A method of estimating a speed of a vehicle is provided. In a first embodiment the device includes an outer housing and an inner shaft member mounted within the housing for relative rotary and telescopic movement. For example, the automated peritoneal dialysis system may include a carriage arranged to receive the one or more lines each having a connector end and a cap. For an electrostatically actuated MEMS structure, these features can be used to limit the range of motion such that pull-in or snapdown is avoided, greatly enhancing the reliability of the device. Further, a space for the manual sheet feeding and the number of parts can be saved to the minimum.