RVDT- Construction, Working, Application, Advantages and Disadvantages


In our previous article, we have already discussed the complete theory of LVDT. Since RVDT is also a Inductive Transducer, not a Transformer. Its design and working are the same as LVDT. So please follow our previous article of LVDT to better understanding the concepts of RVDT. Please refer the following link for detailed concepts of LVDT- Construction, Working, Application, Advantages, and Disadvantages.

    What is RVDT?

    Introduction of RVDT:

    RVDT full form stands for Rotary Variable Differential Transformer. Rotary Variable Differential Transformer (RVDT) is an Electromechanical type Inductive Transducer that converts angular displacement into the corresponding Electrical Signal. As RVDT is AC controlled device, so there is no any electronics component inside it. It is the most widely used Inductive Sensor due to its high accuracy level. Since the coil of RVDT is designed to measure angular position, So it is also known as Angular Position Sensor. Its electrical output is obtained because of the difference of secondary voltages, hence it is called Differential Transformer. Unlike LVDT it is also a Passive Transducer.

    RVDT Construction:

    Design and Construction of RVDT are same as LVDT. The only difference is theshape of the core in transformer windings. LVDT uses the soft iron core for measuring linear displacement whereas RVDT uses cam shaped core (Rotating core) for measuring angular displacement. To understand the Construction of RVDT, please follow the link LVDT Construction.

    RVDT Theory:

    If we denote both the secondary induced voltages by Es1 and Es2 (see in below fig.) and also the sensitivity of RVDT is G. Then the angular displacement of the shaft is varies as:
    The secondary voltage is determined by the help of the equation given below as:
    The differential output Es1 – Es2 will be determined as
    A total sum of voltages will be calculated as a constant C.

    RVDT working principle:

    Working principle of RVDT and LVDT both are same and based on mutual induction principle. When AC excitation of 5-15 V at a frequency 50-400Hz is applied to the primary winding, then a magnetic field is produced. This magnetic field induces a mutual current in secondary windings. Then due to transformer action, the induced voltages in secondary windings (S1 & S2) are Es1 & Es2 respectively. Then the net output voltage will be the difference between both induced voltages (Es1 & Es2) in secondary windings.
    Hence Output will be  E0 = Es1 – Es2

    RVDT Construction and working

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

    Case 1: When the core is at 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 (Es1 & Es2) in both the windings will be the same. Hence the Net differential output voltage E0 = Es1 – Es2 will be zero (E0 = Es1 – Es2 = 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 induced emf in S2. Hence Es1>Es2 and Net differential output voltage E0 = Es1 – Es2 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. This means the emf induced in S2 will be more than induced emf in S1. Hence Es2>Es1 and Net differential output voltage E0 = Es1 – Es2 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

    Fallowing 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 fallowing.
    • 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 nozzle is not possible for all time.

    Applications of RVDT

    RVDT is most commonly used sensor nowadays, and it doesn't experiance any functional problem due to its contactless structure. The main applications of RVDT include the fallowing.
    • Actuators for controlling flight as well as engine.
    • Fuel valve as well as hydrolics.
    • Brake with cable system.
    • Modern machine tools.
    • Nose wheel steering systems.
    • Weapon and Tarpedo system.
    • Engine fuel control system
    • Aircraft and avionics.
    • Engines bleed air systems.
    • Robotics.

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