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Synchros are used to transmit angular data electrically from one location to another, where a high degree of accuracy is required. They are essentially variable transformers in which the coupling between windings varies with the rotor position relative to the stator. Several different types are produced to suit particular applications and whilst their external appearance is similar, the internal construction varies to optimise the unit’s functional requirements. Muirhead’s pedigree and capability in the field of Synchros will ensure the most demanding specifications are met. Typical applications include remote positioning of low torque mechanisms, remote control by servo motor driven mechanism, remote digital measurement of angle via a suitable signal converter, remote pointer indication of angular position.


Slab Resolvers

Variable Reluctance Resolvers


AC Pick Offs

The illustration to the right shows a synchro with a shaft, bearings and stator windings forming part of the constituents of a size 23 synchro. 

Differential Transmitters

It is sometimes necessary to add or subtract additional information from a Synchro Chain and Differential Transmitters serve this purpose. They are similar in construction to the other elements except for a 3-phase winding on the rotor.

Control Synchros

The design principle of a Control Synchro is to minimise errors in the output signal due to current loading, magnetic non-linearity and temperature rise, by the use of high impedance windings and special attention to the magnetic circuits. The Control Transformer, which provides the error signal to a servo amplifier, can be considered a ‘null’ detector and it is most often used in this way. However the ‘null’ is never zero due to residual voltages. This is due to stray couplings within the laminated stator that result in an in-phase voltage, a quadrature voltage, both at fundamental frequency, plus a number of harmonics. These residual voltage levels are quoted in the performance data tables for each unit.

Torque Synchros

The Torque Synchro is designed to provide a light torque output without additional servo components. Current is fed to both the Transmitter and the Receiver from the same source and the winding impedance values are considerably lower than the equivalent control element. Torque is generated as a result of interaction of the stator field and rotor field in the receiver, which drives the rotor of the Receiver into alignment with that of the Transmitter. The torque / misalignment curve takes sinusoidal form through 360 degrees with maximum values of opposite polarity at 90 and 270 degrees.

Resolvers are used to transmit angular data electrically from one location to another, where a high degree of accuracy is required. They are essentially variable transformers in which the coupling between windings varies with the rotor position. Resolvers can be used for the solution of trigonometric problems since their outputs are related by sine and cosine functions to the angular positions being measured. Several different types are produced to suit particular applications and whilst their external appearance is similar, the internal construction varies to optimise performance. Muirhead’s pedigree in this critical field of fine wire technology has achieved for the Company a world leading market position. Typical applications include range and elevation calculation for radar equipment and gunnery, remote digital measurement of angle via a suitable signal converter, conversion of geometric co-ordinates and data transmission in engine fuel control units.

Resolver to Digital Conversion

Resolvers are generally considered to be the most robust of all angular measurement devices and to provide the best long-term reliability, in a wide range of operating environments. Should a digital signal be required, the output from the Resolver can be converted to make the signal compatible.

Temperature Extremes

For Resolvers in aircraft engine applications where critical components may need to survive limited time exposure to flame in emergency situations Muirhead use special materials and lubricants. Also with close attention to tolerances and expansion rates avoids seizure at extremes of the temperature range. Similar considerations apply for low temperatures as required in space applications.

Muirhead Resolvers can be housed in a variety of units to suit customer application and environment. The specification overleaf is for one type of brushless Resolver with a fuel pressure case and pinion gear used for engine fuel metering valve position feedback. Unit parameters can be altered depending on customer application.

Types of Resolvers Units Available:

Computing Resolver (non-compensated)

Generally used for developing trigonometric functions. Normally the stator winding is energised, with sine and cosine signal outputs being produced at the rotor terminals. These are normally used where the supply voltage, frequency, operating temperature and load impedance remains relatively constant.

Compensated Computing Resolver

Designed to overcome the problems associated with varying supply voltage, frequency, temperature and load impedance, this unit incorporates an additional stator winding whose output signal can be used in the computation to minimise variations in the transformation ratio and output voltage.

Data Transmission Resolver

Designed for optimum operation in a Resolver chain. Basically, they are similar to a computing Resolver but generally with lower impedance windings. They have the advantage over synchros that the outputs can be trimmed individually and having 2-phase output, they are particularly suited to transmission of co-ordinate data.

Sweep Resolver

Designed for operation over a very wide frequency range without appreciable variation in transformation ratio. The sweep Resolver is particularly relevant to PPI indicators or similar rotary scanning applications.

Brushless Units

Some versions are produced with a rotary transformer instead of slip rings and brushes. These are free from the limitations of brush wear or the problems associated with brush / commutators in hazardous atmospheres and for long life applications.


This unit is a hybrid synchro/Resolver having 3-phase primary and 2-phase secondary or vice versa. It is used for conversion between 3-line and 4-line data transmission


Single Resolvers

Muirhead have designed a range of Single Brushless Data Transmission Resolvers with an electrical accuracy of six minutes of arc. The units are a derivative of the Muirhead Aerospace range of Double Resolvers used in many gas turbine fuel metering applications and therefore enjoy the same build standard reliability benefits.

