RVDT
Encyclopedia
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.
RVDT’s utilize 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 RVDT are composed 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.
s of the secondary windings,
and 
, vary linearly to the mechanical angle of the rotor, θ:
where
is the gain
or sensitivity. The second voltage can be reverse determined by:
The difference
gives a proportional
voltage:
and the sum of the voltages is a constant:
This constant gives the RVDT great stability of the angular information, independence 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 degrees 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 RVDT’s 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 :
, 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:
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
used for measuring angular displacement
Angular displacement
Angular displacement of a body is the angle in radians through which a point or line has been rotated in a specified sense about a specified axis....
.
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.
RVDT’s utilize 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 RVDT are composed of a wound, laminated stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
and a salient
Salient
Salient may refer to:* See Salients, re-entrants and pockets for the battlefield feature* Salient , part of a discrete territory projecting out of the main portion, bordered by foreign territory on three sides, into which it projects...
two-pole rotor
Rotor (electric)
The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
. 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 RVDT's
The two induced voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s of the secondary windings,
and , vary linearly to the mechanical angle of the rotor, θ:where
is the gainGain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
or sensitivity. The second voltage can be reverse determined by:
The difference
gives a proportionalProportionality (mathematics)
In mathematics, two variable quantities are proportional if one of them is always the product of the other and a constant quantity, called the coefficient of proportionality or proportionality constant. In other words, are proportional if the ratio \tfrac yx is constant. We also say that one...
voltage:
and the sum of the voltages is a constant:
This constant gives the RVDT great stability of the angular information, independence of the input voltage or frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, or temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, 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 degrees 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 RVDT’s 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 laminationsLaminate
A laminate is a material that can be constructed by uniting two or more layers of material together. The process of creating a laminate is lamination, which in common parlance refers to the placing of something between layers of plastic and gluing them with heat and/or pressure, usually with an...
, 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:
See also
- Rotary encoderRotary encoderA rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...
- SynchroSynchroA synchro is a type of rotary electrical transformer that is used for measuring the angle of a rotating machine such as an antenna platform. In its general physical construction, it is much like an electric motor...
- ResolverResolver (electrical)A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analog device, and has a digital counterpart, the rotary encoder.-Description:...
- LVDT, Linear-movement counterpart to the RVDT