Parallel wire suspension mechanisms with a hierarchical structure

Heavy parts handling is a very dangerous tasks, but it has not yet been automated. A wire suspension mechanism is essential for handling heavy parts from the viewpoint of space and payload capacity. However, a conventional crane suspending an object by only a single wire cannot control the orientation of the object. Thus, human help is inevitable in positioning heavy parts. To solve these problems, cranes with multiple parallel wire structures have been developed by several researchers. In general, a wire feed actuator needs a high reduction ratio, such as a worm wheel mechanism, in order to reduce energy consumption while suspending an object. Thus, each wire cannot be tension-controlled but must be length-controlled. However, the number of wires suspending an object is physically limited when the wires are length-controlled. Figure 1 shows a simple example for positioning a mass in a two-dimensional (2D) plane. When gravity exists in the y direction, the position of the mass (xo, yo ) can be determined by two wire lengths. If there are three wires suspending the same object shown as in Fig. 2 and they have mutual length errors, one of the wires becomes loose and the positioning system may become unstable. Theoretically, the maximum number of wires suspending the same object in a threedimensional space is proven to be six. However, we can use as many wires as we want by constructing a wire mechanism with a hierarchical structure. Systems with a hierarchical wire structure have some advantages, such as large payload capacity, positioning accuracy and high stiffness against external forces. We designed a new type of crane mechanism with a hierarchical structure in a 2D space illustrated in Fig. 3. In this system, even if some of the wires have length