Process Planning Based on Feature Recognition Method Fernando Garcia, Minna Lanz, Eeva Järvenpää and Reijo Tuokko Tampere University of Technology Department of Production Engineering Tampere, Finland
[email protected] Abstract—Managing and controlling very complex manufacturing systems and vast volumes of accumulate knowledge, holonic manufacturing system is developed. This paper introduces a method, which utilizes feature-based modeling for defining a pre-process plan. The pre-process plan developed can be linked in a holonic system. This paper, the generic steps for making a product are called “pre-process plan”. The pre-process plan defines the required capabilities on a high level. All the resources have some sort of capability that represents the possible candidates for the product manufacturing. The feature recognition method offers geometric and non-geometric (such as shape, type, tolerance and material) information. Using feature information a preprocess plan can be defined. The fact that the pre-process plan does not strictly define the used processing methods allows the product to be manufactured on different machines based on their availability or other criteria. This is important in dynamic, adaptive production environment. Keywords – feature-base modeling, ontology, resource description, capability, pre-process planning, holonic system.
I.
INTRODUCTION
Products and manufacturing systems are getting more and more complex. This leads naturally to smaller batch sizes and tight production schedules. In dynamic adaptive production environment, the process plan needs to be created rapidly based on available resources. In order to answer the demand in cost effectively the knowledge transfer between product design and process planning needs to become faster. It also sets new requirements for the process planning tools. A selected number of steps have to be followed from design to manufacturing. Making the right selection of steps gives as a result low production costs and efficient results. To find a method where, the steps on manufacturing products are efficient, the industry and academic area are investing in research on “process plan”. Preparatory steps like, process sequencing, tool selection, tool path planning, optimization, NC code generation; are referred to as “Process Planning”. The field of process planning has been researched for a long time and several research studies on process planning have been reported [1][2][10][13]. All of them have the same goal: to create a robust, precise, yet flexible process plan, effectively. Other research papers explain that feature analysis information is an important clue on process plan [1][2]. However, they do not explain which feature information is used and the steps for getting the feature information. This
paper proposes a method for defining a pre-process plan based on feature analysis and explains how the information is collected from feature recognition method. In [5] is mentioned an important point: every manufacturing resource continuously changes over time. The result of this point is that designers do not have updated information about devices that exist in the production system. If a new tool exists in the world or in a specific production system, designers do not have an idea what capabilities or characteristics it has. This problem can end up with an obsolete process plan configuration, and the production system has to be re-designed. This paper proposes a method that defines a pre-process plan and the link between the pre-process planning with the resources existing in the production system. When devices and resources are combined, new capabilities appear. The new capabilities are rarely updated by the company producing the devises or resource, for example a robot with specific gripper from other brand than the robot produce a final capability that is not reported by any of the companies. The final user in the manufacturing system has to define the new capabilities. A method to represent the capabilities of devices and combined capabilities of multiple devices is represented in [9]. Using the capabilities of resources in the creation of process plan, aim to find a robust, precise and flexible process plan. The software tool used for feature recognition, presented in [6] is used for designing a pre-process plan. In this paper, generic process steps are called “Pre-Process Plan”. Using the pre-process plan (combined with the capabilities of resources) a suggested process plan can be sent to production system. The paper is organized as follow. Chapter II will explain the reason of using feature recognition as the core of the research. Then, chapter III will make the link between feature recognition and “pre-process plan.” Chapter IV will explain about the core ontology used for this research. Finally, chapter V will conclude the paper. II.
