a first frame;

a second frame configured to be movable along a first direction or a second direction with respect to the first frame, the second direction being opposite the first direction;

an optical tracking sensor (OTS) mounted to the first frame to face a first surface of the second frame, the OTS being configured to acquire optical pattern information corresponding to an opposing surface;

a driver configured to provide a driving force to move the second frame along the first direction; and

a controller configured to calculate a movement distance of the second frame based on the optical pattern information acquired by the OTS and to control the driver based on the calculated movement distance,

wherein a marker is located at the first surface of the second frame at a position facing the OTS,

wherein the second frame is further configured to move along the first direction to change from a first state, in which the second frame has a maximum overlapping area with the first frame, to a second state, in which the second frame has a minimum overlapping area with the first frame, and

wherein the controller is further configured to adjust a speed of the driver.