RVDT- Construction, Working, Application, Advantages and Disadvantages

RVDT is an Electro-mechanical type Inductive Transducer that converts angular displacement into the corresponding Electrical Signal.

Introduction of RVDT:

RVDT full form stands for a Rotary variable differential transformer. A rotary variable differential transformer (RVDT) is an electro-mechanical transducer that provides a variable AC output voltage that is proportional to the angular displacement of its input shaft.

As RVDT is an AC-controlled device, so there is no electronics component inside it. Also, the electrical output of RVDT is obtained by the difference in secondary voltages of the transformer, so it is also called a Differential Transformer.

As in our previous article, we have already discussed the complete theory of LVDT. . As we know that RVDT is also an Inductive Transducer, not a Transformer. Its design and working are similar to LVDT. So for better understanding the concepts of RVDT, please follow our previous article titled as Construction and working of LVDT.

What is RVDT?

RVDT is an electro-mechanical inductive transducer that converts angular displacement into the corresponding electrical signal. It is the most widely used inductive sensor due to its high accuracy level.

Since the coil of RVDT is designed to measure an angular position, so it is also known as an angular position sensor. Unlike LVDT, RVDT is also a passive differential transducer.

RVDT Construction

The design and construction of RVDT is similar to LVDT. The only difference is the shape of the core in transformer windings. LVDT uses the soft iron core to measure the linear displacement whereas RVDT uses the Cam-shaped core (Rotating core) for measuring the angular displacement.

For understanding the construction of RVDT in detail, please follow our previous article about LVDT construction.

[ ##eye## Causes and Disadvantages of Low Power Factor]

RVDT Theory:

If we denote both the secondary voltages by E_s1 and E_s2  (see in below fig.) and also the sensitivity of RVDT is G. Then the angular displacement of the shaft will vary as:

The secondary voltage is determined by the help of the equation given below as:

The differential output E_s1 – E_s2 will be determined as:

A total sum of voltages will be calculated as a constant C.

RVDT working principle

The working principle of RVDT and LVDT both are the same and based on the mutual induction principle.

When AC excitation of (5-15) Volt at a frequency of 50-400 Hz is applied to the primary windings of RVDT then a magnetic field is produced inside the core. This magnetic field induces a mutual current in secondary windings. Then due to transformer action, the induced voltages in secondary windings (S₁ and S₂) are E_s1 and E_s2  respectively. Hence the net output voltage will be the difference between both the induced secondary voltages.

Hence Output will be  E0 = Es1 – Es2.

Now according to the position of the core, there are three cases that arise. So Let’s discuss these three cases one by one in detail.

[ ##eye## Electrical Bonding]

Case 1: When the core is at the Null position.

When the core is at the null position then the flux linkage with both the secondary windings will be the same. So the induced emf (E_s1 and E_s2 ) in both the windings will be the same.

Hence the Net differential output voltage E₀ = E_s1 – E_s2 will be zero (E₀ = E_s1 – E_s2  = 0). It shows that no displacement of the core.

Case 2: When the core rotates in the clockwise direction.

When the core of RVDT rotates in the clockwise direction. Then, in this case, the flux linkage with S₁ will be more as compared to S₂.

This means the emf induced in S₁ will be more than the induced emf in S₂. Hence E_s1 > E_s2  and Net differential output voltage E₀ = E_s1 – E_s2 will be positive. This means the output voltage E₀ will be in phase with the primary voltage.

Case 3: When the core rotates in the anti-clockwise direction.

When the core of RVDT rotates in the anti-clockwise direction. Then, in this case, the flux linkage with S₂ will be more as compared to S₁.

It means the emf induced in S₂ will be more than the induced emf in S₁. Hence E_s1 < E_s2  and Net differential output voltage E₀ = E_s1 – E_s2  will be negative. This means the output voltage E₀ will be in phase opposition (180 degrees out of phase) with the primary voltage. 

Advantages of RVDT

Following are the main advantages of RVDT:

• High Accuracy.
• Compact and strong construction.
• The consistency of RVDT is high.
• Long life span.
• Very high Resolution.
• Low cost.
• High durability.
• Linearity is excellent.
• The performance is repeatable.
• Easy to handle.

[ ##eye## Construction and Working of LVDT]

Disadvantages of RVDT

The disadvantages of RVDT mainly include the following.

• Since the output of RVDT is linear ( about +40 degrees or -40 degrees), So it restricts its usability.

• The contact among the measuring exteriors as well as the nozzle is not possible for all time.

Applications of RVDT

RVDT is most commonly is used as a sensor nowadays, also it doesn't experience any functional problem due to its contactless structure. Hence the main applications of RVDT include the following.

• Actuators for controlling flight as well as engine.
• Fuel valve as well as hydraulics.
• Brake with a cable system.
• Modern machine tools.
• Nose wheel steering systems.
• Weapon and Torpedo system.
• Engine fuel control system.
• Aircraft and avionics.
• Engines bleed air systems.
• Robotics

Read more articles:

• Theory and working of Star-Delta Starter

• Surge impedance loading (SIL) of Transmission line

• Power Factor Correction techniques

• Notch filter - Theory, circuit design and Application

• Different Parts of transformer and their functions

• What is power factor and why it is important