What is an RVDT? Construction, Principle, Calculation

RVDT or Rotary Variable Differential Transformer is used to generate variable AC voltage which is proportional to its angular shaft position.

How does it work like that?

An RVDT is classified as an electromechanical transducer because its mechanical movement affects its electrical output.

Since it is a transformer, it has primary and secondary windings. Its rotary mechanism is the one that is able to generate variable AC voltage.

An RVDT doesn’t have any electrical component except for conductors, if we count that as an electrical component. Other than that, there is none.

Please read the LVDT (Linear Variable Differential Transformer) first to make sure you can differentiate these two completely.

What is an RVDT?

Just like its name implies, an RVDT is a Differential Transformer (DT) with a Rotary Variable (RV). Its output is generated by the difference of the secondary voltages. Thus, the Differential Transformer made its presence in the electrical world.

Even though both LVDT and RVDT have “Transformer” at their end, they are actually transducers, not a typical transformer which is able to step up or step down AC voltage or current.

The idea behind a transducer is they convert mechanical displacement into the proportional electrical signal. If an LVDT uses a linear movement mechanism, the RVDT uses a rotary movement mechanism.

This transducer is used as a high accuracy sensor.

Since it is a rotary transducer, it measures an angular displacement or position and produces an electrical signal proportionally to its position.

Construction of an RVDT

When talking about the construction of an RVDT, it will be the same with an LVDT except the shape of the core. If an LVDT has a linear shape for its core, an RVDT has an cam shape for its core. This core is connected to an angular mechanism shaft to produce variable electrical voltage.

The angular movement of this core is used to measure angular displacement.

Like a common transformer we have known, an RVDT also has similar primary and secondary windings. The primary winding where we apply AC voltage and the secondary winding where we get our output voltage as a measurement.

RVDT Working Principle and Calculation

After learning about the construction of an RVDT above, we will make it complete with the mathematical calculation to make it easier to learn.

Observe the illustration of an RVDT below.

rvdt 1

Here we have a primary winding and a pair of secondary windings connected together to produce secondary voltage based on the core’s position.

The secondary voltages are VS1 and VS2 and the final output voltage is VO.

Just like an LVDT, the mutual inductance principle also exists on an RVDT. You better read about mutual inductance first to make this thing easier.

Short story, when an AC voltage is applied to the primary winding, the magnetic field is generated inside the core. Furthermore, this magnetic field induces mutual voltage to the secondary windings (S1 and S2). Here we get VS1 and VS2.

Just as stated above, the output voltage is the difference between secondary voltages, VS1 and VS2.

Thus, the output voltage is


We have our output voltage but we haven’t written our angular displacement. This one is as important as the output voltage because the main purpose of an RVDT is to measure angular displacement of the rotary shaft.

The secondary voltages, V1 and V2 will vary depending on the core angle.


Where G is the core sensitivity.

The secondary voltage is


The difference of V1 and V2 produces a proportional voltage:


The sum of the voltage is a constant:


The angular measurement output is independent of the input voltage, frequency, and temperature.

The RVDT working principle is listed below:

Case 1: Null position

When the core is in null position (exactly in the middle of S1 and S2), the flux linkage of these two secondary windings are equal.

Since the flux linkage is equal on those two secondary windings, the induced emf to both secondary windings (VS1 and VS2) are also equal.

Thus, the differential output voltage VO should be zero


This indicates that there is no movement or displacement of the core.

Case 2: Clockwise direction

When the core is rotating in the clockwise direction for a maximum of 40o, the flux linkage of S1 will be bigger than the S2.

We will find that the S1 has higher induced emf than S2. thus,


Difference of the output voltage is


The value of VO will be positive. This indicates that the VO is in phase with input voltage (primary voltage, VP).

Case 3: Counterclockwise direction

When the core is rotating in the counterclockwise direction for a maximum of 40o, the flux linkage of S2 will be bigger than the S1.

We will find that the S2 has higher induced emf than S1. thus,


Difference of the output voltage is


The value of VO will be negative. This indicates that the VO is out of phase with input voltage 180o (primary voltage, VP).

Advantages and Disadvantages of an RVDT

After learning how an LVDT works and the principle behind it, we can list some of its advantages and disadvantages.

The advantages are:

  • Easy to use
  • Durable
  • Stable performance
  • Consistency is high
  • Low cost
  • Linearity is good
  • High life expectancy

The disadvantages are:

  • The angle of an RVDT is limited to 40o or -40o makes the RVDT limited to a certain degree of usability in wide applications.
  • The rotation mechanism is less applicable than the linear mechanism.

Applications of an RVDT

Due to its contactless design, RVDT is currently the most widely utilized sensor and doesn’t encounter any functional issues. Therefore, the following are some of the key uses of RVDT.

  • Actuators for both engine and flying control.
  • Fuel valve and hydraulics both.
  • Utilize a cable system to brake.
  • Contemporary machinery.
  • Systems for directing the nose.
  • System of weapons and torpedoes.
  • Fuel control system for the engine.
  • Avionics and aircraft.
  • Engine air bleeding mechanisms.
  • Robotics

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