A system can include a housing that defines an interior space; a shaft disposed at least in part in the interior space of the housing where the shaft includes a longitudinal axis, a curved surface and an end surface; a submersible electric motor operatively coupled to the shaft where the submersible electric motor includes a cable connector; two proximity sensors where each of the proximity sensors includes a sensor aperture disposed in the interior space of the housing; and circuitry operatively coupled to the proximity sensors that determines position values of the shaft with respect to time based at least in part on output of the proximity sensors.