JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year BASICS OF SENSORS TECHNOLOGY Previous Question Paper (August - 2022 - R18)

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

• Operation: An AC excitation voltage is applied to the primary winding, creating a magnetic flux that links the secondary windings. The secondary windings are connected in series opposition, meaning the output voltage is the difference between the voltages induced in S1 and S2.
• Core Position Dependency: The output voltage depends on the position of the ferromagnetic core:
• When the core is at the null position (center), the flux linkage to both secondary windings is equal. Since they are in series opposition, the output voltage is zero.
• When the core moves upward, the flux linkage to S1 increases and to S2 decreases, resulting in a positive output voltage proportional to the displacement.
• When the core moves downward, the flux linkage to S2 increases and to S1 decreases, resulting in a negative output voltage with a 180-degree phase shift.

The gauge factor (GF) of a semiconductor strain gauge represents its sensitivity. Unlike metal gauges that rely primarily on dimensional changes, semiconductor gauges rely on the piezoresistive effect, where the resistivity of the material changes significantly under mechanical stress.

• Doping Level Influence: The gauge factor is inversely proportional to the doping concentration of the semiconductor material.
• High Doping: At high doping levels, the material behaves more like a metal, resulting in a lower gauge factor. The sensitivity is lower because the carrier concentration is less sensitive to strain-induced changes in the energy band structure.
• Low Doping: At lower doping levels, the piezoresistive effect is much more pronounced, leading to a significantly higher gauge factor. This allows for extremely high sensitivity, often 50 to 100 times greater than metallic gauges.
• Trade-off: While lower doping increases the gauge factor, it also increases the temperature sensitivity of the gauge, which is a significant disadvantage in practical applications.

The relationship is generally expressed as:

where ΔR/R is the fractional change in resistance and ε is the strain.