Sensing the proximity of objects via electromagnetic waves is not a new idea. Radar functions off of that basic premise to detect objects flying in the sky, and that is a technology we have possessed since the 1940’s. When we talk about proximity sensors in this day and age however, we are usually referring to the small sensors found in objects like modern mobile devices, parking sensors on automobiles, and automatic faucets. Small, inconspicuous devices we barely think about, but incredibly useful, nonetheless.
Proximity sensors, as their name implies, detect the proximity of nearby objects without requiring physical contact by flashing out an infrared beam and monitoring the reflection of those beams by objects that they bounce off of. One major advantage of proximity sensors is that they are highly reliable and have a long functional life due to their absence of mechanical moving parts, and the lack of contact between the sensor and the sensed object.
For a closer look at how proximity sensors work, we will use an example of an inductive proximity sensor. Inductive proximity sensors consist of an oscillator, a ferrite core with a coil, a detector circuit, an output circuit, housing, and a cable or connector for power. The oscillator generates a sine wave of a fixed frequency, which is used to drive a coil. The coil in conjunction with a ferrite core induces an electromagnetic field. When the field lines are disrupted by a metal object, the oscillator voltage is reduced, proportional to the size and distance of the object from the coil. The reduction in the oscillator’s voltage is caused by eddy currents created by the metal interrupting the field lines. This reduction in the oscillator’s voltage is then detected by the detecting circuit.
There are numerous types of proximity sensors, each with its own characteristics, advantages, and weaknesses. Inductive sensors, for example, are very robust and accurate, but are also bulky, heavy, and expensive to produce. Capacitive sensors, such as those found in smartphone touchscreens, can detect metal, as well as resins, powders, and liquids. However, they are susceptible to temperature variances, and interference from nearby objects. Magnetic sensors, also known as Magnetic Contacts detect magnetic field variations, and do not suffer from any electrical noise effect. However, they suffer from similar issues as capacitive sensors, including interference from temperature and surrounding objects.
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