A wearable infusion pump assembly. The wearable infusion pump assembly includes a reservoir for receiving an infusible fluid and a fluid delivery system configured to deliver the infusible fluid from the reservoir to an external infusion set. The fluid delivery system includes a controller, a pump assembly for extracting a quantity of infusible fluid from the reservoir and providing the quantity of infusible fluid to the external infusion set, the pump assembly comprising a pump plunger, the pump plunger having distance of travel, the distance of travel having a starting position and an ending position, at least one optical sensor assembly for sensing the starting position and ending position of the pump plunger distance of travel and sending sensor output to the controller, and a first valve assembly configured to selectively isolate the pump assembly from the reservoir, wherein the controller receives the sensor output and determines the total displacement of the pump plunger.