RBO Hand 2, soft hand, Robotics and Biology Laboratory, Technical University of Berlin, Berlin, Germany

ABSTRACT: The usefulness and versatility of a robotic end-effector depends on the diversity of grasps it can accomplish and also on the complexity of the control methods required to achieve them. We believe that soft hands are able to provide diverse and robust grasping with low control complexity. They possess many mechanical degrees of freedom and are able to implement complex deformations. At the same time, due to the inherent compliance of soft materials, only very few of these mechanical degrees have to be controlled explicitly. Soft hands therefore may combine the best of both worlds. In this paper, we present RBO Hand 2, a highly compliant, underactuated, robust, and dexterous anthropomorphic hand. The hand is inexpensive to manufacture and the morphology can easily be adapted to specific applications. To enable efficient hand design, we derive and evaluate computational models for the mechanical properties of the hand’s basic building blocks, called PneuFlex actuators. The versatility of RBO Hand 2 is evaluated by implementing the comprehensive Feix taxonomy of human grasps. The manipulator’s capabilities and limits are demonstrated using the Kapandji test and grasping experiments with a variety of objects of varying weight. Furthermore, we demonstrate that the effective dimensionality of grasp postures exceeds the dimensionality of the actuation signals, illustrating that complex grasping behavior can be achieved with relatively simple control.

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The passive compliance of the soft robotic RBO Hand 2 allows for easy in-hand manipulation. In this video the hand repeats a simple, pre-defined motion to turn a plastic ball it holds between its fingers.

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The capabilities of the soft fingers can be increased by being able to change their relative positioning. In this video, this is accomplished by arching the back of the hand with a single pneumatic actuator. In this prototype, this actuator is attached to the back of the hand, but it can easily be integrated into the palm. The video shows three automatic grasping sequences: 1) grasping without arching, 2) grasping after arching, 3) grasping without arching, then arching, releasing the grasp, and re-grasping.

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The video demonstrates the capabilities of a pneumatic soft hand when it is operated with a mass controller. Mass control enables the effective use of actuator compliance while maintaining continuous and reactive control of the complementary preset position. Additionally it gives an intuition on the attainable performance of mass control in terms of speed, stability and precision.

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