A novel algorithm is proposed to solve the direct kinematics problem of arbitrary planar three-legged parallel mechanism composed of revolute joints R and prismatic joints P based on conformal geometric algebra(CGA). It is proved that there exist 1 140 possible forms of planar three-legged parallel mechanisms composed of revolute joints R and prismatic joints P by using combinatorics
and the equivalent mechanism model covering all these 1 140 forms is presented in terms of the constraints of each leg length. The mathematical model of the equivalent mechanism is built by using the expressions of rotation and translation under the CGA framework. Using the simple linear elimination
the symbolic input-output equation with 6th degree can be deduced from the mathematical model and all solutions can be obtained. Two numerical examples are given to demonstrate the efficiency of the algorithm based on Maple16. The results show that this algorithm can simplify the computation and is suitable for computer programming.
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