Control concept for PKM considering the mechanical coupling between actors

In machines with parallel kinematic structures, the actors will interfere with each other depending on the mechanical setup. Thus, movements of one actor will directly cause disturbances on the other actors and position errors at the tool centre point. Within a research project it was shown by simulations and measurements on a hybrid kinematic machine that this interference limits the possible control loop settings.The first part of this paper discusses the possibility to increase the damping of a parallel kinematic structure, if the transformation of the kinematic coordinate systems is moved into the control loops. If this shift leads to an increased damping in the control loops, it can reduce the influence of the mechanical coupling. The project results yielded that the transformation in the control loops only has scaling effects and that these effects are different with regard to every specific transformation. Thus, the results are not commonly applicable for machine tools and manufacturing.The second part describes another research approach to increase the damping. The coupling of the actors causes the force of the first drive to act as a disturbance on the second drive. With the known disturbance and the use of a feed forward control the position error due to the coupling can be reduced by nearly 60%.