Developing an integrated STEP-compliant CNC prototype
Suh, S H
Abstract
STEP-compliant CNC is the next-generation CNC controller, taking the STEP-NC data model as the interface scheme between CAM and CNC and carrying out various intelligent functions. At the moment, efforts are being made worldwide to establish an international standard for the new interface scheme (so-called STEP-NC), formal– ized as ISO 14649. In the near future, the new interface scheme will be completed and announced as the international standard. Upon completion, the standard will replace the conventional scheme based on ISO 6983, so-called M&G codes. Further, the new interface scheme will impact the CAD-CAM-CNC process chain and the advancement of the CNC controller. This paper develops an integrated STEP-compliant CNC system (or STEP-CNC) based on the new interface scheme. The system is composed of five modules: (1) Shop Floor Programming System (PosSFP), (2) Tool Path Generator (PosTPG), (3) Tool Path Viewer (PosTPV), (4) Man Machine Interface (PosMMI), and (5) CNC Kernel (PosCNC). The developed system is a prototype but very comprehensive, including all the modules required for realizing “art-to-part” through the new CADCAM-CNC chain. Architecture and functional details are presented together with a realistic demonstration.
Keywords: STEP-NC, ISO 14649, STEP-Compliant CNC, Shop Floor Programming, Soft-NC, Intelligent CNC, STEP Manufacturing
Introduction
As the brain for industrial machinery, computer numerical control (CNC) is the core element in modern manufacturing systems. In spite of a great deal of technological achievement, contemporary CNC still needs further enhancement to overcome the existing drawbacks; that is, (1) it is basically an executing mechanism without intelligence, (2) it is based on low-level language [ISO 6983 (ISO 1982), so-called M&G codes], and (3) its architecture is vendor specific and black-box-styled proprietary without allowing user access.
Therefore, the next-generation CNC is required: (1) to use high-level language for seamless integration in the CAD-CAM-CNC chain, (2) to be multifunctional, intelligent, and autonomous, and (3) to have an open architecture based on modular/software implementation technology. These requirements should be accounted for in developing the next-generation CNC, for which researches are being undertaken in various aspects. In particular, a new interface scheme between CAM and CNC, often called STEP-NC, is under active progress by ISO TC 184 SCI and SC4 (ISO 2000). The new scheme is based on the internationally standardized product model (STEP: STandard for the Exchange of Product model data), formalized as ISO 10303, as well as the process plan information. As described in the next section, ISO 14649 specifies information contents and semantics (ICS) for various CNC manufacturing processes. It is currently available in FDIS (Final Draft International Standard) version, with its final version to be completed in the near future. Upon completion, it will become a new CNC language replacing ISO 6983. The impact of the new interface scheme can be visualized in many ways. As the new data model will be an information highway for e-manufacturing, encompassing CAD, CAM, and CNC, the `art-to-part’ dream (Albert 2000) can be realized, thereby producing 3-D models in physical parts by CNC, like producing a hard copy from the printer. Significant gains are expected in the process chain of CAD, CAM, and CNC, as will be discussed in the next section. Furthermore, complete elimination of postprocessing is possible (Albert 2000). Soon, the new interface scheme will be used as a means for implementing the Internet’s B2B activities, e-design, and e-manufacturing (Hardwick 2001a, Leyrich 2001, Albert 2001, Maniscalo 2001, Teresko 2001, Hardwick and Loffredo 2001, American Machinist 2000, Hardwick 2000).
From the perspective of CNC, the new data model is very significant, providing CNC with all the information about `what-to-make’ (geometry) and `how-tomake’ (process plan) with its machine tools. In other words, depending on how the new data model is implemented, CNC would be able to incorporate various intelligent functions, which is not feasible in the conventional control based on ISO 6983. Thus, as the new language is under establishment, increasing attention has been paid to the development of new CNC based on the new interface between CAM and CNC. Major developments include Super Model in the United States and NC Prototype in Europe.
The U.S. Super Model, whose prototype was presented at the 2001 SC4 meeting held in San Francisco (Hardwick 2001b), places emphasis on the development of an `intelligent interface’ between the ISO 14649 database and CNC via XML and is implemented on the commercial systems of FB Mach and Virtual Gibbs. The demonstration was actually done with a Bridgeport CNC via G-code conversion. The European prototype, NC Prototype, presented at the 2000 SC4 meeting held in Charleston, South Carolina (Glantschnig 2000), focuses on CNC execution based on the part program written in the ISO 14649 physical file. The STEP-NC was implemented on a Siemens 840D NC Kernel interfaced with the commercial systems of Open Mind and CATIA. Interface between ISO 14649 and CNC was made via Interpreter built in CNC.
This paper presents a prototype STEP-NC, namely Korea STEP-NC, recently developed by the POSTECH research team. The Korea STEP-NC prototype is an integrated STEP-compliant CNC controller comprehensively incorporating all the crucial components for realizing the full benefit of the STEP– NC paradigm, without using any existing commercial CAD/CAM and CNC controller. The Korea STEP-NC prototype was first demonstrated at the KoreaGermany Workshop on STEP-NC Technology held at POSTECH in Korea in 2001 (Suh 2001a) and presented at the 2001 SC4 meetings in San Francisco (Suh 2001b); Fukuoka, Japan; and Myrtle Beach, South Carolina.
