An overconstrained mechanism is a linkage that has more degrees of freedom than is predicted by the mobility formula. The mobility formula evaluates the degree of freedom of a system of rigid bodies that results when constraints are imposed in the form of joints connecting the links.
If the links of the system move in three dimensional space, then the mobility formula is
where N is the number of links in the system, j is the number of joints, and fi is the degree of freedom of the ith joint.
If the links in the system move planes parallel to a fixed plane, or in concentric spheres about a fixed point, then the mobility formula is
If a system of links and joints has mobility M=0 or less, yet still moves, then it is called an overconstrained mechanism.
Read more about Overconstrained Mechanism: Sarrus Linkage, Bennett's Linkage
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