Assembly Planning of Mini and Micro Products Enhanced by an Integrated Product and Process Model

In recent years, a significant growth of mini and micro products’ market shares has been detected. However, the assembly systems and assembly processes of larger products cannot be transferred to mini or micro products in an acceptable way, which results of the special physical properties of small components. Apart from new materials, special problems with handling and tolerances of these products arise compared to larger products. Further more, new problems like adhesion forces come up when handling products outside of the dimension of macro systems. In order to embrace these requirements the total system’s behavior and characteristics and the single parts’ behavior and characteristics have to considered in product design as well as in assembly planning. This paper presents a new approach towards the improvement of assembly planning of mini and micro products by an integrated product and process model. With this model, it is possible to regard and to evaluate the critical design and process parameters like material properties of sintered parts over the whole production planning process. The integrated product and process model is based on the SADT method (Sequence Analysis and Design Technique), which enables to consider all necessary information about the products’, components’ and processes’ behavior and characteristics. INTRODUCTION The constantly growing market share of mini and micro products and components causes a rising of requirements of suitable production facilities. As a consequence, a transition from manual assembly towards automated assembly has been detected in order to increase the productivity of the production. Therefore, the production activities have to show an acceptable quality level. The present assembly processes of larger products cannot be transferred to mini and micro products in an adequate way, thus, it is necessary to develop new methods and procedures to consider the specific properties of small components in the production process. A new integrated product and process model can support the planning of the assembly process to solve the typical problems of these product classes. Mini and micro products At present, there is no consistent definition about the dimensions of mini and micro products [1]. The following definition can be considered as a common understanding among all authors. Mini products are components whose dimensions are in the range of millimeters. So, these products adjoin to fine mechanics on the one hand and to micro products on the other hand. Consequently, the dimensions of micro products are in the range of micrometers. The border between mini and micro products is not exactly specified, as the transitions are fluent. Further more, mini and micro products combine several functionalities on smallest range. Saarland University, Institute of Production Engineering Process Planning In process planning as a part of assembly planning, the assembly processes and their sequence are defined. As a first step, the design-oriented engineering bill of material (E-BOM) is transformed into a production oriented-manufacturing bill of material (M-BOM). Based on the components and single parts, the technological activities to join these products are determined. Further more, these activities or processes are arranged according to their sequence. Part Production In this paper, the term part production implies the addition of all manufacturing operations, not only the result of the finishing operation. In this case, all technologies that are involved in the single part production have to be considered. STATE OF THE ART Design The class of mini and micro products is still a young discipline. The use of established software solutions for designing macro products is basically possible, for example CAD or FEM. But standards are not yet defined and state of the art often changes. The technologies in materials, single part manufacturing, assembly processes and assembly systems are still not established. This causes severe problems for the designer, as optimal design often is not possible. Assembly Planning In general, production planning and also assembly planning as a special area of production planning are only sporadically supported by IT-tools. For example, the motion path and speed of an assembly process can be visualized and optimized by Digital Mock-Up (DMU). In addition, the layout, dimensioning and optimization of a technological process can be supported by process simulation [2]. However, process simulation only concerns manufacturing processes like cutting or deep drawing. Complex assembly processes like welding or gluing are only rudimentarily supported by process simulation tools. Thus, there are a lot of open questions how to plan complex assembly processes of mini and micro products. A systematic method regarding all necessary influences of the product, of its operation behavior and of the technological process itself is needed in assembly planning. At present state, planning activities are largely accomplished with the aid of pencil, paper and office software like Excel and Powerpoint. In this way, a methodology that supports the planning activities should be applicable without complex and complicated software tools. The planner has to define what should be produced or assembled, how and wherewith [3]. Thus, based on the product structure in the manufacturing bill of materials, the assembly processes that are necessary to produce the product are determined. Further more, the required resources are allocated to the assembly processes. In this so called PPR-view (Product, Process and Resource view), no interactions between the objects are considered. For example, there is no information about the quality and quantity of the influence that certain product parameters have on the assembly process. Information about the interaction of a process with preliminary and downstream processes is also not available. In assembly planning of mini and micro products, the assembly systems also play an important role because the systems that are used for the production of larger products can not be miniaturized in an ordinary way. Saarland University, Institute of Production Engineering Assembly Systems The assembly systems have to realize the guidelines that are created by the process planning. The required assembly devices like conveyors, transport facilities and handling devices should be integrated. The know-how from planning macro assembly systems is not applicable in a simple way, because the influence of the micro parts’ properties are much more critical than in macro systems [4]. Similar problems occur when configuring assembly processes like welding or soldering. At the moment, there are only few relevant publications to this topic, since we are talking about a young domain. Most publications concerning mini and micro systems treat for example specific handling problems [5] or solutions that are adapted especially to one product. The forces when handling such a part are very small and the parts are easy to damage. Complex shapes also complicate the handling, e.g. oriented supply or deposition. Thus, the shape of the gripper must be adapted. An additional problem is caused by adhesion forces, as the single part should be easily detached from the gripper. So, detailed product and process information is necessary for setting up the assembly system. stable assembly processes are one important aspect of assembly systems. High quality is required, because this quality is part of the total product’s quality. The quality of the assembly processes is also responsible for the product’s properties in later operation. The choice of the assembly technology depends on the function and material of the components. In mini and micro dimensions additional influences have to be evaluated. One main problem is the lack of standards. Materials, single part manufacturing and process technology are about to make considerable progress. However, there are no experiences in applying these technologies in large batch production. The main problem of any assembly process is the change of the single part’s properties or the degradation of this part. This can be a result of the influence of temperature. Thus, the position of the single part should not be changed by forces that appear in the assembly process and the junction should be consistent. The causes of these difficulties are insufficient knowledge about the effects of the process and inaccurate process settings, because there are no reference data. Other reasons for process problems can be put down to the single part manufacturing. An example for such a process is an incontrollable sinter process that does not create an optimal structure. Advanced, but not sophisticated micro chipping technologies for single parts enlarge the design fetch concerning dimensions and material. This results in changes in edge layer areas or residual stresses that can generate instabilities in junction areas. All these aspects have to be evaluated in an holistic way, so an integration of product and process parameters in assembly planning is needed. METHODS AND TOOLS Before explaining the new approach towards an integrated product and process model, the SADT method that underlies this model is presented. Further more, the vision of the Digital Factory, a virtual representation of a real factory, is presented. SADT System analysis and process modeling are important aspects of understanding and dimensioning complex processes. A suitable, parameter-oriented method for this purpose is modeling according to the Structural Analysis and Design Technique (SADT) that integrates product and process data in one model. An important aspect is the division of the problem by a top-down method. One great advantage of this method is an increased level of detail. This helps the planer to detect little influences that can cause severe problems. The structure Saarland University, Institute of Production Engineering determined for the designed model divided into subsystems, elements, connections, mechanisms, control data and input/output data presents important characteristics of the system. In past projects, this model was modified and enhanced [6] (Fig 1). The control data are divided into two classes, influencing data and the classical control data. Control data can be set or changed within an activity by the user, influence data affect the result of the process, but they cannot be changed within this step of activity, although they can be altered eventually in preceding steps. Input data are transformed into output data by the activity or they are consumed by the activity. Data that are not changed in the viewed activity are assigned as input data to the one of the following activity steps that will change this data. Mechanisms are instruments that support the activity. Examples for mechanisms are machines, tools or handling devices. Output data characterize the result of a process step and serve as input or influencing data in following process steps or they represent final product requirements. So, many outputs must end as inputs or control data in a subsequent process step or as a final product characteristic. input output influence manipulated variables