Overview of ISO 14649
Brief Background
ISO 14649 is often called STEP-NC, an acronym originated from the STEP-NC Project carried out in Europe, ESPRIT IV 29708. The initial effort on the new data model was made by the Laboratory for Machine Tools and Production Engineering (WZL) at the Technical University of Aachen between 1994 and 1996 as the European Project called OPTIMAL (formally ESPRIT III 8643). In this project, the data model for 3-D milling was investigated based on the STEP paradigm, in which STEP data was first used as the basis of the interface scheme between CAM and CNC. The STEP-based interface scheme was extended to 2.5-D milling and other operations, such as turning and EDM, in the subsequent European Project ESPRIT IV 29708 between 1999 and 2001. The new interface scheme, having gained worldwide consensus, will be completed by an international project IMS STEP-NC starting from early 2002 by the international consortium composed of Europe, the United States, Switzerland, and Korea. Research results have been formally documented as ISO 14649 by ISO TC 184 SCI and SC4. Currently, the overall framework together and the milling process are available in FDIS (Final Draft International Standard) version, and other processes will be finalized in the near future (see Phase 2 in Table 1).
Information Contents and Structure of ISO 14649
The data model incorporated in the Korea STEP– NC is based on the DIS-ballot version of September 2000 (ISO 2000). The structure and information contents will be basically kept with some details to be changed in the future version. ISO 14649 is basitally a structured feature-based representation of process plans for such manufacturing processes as milling operations, turning operations, EDM, and so on. Currently, ISO 14649 is under development for milling operations based on the geometric information of ISO 10303, such as AP 203, AP213, and AP 224. As illustrated in Figure 1, information contents of ISO 14649 are composed of: (1) task description, (2) technology description, (3) tool description, and (4) geometry description. Task description describes the logical sequence of executable tasks (e.g., machining_ workingstep, NC unction) and data types. Details of each workingstep are covered in the technology description in reference with the tool description and the geometry description.
Specifically, the workingsteps include manufacturing features for 2.5-D (two5D-manufacturing_ feature) and 3-D milling operations (region), and each workingstep has its subordinate subfeatures (such as planar-face, pocket, step, slot, round_hole, and general-outside-profile) together with cutting condition information. It is important to note that the tool path specification in ISO 14649 is `optional,’ unlike in the current NC programming where the tool path in terms of machine axes is the main information content. Thus, the capability of tool path generation is essential for STEP-compliant CNC. Programming of ISO 14649
Compared with ISO 6983, ISO 14649 has a variety of information with a rather complex structure, whose programming is not an easy task if prepared manually. It is thus required to develop a computerassisted part programming system. ISO 14649 code can be prepared either by the shop floor programming (SFP) system installed in the STEP-compliant CNC, or by the off-line programming (OLP) system. In either case, a programming system should be developed so that appropriate information can be retrieved from the CAD/CAPP/CAM kernels. For instance, as illustrated in Figure 2, the geometry description of ISO 14649 can be retrieved from the CAD kernel with built-in AP 203 and AP 224 interface, technology description and tool description from the CAPP kernel with built-in AP 213 and ISO 13399 (ISO 1997) interface, while tool path from the CAM kernel. Based on the interface scheme with user interface modules, the ISO 14649 programming system can be developed either in SFP or OLP fashion.
Impact of STEP-NC
The impact of the new interface scheme is most felt in the CAD-CAM-CNC chain. In the sense that the STEP-NC data model is an extended product data model including process plan information, it can be used as an information highway encompassing CAD, CAPP, CAM, and down to CNC, thereby enabling the so-called `art-to-part’ dream to come true; that is, the 3-D model (of part DB) can turn into a physical part by CNC, like producing a hard copy from a printer. Specifically, at the CAD-to– CAM level between design and manufacturing, the NC extension parallels STEPs overall ability to seamlessly facilitate data flow in 13213 situations. The savings derived from it have a synergy effect, complementary of each other. Because the 3-D model can be sent directly to manufacturing, a time savings of 75% [according to an analysis by Hardwick (2001a). Note that `STEP-NC (ISO 14649)’ in Figure 3 is represented as AP238 in Hardwick (2001a), which is the AIM version of ISO 14649.] can be easily achieved over the current process, where conversion into a drawing should take place before it is sent to manufacturing.
Further, because the new data model defines all the information for process planning, the process planning step can be greatly simplified, sparing 35% to 60% from the time normally required for the step. By implementing feature recognition capability in a CAM system based on the STEP– NC data model, the process planning task can become a `push-button’ task for producing `universal’ part programs, nullifying any postprocessing for CNC to be used for machining. In the near future, the new interface scheme will be used as a means for implementing the Internet’s 13213 activities, e-design, and e-manufacturing.
