Integrated Cost Estimation Based on Information Management

Transcription

1 University of Twente; Laboratory of Design, Production and Management Integrated Cost Estimation Based on Information Management Hubert Kals; Erik ten Brinke; Eric Lutters; Ton Streppel Abstract: For adequate Cost Engineering, information generated and affected by different engineering tasks as well as effective communication are prerequisites. In the context of Concurrent Engineering, Cost Engineering requires an information management system. The Manufacturing Engineering Reference Model incorporates an appropriate basis for the management of cost information. All cost information is stored, based on information structures related to the Reference Model. This enables the storage of cost information differentiated according to cost driver and aggregation level. An important representation aspect is the ability to construct different views with the same information. For different types of cost estimation, the role of information management and the application of the information structures are described. An architecture for cost estimation, employing a previously developed information management method, is proposed and the use of this architecture is explained. It appears that the employment of the cost estimation architecture and the application of the information management method make a truly integrated cost control system possible. Keywords: Computer Aided Engineering, Cost Estimation, Information Management, Concurrent Engineering Introduction Concurrent Engineering, the simultaneous execution of shared tasks by separate departments, has emphasised the need for good interaction and communication between diverse disciplines. The basis for adequate communication is availability and accessibility of information. In particular, meaningful representations of information reflecting the current state of affairs are more desirable than the sole exchange of data [1]. Cost Engineering must use information covering the entire product life cycle. This information is generated and affected by different engineering tasks like design, process planning and production planning. Since all the information required for cost engineering is not always available at the desired time, historic information is also of major importance. For cost engineering in particular, but also for the control of the entire engineering cycle the use of an information management system is indispensable. Recognition of the fact that information management is a key item in the control of the engineering life cycle has lead to new perceptions about the structure of, and the interaction between, the engineering tasks. This paper concentrates on these aspects from the point of view of costs. The Manufacturing Engineering Reference Model [1] is used as the basis for information management. The principles of information management and the structuring of cost information related to this model are explained. The role of information management and the application of the related information structures are described for different types of cost estimation, being variant cost estimation, generative cost estimation and hybrid methods of cost estimation. After considering the principles of cost control, an architecture for cost estimation is presented. The use of this architecture and its interaction with information management is explained. Because information management is put centrally in the development of an integrated cost estimation system, it is possible to achieve integration in the entire engineering cycle. This surpasses partial solutions as the integration of process planning and cost estimation [2] or the integration of cost estimation and CAD/CAM systems [3]. The architecture for cost estimation is general applicable, it is not based on e.g. one cost estimation method [4], one production environment [4], one product type [3], one product [5] or one production process [2]. Information management In order to deal with the availability of meaningful representations of information, reflecting the current state of affairs, the manufacturing system is represented by a reference model. A reference model represents a system as an organisation in terms of relatively independent, interacting components and the globally defined tasks of these components. The Manufacturing Engineering Reference Model is depicted in fig. 1; it emphasises the equivalent importance of products, orders and resources in the manufacturing cycle. Fig. 1: The Manufacturing Engineering Reference Model Company Management is responsible for the strategic objectives of a company. It determines the range of products and the required processes and resources, and it controls the customer orders. Product Engineering performs the design and development of a product type from functional requirements up to the specification of the final recycling or disposal. The in-time execution of orders is the task of Order Engineering, which determines the production sequence and the applied resources. Resource Engineering refers to all activities concerning the specification, design, development, acquisition, preparation, use and maintenance of resources. The production plans generated by the engineering tasks are actually executed by Production. The kernel of the reference model is