BACKGROUND
A. Feature Recognition Methods Before the computer-aided design (CAD) tools, the design of any prototype was time-consuming, process fraught with trial and error. After the availability of CAD software, designers use CAD in all industries where the precision
construction and assembly of parts are required. The CAD software solves several problems of designing; however, it as well opens the door for new problems. Exporting the CAD files into other formats usually ends up with missing information [6]. From the design offices to manufacturing system, the CAD file has to pass several modifications and format conversions. When the data is missing, manufacturing engineers have to manually add that missing data. This results in time consuming manual work. The feature-based model was developed in order to fill the gap between detailed geometry information (expressed in CAD files), the elementary relations (expressed in eBOMs and mBOMs) and abstract functional information (other design documentation) [4][6]. Features include both geometric and non-geometric information. The non-geometric information can be functional characteristics of the product such as shape, type, tolerance and material. By including features into the model, the model can be represented in a higher abstraction level than just a pure geometrical model [7][12]. The process plan is traditionally produced by human experts, and often it has to be adjusted several times in trialand-error process [11]. The Pre-process plan or process plan are based on geometric information and the functional characteristics of the product. B. Pro-FMAextended Tool Software In order to answer the problem regarding the meaning of models a tool named Pro-FMA (Professional Feature Modeling and Analysis Tool) was developed. The tool focused on the re-creation of the lost geometric features adding the extra information needed for other process steps. The tool used formalized ontology for describing the structure of the information model and for representing the instances [8]. Pro-FMA is used to define the product requirements. Product requirements are those product characteristics or features which require a set of process in order for the product to transform towards finished product. These processes are executed by devices and combination of devices possessing suitable capabilities. The product requirements can be expressed by the required capabilities and their temporal and logical order [9]. Feature recognition is done with vertices and faces or triangles data only. The reason behind is that this is the only data that is common in different CAD models and general 3D models. The data is exported from any CAD system or 3D studio as VRML (Virtual Reality Modeling Language). Pro-FMA can read either VRML or X3D (eXtensive 3D), which is a successor of old VRML format [15][16]. Depending on the used system the data can vary from a set of polygons to more complex faces even though the format is same. Each Face-Extended contains enough information to re-create one feature-set, their vertices and edges are linked forming it.
Figure 1 shows an example of feature analysis, where a face is separated in two faceExtededs and the part is split into their features set. The detailed steps for analysis of featureset are presented in [6][8] .
Figure 1, Face to FaceExtended and part to features However, geometric data is not enough for selecting devices and resources that can produce that specific feature. Material, tolerance and shape information, are needed as well in the task of selecting the right resources and appropriate devices. Pro-FMA has been developed in several versions. In this research Pro-FMAextended is an extension from Pro-FMA dedicated for the recognition of the features and creation of pre-process plan. The pre-process plan is a generic recipe on how to build the part. This topic will be explained in depth later. Figure 2, shows the graphical user interface of ProFMAextended.
Figure 2, Pro-FMAextended GUI Figure 2, shows an example of a part analyzed. The part named “Hinge1_Part1” was analyzed by Pro-FMAextended. The results are displayed inside of the boxes around the 3D model image. The feature tree contains as a root, a cylinder that is the main feature. The root has several children; two holes, two chamfers and one cylinder with two children more, two chamfers. Each feature contains their characteristics: shape,
type, material, tolerance and geometric dimensions. The pre-process plan can be created using these basic elements. III.
LINKING THE FEATURES AND PRE-PROCESS PLAN
A. Pre-Process Plan To create a final process plan is like a journey and it is not an easy task. The starting journey is collecting the needed information. A generic process plan consists of two parts [1], generic data and machine-specific data. The generic data consists in a basic sequence. In this research, “generic data” is the main body of the pre-process plan. Pre-process plan includes a generic list of steps that are needed to realize the part. The pre-process plan created with the Pro-FMAextended, uses generic process names. The generic process names are the most basic operations that can be performed in almost any product, such as: • Material removing, when a feature needs for example, drilling, turning or any removal of material. • Material adding, when a feature is added to a surface that requires such tight tolerances or surface parameters that the adding is one of the only options • Material joining, if the feature requires something basic operation like assembly. • Material conserving, this is for basic operation like which transforms material without adding or removing, like bending. • Material transportation, like the name says, the basic operation of moving the part • Surface finishing, if the feature or the part needs a special finishing
Figure 3, Pre-Process plan example The pre-process plan diagram follows different rules, according the features shape and the position of each feature in the feature tree. The basic rules are: • The first feature to be processed is the deepest in the feature tree and has to be a basic shape (cylinder, box) • Hole with cylindrical shape are the last to be done. But, if the part is a sheet metal, holes are made first. • Chamfer, rounded face are performed in the list after its parent shape. • If the feature shape is hole, chamfer, rounded face, the generic name is “remove material” • If the feature is bended, the generic name is “material conserving” • If the feature needs any rotation or moving somewhere else, the generic name is “material transporting” Pro-FMAextended uses the list mentioned before for creating the pre-process plan. Pro-FMAextended does not know what kind of resources exist in the manufacturing system before connecting with a Knowledge base explained in the next section. So, instead to name the process as drilling or turning, the generic names are added into preprocess plan. Figure 4 shows the GUI of the ProFMAextended with the pre-process plan obtained from the part showed in figure 3.
