Rotary variable differential transformer

A rotary variable differential transformer (RVDT) is a type of electrical transformer used for measuring angular displacement.

More precisely, a Rotary Variable Differential Transformer (RVDT) is an electromechanical transducer that provides a variable alternating current (AC) output voltage that is linearly proportional to the angular displacement of its input shaft. When energized with a fixed AC source, the output signal is linear within a specified range over the angular displacement.

RVDTs use brushless, non-contacting technology to ensure long life and reliable, repeatable position sensing with infinite resolution. Such reliable and repeatable performance assures accurate position sensing under the most extreme operating conditions.

Most RVDTs consist of a wound, laminated stator and a salient two-pole rotor. The stator, containing four slots, contains both the primary winding and the two secondary windings. Some secondary windings may also be connected together.

Operation of RVDTs

Scheme

Characteristics

The two induced voltages of the secondary windings, $V_{1}$ and $V_{2}$ , vary linearly to the mechanical angle of the rotor, θ:

\theta \ =G\cdot \ \left({\frac {V_{1}-V_{2}}{V_{1}+V_{2}}}\right)

where $G$ is the gain or sensitivity. The second voltage can be reverse determined by:

V_{2}=V_{1}\pm \ G\cdot \ \theta \

The difference $V_{1}-V_{2}$ gives a proportional voltage:

\Delta \ V=2\cdot \ G\cdot \ \theta \

and the sum of the voltages is a constant:

C=\sum \ V=2\cdot \ V_{0}

This constant gives the RVDT great stability of the angular information, independent of the input voltage or frequency or temperature, and enables it to also detect a malfunction.

Putting the above mathematical equations in some theoretical form, the working of RVDT can be explained as below.

Basic RVDT construction and operation is provided by rotating an iron-core bearing supported within a housed stator assembly. The housing is passivated stainless steel. The stator consists of a primary excitation coil and a pair of secondary output coils. A fixed alternating current excitation is applied to the primary stator coil that is electromagnetically coupled to the secondary coils. This coupling is proportional to the angle of the input shaft. The output pair is structured so that one coil is in-phase with the excitation coil, and the second is 180° out-of-phase with the excitation coil. When the rotor is in a position that directs the available flux equally in both the in-phase and out-of-phase coils, the output voltages cancel and result in a zero value signal. This is referred to as the electrical zero position or E.Z. When the rotor shaft is displaced from E.Z., the resulting output signals have a magnitude and phase relationship proportional to the direction of rotation. Because RVDTs perform essentially like a transformer, excitation voltages changes will cause directly proportional changes to the output (transformation ratio). However, the voltage out to excitation voltage ratio will remain constant. Since most RVDT signal conditioning systems measure signal as a function of the transformation ratio (TR), excitation voltage drift beyond 7.5% typically has no effect on sensor accuracy and strict voltage regulation is not typically necessary. Excitation frequency should be controlled within ±1% to maintain accuracy

Although the RVDT can theoretically operate between ±45°, accuracy decreases quickly after ±35°. Thus, its operational limits lie mostly within ±30°, but some up to ±40°. Certain types can operate up to ±60°. The advantages of the RVDT are:

low sensitivity to temperature, primary voltage & frequency variations
sturdiness
low cost
simple control electronics
small size

RVDT varieties

An RVDT can also be designed with two laminations, one containing the primary and the other, the secondaries. These types can operate on larger rotations.

A similar transformer is called the Rotary Variable Transformer and contains only one secondary winding giving only one voltage:

V=G\cdot \ \theta \

External links

Selection of Rotary Sensors

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Transformer
Types	Amorphous metal transformer Austin transformer Autotransformer Buck–boost transformer Capacitor voltage transformer Current transformer Distribution transformer Delta-wye transformer Energy efficient transformer Flyback transformer Grounding transformer Instrument transformer Isolation transformer Dry isolation transformer Linear variable differential transformer Modulation transformer Neon-sign transformer Padmount transformer Paraformer Planar transformers Rotary transformer Rotary variable differential transformer Scott-T transformer Solid-state transformer Trigger transformer Variable-frequency transformer Zigzag transformer
Coils	Hybrid coil Induction coil Oudin coil Polyphase coil Repeating coil Tesla coil Trembler coil
Topics	Agbioeletric Balun Buchholz relay Bushing Center tap Circle diagram Condition monitoring of transformers Copper loss Dissolved gas analysis Electrical insulation paper Growler High-leg delta Induction regulator Leakage inductance Magnet wire Metadyne Open-circuit test Polarity Polychlorinated biphenyl Quadrature booster Resolver Resonant inductive coupling Severity factor Short-circuit test Stacking factor Synchro Tap changer Toroidal inductors and transformers Transformer oil Transformer oil testing Transformer utilization factor Vector group
Manufacturers	ABB Prolec GE Siemens TBEA

