Three problems arise in this context:1. Hysteresis: the inhalation curve may differ from the exhalation curve.2. The direction of the internal target motion may not be the same as the direction of the surface motion (e.g., consider the diaphragm, moving in inferior–superior direction, while the abdomen surface moves in anterior-posterior direction).3. The velocity of the motion may vary along the motion curve.Our previous work has investigated the design of a new sensor method for tracking respiratory motion