Inductive proximity sensors are a type of sensor that allows non-contact detection of metal objects. They contain a coil and an oscillator that create an electromagnetic field close to the detection surface. When metal enters this field, there is a change in the oscillating amplitude. Which causes the sensor output to change, thus showing the user and/or associated equipment that metal has been detected. So we always work with 2 sensors so that detection for open and closed state can be determined.