By your question it appears that you are mixing two different terms.
1. sensors that work with the principle of induction are called inductive sensors, they reacts on metal (in reality they react on electrically good conducting materials). The working principle is that the sensor generates a oscillating magnetic field. When something conducting electrical current comes enough within this magnetic field currents are induced in the object. The object itself acts as a secondary winding om a short-circuited transformer (the primary winding is inside the sesor, it is the one generating the magnetic field in the first place). This leads to a higher current drawn by the primary winding (as it would in any short-circuited transormer) and this rise in current is what really is detected.
2. npn/pnp, this have to do with if you send a positive voltage to the input to indicate an activated sensor (pnp) or if you ground the input to indicate an activated sensor (npn). The names comes from what transistor you would need at the output stage to perform the selected task (completely regardless of if there actually is
a transistor of that type there or not)
An inductive sensor can be bought with either function (pnp or npn) and some models does actually have both functions and you wire them
diffrently depending on what function you want.
Other types of sensors exist, such as capacitive (sensing all solid materials), optical (sensing if light can pass or not) and a lot of others. All these sensor types can have either function (pnp or npn) in the same way as inductive sensors.
/Johan Bengtsson
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