
a) simple resolver (internal pole machine);
b) simple resolver (external pole machine);
c) differential resolver;
d) + e) ​​Function resolver;
f) Resolver without sliding contact

By their construction, resolvers are low-power, high-precision AC machines. The stator and the rotor consist of packages made of thin electrical steel , the individual layers of which are packed in a mutually insulated manner. Such a construction causes a significant reduction in eddy current losses . To ensure the required accuracy, the precision in the layering of the laminated cores, in the winding design and in the ball bearings is significantly higher than with normal AC machines. Due to these high requirements, resolvers of each design are offered in several accuracy classes. Resolvers with three windings offset by 120 ° (three-phase system) are sometimes referred to by their English names as Synchro or Selsyn (English case word from sel f and syn chronizing ); Resolver with a two-phase system offset by 90 ° as a functional resolver or resolver .


Sections of different designs of resolver rotors
I-shaped (with only one winding, mostly for resolver encoders);
H-shaped (with only one winding, mostly for rotary signal receivers);
Y-shaped (for three windings, mostly for differential detectors);

In the simplest case, the rotor is a dumbbell-shaped core with often only one winding, or with two windings that are connected in series. The pole edges are rounded so that they only have a narrow air gap to the stator. An axle is pressed into the rotor package, on which the slip rings for the power supply to the rotor, ball bearings as well as gears or dial pointers are placed. The rotor windings are evenly inserted into the grooves of the rotor core profile. The width of the pole edges can be more or less pronounced. Often, a stronger shape of the pole edges is used in the resolver receiver than in the resolver encoder.

If more than one coil is attached to the rotor, the core must have correspondingly more pole edges: with differential detectors , the windings on the rotor and the stator consist of three single-phase windings that are offset by 120 ° and connected to form a star. With function detectors , the number of windings on the rotor or the stator can differ, which are wired according to their function and number, for example rotated by 90 ° according to their function. The rotors are electrically connected via slip rings or magnetically separated transformer windings.

For resolvers specifically designed as a resolver, an I-shaped rotor core is preferred because it enables more precise positioning. More an H-shaped design is used for rotary signal receivers. This design distributes the amount of torque more favorably and prevents overshooting or vibration around the zero point. Another possibility to suppress such vibrations is to give the rotor as much mass as possible in order to utilize the moment of inertia . For this purpose, additional weights can be mounted on the rotor of the rotary signal receiver.


The stator of a resolver is usually a cylindrical body that encloses the rotor. It also contains a core of sheet metal on which at least one, but mostly three coils linked in a delta connection or star connection are wound. An insulated holder for the sliding contacts transfers the voltages to the rotor.

The windings on the stator can be divided into several pole pairs . This corresponds to an electrical translation of the speed of this resolver. At the same time, this creates several zeros, which, if necessary, have to be reduced to a clear zero by superimposing them in a multi-channel resolution system (see below). However, 18 to 36, in individual cases 648 pole pairs would be required for a highly precise determination of an angular position. This high number of windings cannot be implemented in the limited space available, which is why these multi-channel speed reporting systems usually use a precision gear. Resolvers with a multiplication of pole pairs are therefore only used where either the stator can have very large geometrical dimensions or where only a small number of pole pairs is sufficient (for example for exclusive rotational speed transmission).


Resolving voltages as a function of the rotation of the rotor of a resolver:
An alternating voltage is measured at the stator coils with an effective value specific for this position (angular position of the rotor). (The more strongly drawn envelope curve is not synchronous with the AC voltages included in it!)

The rotor winding and the stator windings each form a transformer , the transmission ratio of which depends on the angular position of the rotor to the stator winding. A coil is fed with an alternating voltage, which is called backup voltage here . The effective value of the backup voltage used is in the range of a few tens of up to 115 volts, typically with the existing line frequency of 50 Hz or 400 Hz. A higher frequency of this backup voltage (up to 12 kHz) results in a much smaller design of the resolver, so that Blocks with a large number of resolvers and their mechanical connection via gears and adjusting wheels no longer formed such a large, bulky unit.

