When lifting a load using a crane, it must be hoisted directly above its center of gravity (COG) to prevent tilting and swinging, which can lead to serious accidents. Therefore, accurately identifying the COG before lifting is essential for crane safety and is a key technology for crane-lift automation. Although many methods exist for measuring the COG after the load is lifted, they often fail to prevent potential risks. Therefore, for loads with a rectangular bottom, we propose and develop a sensor system capable of detecting COG “before” the load is lifted. A depth camera and a lifting force sensor are mounted on a crane hook. The lifting force sensor also functions as a spring, allowing the lifting force to increase gradually when a crane wire is wound. The direction of the tilting of the load, that is, the direction of the COG, is detected from the change in the depth image of the top surface of the load when the load is lifted only slightly. The horizontal distance to the COG is determined by observing the changes in the lifting force, which varies before and after any edge of the load bottom is slightly lifted. Unless the COG of the load is directly beneath the lifting position, the load typically tilts toward one of the four edges of the rectangular bottom. Subsequently, to determine the COG in a two-dimensional plane, two lifting trials are required when the weight of the load is known, whereas three trials are required if the weight is unknown. Experiments were conducted to detect the COG of loads, confirming the effectiveness and sufficient accuracy of the proposed detection method.

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