Apparatus are provided for infusion devices and related control systems and methods. In one embodiment, an infusion device includes a voided portion adapted to receive a shaft portion that includes a shaft coupled to a plunger of a reservoir. The shaft portion includes a detectable feature having an optically detectable pattern such as gray code, and the infusion device includes a sensing arrangement having an optical sensor proximate the voided portion to sense the detectable feature. In some embodiments, a control module is coupled to the sensing arrangement to determine a remaining amount of fluid in the reservoir based at least in part on the sensed position of the detectable feature. In embodiments, the optical sensor detects the sensed position of the shaft based on the detectable feature only when the reservoir is replaced and/or after each delivery of fluid to reduce power consumption.