In the other, not primarily fed coils of the resolver, a secondary voltage dependent on the angle to this fed coil is induced. Usually these are attached to the stator. The voltage curves on the stator windings are almost in phase with the excitation voltage of the rotor winding and differ in amplitude, which depends on the sine of the angle between rotor and stator winding:

Where K is the maximum transmission ratio between rotor and stator; ϑ the detuning angle ; U L is the voltage at the output of the respective winding (S 1  … S 3 ); U R is the backup voltage (fed in here at the rotor).

Almost every resolver can be used both as a resolver encoder and as a resolver receiver. In practice, however, special devices are designed for use as resolver encoders which, for example, provide a higher current on the lines in order to be able to operate several resolver receivers installed at different locations. Higher demands are often placed on the accuracy of the response receiver. With a backup voltage of 110 V at 50 Hz, the maximum secondary voltage is around 50 V. The feed power of a resolver depends on the number of resolver receivers that can be connected and is typically between 2 watts and 20 watts. The accuracy of the angle transmission can reach down to the range of a few arc seconds.

When a resolver is set to zero, a voltage minimum must be measured between windings S 1 and S 2 . This is the case when the stator field of winding S 3 is parallel to the rotor winding. In the rotated state of the resolver, the error voltage U f induced in the rotor is minimal when the rotor direction is perpendicular to the resulting stator field, i.e. there is a very loose coupling between the two windings.

So-called brushless resolvers are produced to reduce maintenance. These can transfer the backup voltage to the rotor by means of a transformer. Another possibility is that the support voltage is transmitted through permanently connected and slightly movable cables. Such an application is useful for the angular transmission of a periodic pivoting movement. A rotation of more than 360 ° must be prevented mechanically here.

A decisive disadvantage of rotary reporting systems is that their function is based on the flow of current and that they cannot bridge large distances with at least five current-carrying lines. In practice, distances in the range of 500 m to 1000 m were mostly achieved here.

Apparent change in direction of rotation by changing the viewing direction

The direction of rotation of a resolver is determined by looking at the side of the connection terminals. If the rotor axes of the resolver encoder and resolver receiver are on one line and the connections are wired in the same way, the direction of rotation in this system is the same: however, the different viewing direction to the connection terminals results in an apparent change in the direction of rotation.

For mechanical reasons, a resolver can be mounted counterclockwise on a clockwise rotating system. In this case, the direction of rotation is changed electrically. Interchanging two connections of the stator windings at any point in the system reverses the direction of rotation of the rotary receiver.

Resolution systems

There are two main possible uses for resolvers: remote display operation or indicator operation for a system that transmits only low forces and has only a display function; and transformer operation in which larger loads such as antennas weighing several tons, ship rudders or complete weapon systems can be moved. In indicator mode, a reference angle is easily set mechanically by turning the pointer on the rotor axis. The rotors of the resolver encoder and the resolver receiver point approximately at the same angle to the respective stator. In applications involving transformer operation, the rotor of the resolver receiver is usually rotated by 90 ° in relation to the rotor of the resolver. Instead of applying a backup voltage to the rotor of the rotary signal receiver, an error voltage U f is tapped, electronically processed and used in an electromechanical control circuit for tracking to the target angle.

Single-channel resolution systems

Resolution system made up of two identical resolvers with one rotor winding and three stator windings that are spatially offset by 120 °. The resolver receiver (rear) is only electrically connected to the resolver encoder, its rotor winding follows the rotor of the resolver encoder.

Single channel resolver systems only use a single resolver and can control one or more resolver receivers located in different locations. When using multiple resolver receivers, the resolver encoder must be able to provide the power for this number of receivers on the signal lines. The resolver is therefore usually larger than the receiver. The accuracy of single-channel resolver systems is in the range of about one degree.