2 Information Management; it controls the accessibility, availability and different representations of information. In correspondence to the reference model, three information structures are distinguished, namely the Product Information Structure, the Resource Information Structure and the Order Information Structure (fig. 2). Each structure has different types of objects to which information is attached. For the Product Information Structure, the objects are depicted in fig. 3. The structures can evolve independently, while their relationship remains the same. Because of this, the entire range of manufacturing environments can be described. In an engineer-to-order environment, the Order Information Structure and the Product Information Structure can evolve simultaneously while the Resource Information Structure can remain relatively unchanged. this way, a product can be described with the fundamental structure depicted in fig. 4. An element is part of an aspect system representing a product (e.g. functional system, physical product definition). The number of aspect systems is limited and, within the information management model, they are referred to as domains. An information structure can have several domains. Fig. 4: The fundamental structure Fig. 2: The three information structures The Resource Information Structure contains the abilities, occupation and condition of all the resources in a company at any given time. The Order Information Structure deals with all the information that indicates which product element is manufactured when and with which resources. The Product Information Structure contains all the information concerning a product type. For a finished product, it contains for instance the description of function, materials, dimensions, tolerances, used resources, assembly sequence, production times, costs, etc. Because different interpretations of a sovereign domain are possible, the concept of different views is introduced. A view furnishes a focussed, partial representation of the information in a certain domain of an information structure. For example, a designer and a process planner can visualise different interpretations of the same geometry with the help of a view. The design view shows e.g. the design features while the process planning view shows manufacturing features. The features are based on the same geometrical elements, but their arrangement is subjective (fig. 3). To be able to focus on a part of all information, filters are used to show only those elements and relations that are relevant to the user. The use of elements and relations as described previously is very suitable for Object Oriented Programming. The elements and relations have characteristics as shown in fig. 5. Every element and relation must have a unique ID. For every element and relation, the type has to be specified and the views and domains they belong to have also to be specified. Further, it is possible to assign a value to an element. For relations it is also necessary to indicate the IDs of the elements between which the relation exists. Fig. 5: Characteristics of elements and relations Based on the Reference Model and the information structures, presently an information management system is being developed. Cost related information Fig. 3: The objects in the product information structure, with the distinction between objective and subjective constituents The information is stored using elements, relations and attributes. The elements are the objects and the relations represent interactions between the objects. The attributes are the characteristics of an object or the characteristics of a relation between objects. In A major advantage of the information structures is the use of views and filters. A cost view can be constructed, being a helpful aid in the calculation and analysis of costs. The cost view can be used by other engineering tasks in order to analyse the consequences of decisions to be made. Further, the associated information structures enable a differentiated view of cost information. The first requirements for cost control are knowledge of the costs and a generic method to store cost information. Furthermore, it is

3 advantageous to have a differentiated view of the costs. This can be achieved by dividing costs in separate constituents. The next cost drivers, i.e. cost driving product characteristics, are used [6]: Geometry Material Production Process Production Planning The cost drivers can be related to the Product Information Structure. They become attributes and the cost carriers become elements in the fundamental structure (fig. 6). For reasons of function integration, standardisation and modularization, costs are only allocated to physical product elements. The costs for a relation are accounted for on a higher aggregation level. For instance, the costs for connecting two components are accounted for on assembly level (see fig. 3). An arrangement of the objects in the other information structures for cost related information is proposed by Liebers [7]. these features and the cap itself. The relations between the features are not depicted in the figure. Fig. 8: The global structure of the computer power box The slot consists of three faces and carries some dimensions (L, R). The slot does not impose restrictions on the material type. The possible production processes for the slot are nibbling and lasercutting. For the time being, all resources for nibbling and laser cutting are available. The final selection of the production process can be made based on the cost estimates (assuming the availability of the resources does not change). The cooling holes have a diameter (D) and they are arranged in a pattern, indicated with a distance between the holes. The holes don t impose any restrictions on the material type. The resources for the only possible production process "nibbling" are available at the time. The bending lines have a radius and they don t impose restrictions on the material. The production process is bending and the resources are available. Fig. 6: The fundamental cost structure Example cost related information The principles of structuring cost related information, described in the previous section, will be explained with the aid of the computer power box, depicted in fig. 7. Fig. 7: A computer power box [6] The power box has to protect the electrical components inside and is made of sheet metal. The box is an assembly consisting of three components with several features. The holes have different functions. They are used to fix the components, they are needed for cooling and they enable connectors to be plugged in. The amount of holes for cooling is determined by the amount of heat to be transferred. The global structure of the unit, with the cost drivers allocated to every object, is given in fig. 8. Zooming in on the cap, results in the structure given in fig. 9. In this figure, the central face of the cap (fig. 7: A) with the cooling holes and a slot for a connector is depicted. The grey areas around the faces indicate features and the cost drivers are allocated to Fig. 9: Detail of the cost drivers of the cap of the computer power box The material type (Al) and the accompanying material costs are allocated to the cap. The dimensions and tolerances between the features are related to the cap as well. For instance, the bending sequence determines the achievable bending accuracy. If the required tolerances are not met, another bending sequence has to be selected. The costs for changing resources and set-ups are also allocated on component level. For example, a set-up change would be required if the radii of the bending lines would not be the same. Furthermore, the availability of the resources required for the cap is allocated to the cap.

