A method includes moving a wafer transport device to a position above a load port; lowering a hoist unit of the wafer transport device above the load port, wherein the wafer transport device has a plurality of belts, each of the belts is connected to the hoist unit and wound around a respective belt winding drum; detecting sound waves from the belts by using at least one acoustic sensor to measure tensions of the belts; and comparing the tensions from the belts to determine an inclination of the hoist unit.