Remote display operation

The stator windings and those of the rotor are connected in parallel with the corresponding windings on the opposite side, which requires five wires. If the angles of rotation of the transmitter and receiver match, the same voltages are induced in the corresponding stator windings and no current flows.

In the event that the angles of rotation on both rotors are not the same, the induced voltages in the windings are not the same and a compensating current flows depending on the complex resistance of the circuit of the winding and the voltage difference. These currents generate magnetic fluxes in the windings of the resolver receiver, which counteract the magnetic flux of the field winding of the stator. This creates a synchronized torque that turns the rotor of the resolver receiver in the direction and up to the point in time until the angular position of the resolver encoder and the resolver receiver are the same again. This means that the induced voltages are the same and compensating current can no longer flow; the correct angle is displayed.

When rotating the axis of the resolver at a constant angular velocity, the axis of the resolver will rotate at the same speed. However, a residual error arises, since the mechanical load on the rotor of the rotary signal receiver counteracts the rotation.

This simple form of a speed reporting system is only suitable for low load torques, for example for instrument pointers or measuring device scales. The adjustment is done mechanically by turning the display element on the rotor axis of the rotary signal receiver. Electrical coordination is not necessary.

Transformer operation with tracking by servo motor

Single-channel resolution system in transformer operation with servo motor

The limited actuating force of the indicator circuit can be increased with a servo motor. In such a system, the resolver encoder is also excited via the rotor, the resolver receiver only on the stator side with the signals from the resolver encoder. The magnetic field in the resolver receiver is aligned in the same way as in the resolver encoder. A sinusoidal voltage with the frequency of the excitation and a sinusoidal amplitude dependent on the difference in angle of rotation results on the rotor of the rotary signal receiver. The working point is at one of the zero crossings of the amplitude and is rotated by 90 °. The amplified error voltage is fed to a servo motor. Systems that had to move very large antennas weighing several tons used electromotive amplifiers : a powerful asynchronous electric motor drove a generator, whose excitation winding was fed with the fault voltage. The drive motors of the antenna were operated as a load on this generator, turning it vigorously into the angle at which the error voltage has a minimum value.

In practice, resolution systems in transformer operation are easier to set up if, with an error angle equal to zero, the output voltage, instead of following a maximum, is also equal to zero. Therefore, the zero position of the resolver receiver (in contrast to the indicator principle) is defined in such a way that the windings of the stator of the resolver receiver must be offset by 90 ° to the windings of the stator of the resolver receiver.

Transformer operation without tracking

Such a resolution system can be used as a transformer or converter. If the rotor of the resolver receiver does not track the resolver encoder, the effective value of its output voltage is proportional to the sine of the angular difference between the two rotors. This is used in all-round radar devices to obtain a correction voltage for wind compensation in the case of fixed target suppression . This detunes the coherent oscillator of a pseudo-coherent radar minimally . Instead of the fixed targets, all targets that move with this radial component of wind speed are now suppressed: for example the echo signals from strong cloud fields, from false echoes from swell or even from chaff .

Application for angular velocity transmission

In the indicator principle, the transmission of very small angular differences is characterized by only very small equalizing currents, with the result that their display in this area is quite imprecise. If the torque principle is used for the transmission of an angular speed (as an "electrical speedometer cable "), the strength of the transmission can be improved by increasing the number of pole pairs , since the equalizing currents multiply in relation to the number of pole pairs with small angular differences. However, in this case, depending on the number of pole pairs, there are several zeros (ambiguities) that cannot be resolved with single-channel transmission. A maximum transmittable angular velocity is limited by the frequency of the backup voltage.

Two-channel resolution systems

Basic circuit of a two-channel slave system with resolvers, example from the Russian radar P-37

With a two- channel torque system (coarse channel and fine channel) , the accuracy of the subsequent system can be significantly improved. Two channels of resolver systems are coupled via a high-precision gear with a ratio of 1:25 or 1:36. The coarse channel responds to large angle differences, the fine channel works in the area of ​​small angle differences. Normally the following system only works with the fine channel and thus achieves greater accuracy. In this transmission, for example, the accuracy is increased by preventing any backlash between the gears by using two identical gears. One of them is firmly attached to the shaft, but the second lies loosely next to the fixed gear. Both gears are spread apart from one another by means of a strong spring, lie force-fittingly on the left and right of the opposite tooth and thus prevent play between the gears of the transmission.