4 From the explanation of fig. 9, it is clear that the cost drivers are influenced by the decisions of the engineering tasks design, process planning and production planning. Opposite of that, the decisions of an engineering task can be supported with cost information. That the difference between the allocation of costs in case a design feature is not equal to that of a manufacturing feature becomes visible in fig. 10 (presuming that the designer did not have a design feature describing the strip). Generally, if the strip is produced in small batches, the features of the strip will be produced with different tools. Therefore, the costs are allocated to the separate features. In case of mass production, a special tool is likely to be available. In that case, the costs are allocated to the group of features. relatively high. Based on this knowledge a redesign of the feature or the entire component may be justified. Another way of cost control can be achieved by introducing option points. For one of the components of the assembly in fig. 12 three alternatives exist. For every alternative, the costs can easily be calculated. When a choice between the alternatives has to be made, the cost estimates can be used to compare the costs of the three components. An alternative can also be to buy such a part instead of to make that part. The buy-part is not specified in detail, only the price of the supplier is allocated to the component. Fig. 12: Cost calculation of the computer power box Fig. 10: The cost attributes of the strip Generative cost estimation With the cost drivers of an element, the costs can be calculated and be attached to the element. The costs and the cost drivers can be used to analyse the costs of a product. In the upper part of fig. 11, the global structure of the computer power box is given, together with the costs for some elements and some cost drivers. The costs of the component on the right (10), is the summation of the process costs of the two features (2 and 4) and the material costs of the component (4). The total costs (80) equals the costs of the components (50, 10 and 10) and the assembly costs of the components (10). During every engineering task, the costs can be monitored through the cost view. Whenever the product information is modified, the effects on the costs can be made visible by redoing the cost calculation for the new situation. Additionally, during every engineering task one can easily compare alternatives based on cost information. Variant cost estimation The product information structures of instantiated products, containing cost information, are stored in the order information structure. When a new product has to be designed or manufactured, the (partial) product structure of the new product can be compared with previously manufactured products. When the new product corresponds to a previously manufactured product, in a sufficient manner for certain characteristics, the previously manufactured product can be used to estimate the costs for the new product. The basic principle is to compare the elements of a new product with the elements of previously manufactured products [9]. The comparison is type based (see fig. 5). Depending on the available information of the new product, the characteristics of the type of element are also compared. Based on this comparison, the level of similarity is calculated. Fig. 11: Cost calculation and cost control for the computer power box When the costs of a product are known, it is relatively easy to control the costs by means of the product information structure. It is relatively easy to find "irrational" designs [8]. Fig. 11 reveals that the component on the left side of the figure constitutes more than 60% of the total costs. When zooming in on this component, it becomes clear that the production costs of one of the features is The comparison algorithm can be controlled by setting some preferential variables. The extend of requested similarity has to be set. When the similarity between a previously manufactured product and the new product is lower than the requested extend of similarity, the product is not considered in the cost estimation. For every engineering task not every product characteristic is evenly important. Therefore, it is possible to indicate, for a group of related characteristics, the extend of similarity. If a previously manufactured product does not meet the requested similarity, it must be possible to leave out the elements that cause the low extend of similarity. Since the costs of a product are known for every product element, a temporarily cost value can be

5 calculated without including the elements that are not similar. In that case, that product can still be used for the cost estimate. Hybrid cost estimation For the structuring of the costs, it is not important how they are calculated. Because the costs are attached to product elements, it is possible to calculate them with different calculation methods. So, generative cost estimation and variant cost estimation can be applied to the same product. In addition, other types of cost estimation, e.g. based on neural networks, can easily be integrated. The only condition is the existence of an interface to the information management system. For the engineering tasks that employ cost estimates for decision making, it is not important to know how the cost estimates were calculated. The information needed is a cost estimate with an indication about the reliability of the estimate. The way in which the cost estimates are determined depends on the available information, the available time and the required accuracy of the estimate; a different choice can be made for every different product element. In case the available information of a section of a product is low, variant-based cost estimation has to be used for that section. If sections of the product are specified in more detail, generative cost estimation can be used for these sections. When the available information is low and the available time for the cost estimation is high, the cost estimation system can activate another engineering task to generate more detailed manufacturing instruction information of the product. For instance, when a cost estimate for a newly designed product is requested, process planning can be activated in order to generate more information for the cost estimation process. The advantages of a modular system are the possibility of integration with other software packages, the possibility of extending the system and an appropriate environment for maintenance [5]. transparent; The use of the system should be easy to understand. highly automatic; Nevertheless, the user must have the possibility to intervene the cost calculation at any time. With these functional specifications and the functional modules of the cost control architecture, a cost estimation architecture has been developed (fig. 13). Six functional modules are distinguished, which are arranged around the Information Management kernel. The six functional modules are: Cost Models (CM): This module is used to define the cost models. The module has to be generic to enable the definition of different cost models and the use of different types of costs and formulas. The storage and retrieval of the cost models must be dealt with by the information management system. Cost Determination (CD): This module has to carry out the actual calculation of the costs, based on the selected cost model. Depending on the type of calculation, actual production data or estimated data are used. Cost Reports (CR): This module is intended to define different cost reports. The type of information in the report must be indicated so that the Data Tuning module can collect the necessary data and deduce the requested data. Cost estimation architecture The information management system described above deals with the manipulation of information and the representation of the different views on the information. It is used as the basis for a cost estimation system. In this way, the functional modules of a cost estimation system don t need to incorporate database functionalities, but solely the cost estimation functionalities. Before discussing the functional modules of the proposed cost estimation architecture, the functional specification of a cost estimation system is listed. A cost estimation system must be able to: calculate costs in a generative, variant based or hybrid way; deal with different cost models; handle different cost types; apply different analysis tools for cost related information; calculate costs on different aggregation levels; support other engineering tasks, such as: - order acceptance; - design; - process planning; - production planning; - calculation of the actual costs; - management. generate reports for other engineering tasks; communicate with the information management system. A cost estimation system has to be: modular; Fig. 13: The cost estimation architecture Data Analysis (DA): Historic data needed for calculations have to be analysed, e.g. averages, variances and trends are needed to be able to use historic data in the right way. This module must also compare the estimated costs with the actual costs in order to improve the cost models. Risk Analysis (RA): An important aspect of the costs is an indication of the accuracy of the estimation. The sensitivity of the costs to changes of the parameter values in the cost model must also be known [2]. This module must take care of all the aspects concerning quality, accuracy and sensitivity of cost estimates. The data needed for these aspects is obtained through the information management system. Data Tuning (DT): In many cases, data first have to be transformed to be able to use it. Some examples are the tuning of currencies and the correction for inflation. Another aspect is to arrange data for a certain selection e.g. for a time interval or a product family. When (non-

