| 摘要 |
PROBLEM TO BE SOLVED: To precisely detect the whole length of an extension boom without being affected by snowfall, rainfall and wind, in a crane wherein a boom body is supported onto an equipment body to be freely turned up and down by a hydraulic cylinder, and wherein the extension boom extended multistagedly along an axial direction is stored in the boom body to be freely extendable. SOLUTION: A target 24 for reflecting an electromagnetic wave is provided in one side out of a boom body 3A and a tip part of an extension boom 5, and a transmitter 23 faced to the target 24 to emit an electromagnetic wave for removing light and a receiver 25 for receiving the reflected electromagnetic wave reflected by the target 24 are provided in the other side. A boom length of the extension boom 5 is operated by a controller 9A based on a required time from an electromagnetic wave transmission starting time in the transmitter 23 to a reflected wave receiving time in the receiver 25, and based on a velocity of the electromagnetic wave.
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