Topology synthesis based on FIS theory of double-layer parallel mechanism with all drives fixed

The manipulation of large workpieces often requires manipulators with complex structures and high stiffness to ensure stability and precision during operation. In situ processing equipment is typically employed for this purpose, comprising a mobile carriage (CGA), mechanical arms, and parallel processing units. The end mechanism of in situ processing equipment must exhibit high rigidity and a wide range of motion in order to effectively satisfy the high-efficiency processing requirements of large and complex structural components. This paper presents a comprehensive process for the topology synthesis of double-layer parallel mechanisms and conducts research on its topology synthesis. Firstly, a comprehensive analysis is conducted on the number and types of degrees of freedom required for basic tasks such as stretching, derotating, twisting, and grasping. This results in the simplest mathematical expression for continuous motion corresponding to the processing tasks. Subsequently, the double-layer superimposition principle of the parallel mechanism is elucidated in accordance with the requirements of the processing tasks. Proposed are anticipated motion patterns and allocation methods. Furthermore, the standard chains are analyzed and characterized based on the desired motion patterns. Derived standard chains that satisfy the desired motion patterns are obtained through joint equivalent transformations. Finally, the assembly conditions are determined in order to obtain the various available configuration structures that satisfy the processing requirements. This study proposes a novel manipulator design that can precisely control the motion of the end platform using only one set of drives, significantly improving the stability and precision of large workpiece manipulation, a challenge that has not been fully addressed in the existing literature. This provides a robust theoretical foundation for the subsequent development of in situ processing equipment.