Jawaharlal Nehru Technological University Hyderabad B.Tech II TRANSDUCERS ENGINEERING Previous Question Paper (March - 2021 - R18)

A generalized measurement system consists of several functional stages that convert a physical quantity into a readable output. The block diagram follows this sequence: Primary Sensing Element → Variable Conversion Element → Variable Manipulation Element → Data Transmission Element → Data Presentation Element.

1. Primary Sensing Element: Receives the physical quantity (measurand) and produces an output that is a function of the measurand.
2. Variable Conversion Element: Converts the output of the primary sensing element into a more suitable electrical form (e.g., resistance, voltage, or current) without changing the information content.
3. Variable Manipulation Element: Changes the magnitude or nature of the signal (e.g., amplification, filtering, or attenuation) to suit the next stage.
4. Data Transmission Element: Transmits the signal from one location to another, often over long distances, using cables or wireless links.
5. Data Presentation Element: Provides the final output in a human-readable format, such as a digital display, recorder, or computer interface.

A Linear Variable Differential Transformer (LVDT) is an electromechanical transducer that converts linear motion into an electrical signal. It consists of a primary winding and two secondary windings (S1 and S2) wound on a hollow cylindrical former, with a movable soft iron core positioned inside.

The primary winding is excited by an AC source. The output voltage is the difference between the voltages induced in the two secondary windings. The core position determines the magnetic coupling between the primary and the secondary windings:

1. Null Position: When the core is at the center, the flux linkage with both secondary windings is equal. Thus, the induced voltages are equal, and the net output voltage is zero.
2. Displacement to the Left: The core moves toward S1, increasing the flux linkage with S1 and decreasing it with S2. The output voltage becomes positive and proportional to the displacement.
3. Displacement to the Right: The core moves toward S2, increasing the flux linkage with S2 and decreasing it with S1. The output voltage becomes negative and proportional to the displacement.

The magnitude of the output voltage indicates the amount of displacement, while the phase shift (relative to the primary excitation) indicates the direction of movement.