The resulting pre-process plan is a list of generic elements that it is able to be used in the manufacturing system. As well if the manufacturing system is upgraded, the preprocess plan can be used again. Figure 3, shows an example of features linked with a pre-process plan.
Figure 4, Pro-FMAextended Pre-Process Plan GUI
B. Linking Pre-Process plan with capabilities Using the capability taxonomy, presented in [9] the preprocess plan, expressed with generic process names, can be matched with suitable resources possessing required capabilities. The pre-process plan is not defining the actual processing method to be used, but define the required capabilities on a high level. This way, all the resources having some sort of material removing capability (e.g. milling, turning, drilling)
represent the possible candidates for the product manufacturing. The final decision on, which resources to select, is done based on the capability parameters. For more information, please refer to [9].
the knowledge bases are structured with Pro-FMA and other software tools.
The generic process in the pre-process plan have a reference to the same capability taxonomy where the devices are referring, allowing the suitable resource to be found based on their capabilities [9]. The fact that the pre-process plan does not strictly define the used processing methods allows the product to be manufactured on different machines based on their availability or other criteria. This is important in dynamic, adaptive production environment.
Figure 6, Pro-FMA structured with KB and other tools V.
Figure 5, Pro-FMAextended connection with KB IV.
CORE ONTOLOGY
Feature recognition and pre-process plan results are saved into core ontology. The Core Ontology, explained in detail in [3][8] and [14] is used as the main saving format for the recognized features and the pre-process plan. The Core Ontology includes three sub-domains; product, process and system domains. The geometric and non-geometric feature information is stored into the product domain and preprocess plan is stored in the process domain. The Core Ontology uses OWL DL (Web ontology Language, Description logics) and RDF (Resource Description Framework) for formalizing the knowledge. A method to represent the capability information of resources within the ontology is explained in [9]. The Web Services interface is used to post and retrieve requested knowledge to the Knowledge Base (KB). Figure 5 shows a diagram of connection with the KB. These requests are responded with information about an object or several objects in a desired format (RDF, JSON, etc). ProFMAextended can request a list of products available in the KB in RDF format and visualization of each product in X3D (eXtensive 3D) and download those models for feature recognition and resources. Figure 6 shows a diagram how
CONCLUSIONS
Due to the fact that modern CAD system do not allow feature-based models (VRML, STEP, and other generic product data models do not include feature or nongeometrical information) to be shared outside the system boundaries the developed Pro-FMAextended is seen as higly important tool that allows the utilization of input data to be used as a basis for the analysis. The Pro-FMAextended parses the missing data and allows new knowledge generation to be applied based on other resources. The connection to the manufacturing system capabilities allows freedom to plan and later on simulate the different manufacturing scenarios. Figure 6, also illustrate the information architecture of a holonic manufacturing system, where the actual process plan is based on resources availability. In this scenario, there does not exist a process plan before the production takes place. In this scenario the importance of Pro-FMAextended becomes quite clear. It allows the definition of needed process steps and their characteristics in a form of pre-process plan, since the preprocess plan defines requirements yet allows flexibility towards feasible manufacturing system combinations. REFERENCES [1]
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