The dynamic model of a manipulator described by second-order differential equations suggests that the couplings between its links are strong. Consequently, any control task (position regulation or desired trajectory tracking) is imprecise. Hence, the decoupling of dynamic equations in robotic mechanisms has attracted many researchers in recent years and various solutions have been proposed. This paper deals with a new dynamic decoupling principle, which involves connecting to a serial manipulator a two-link group forming a Scott-Russell mechanism with the initial links of the manipulator. The goal is to simplify the controller design by reducing the effects of complicated manipulator dynamics. The opposite motion of links in the Scott-Russell mechanism combined with optimal redistribution of masses allows the cancellation of the coefficients of nonlinear terms in the manipulator's kinetic and potential energy equations. Then, by using the optimal control design, the dynamic decoupling due to the changing payload is achieved. The proposed approach, which is a symbiosis of mechanical and control solutions, improves the known design concepts permitting the dynamic decoupling of serial manipulators taking into account the changing payload. The suggested design methodology is illustrated by simulations carried out using ADAMS and MATLAB software, which have confirmed the efficiency of the developed approach.

It is known that a parallel manipulator with a singular configuration can gain one or more degrees of freedom and become uncontrollable. That is it might not reproduce a stable motion under a prescribed trajectory. However, it is proved experimentally that there is possible passing through the singular zones. This was simulated and shown on numerical examples and illustrated on several parallel structures. In this paper, we determine the optimal dynamic conditions generating a stable motion inside the singular zones. The obtained results show that the general condition for passing through a singularity can be defined as follows: the end-effector of the parallel manipulator can pass through the singular positions without perturbation of motion if the wrench applied on the end-effector by the legs, and external efforts of the manipulator are orthogonal to the twist along the direction of the uncontrollable motion. This condition is obtained from the inverse dynamics and analytically demonstrated by the study of the Lagrangian of a general parallel manipulator. Numerical simulations are carried out using the software ADAMS and validated by experimental tests.