I am attempting to create a robot that will sort aluminum from steel automatically via an arduino. The code is essentially: if steel, do X, if aluminum, then do Y.

I am using an analog inductive proximity sensor (NPN) made by IFM ([product page here](https://www.ifm.com/products/us/ds/IFS258.htm)). 

I have managed to read values from the sensor with analogRead, but the values for aluminum and steel are exactly the same (around 165). I am using the arduino's native INPUT\_PULLUP function since the sensor is sinking. 

I am using a 27 volt power source (3x 9 volt batteries) sent through a voltage regulator set to output 25 volts.

How can discrimination be achieved? is this even possible with the current hardware? I was lead to believe that this was possible, but I am not very experienced. 

Thanks

EDIT: I purchased this sensor believing that it was an analog sensor, but it is not.