Parallel mechanisms (PM) with two rotations and one translation (2R1T) have been used extensively. The parallel module of the Exechon robot is one of the most successful cases, which is equal to a 2-SPR-UPR PM. Few researchers mentioned what would happen to the PM's DOF if the arrangement of limbs were changed. Using screw theory, we investigate the mobility of the 2-SPR-UPR PM with two different arrangements of limbs. It is shown that the 2-SPR-UPR PM has two DOFs when its limbs are arranged to be symmetrical or to be parallel.
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