6 repetitive) disturbances occur during production, the production data have to be corrected, when possible. With these six modules, all the functional specifications of the cost estimation system can be realised. Furthermore, practical characteristics such as time saving, flexibility, user-friendliness for inexperienced users and unification of the cost estimation process [2] must be achievable. After the three main modules (CM, CD, CR) are implemented, the integration of the cost architecture can be verified. Similar architectures, i.e. functional modules arranged around the Information Management kernel, can be created for every engineering task [10]. An architecture for computer aided process planning for sheet metal was presented by Lutters [11]. For example, environmental control and quality control will have an architecture very similar to cost estimation. Cost control architecture The major function of cost control is the feedback of cost related information. Actually, two feedback loops can be distinguished. The engineering and planning tasks generate (alternative) solutions to manufacture a product. Based on a solution, a cost estimate can be made. Depending on the outcome of the estimate, an alternative solution can be chosen or the solution can be adapted. This applies to a short-term cost control loop. If an economic solution is found, the accompanying production plans can be executed. During the production, the actual production data have to be collected. These data are used to determine the actual manufacturing costs. The data also have to be analysed to be able to use them to create or to adapt the cost model. With a new or adapted cost model, new cost estimates can be made. This is a long-term cost control loop. Fig. 14: The cost control architecture In fig. 14, the generic cost control architecture of Liebers is depicted [12]. The architecture is valid for any kind of information used in the design and manufacturing phase. It contains four functions needed to execute the two control loops: cost estimation (F1): determination of the costs before the actual production; monitoring (F2): acquisition of relevant data at the time of execution of the production process; diagnostics (F3): interpretation of data of the monitoring process; cost modelling (F4): creating or adapting cost models with the help of data of diagnostics and monitoring. Loop 1 is the short-term control loop and loop 2 is the long-term control loop. Conclusion Information management and the use of information structures enable the construction of a cost view and differentiated representations of the costs. The costs can be calculated for every aggregation level of a product. The characteristics of the information structures enable adequate support of design and engineering tasks with cost information. By means of the product information structure, the origin of costs is exposed and the cost drivers indicate the cause of the costs. The consequences of decisions made by designers and planners are directly available and the choice between design alternatives can be made based on the estimated costs. Because information management deals with all the handling of information, the functions of the cost estimation architecture need only to be focussed on cost estimation. Given that the Information Management System is suitable for every manufacturing environment, the cost estimation system built on it is also applicable in every manufacturing environment. Based on the Information Management System and the cost estimation architecture, the development of a truly integrated cost control system is possible. References 1 Lutters, D.; Streppel, A.H.; Kals, H.J.J.: The role of Information Structures in Design and Engineering Processes, Proc. of the 3rd WDK workshop on Product Structuring, (1997), pp Luong, L.H.S.; Spedding, T.: An integrated system for process planning and cost estimation in hole making, Int. 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