Component Based Mechatronics Modelling Methodology R.Sell, M.Tamre Department of Mechatronics, Tallinn Technical University, Tallinn, Estonia ABSTRACT There is long history of developing modelling systems in the fields like mechanics, electronics and control. Modelling a mechatronical system needs a sophisticated approach in modelling methodology especially at early stages of the design process. There are modelling tools in the market for mechatronics systems based on the general physical similarity principles of both mechanical and electrical components. It is well known that most of the later design constraints are designed into a product at the very first stages of the product development process. Therefore the concept design stage is of main interest though with strong links and sights to prospective end product. 1 INTRODUCTION It is generally realised that up to 70% of a product s cost is being determined during the first design phase and only 20% during the actual production. Improvements can be reached by avoiding improper component selection, too complex part design, etc. Thus a good starting point for resources reduction and quality improvements within a product life cycle is the very firs phase of the idea formation, and it become essential to consider operability and manufacturability early in the design process (1). The research on mechatronic systems has traditionally focused on functionality, performance, and dependability. However, as the systems become increasingly more complex and widely used, complexity and flexibility becomes critical as well. Currently, for many industry products, the success of complexity and flexibility management turns out to be one of the major competitive edges. In general, complexity management is concerned with how to make a system comprehensible, communicable, and analyzable. It is a quality attribute that impacts all activities in system development, maintenance, and modification, etc. Flexibility refers to the ability to make changes with respect to functionality, performance, dependability, or target platforms quickly, easily, and cost
effectively. It is important due to cost, time, changes in technology, as well as other practical reasons (3). The aim of the work is to analyse the specifics of a mechatronics system design early stages, to outline the general design features and to develop an approach could be used through all the design phases. The paper discusses initial conditions and constraints at developing methodology for mechatronics modelling. Considering the methodology the approach for creating an environment for mechatronics modelling are introduced and discussed. The main emphasis of the approach is in creating an object-oriented environment, which allows integrating different components from different engineering fields employing advantages of the object-oriented design. 1.1 Model Based Methodology The developed methodology is bases in great extent on the model based reasoning (MBR), which is an inferring process using models abstracted from the reality of a physical system. MBR exploits the symbolic processing of an explicit representation of the internal working of a system in order to predict, simulate and explain the resultant behaviour of the system from the structure, causality, functional and behaviour of its components (2). The advantages of the MBR are that developed systems or sub-systems can be reused in the design, analysis, simulation, diagnosis and prediction of a technical system. This ensures the quality and consistency of a solution in carrying out a task. 1.2 Phases of development Currently, the most widely used overall process model is the V-lifecycle model (Figure 1.) (3) The model is based on the traditional waterfall process model but has a set of useful properties in description in the sense that relations between different design stages can be easily illustrated. Fig. 1 A conceptual illustration of the V-lifecycle The effects of using a component-based system development are depicted as short circuit. This paper focuses mostly on the first and the second phase of V-lifecycle (conceptual phase) as emphasised on above. Our main goal is to find an efficient tool to help designers in this phase.
