Seven Common Object Detection Sensors

Although sensing devices cannot measure, inspect or quantify objects in every way, they must reliably convey the presence or absence of a target to machines via electrical signals. There are many object sensing technologies. Below we discuss seven common types with brief descriptions of their operation, advantages and limitations.

1. Electromechanical
The most basic sensors are electromechanical limit switches. These devices include a sensitive microswitch whose state changes when an actuator is displaced by a detected object. Rollers, whiskers and levers are common actuator variants from different manufacturers. Because these devices have moving parts, they are subject to wear and failure. Also, physical contact with the target may not always be desirable or possible.

2. Pneumatic
These sensors use compressed air and a sensitive diaphragm valve to detect objects. Air flows through a small orifice until the target blocks the flow, causing a slight pressure change detected by a downstream diaphragm switch that produces an electrical control signal. Pneumatic sensors suit environments requiring explosion-proof devices or very dirty conditions.

3. Magnetic
Magnetic sensors are driven by permanent magnets within the detection range. Two common principles are reed switches and Hall-effect sensors. In both cases, the presence of a magnetic field changes the electrical signal state. Reed and Hall sensors are often used to detect piston positions inside cylinders. Because reed switches have higher failure rates, engineers often specify Hall-effect sensors when possible.

4. Inductive
Inductive proximity sensors detect metallic objects that disturb the electromagnetic field emitted by the sensor. Reliable detection distance depends on metal type and the amount of metal within the sensing range. These sensors come in many sizes and shapes. They are reliable and cost-effective, making them widely used in automation and process equipment.

5. Capacitive
Capacitive proximity switches detect non-metallic objects with dielectric constants different from air. They are ideal for materials like wood, paper, fabric, liquids and plastics. Their principle is similar to inductive sensors but uses electrostatic fields rather than electromagnetic fields.

6. Photoelectric
Photoelectric sensors use different techniques for various application configurations. They all emit a beam of light and detect changes in returned light. The most popular types are diffuse-reflective, retro-reflective and through-beam. Light source choice—visible light, infrared, LED or laser—affects sensing distance. In diffuse-reflective sensors, the presence of an object in the optical field reflects light back to the receiver. Retro-reflective and through-beam sensors generate a beam and detect objects that break it. Laser sensors can project beams of 50 meters or longer. Transparent objects or surfaces with varying gloss may present challenges for photoelectric detection.

7. Ultrasonic
These devices send short ultrasonic pulses to the target and receive echoes back at the sensor. Sound waves reflect from nearly all dense materials (metal, wood, plastic, glass, liquids, etc.) and are not affected by color, transparency or surface gloss. Foam-like materials that absorb sound are not suitable for this sensor type. Ultrasonic sensors are commonly used to determine liquid levels in process tanks.