At present, most industrial robots are controlled with a semi-closed loop for controlling the movement of a robot arm by detecting the rotational angle and/or angular velocity of a driving motor. A robot system with such a structure is comprehensively analyzed by a computer simulation, considering not only an arm-dynamics system, but also a control system, driving system, and transmission system. Robot arm vibrations, and methods for their reduction are investigated by a parametric study. As a result, the following conclusions are obtained. (1) Characteristics of robot arm vibration. If the stiffness of the transmission system is thoroughly weaker than that of the control system, the lowest natural vibration of the arm-dynamics system becomes predominant. The arm motion can be described by primary natural vibration of a mathematical model with spring-connected rigid links, in which the stiffness of the joints are equal to those of the transmission system. If the stiffness of the control system is sufficiently lower than that of the transmission system, the vibration derived from the stiffness of the control system occurs. (2) Reduction of robot arm vibration. In the first place, it is necessary to clarify whether the vibration is derived from the stiffness of the control system or that of the transmission system. If the vibration is derived from the stiffness of the transmission system, the effective way to reduce the vibration is to raise it or lower the stiffness and damping of the control system. If the vibration is derived from the stiffness of the control system, it is most effective to raise the damping of the control system.
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