Good resolvers could achieve an accuracy of up to 0.25 ° for the resolver encoder and 0.75 ° for the resolver receiver per channel. Due to inaccuracies in the gearbox, this could not be improved simply by changing the gear ratio. However, the two-channel torque system of the P-37 radar device, for example, provided an overall accuracy of less than 6 angular minutes when properly coordinated.

The servomotor either receives the sum of the two error voltages of the channels, or the coarse channel is only switched on when a threshold value is exceeded (for example, if the angle error is greater than 3 °). If the position is ambiguous, this zero point of the fine channel is covered by the coarse channel and the system leads via the coarse channel to the correct zero point of the fine channel. With an even-numbered gear ratio (1:36), an apparent zero is created at 180 ° with an unstable zero point of the coarse channel (unstable: a deviation from zero leads to a control that leads away from zero) and a stable zero point of the fine channel (stable: a deviation from zero leads to towards a regulation towards zero). Overall, the system could only work on the wrong zero, controlled by the fine channel, if the error voltage is too small for a long time to switch on the coarse channel. To suppress this zero point, a constant, small alternating voltage is added in phase to the voltage of the coarse channel. This leads to a shift in this zero point of the coarse channel, which now no longer corresponds to a stable apparent zero point of the fine channel. The following system can therefore no longer adjust to this wrong angle.

Highly accurate systems use additionally a tachogenerator on the rotary message receiver circuit that measures the rotation speed of the subsequent system, converts into a voltage proportional to the rotational speed and this regulates the gain of the error voltage amplifier of the servo system. With such a circuit a possible overshoot of the system is prevented and a uniform follow-up to the target angle is effected.

Three-channel resolution systems

For very far-reaching weapon systems, the already high accuracy of two-channel rotary reporting systems was not yet sufficient. The angular accuracy could be increased by using a third rotation speed. Common ratios of the number of revolutions in the individual channels were 1: 1, 1:36 and 1: 180 or 1: 360 as well as 1: 1, 1:18 and 1: 648.

Differential resolver

How a differential
resolver works 1. Torque encoder with input of the first angular position;
2. Differential torque indicator with input of a second angular position;
3. Rotating signal receiver shows the difference between the two angular positions.

In a differential resolver, the stator and the rotor is a three-phase system in a star connection . If this differential resolver is switched between the resolver and the resolver receiver, the output voltage at the resolver receiver is proportional to the angular position of the resolver and the differential encoder . With the help of this circuit, the output signal of the overall system can be influenced from several locally different points, for example for the correction of the display of an antenna direction on a ship relative to the direction of travel ( English relative bearing ) by a gyro compass to display the antenna direction relative to the direction of the local one Meridians (English true bearing ).

So-called power resolvers have the same structure as a differential resolver. The backup voltage is provided here as a three-phase alternating current . For transmission, the backup voltage (instead of two) must then be transmitted with three lines to the resolver receiver or vice versa to the resolver encoder. This is often done synchronously with the network , so that these lines also represent the power supply for the speed reporting system. These resolution systems can already transmit higher torques in the indicator circuit, but do not achieve the performance of resolution systems with servo motors. In multi-channel speed reporting systems, they achieve slightly higher accuracy than single-phase backup voltage.

Function resolver

Functional speed indicators or resolvers work in transformer mode and convert mechanical quantities into electrical quantities. The stator is constructed like a normal resolver. For example, the rotor of a function detector contains two windings offset by 90 °, which emit an output signal proportional to the sine and cosine of an angle. With these voltages, the angular deflection of a deflection beam of the cathode ray tube of a panoramic device can be electronically controlled without further mechanical components . In the past, special function resolvers were used in older analog computers to carry out complex arithmetic operations such as trigonometric functions , for example for coordinate calculations in missile control systems or the conversion of polar coordinates into Cartesian coordinates .