Tandem Resolvers for Engine Fuel Control

This is a size 11 Tandem Resolver with a newly designed housing and cable assembly as the Resolver was relocated to the cockpit area. The Resolver is used to sense the position of the Pilot Lever Angle Unit, which forms part of the throttle control fuel system.

The new unit has a special housing that is sealed against a full range of environmental conditions, together with a newly designed cable assembly for each Resolver lane. These cable assemblies are compliant with the EMC requirements of cockpit mounted equipment.

Triple Brushless Resolvers for Flight Control

The unit shown is a Triple Resolver Position Transducer, providing three sets of output information as a function of input rotor shaft position. In operation the unit transmits collective pitch demand on a helicopter, within the aircraft engine FADEC system.

Slab Resolvers

Slab Resolvers can be used in many applications including the Hydraulic Slat Drives for aircraft. The Resolvers can be installed inside a motor casing and fully submerged in a hydraulic fluid, such as “Skydrol”. Special processes have been developed to enable this technology to operate reliably in hostile environments. Resolvers can be used to measure the hydraulic motor shaft speed, direction, position and the motor yoke.

A Variable Reluctance Resolver (VRR) is a Brushless Resolver with no windings on the rotor.  The VRR is a type of variable reluctance Resolver, which has high reliability, and is suitable for wide temperature ranges and difficult environmental conditions.  This sensor is particularly suitable for brushless motor commutation where the number of pole pairs is equal to the number of output cycles of the VRR. Comparisons between Resolvers and VRRs can be shown below.


  • Four wound components per Resolver
  • Resolver and rotary transformer sections
  • Active parts approx. twice length for Field Director
  • Tight control of end-play required
  • Accuracy would be badly compromised in very short lengths
  • High cost  - by design construction


Variable Reluctance Resolver

  • Single wound component per Resolver
  • No rotary transformer section
  • No windings on rotor
  • Active parts approx. half length for Brushless Resolver
  • Sine/Cosine outputs, as Resolver
  • Multi speed possible and desirable
  • Relatively insensitive to axial play
  • High speed operation
  • Lower cost construction than Resolver


Slab Resolvers

Slab Resolvers are used to transmit angular data electrically from one location to another, where a high degree of accuracy is required. Slab devices are specifically designed for custom applications where weight and size are key design requirements. They are essentially variable transformers in which the coupling between windings varies with the rotor position relative to the stator. Several different types are produced to suit particular applications. Typical applications include Gimbal Systems and Gyro Platforms.

Resolvers are one from the many different ranges of servo components designed and manufactured by Muirhead to meet the demanding requirements of the aerospace, defence and high technology industrial market sectors. A number of units are available as separate rotor and stator, usually required for applications where space or weight is a limitation and the rotor and stator can be built in as structural components of the equipment. This often eliminates the need for couplings, gears and their associated bearings together with an increase in measuring accuracy.

Rotary Variable Differential Transformers (RVDTs) are devices that function by the motion of the rotor within the electro-magnetic field of the stator. This produces an electrical output signal, which has a known relationship to the mechanical motion. In simple terms the RVDT measures angular or rotary motion. These devices are available in an AC to AC configuration, giving an AC output signal at the same frequency as the input signal but with amplitude proportional to the rotor angular position. Operation is based upon the principle of a variable coupling transformer with one primary and two secondary windings. A “soft iron core” occupies a portion of the space inside the wound stator and when positioned equally between the two secondary coils the coupling is balanced and the induced voltage in each secondary coil is equal and opposite, resulting in a “null” (zero) output. The magnetic core is fixed to a non-magnetic shaft, which is the mechanical input. By rotary motion the core will be displaced from the central-null-position. The coupling to one coil will increase whilst coupling to the other coil will decrease, the differential output voltage is related linearly to the mechanical displacement. When the core is moved through the null position in the opposite direction, the output signal will have a similar characteristic but with its sign reversed. The sum of the output voltages of the two secondary coils results in a signal of constant amplitude over its rated angular range. This signal can be used to monitor the “health” of the RVDT.

AC Pick Offs

These devices give an AC output signal, the magnitude of which varies with the angle of rotation over a limited range. Both input and output are via a single phase winding for each. AC pick-offs are used where weight reductions, a reduced number of connector lines, non-ratiometric signal conditioning and general simplicity are paramount design requirements.

Considerable care is taken in the manufacture of the sensor to ensure a high degree of uniformity in the width, roundness, and concentricity of the air gap between the rotor and stator. The rotor magnetic surfaces are accurately ground with respect to the bearing journals, and the stator magnetic surfaces are accurately honed with respect to the stator bearing seatings. The rotor assembly is mounted in the housing in precision stainless steel bearings. Slab and fully housed pick offs are available.

AC Pick-off for Flight Critical Position Data

The unit shown opposite was designed to monitor an aircraft engine inlet guide vane position and transmit the data to the pilot's display.

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Muirhead Aerospace

Oakfield Road, Penge, London, SE20 8EW, England, UK.
Tel: +44 (0) 20 8659 9090
Fax: +44 (0) 20 8659 9906

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