I have an application where a rotating plastic wheel has ferromagnetic inserts on the circumference of the plastic wheel (they don't stick out). Shown below in the picture is the plastic wheel (black) with ferromagnetic inserts (red). This could be accomplished with a hall-effect edge detection sensor like the Allegro ATS616LSGTN-T but I don't want to use it because the magnet in this sensor will cause drag (due to the reluctance effect); this is my only reason for wanting to avoid using hall-effect sensors.
I am wondering if it would be possible to create a capacitive switching sensor to distinguish between plastic and metal given the wheel is rotating around 3500 rpm.