This paper proposes a stability analysis criterion for the multiple objects handling operation of dual arm manipulator to make optimal postures. When the dual-arm manipulator grasps a fixed object to carry it to a desired location, the dual-arm manipulator should hold the object stably and efficiently without losing the control. As a performance evaluation parameter, joint torque data from current sensors are utilized. From the current information, torque and power consumption in each joint are estimated. In this paper an optimal posture for an unstructured object is defined by a cost function formed by three factors: the holding forces, manipulability measure, and the power consumption. For the stable grasp, the forces of the dual-arm manipulators and the gravity force need to be well-balanced to make it minimum. The grasping force to the object is computed by using the pointing vector to the object and the joint torque. Manipulability measure which implies how efficiently the dual -arm manipulator can move the holding object together, can be represented by the intersection of the two manipulability ellipsoids for the left and right arms. Effectiveness of the proposed algorithm has been verified through the real experiments.
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