In this article, the structural vibration controllability of flexible-link manipulators is discussed. For some spatial flexible-link manipulators, the structural vibration controllability is configuration-dependent. Therefore, the flexible-link manipulator may have some vibration-uncontrollable configurations. To understand the physical interpretation of vibration-uncontrollable configurations, we propose the modal accessibility concept, which indicates how well the actuators can affect the structural vibration modes. The configuration in which all of the actuators cannot affect at least one of the manipulator's vibration modes is vibration uncontrollable. Main contributions of this article are the following two points: first, interesting structural vibration-uncontrollable configurations are found within the two-link, three-joint-type manipulator's workspace, and are verified experimentally; second, the modal accessibility index is introduced to indicate how well the corresponding vibration mode is controllable. Experimental results show that even in the controllable configurations, it becomes difficult to suppress vibration if the modal accessibility is small.
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