Design and analysis of a hybrid serial-parallel manipulator

This work presents a novel design of a hybrid serial-parallel Stewart like mechanism (HS-PM). This design presents a compromise between high rigidity of-fully parallel manipulators;and extended workspace of serial manipulators. This mechanism is made of a base and two platforms in series. The first platform, called the mid platform is linked to the base by three legs in parallel. Its motion is restricted to only three translations using three successive prismatic joints linking the mid platform to the base. The second platform, called the top platform, has a spherical joint with the mid platform and three legs in parallel are used to impart the relative motion between these two platforms. Therefore, the top platform has 6 degrees of freedom with respect to the base. The nature of this design presents several advantages. Firstly, a closed-form solution is obtained for this type of mechanism allowing the determination of all possible solutions of the forward analysis problem. This characteristic is a great help in investigating the capabilities of this type of platforms and it is a valuable tool for the engineer to derive its controller. It is also shown that for the lower platform, formed by the base and the mid platform, a maximum of two solutions exist, whereas eight solutions were found for the upper platform, which is made from the mid platform and the top platform. This second result confirms the numerical solution of the upper platform found in recent literature. Secondly, this design presents the particularity of having the size of the orientation workspace independent of the position of the top platform, unlike the fully parallel mechanisms. This characteristic is achieved by separating the actuators responsible for each type of motion. Indeed, the three legs of the lower platform are responsible of the linear displacement of the top platform, whereas the three legs of the upper platform cause the change of orientation of the top platform. Finally, a numerical example is treated to demonstrate our analysis and to determine the range of permissible extension of the legs. (C) 1999 Elsevier Science Ltd. All rights reserved.