Introduction of RVDT:
RVDT full form stands for a Rotary variable differential transformer. A rotary variable differential transformer (RVDT) is an electromechanical transducer that provides a variable AC output voltage that is proportional to the angular displacement of its input shaft. As RVDT is an ACcontrolled 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
It is an electromechanical 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 Camshaped core (Rotating core) for measuring the angular displacement. For understanding the construction of RVDT in detail, please follow our previous article about LVDT construction.
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 515V at a frequency of 50400 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 (S_{1} and S_{2}) 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.
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 E_{} = E_{s1} – E_{s2} will be zero (E_{} = 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 S_{1} will be more as compared to S_{2}. This means the emf induced in S_{1} will be more than the induced emf in S_{2}. Hence E_{s1} > E_{s2} and Net differential output voltage E_{} = E_{s1} – E_{s2} will be positive. This means the output voltage E_{} will be in phase with the primary voltage.
Case 3: When the core rotates in the anticlockwise direction.
When the core of RVDT rotates in the anticlockwise direction. Then, in this case, the flux linkage with S_{2} will be more as compared to S_{1}. This means the emf induced in S_{2} will be more than the induced emf in S_{1}. Hence E_{s1} < E_{s2} and Net differential output voltage E_{} = E_{s1} – E_{s2} will be negative. This means the output voltage E_{} 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.
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