The determination of virtual constraints is always one of the key and difficult problems in traditional mobility calculation.To make mobility calculation simple,considering avoiding virtual constraints,some new formulae have been presented,however these formulae can hardly intuitively reflect general link group’s restrictions on output member and its influences on independence of output parameters,which is premise to the judgment of the properties of mobility.Towards the problem to reveal the intrinsic relationship between the degree of freedom(DOF)of a mechanism,the link group,and the dimension of output parameters,also to avoid determination of virtual constraint,based on the new concepts of the"DOF of general link group"and"node parameters",a new formula in the calculation of the mobility of mechanisms is presented that is expressed with DOFs of the general link groups and rank of motion parameters of base point of the output link.It is named GOM(mobility of groups and output parameter)formula.On the basis of new concepts of"effective parameters"and"invalid parameters",a rule is put forward for solving the DOF of mechanisms with invalid parameters by GOM formula,that is,the base point parameters are the subset of effective parameters of link group.Thereafter,several examples are enumerated and the results coincide with the prototype data,which proves the validity of the proposed formula.Meanwhile,it is obtained that the necessary and sufficient condition for the judgment of output parameters independence is that each of the DOF of the link group is not less than zero.The proposed formula which is simple in calculation provides theoretical basis for the judgment of independence of output parameters and provides references for type synthesis of novel parallel mechanisms with independence requirements of their output parameters.
