Which type of sensor should be used to detect both metallic and nonmetallic objects?

• A. Capacitive proximity
• B. Inductive proximity
• C. Optical sensor
• D. Ultrasonic sensor

Answer: (A)

Capacitive proximity sensing works by detecting changes in capacitance between two conductors when an object enters the sensor’s electric field. That change happens for many different materials because objects—whether metal or nonmetal—affect the dielectric environment around the sensing plates. Metals alter the field due to their conductivity, and nonmetals (like plastic, wood, glass, liquids) alter it because they have different dielectric constants than air. So this type of sensor can detect both metallic and nonmetallic objects.

Inductive sensors rely on metal-specific eddy currents, so they won’t reliably detect nonmetals. Optical sensors depend on light reflection or interruption, which can be inconsistent with certain colors, transparencies, or dirty surfaces. Ultrasonic sensors use sound waves and can detect a broad range of materials, but performance can vary with target size, shape, and surface condition. Because capacitive sensing inherently responds to a wide range of materials, it’s the go-to choice when you need to detect both metal and nonmetal objects.