To reliably detect materials with very low dielectric constant, which sensor variant is appropriate?

• A. Inductive proximity sensor
• B. Photoelectric sensor
• C. Unshielded capacitive sensor
• D. Shielded capacitive sensor

Answer: (D)

The important idea is that detecting materials with very low dielectric constant relies on focusing the sensor’s electric field so it interacts primarily with the target, while minimizing interference from the surroundings. A shielded capacitive sensor accomplishes this by surrounding the sensing electrode with a grounded shield. This keeps most field lines inside the sensing zone and away from nearby objects, so the change in capacitance caused by a low-permittivity material becomes more detectable. In practice, that means more reliable sensing of weakly permittive targets, such as certain plastics or foams, even when there’s background clutter or moisture.

Unshielded capacitive sensors spread the field more broadly, so stray capacitance from the environment and nearby materials can swamp the small signal from a low-dielectric target, reducing reliability. Inductive sensors detect metals, not plastics or low-dielectric materials. Photoelectric sensors rely on light and surface properties rather than permittivity, so they aren’t optimized for detecting low-dielectric targets.