Degrees of freedom/mobility of a mechanism: It is the number of inputs (number of independent coordinates) required to describe the configuration or position of all the links of the mechanism, with respect to the fixed link at any given instant.
Grubler’s equation: Number of degrees of freedom of a mechanism is given by
F = 3(n-1)-2l-h. Where,
F = Degrees of freedom
n = Number of links = n2 + n3 +……+nj, where, n2 = number of binary links, n3 = number of ternary links…etc.
l = Number of lower pairs, which is obtained by counting the number of joints. If more than two links are joined together at any point, then, one additional lower pair is to be considered for every additional link.
h = Number of higher pairs
Examples of determination of degrees of freedom of planar mechanisms: