Du Xu and Xinjiang Lu
Assembly, compliant wrist, robot, variable stiffness
Automatic assembly using a manipulator often produces a large contact force, owing to the deviated position and angle, which occurs because the manipulator is fully rigid. This large contact force can cause assembly failure and even damage the manipulator itself. To avoid this potential problem, we have designed a continuously variable stiffness wrist to achieve reliable assembly. The wrist consists of four hyper-elastic nickel–titanium (Ni-Ti) alloy wires, two guide rods, one screw, one slider, and one servo actuator. The guide rods and the screw are rigid and only connected with the robot. The four Ni-Ti alloy wires are flexible and designed to connect the robot and the assembled shaft. When the slider moves along the screw driven by the servo actuator, the upper part of the slider remains rigid. This is due to the rigidity of the guide rods and the screw. Comparatively, the lower part remains flexible owing to four super elastic and slender Ni-Ti wires (length/radius >10). The stiffness of this designed wrist can continuously change with the position of the slider, producing translational and rotational deformation. Due to this variable stiffness and deformation, the contact force is significantly reduced. After design and fabrication, the stiffness of the model was then derived and analysed. Finally, experiments assessing a robot assembly with this designed wrist were conducted, using different position deviations to demonstrate its reliable assembly performance.
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