A method for easing data transmission congestion in a switch device having a plurality of ports. The windmill includes a main vertical shaft, a rotor, and a wind-receiving blade. The priority metric is determined based on estimated performance of the client request and load on the pending request list. Further, the abnormal state can be determined through the difference between the first and second detection values, and therefore, errors resulting from problems such as variations in gears and speed reducers due to temperature changes of the operational state and disposition environment of the robot can be avoided. The warpage control structure may be a layered structure comprising at least one layer of high coefficient of thermal expansion material, including but not limited to a filled epoxy material, and at least one high elastic modulus material layer, such as a metal layer. The second lighting device is mounted on the second end of the pivoting arm.