Rotary reporting systems can be designed for extreme temperatures, radioactive “hot zones” or flooded operation (for example in transformer oil ). They are low-maintenance and have a long service life. These advantages result in a wide range of applications for rotary reporting systems in modern systems. The application principle is not only reduced to rotary movements. The differential transformer is a sensor that works on the same inductive principle and is designed for linear distance measurement.

Digital displays have established themselves for display operation. This bypasses the disadvantages of the current-controlled resolver receiver (high power requirement on the receiving side and limited distance between the resolver encoder and the resolver receiver). Existing old, durable and robust resolver systems are often upgraded with modern computer-aided displays or control loops. The converters for the resolver voltages measure the peak voltage on the three lines of the resolver encoder in each period of the support voltage frequency using a sample-and-hold circuit . If the frequency of the backup voltage is higher or if the accuracy requirements are lower, the analog-digital converter used can measure the rms value of these signal voltages after rectification. A microcontroller calculates the exact angle to be displayed from the ratio of the three voltage values . This measuring principle can be set up with multiple channels with the resulting increased angular accuracy.

Modulation methods were used to increase the range . For a transmission of the resolution voltages over the radio link RL-30, the resolution voltages were converted into a pulse phase modulation and transmitted over a distance of up to 15 km. At the destination, these impulses were converted back into resolution voltages in order to synchronize the rotation of the deflection beam in the radar display devices.

The Salford Quays Millennium Lift Bridge in Manchester: The lifting technology is controlled with resolvers.

Resolution systems are or were used for:

  • Position indicators and tracking systems for antennas and automatic weapons, mainly in military technology of the Second World War and in the post-war period
  • Position indicators for high-voltage switches in fully automatic substations
  • Synchronization of the drives of swing and lift bridges
  • Synchronization of the rotation of several radars
  • Wind compensation for interference suppression in radar devices
  • Transmission of rudder positions in ships, as a display of the rudder position indicator , for example as a connection between steering wheel and steering gear
  • As a machine telegraph between the engine room of a ship and the bridge (before fully automatic ship controls could be used). The telegraph consisted of two independent rotary reporting systems, whose pointers turned on the same axis. Here, the second pointer was adjusted manually by the machinist as an acknowledgment for the command (for example in the "Half speed ahead" position)
  • Replacement of a flexible speedometer cable for longer transmission distances (so-called electrical shaft )
  • The radio magnetic indicator (RMI) contains three resolvers, one to drive the compass rose and one each for the pointers of the radio navigation receiver
  • Control of an ion source that is at high potential in an electrostatic particle accelerator

In less extreme environmental conditions, the rotary encoders were largely replaced by optical and magnetic angle encoders or incremental encoders , the rotary receivers by stepper motors and other brushless drives or digital displays.


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  • United States Navy Bureau of Naval Personnel (Ed.): Submarine Electrical Installations . Chapter 10: Self-Synchronous Transmitters and Indicators, Chapter 11: Selsyn-Operated Systems. NavPers 16162, 1946 ( Chapter 10 , Chapter 11 [accessed May 26, 2013]).
  • Radar Circuit Analysis . In: Department of the Air Force (Ed.): Air Force Manual No. 52-8 . Chapter 13: Selsyns and Servomechanisms. Washington, DC 1951 ( [PDF; 10.1 MB ; accessed on May 26, 2013]).
  • Jeffrey J. Keljik: Electricity 4: AC / DC Motors, Controls, and Maintenance . 9th edition. Delmar, 2008, ISBN 978-1-4354-0031-3 , pp. 311–316 ( limited preview in Google Book search).

Web links

Commons : Synchro  - collection of images, videos and audio files
Wiktionary: resolver (noun)  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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