Mobility is the number of degrees of freedom in a mechanism.
It corresponds to the number of independent parameters required to define the position of the mechanism in a unique way.
A mechanism is built from links.
Let L be the number of links. Each link has 3 degrees of freedom. In all, there is 3 times L available degrees of freedom.
One link is the ground, which is static, it consumes then three degrees of freedom.
Links are connected together with lower and higher kinematic pairs.
Each lower pair blocks two degrees of freedom. If j 1 lower pairs are used, 2 times j 1 degrees of freedom are consumed by the lower pairs.
Each higher pair blocks 1 degree of freedom. If j 2 higher pairs are used, j 2 degrees of freedom are consumed by the higher pairs.
Mobility M is then equal to 3 times L minus 1 minus 2 j 1 minus j 2.
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