Elevating billboard and control method thereof

An elevating billboard, comprising:

• a hollow column;
• a billboard stent;
• a windlass comprising:
• a motor, a forward relay controlling said motor to rotate forward and a backward relay controlling said motor to rotate backward;
• a hollow cylinder fixed with said billboard stent and ringing said column so as to slide along said column up and down;
• and
• a fixed pulley disposed on a top of said column, wherein a free end of a guy cable on said windlass connecting with said hollow cylinder via said fixed pulley,
• wherein a locking assembly is disposed between said hollow cylinder and said column, such that when said hollow cylinder is positioned on a top of said column, said locking assembly bears as weight of said billboard stent,
• wherein said locking assembly comprises two locating pieces respectively having a keyhole and fixed on an outside wall of said column, a locking pin, an electromagnetic driver, and a spring, wherein a sliding path is formed between said two locating pieces, a lockplate is axially disposed on an inside wall of said hollow cylinder and is capable of inserting into said sliding path, said lockplate has an axial slotted hole corresponding to said keyhole, a head of said locking pin corresponds to said keyhole and a tail of said locking pin contacts with said spring, when said electromagnetic driver loses power, said locking pin inserts into said keyholes of said locating pieces, when said electromagnetic driver gets power, said locking pin disengages from said sliding path,
• wherein a first trigger switch is disposed on an outside wall of an lower end of said column and is capable of controlling said motor to stall while said hollow cylinder is falling, a third trigger switch is disposed on an upper end of said column, while said hollow cylinder is falling, said third trigger switch is capable of controlling said motor to stall and controlling said electromagnetic driver to lose power at the same time.