Re: Approximate sensor
Would be probably better solved by designing the sensor function into the construction from the start rather than supplementig it later, as it seems to be the case now. Some material choices or form factors may be adapted to the measurement requírements then. Integrating the measurement in the shaft is one option.
Generally, if optical sensing is regarded unsuitable, a capacitive proximity switch would be the next obvious solution for plastic objects, apart from ultrasonic or mechanical feelers.