Inductive Proximity Sensor
Inductive proximity sensors depend on "changing a magnetic field surrounding a sensor element by inducing eddy current in a ferrous or nonferrous target brought into close proximity with the sensor element".
This type of sensor is also sometimes called a proximity switch.
It offers fast, accurate, and highly repeatable operation at relatively low cost.
The-eddy-current inductive sensor circuit consists of an AC oscillator circuit, an inductor coil L1, and an output voltage detector (or demodulator circuit) that senses the amplitude of the output waveform produced by the oscillator.
Oscillator circuit implementation
When nothing is near the inductor sensor head, the oscillator amplitude will stabilize to some nominal value.
When a metallic target is in the field of the coil, the magnetic field of the coil will induce eddy currents in the target. These eddy currents create a magnetic field around the target, and this field will change the resistance component of the impedance of the sensor coil. This interaction will cause a sudden drop in the output amplitude of the oscillator, and can be used to indicate the presence of the object.