2 METHODOLOGY There are several modelling methodologies in the field of mechatronics under the development today. Most of them base on heuristic or systematic approach. There are many limitations of this kind of methodologies. The model-based approach to design has several advantages over the traditional heuristic approach. 2.1 Background of the methodology The main idea of the methodology is to develop an approach dealing with up-down synthesis of multidomain mechatronics system involving components for the whole mechatronics domain. The interaction of the components is realised by a specific universal interface. Developing a new mechatronics system, designer can use existing components or create new ones. The approach is discussed on the basis of applying agent systems for feedback and neural networks as the main tool. The created methodology involves description rules for creating mechatronics components and universal interfaces for communication. This helps the designer to create more reliable products with fewer resources. The whole process and decisions can be documented automatically. The new methodology should increase productivity, maintaining a complete overview over the design process at the same time. 2.2 Description of the methodology The methodology exploits UML language to define a mechatronical problem and the constraints. The model is generated by the neural network scheme, which is also responsible for the result generation (6). Pre-defined components and rules database is used by the neural network and this scheme creates capability to develop new components concurrently. An open standard like XML is used to describe components and interfaces in the software environment. Pre-defined components and userdefined components are associated with mathematical functions in form compatible with MatLab. Problem definition Constraints DB of principles Verifiction Adjustment Modifications Specification Fig. 2 Methodology diagram
2.2.1 Problem definition An engineering problem is described in very simple and robust way. The UML - open standard language is used supported by the mechatronics profile. This is convinient and intuitive way to describe an engineering problem have to be solved. Additionaly the design limitations and basic rules have to be described on this stage. UML describes system interactions with the user and environment (use case model), system statics (class model), system dynamics (sequence, collaboration, state, activity) in this stage (see Fig. 3 and 4). The UML profile for mechatronical applications, in development stage currently, is a necessary tool for this methodology. Approach above has some analogy with the UML profile for business modeling (5). The approach gives opportunity to concentrate for basic needs and constraints of the system and describe these in a simple and intuitive way. Unknown or uncertan information can be discarded in this stage. Fig. 3 Case study use of a Mobile Robot
Fig. 4 State diagram of the system model 2.2.2 Solution generation Next step following the problem and limitations adjustment is generation of a set of potential solutions, which meets the problem definition and limitations. The generation process is realised by the neural network, which exploits rules and pre-defined components from the meta-model database. The neural network schema attempts to find the system specification optimized as a whole. The result from the neural network loop is an unique combination of pre-defined components and/or completely new components created concurrently consisting of combined sets of predefined properties as carriers of certain component sub-functions. The result is represented, as a block diagram of the mechatronic system compatible in structure with bond graphs. The basic elements in a block diagram are components and connections between them. The components have universal energy flow ports, which connects them to and from the outside world. A component can contain sub-components or mathematical functions and can have different realizations with different description as long as the basic functionality and the port specification are identical. The result diagram describes system principles and basic solutions. 2.2.3 Adjustment The adjustment stage gives the possibility to modify the generated solution. The result of the previous stage is adjusted by the designer discarding evidently inappropriate solutions and adding descriptions of additional requirements for the system if needed. Missing or additional functionality parameters revealed on the last run can be added and iteration parameters adjusted. 2.2.4 Verification The modified result from the previous stage is verified and the result of the verification is generated. Verification process tests the integrity of the system and conformity of the initial requirements. If these conditions are not satisfied the engineer has to modify the result of the previous stage, block diagram or initial requirement. To modify block diagram, there is two possible ways: the system offers verified solution, which meets changes done by the engineer or engineer re-modifies the diagram by himself. The overall result of the process is a specification of the system optimized block diagram that meets the initial requirements, specified at the very beginning by UML. Onward design process is not supported by this methodology at the moment. This can be done by well-known software like 20- sim, Dymola, etc. Key benefits of our approach: Faster product development process More intuitive - meets better human thinking manner Does not depress the creativity Modular meets the changing requirements. Easy to handle complex systems System quality is easier to determine Automated documentation (created by software)
3 CONCLUSIONS Described approach follows natural way of human working and thinking. The methodology follows practical reasoning, which means step-by-step advancing. Practical reasoning includes solution for two important questions: What are our goals? and How to reach these goals?. The new approach is trying to solve these questions by following human thinking and working, using at the same time the well-known UML standard as a goal descriptor and neural networks as a solution generator. The further aims are to prove the quality of the system and use the multiagent system for more complex solution. REFERENCES 1 Bauert F., Chen W. J., Green (1992) Knowledge based assistance to support design for Manufacture, ECAI Workshop of Concurrent Engineering. 2 Monet European Network of Excellence. Artificial Intelligence into Industry, Model Based Systems FAQ (2002). http://monet.aber.ac.uk:8080/monet/technology/faqs.htm#mbr 3 Chen D.J. (2001) Architecture for Systematic Development of Mechatronics Software Systems. Licentiate Thesis, Royal Institute of Technology, Stockholm, 117 p. 4 Weiss G., ed. (1999) Multiagent Systems - A Modern Approach to Distributed Artificial Intelligence, The MIT Press, 620 p. 5 Unified Modelling Language Specification (2002) OMG, 737 p. 6 Rafiq, M., Y., Bugmann, G., Easterbrook, D., J. (2001) Neural Network Design for Engineering Applications, Computers & Structures Vol. 79 pp.1541-1552. 7 Amerongen, J.,V., Breedveld, P. (2001) Modelling of Physical Systems for the Design and Control Mechatronic Systems, IFAC PROFESSIONAL BRIEF, pp. 8-18.