An autonomous transport vehicle including a transfer arm including at least one finger and a movable finger support member, at least one sensor configured to detect movement of the at least one finger, the at least one sensor having a registration member and a detection member where one of the registration member and detection member is mounted to each of the at least one finger so as to be movable with a respective one of the at least one finger and the other one of the registration member and detection member is stationary relative to the at least one finger, and a controller in communication with the at least one sensor, the controller being configured to determine a position of the at least one finger along the second direction based on a proximity of the registration member relative to the detection member.