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3. Synthesis of Mechanism Systems 3.1 Synthesis Methods for Mechanism Design

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Abstract The aim of this thesis is to provide a basis for efficient modelling and software use in simulation driven product development. The capabilities of modern commercial computer software for design are analysed experimentally and qualitatively. An integrated simulation model for design of mechanical systems, based on four different ”simulation views” is proposed: An integrated CAE (Computer Aided Engineering) model using Solid Geometry (CAD), Finite Element Modelling (FEM), Multi Body Systems Modelling (MBS) and Dynamic System Simulation utilising Block System Modelling tools is presented. A theoretical design process model for simulation driven design based on the theory of product chromosome is introduced. This thesis comprises a summary and six papers. Paper A presents the general framework and a distributed model for simulation based on CAD, FEM, MBS and Block Systems modelling. Paper B outlines a framework to integrate all these models into MBS simulation for performance prediction and optimisation of mechanical systems, using a modular approach. This methodology has been applied to design of industrial robots of parallel robot type. During the development process, from concept design to detail design, models have been refined from kinematic to dynamic and to elastodynamic models, finally including joint backlash. A method for analysing the kinematic Jacobian by using MBS simulation is presented. Motor torque requirements are studied by varying major robot geometry parameters, in dimensionless form for generality. The robot TCP (Tool Center Point) path in time space, predicted from elastodynamic model simulations, has been transformed to the frequency space by Fourier analysis. By comparison of this result with linear (modal) eigen frequency analysis from the elastodynamic MBS model, internal model validation is obtained. Paper C presents a study of joint backlash. An impact model for joint clearance, utilised in paper B, has been developed and compared to a simplified spring-damper model. The impact model was found to predict contact loss over a wider range of rotational speed than the spring-damper model. Increased joint bearing stiffness was found to widen the speed region of chaotic behaviour, due to loss of contact, while increased damping will reduce the chaotic range. The impact model was found to have stable under- and overcritical speed ranges, around the loss of contact region. The undercritical limit depends on the gravitational load on the clearance joint. Papers D and E give examples of the distributed simulation model approach proposed in paper A. Paper D presents simulation and optimisation of linear servo drives for a 3-axis gantry robot, using block systems modelling. The specified kinematic behaviour is simulated with multi body modelling, while drive systems and control system are modelled using a block system model for each drive. The block system model has been used for optimisation of the transmission and motor selection. Paper E presents an approach for re-using CAD geometry for multi body modelling of a rock drilling rig boom. Paper F presents synthesis methods for mechanical systems. Joint and part number synthesis is performed using the Grübler and Euler equations. The synthesis is continued by applying the theory of generative grammar, from which the grammatical rules of planar mechanisms have been formulated. An example of topological synthesis of mechanisms utilising this grammar is presented. Finally, dimensional synthesis of the mechanism is carried out by utilising non-linear programming with addition of a penalty function to avoid singularities. Keywords: Design, Simulation, Optimisation, Control Systems, Modelling, Computer Aided Design, Computer Aided Engineering, Multi Body Systems, Finite Element Method, Backslash, Clearance, Industrial Robots, Parallel Robots. On Multi Body Systems Simulation in Product Design Makkonen 1. Introduction 1.1 Background When designing mechanical systems, the dynamic behaviour is often difficult to foresee intuitively just from the major design parameters, e.g. from manufacturing drawings and bill of materials. In general, there is no possibility to synthesise a new product concept directly from the specified functional and performance requirements. The design process will then be iterative, one or several concept solutions that should meet the specified requirements are proposed by the designer and must then be analysed and verified, to determine if the specified functional requirements are satisfied. If not, the concept has to be modified or completely redesigned. In this iterative design synthesis-analysis loop, analysis for verification of product performance could be carried out in many different ways, e.g. by isolated functional rig tests in the laboratory, by complete prototype testing, by rough calculations or by mathematical modelling and computer simulation at varying degree of detail. Synthesis Design Creation process of information; Specification Design concept Data generation Specification change Concept change Analysis Calculation and testing Evaluation Data reduction Design solution Documentation Fig 1. Synthesis and analysis loop in design. Traditionally, prototype testing has been the predominating method for analysis and verification of product performance. Manufacturing of hardware prototypes and laboratory testing is however, in most cases, very time consuming and expensive. Hardware prototypes also suffer from the problem that the design parameter values, like geometry and configuration, main dimensions and materials are relatively fixed. The product concept can then be evaluated only for one set of parameter values at a time, i.e. in one point in the design parameter space. If the products performance does not meet the specification, the hardware parameters contributing to the non feasible performance must be identified and varied until a satisfactory solution has been obtained. For each parameter change, the design has to be changed and the prototype must be modified. Mathematical modelling and computer simulation of product concepts and detail design solutions, for prediction of product properties and performance, has been used since the mid-sixties. At that time, time-consuming programming and debugging of product specific code, usually in FORTRAN, had to be developed. For this reason, simulation was most often not considered an alternative for evaluation during the design of new products. Computer simulation was then used primarily for 1 On Multi Body Systems Simulation in Product Design Makkonen refinement and optimisation of mature, ”static” and long-life product families. It is not until the last decade, since the introduction of advanced generalised software packages for CAE (Computer Aided Engineering) and of high-performance work stations, that simulation has proved to be a powerful tool in new product design. The possibilities for rapid development of product specific and product family specific parametrised models has then simplified modelling work. There is now a considerable potential for shortening the cycle time during design iterations. By speeding up the synthesis-analysis loop by means of powerful computerised analysis tools, computer simulation could now be used also during the early conceptual design phase, for quick evaluation, modification and optimisation of design concepts. Current general CAE software packages are however very complex, and it is not obvious for the designer or for an industrial corporation how to fully utilise the potential of these new tools. Several important questions are still under discussion in industry, e.g. whether an integrated environment or a distributed simulation with communication between different types of simulation software should be preferred, if simple geometry models should be created directly in the simulation software or if a CAD solid model always should be used as a master model for all analyses etc. There is obviously a need for a systematic approach in modelling and simulation. The use of simulation software should be different during the various design phases for a product development project. Design theory and product development models from design research have an obvious potential and should be used as a basis for an improved and efficient methodology and approach to modelling and simulation in the design of mechanical products. The optimal combination of simulation and physical prototype testing is another important issue in industry. Simulation results should be utilised to minimise testing effort and to concentrate the expensive testing to the most relevant cases. Test results on the other hand, should be used to improve mathematical models of basic or product specific phenomena and to incorporate empirical knowledge in the simulation models. This situation in industry, concerning the potential - that is still far from fully utilised - for modelling and simulation as a tool for efficient and rapid development of mechanical products, formed the basis for this PhD project at the Department of Machine Design at KTH. 1.2 Scientific approach The objective of this thesis work is to study how multi body systems simulation should be efficiently used in product development. Multi body systems simulation is a modelling technique closely related to other similar modelling techniques in Computer Aided Engineering (CAE). A study of the relative strengths and weaknesses of these methodologies is then motivated, to determine the most feasible modelling methodology
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