An inductive sensor is an electronic device that senses the presence of metal. These devices were introduced in the 1960s and have rapidly replaced mechanical switches, especially in dirty or wet environments. Inductive sensors, also known as proximity sensors, use a magnetic field that is affected by nearby metals. When metal is present, electronic circuits detect the field changes and send a signal that can be used to operate other machinery. The effect of metal on a magnetic field is called an induction effect, and a sensor uses an induction loop.
Four main parts make up an inductive sensor. An oscillator changes direct current (DC) to alternating current (AC), although some sensors are AC-powered and do not need this part. A wire-wrapped iron core or coil creates a magnetic field that will be affected by the presence of metal. The sensing circuit monitors that magnetic field and senses field changes caused by metal passing nearby. An output processor takes the sensing circuit information and sends out a signal to other equipment.
Inductive sensors have a wide range of uses. These sensors are normally produced as normally-open or normally-closed devices. A normally-open inductive sensor will not permit an electrical current to flow unless the sensor is activated. One simple example of this is a garage door opener, where the proximity sensor will stop the door motor when the opener assembly approaches the motor hanging from the ceiling.
In contrast, normally-closed sensors allow current to flow until the metal is sensed, which causes the switch to open, interrupting the flow. They are used when a metal object needs to turn on a circuit or operate machinery. Applications can include position sensors for remote valves, sorting machinery to separate metal from other materials, or car washes where the passing car or guide roller turns on the car wash equipment.
An inductive sensor is favored over mechanical switches for several reasons. They have no moving parts, and need no preventive maintenance. There is no physical contact between the metal object and the inductive sensor, and therefore products won’t be damaged during production or assembly operations. Inductive sensors are rugged and unaffected by a wide range of temperatures or environmental conditions, including dirt or chemicals.
Iron is most responsive to the magnetic field created by an inductive sensor, but other metals can be detected as well. Stainless steel, brass and even aluminum can be sensed, but magnetic field effects are less with these metals and calibration may be needed to ensure reliability of the signal. Sensors used for non-iron metals may need to be placed very close to the metal object, which can reduce their advantages over other types of sensors.