Analysis and design of parallel mechanisms for manufacturing

This thesis is focused on the analysis and kineto-static behavior of Parallel Kinematic Machines (PKMs) for manufacturing applications. The scope is to provide general tools and optimization criteria to support the design of PKMs, from the very first phases of the conception of a new architecture, given the industrial requirements and process needs. Furthermore, the theoretical studies here introduced have been applied to the design of two PKM prototypes.
In particular, the thesis is organized as follows:
Chapter 1 is intended to give a brief overview on the existing PKM architectures in the manufacturing fields, and their main characteristics are discussed with reference to traditional multi axis machining and serial devices. Many issues, such as optimisation, dynamics and control, calibration and error compensation, are still research topics and the general concepts underlying these researches are introduced.
Chapter 2 addresses the problem of kineto-static optimization of PKMs. First, the kinematics and statics are discussed and an analysis methodology for the workspace evaluation is presented. Then, the stiffness mapping and singularity analysis is described. Finally, the kineto-static performance indices are introduced, highlighting the importance to have ‘non dimension sensitive’ indices and introducing the so called ‘isotropy indices’. Several examples are provided to compare commonly used performance indices with the newly introduced isotropy indices.
In Chapter 3, the isotropy criterion is applied to the conceptual design of two PKMs addressed to different industrial applications. The selected architectures – a 3 dof Tsai like mechanism, and a 6 dof hexapod – are optimised, in order to obtain an isotropic behaviour of the machine overall the workspace. The latter architecture has been considered suitable for applications in the sector of 5 axis machining, and its conception have been carried on within the framework of a National Research Project on Innovative Production Systems, Theme 1 (SPI1). Different industrial segments are investigated and finally the machine specifications are given. Conversely, the optimised Tsai like architecture is intended to exploit the advantages of PKMs over serial robots, in term of higher stiffness at the tool tip, and better payload to weight ratio, and it is devoted to the shoe manufacturing industry (leather machining). Indeed, this PKM constitutes a result within the National Research Project on Innovative Production Systems, Theme 6 (SPI6). The industrial context is described and then the final layout of the Dragon Fly PKM is obtained, in terms of position of the joints’ ideal center and legs stroke. However, the conceptual design is not enough to completely characterize the mechanical behavior of the machine, especially when the legs are subjected also to torsion and flexural loads.
Thus, in Chapter 4 the mechanical design steps of the Dragon Fly prototype are described. In the first part of the Chapter, several analyses are performed, in order to evaluate the actuator efforts, due not only to external loads but also to gravity and inertial effects and to compute the internal loads. Furthermore, the effects of manufacturing and assembling errors on the accuracy of the machine and, dually, the compliance at the tool tip due to lumped compliances in the structure are estimated, to verify if the prototype meets the initial requirements. The main mechanical design issues are introduced, and a FEM analysis is performed on the kinematic chain: joint-leg-joint to individuate the critical elements to stiffen. Next, some considerations on the side roughing and gluing work cell are done and its whole control system is briefly described. Finally, a calibration strategy is proposed to evaluate and compensate the predictable kinematic errors.
Chapter 5 will draw some conclusions forwarding the future developments of the work here presented.
Appendix 1 gives an up to date review of the most successful PKMs, and of the most promising ones, ending up with a comparison table specifying the available technical characteristics of 26 prototypes.
Appendix 2 presents some pictures of the Dragon Fly prototypes, during the assembly phase and then installed in the pilot plant, final result of the research project SPI6.