Proximity Sensor

Proximity Sensor (semiconductors)

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Overview of the proximity sensor

Although there are numerous different types of proximity sensor available today, all share the following basic characteristics. By emitting a field or beam of electromagnetic radiation, proximity sensors detect objects (or “targets” in technical terms) in their near vicinity by measuring alterations in the return signal or field. As no physical contact is required for this detection, proximity sensors require relatively few mechanical parts, thereby reducing wear and tear so that the equipment tends to have a long lifespan.

Depending on the material composition of the target, different sensors will be required. Plastic targets will be “invisible” to a photoelectric sensor, as will metal targets to an inductive sensor, for instance.

Different proximity sensors vary in the size of their field of vicinity, with the maximum reach for detection being the sensor’s “nominal range”. The nominal range can be adjusted, however, on some sensors to detect targets either more remotely or much closer to the device. Those which are capable of being adjusted to detect at exceptionally close range are often employed as touch switches.

Proximity sensors can be grouped according to their sensing mechanisms. Although a vast array of models exists in each category, the broad types of proximity sensor are Acoustic, Capacitive, Inductive, Infra-red, and Piezoelectric.

Other proximity sensors include laser rangefinders, Doppler effect sensors, radar, passive thermal infrared sensors, active or passive sonar sensors and charged-coupled passive optical sensors.


History of the proximity sensor 

Human beings are themselves proximity detectors: eyes, ears, and noses detect “targets” visually, audially and through olfaction. Early warning systems involving “tripping” string or wires to warn of animals or intruders will date back through the millennia. As technology advanced and electrical means became available, so the conduction of more sophisticated proximity sensors tailored for specific purposes and targets became possible. The American physicist Edwin Hall discovered the Hall Effect in 1869, which remains the basis of many magnetic field proximity sensors today. The nineteenth and twentieth centuries saw a major rise in the development of various kinds of proximity sensor, driven by the needs of industry and commerce as well as by technological innovations – developments that are still proceeding in the twenty-first century.


Technical aspects of the proximity sensor

·         Ultrasonic Proximity Sensors generate high frequency sound waves from a piezoelectric transducer and then measures changes in the echo when it bounces back off an approaching or static target.

·         Capacitive Proximity Sensors detect changes in the electrical capacitance of both metallic and non-metallic targets as they move toward or away from the device’s own capacitor;

·         Inductive Proximity Sensors emit a high frequency magnetic field vial a coil and then detect the presence of metallic (especially ferromagnetic) targets when they enter the field and absorb some of its energy;

·         Photoelectric Proximity Detectors emit a beam of light from one component and detect the presence of both metallic and non-metallic objects in the other component (the detector) when the target moves into the beam (i.e., between the beam emitter and the remotely positioned detector); another variant incorporates the emitter and detector in the same “box” and receives the reflection of the beam as it bounces back from the target to the sensor;

·         Hall Effect Proximity Sensors, which employ thin semiconductors across which a current is passed; if a magnetic field is placed perpendicularly to the flow of current, the voltage increases slightly from 0V, so that the device measures changes in voltage (usually measured in microvolts) as a magnet approaches or recedes from the semiconductors. The device is widely employed as a fuel level detector: as a magnet mounted onto a floating object on the surface of the fuel rises or falls, it affects the voltage in a fixed semiconductor mounted at right angles to the magnet on the top of the tank’s lining.


Where proximity sensors are used in manufacturing

Proximity sensors are now used ubiquitously in manufacturing, from the “Parktronics” technology integrated into vehicle bumpers to prevent collisions while reversing, to automatic doors which respond to the pressure of footfalls on a piezoresistive sensor plate beneath a mat, to infrared security alarms.

In industry, they are employed to measure the vibration in machinery with rotating shafts and to monitor the stability of top dead centre (TDC) camshafts on reciprocating engines.


How the proximity sensor differs from other sensors

Like all sensors, proximity sensors convert an input energy into another medium for reading and measuring. Proximity sensors are unique in that, although they may employ energy used in other sensors such as thermal radiation, for example, they are not primarily heat measuring sensors such as the thermometers: their chief purpose is to detect the presence of a physical object or target into a spatial field which the sensor is monitoring.