Application of Neural Networks to Modeling and Control of Parallel Manipulators

There are mainly two types of the manipulators: serial manipulators and parallel manipulators. The serial manipulators are open-ended structures consisting of several links connected in series. Such a manipulator can be operated effectively in the whole volume of its working space. However, as the actuator in the base has to carry and move the whole manipulator with its links and actuators, it is very difficult to realize very fast and highly accurate motions by using such manipulators. As a consequence, there arise the problems of bad stiffness and reduced accuracy. Unlike serial manipulators their counterparts, parallel manipulators, are composed of multiple closed-loop chains driving the end-effector collectively in a parallel structure. They can take a large variety of form. However, most common form of the parallel manipulators is known as platform manipulators having architecture similar to that of flight simulators in which two special links can be distinguished, namely, the base and moving platform. They have better positioning accuracy, higher stiffness and higher load capacity, since the overall load on the system is distributed among the actuators. The most important advantage of parallel manipulators is certainly the possibility of keeping all their actuators fixed to base. Consequently, the moving mass can be much higher and this type of manipulators can perform fast movements. However, contrary to this situation, their working spaces are considerably small, limiting the full exploitation of these predominant features (Angeles, 2007). Furthermore, for the fast and accurate movements of parallel manipulators it is required a perfect control of the actuators. To minimize the tracking errors, dynamical forces need to be compensated by the controller. In order to perform a precise compensation, the parameters of the manipulator’s dynamic model must be known precisely. However, the closed mechanical chains make the dynamics of parallel manipulators highly complex and the dynamic models of them highly non-linear. So that, while some of the parameters, such as masses, can be determined, the others, particularly the firiction coefficients, can’t be determined exactly. Because of that, many of the control methods are not efficient satisfactorly. In addition, it is more difficult to investigate the stability of the control methods for such type manipulators (Fang et al., 2000). Under these conditions of uncertainty, a way to identify the dynamic model parameters of parallel manipulators is to use a non-linear adaptive control algorithm. Such an algorithm