A robotic manipulator should be able to obtain an arbitrary orientation of the end-effector, action which needs a wrist mechanism with minimum two independent rotations about two intersecting axes, generally perpendicular. If the task performed by the manipulator is more complex, it may be necessary for the end-effector to rotate about its axis, it means that the wrist mechanism needs three degrees of freedom. Planetary bevel-gear trains can be used in the design of wrist mechanisms. The wrist mechanisms using bevel gears have been shown to have some advantages. More, a nutating gear drivetrain has some advantages over a conventional planetary gear train, including: a decreased number of drivetrain parts, an ability to use lower-strength gear materials, a higher overall reliability, and a decreased manufacturing cost. Based on some nutation geared mechanisms using bevel internal gears having small difference in numbers of teeth, in this paper some robotic wrists will be proposed.
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