This paper studies planar motion approximation problems in the context of a kinematic mapping. Since a planar displacement is determined by three parameters, it can be mapped into a point of a three-dimensional space. A (single-degree-of-freedom) planar motion can, therefore, be represented by a space curve in the space of the mapping and the problem of motion approximation becomes a curve fitting problem in this space. A mapping introduced by Blaschke is used and a general theory for planar motion approximation is developed. The theory is then applied to dimensional synthesis of four-link mechanisms. Furthermore, since the structural error (i.e., the quality of motion approximation) is dependent on the closeness of the fit in the space of the mapping, a general algebraic theory for determining closest fits to points in this space is developed. The theory is illustrated by a numerical example.

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