JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III INSTRUMENTATION AND CONTROL SYSTEMS Previous Question Paper (September - 2021 - R18)

A generalized measurement system consists of several functional stages that transform the physical input variable into a usable output. The block diagram follows this sequence: Primary Sensing Element → Variable Conversion Element → Variable Manipulation Element → Data Transmission Element → Data Processing/Presentation Element.

1. Primary Sensing Element: Detects the physical quantity (measurand) and converts it into a usable electrical or mechanical signal.
2. Variable Conversion Element: Converts the signal from the primary sensor into a more suitable form (e.g., resistance to voltage) without changing the information content.
3. Variable Manipulation Element: Modifies the signal to a desired level (e.g., amplification, filtering, or attenuation) to suit the requirements of the next stage.
4. Data Transmission Element: Transmits the signal from one location to another, often over long distances, using telemetry or cables.
5. Data Processing and Presentation Element: Performs complex operations like computation or storage and presents the final data via indicators, recorders, or digital displays for human interpretation.

A capacitive transducer operates on the principle of variable capacitance. The capacitance C of a parallel plate capacitor is given by the formula:

where ε₀ is the permittivity of free space, εᵣ is the relative permittivity of the dielectric, A is the overlapping area of the plates, and d is the distance between the plates.

The transducer works by converting a physical displacement into a change in capacitance. This can be achieved through three methods:

• Change in distance (d): One plate is fixed while the other moves, causing an inverse change in capacitance. This is highly sensitive for small displacements.
• Change in area (A): The overlapping area of the plates is varied by lateral movement, resulting in a linear change in capacitance.
• Change in dielectric (εᵣ): A dielectric material moves between the plates, altering the permittivity and thus the capacitance.

These changes are typically measured using an AC bridge circuit, which converts the capacitance variation into a measurable voltage signal.