Sensors
Acoustic, sound, vibration	Geophone Hydrophone Microphone Seismometer
Automotive, transportation	Air–fuel ratio meter Blind spot monitor Crankshaft position sensor Curb feeler Defect detector Engine coolant temperature sensor Hall effect sensor MAP sensor Mass flow sensor Omniview technology Oxygen sensor Parking sensors Radar gun Speed sensor Speedometer Throttle position sensor Tire-pressure monitoring system Torque sensor Transmission fluid temperature sensor Turbine speed sensor Variable reluctance sensor Vehicle speed sensor Water sensor Wheel speed sensor
Chemical	Breathalyzer Carbon dioxide sensor Carbon monoxide detector Catalytic bead sensor Chemical field-effect transistor Electrochemical gas sensor Electrolyte–insulator–semiconductor sensor Electronic nose Fluorescent chloride sensors Holographic sensor Hydrocarbon dew point analyzer Hydrogen sensor Hydrogen sulfide sensor Infrared point sensor Ion selective electrode Microwave chemistry sensor Nitrogen oxide sensor Nondispersive infrared sensor Olfactometer Optode Oxygen sensor Pellistor pH glass electrode Potentiometric sensor Redox electrode Smoke detector Zinc oxide nanorod sensor
Electric, magnetic, radio	Current sensor Electroscope Galvanometer Hall effect sensor Hall probe Magnetic anomaly detector Magnetometer MEMS magnetic field sensor Metal detector Planar Hall sensor Radio direction finder Test light
Environment, weather, moisture	Actinometer Bedwetting alarm Ceilometer Dew warning Electrochemical gas sensor Fish counter Frequency domain sensor Gas detector Hook gauge evaporimeter Humistor Hygrometer Leaf sensor Psychrometer Pyranometer Pyrgeometer Rain gauge Rain sensor SNOTEL Snow gauge Soil moisture sensor Stream gauge Tide gauge Weather radar
Flow, fluid velocity	Air flow meter Anemometer Flow sensor Gas meter Mass flow sensor Water metering
Ionising radiation, subatomic particles	Bubble chamber Cloud chamber Geiger–Müller tube Geiger counter Ionization chamber Neutron detection Particle detector Proportional counter Scintillation counter Semiconductor detector Scintillator Thermoluminescent dosimeter Wire chamber
Navigation instruments	Airspeed indicator Machmeter Altimeter Attitude indicator Depth gauge Fluxgate compass Gyroscope Inertial navigation system Inertial reference unit Magnetic compass MHD sensor Ring laser gyroscope Turn coordinator Variometer Vibrating structure gyroscope Yaw-rate sensor
Position, angle, displacement	Accelerometer Angular rate sensor Auxanometer Capacitive displacement sensor Capacitive sensing Gravimeter Inclinometer Integrated circuit piezoelectric sensor Laser rangefinder Laser surface velocimeter Lidar Linear encoder Linear variable differential transformer Liquid capacitive inclinometers Odometer Photoelectric sensor Piezoelectric accelerometer Position sensor Rotary encoder Rotary variable differential transformer Selsyn Sudden Motion Sensor Tachometer Tilt sensor Ultrasonic thickness gauge Variable reluctance sensor Velocity receiver
Optical, light, imaging	Active pixel sensor Angle–sensitive pixel Back-illuminated sensor Charge-coupled device Contact image sensor Electro-optical sensor Flame detector Infrared Kinetic inductance detector LED as light sensor Light-addressable potentiometric sensor Nichols radiometer Optical fiber Photodetector Photodiode Photoelectric sensor Photoionization detector Photomultiplier Photoresistor Photoswitch Phototransistor Phototube Position sensitive device Scintillometer Shack–Hartmann wavefront sensor Single-photon avalanche diode Superconducting nanowire single-photon detector Transition edge sensor Tristimulus colorimeter Visible-light photon counter Wavefront sensor
Pressure	Barograph Barometer Boost gauge Bourdon gauge Hot-filament ionization gauge Ionization gauge McLeod gauge Oscillating U-tube Permanent Downhole Gauge Piezometer Pirani gauge Pressure gauge Pressure sensor Tactile sensor Time pressure gauge
Force, density, level	Bhangmeter Force gauge Hydrometer Level sensor Load cell Magnetic level gauge Nuclear density gauge Piezoelectric sensor Strain gauge Torque sensor Viscometer
Thermal, heat, temperature	Bimetallic strip Bolometer Calorimeter Exhaust gas temperature gauge Flame detection Gardon gauge Golay cell Heat flux sensor Infrared thermometer Microbolometer Microwave radiometer Net radiometer Quartz thermometer Resistance thermometer Silicon bandgap temperature sensor Special sensor microwave/imager Thermistor Thermocouple Thermometer
Proximity, presence	Alarm sensor Doppler radar Motion detector Occupancy sensor Passive infrared sensor Proximity sensor Reed switch Stud finder Touch switch Triangulation sensor Wired glove
Sensor technology	Active pixel sensor Back-illuminated sensor Biochip Biosensor Capacitance probe Carbon paste electrode Catadioptric sensor Digital sensors Displacement receiver Electromechanical film Electro-optical sensor Fabry–Pérot interferometer Fisheries acoustics Image sensor Image sensor format Inductive sensor Intelligent sensor Lab-on-a-chip Leaf sensor Machine vision Microelectromechanical systems Photoelasticity Quantum sensor Radar Ground-penetrating radar Synthetic aperture radar Radar tracker Sensor array Sensor fusion Sensor grid Sensor node Soft sensor Sonar Staring array Transducer Ultrasonic sensor Video sensor technology Visual sensor network Wheatstone bridge Wireless sensor network
Related	List of sensors

Rotary variable differential transformer

Operation of RVDTs

RVDT varieties

See also

External links