An inductive proximity sensor is one of a class of sensors, called proximity sensors, which detects objects without actually touching them. Photoelectric sensors and capacitive proximity sensors are also found in this category. The inductive proximity sensor only detects metallic objects and ignores all others, and has many uses which capitalize on this feature.
Inductor
The heart of an inductive proximity sensor is an inductive loop. An inductor is one of the three basic passive elements in electric circuitry, along with capacitors and resistors. While resistors resist electric current and capacitors store electric charge, inductors store electric current. An inductor takes the form of a simple loop of wire. Current flowing through this loop creates a magnetic field in the loop. This magnetic field is what stores the electric current, and if the source of electricity is cut off the magnetic field will collapse and release its energy as a current falling asymptotically toward zero from its initial level. Storing and releasing current has the effect of dampening out changes in current in the circuit.
Inductance
The capacity for an inductor to store current as a magnetic field is called inductance, and is measured as a unit called the Henry, after American physicist Joseph Henry (1797–1879), who conducted groundbreaking research in the field of electromagnetism. Among other things, inductance is influenced by the volume enclosed by the inductor, the number of wraps of wire it consists of, and the material inside the wire wraps (called the "core"). Metals are a particularly good material to increase inductance, and air is a fairly poor material for this purpose. Most other materials, such as plastic, water, and human flesh, are also poor inductive materials.
Inductive sensor
The inductive sensor is a simple enough device at the fundamental level. It is simply a coil of wire with a current passing through it. It completely ignores most objects which pass near this coil. However, when a metallic object passes near it, it acts as a core material in the inductive loop which increases its inductance significantly. Most non-metallic objects have a negligible effect on its inductance. The complex part of the sensor is the sensing circuitry which detects this change in inductance by monitoring the electric current in the loop. When the inductance changes enough, it triggers the sensor's output, which sends a signal to some other machine to do whatever it is it is supposed to do when a metallic object is near the sensor.
Traffic lights
Please excuse the US-centric content
The most public use of an inductive sensor is for traffic lights. Not all controlled intersections have inductive loops, but many busy ones do. These loops are buried in the asphalt and are usually noticeable by the square shaped cuts in the pavement the loop was inserted into. When a car, truck, or other large metal vehicle drives over the loop, it changes the loop's inductance and triggers the sensor, alerting the traffic light controller that there is a car waiting for the light to change. Intersections with protected turn lanes use these loops to detect if a car is waiting to turn left or not. Sometimes, if you are waiting a long time for the light to change, you may not be far enough over the inductive loop and driving forward a little may trigger it. Unfortunately, sometimes these are not sensitive enough to detect motorcycles. Other traffic lights are controlled by timers and have no sensors.
Similar inductive loops are used to detect cars in many other circumstances, such as drive-through fast food windows and automatic car washes.
Industrial automation
The inductive proximity sensor is one more item in the massive line of sensor packages used in industrial automation. In this case, they are usually connected to a relay or PLC to provide sensing as part of a larger automated process. The major advantage of an inductive proximity sensor (often called a prox switch, although this is really a generic term for any sensor that detects objects close to it without physical contact) is the fact that it ignores nonmetallic objects. This is useful for positioning metal parts, such as conveyors, lift tables, tipples, and machine parts, while ignoring any nonmetallic objects around, such as the goods being manufactured or packaging material like cardboard boxes or loading pallets, or the operator running the machine.
The second advantage of an inductive proximity switch is that, due to the way it ignores most substances, it can get very dirty and still work properly. This, combined with their solid state circuitry and wear-free operation due to not making physical contact with the object it senses, allows them to provide years of nearly maintenance-free operation. They work very well in applications where it would be difficult to get at them to clean or replace them, such as in elevator shafts.
Disadvantages
These rugged, reliable, special-purpose sensors do have a couple of disadvantages though. First, they are omnidirectional. They will not know or care if a piece of metal is in front of it, or to the side, or above or below, it will trigger all the same. Second, they have a very short sensing range, usually less than an inch. This doesn't leave much room for error with fast-moving machinery that could damage the sensor if it gets hit. The trick, as always, is to weigh the benefits and disadvantages and select the proper sensor for the job.