A system and method reduce starting torque for initial rotational displacement and assure smoothness of motion for continuing displacement of a rotary joint. The process circuit generates light receiving information showing the light receiving state of the ambient light with respect to each predetermined number of light receiving elements among the plurality of light receiving elements. The motherboard state signal and the power state signal are at low level in response to the computer changing to a sleep state, the second switch is turned off, the first and third switches are turned on, and power of the computer component is stably supplied by a standby power supply. The first end of the support is adapted to engage the top end of the frame.