From the perspective of CNC, the new data model is very significant, providing CNC with all the information about `what-to-make’ and `how-tomake’ with its machine tools. In other words, depending on how the new data model is implemented, CNC would be able to incorporate various intelligent functions, which is not feasible in the conventional control based on ISO 6983. According to a survey (Hardwick 2001a), a time savings of 50% is reported. This is a rough estimate mainly considering the machining time with adaptive control based on the STEP-NC model. Besides the time savings, there can be many other tangible benefits that cannot be measured by time, such as machining accuracy, quality improvement, automatic part setup, on-machine inspection, and automatic collision avoidance, among others.
Three Types of STEP-Compliant CNC Depending on how ISO 14649 is implemented on CNC (STEP-compliant CNC), there are three types: (1) Conventional Control, (2) New Control, and (3) New Intelligent Control, as shown in Figure 4. Type 1 simply incorporates ISO 14649 to a conventional controller via postprocessing. In this case, conventional CNC can be used without modification. But this cannot be considered as a STEP-compliant CNC because the STEP-compliant CNC should at least be able to read ISO 14649 code. Type 2, the New Control type, has a STEP-NC interpreter in it by which the programmed workingstep is executed by CNC kernel with built-in tool path generation capability. Unlike Type 1, Type 2, having two additional built-in functions of Interpreter and Tool Path Generator, is in essence a `basic type’ where CNC motions are ‘faithfully’ executed based on the machining strategy and sequence specified by the ISO 14649 part program. In other words, Type 2 does not have intelligent functions other than the tool path generation capability. Most of the STEP– NC prototypes developed up to the present time fall into this New Control category.
The third type, which is much more promising than the above, is the New Intelligent Control (Figure 4) in which CNC is able to perform the machining task ‘intelligently’ and ‘autonomously’ based on the comprehensive information of ISO 14649 (Suh, Cho, Hong 2002). Some examples for intelligent functions are automatic feature recognition, automatic collision-free tool path generation including approach and retract motion, automatic tool selection, automatic cutting condition selection, status monitoring and automatic recovery, and machining status and result feedback. These functions are shown in the functional structure of Figure 5, and the Korea STEP-NC is aiming to achieve them. Type 3 is far from a complete implementation. Nevertheless, it presents a direction for future researches on STEP-NC. For the full realization of Type 3, however, the current ISO 14649 data model needs to be updated as discussed at the ISO TC 184 SC 1 meeting held in Frankfurt in May 2001.
Korea STEP-NC The Korea STEP-NC, aiming to eventually become an intelligent STEP-CNC of Type 3, is structured into: (1) human-machine interface (HMI), (2) control functions, and (3) data processing within CNC. The functional architecture shown in Figure 5 is derived from the requirement analysis on a functional level, data-interface level, and implementation level [see Suh and Cheon (2002) for details]. It is composed of: (1) SFP/TPG (Shop Floor Programming/Tool Path Generation) modules, which are extensions of HMI, completely covering part programming and tool path generation based on the STEP-NC data model and data-interface-level requirements, (2) control modules covering various intelligent control functions, reflecting function– level requirements, and (3) common DB modules, providing comprehensive data for SFP/TPG and control modules.
The presently developed Korea STEP-NC is a prototype covering some of the functions shaded in Figure 5. The functional architecture is realized in five `big modules’ as follows: (1) Shop Floor Programming system called PosSFP, (2) Tool Path Generator called PosTPG, (3) Tool Path Viewer called PosTPV, (4) Man-Machine-Interface called PosMMI, and (5) CNC Kernel called PosCNC. As indicated in Figure 6, the modules are interfaced by various DBs and communicated by CORBA protocol. The prefix ‘Pos’ represents our institution name, POSTECH, indicating that the modules are independently developed by our research team for the Korea STEP-NC without using any commercial software system. These five modules are the minimum elements essentially required for realizing the genuine STEP-CNC.
PosMMI
PosMMI is the module for interfacing the human operator and the STEP-CNC system. In essence, it is the console that the operator of CNC system uses: (1) to load the STEP-NC part program, (2) to set up the machine tool, (3) to execute the machine tool operation, and (4) to monitor the operation status.
Compared with conventional MMI, PosMMI is characterized that: (1) it has Internet interface to access various databases (such as STEP-NC part program DB and STEP-repository), (2) it is based on Java/JaveBeans technology so it can operate in various shop-floor environments regardless of the operation system, (3) it communicates with other modules (such as PosSFP, PosTPG, PosTPV, PosCNC) and plays the role of `communication bridge’ between modules via CORBA technology, and (4) it provides the operator with configuration service capability to set the attributes of the connection between modules. These are the implementation platforms on which STEP-CNC should be built in order to achieve operability in STEP-based, Internet-based, and distributed control environments. Figure 7 shows example screens for viewing the part program and the operation status monitoring, respectively.
PosSFP
PosSFP is the shop floor programming system in Korea STEP-NC. It takes the geometry file in AP203 file format created by any CAD system and outputs the ISO 14649 part program in physical file format. The architecture of the PosSFP in the Application Activity Model (AAM) of SFP is shown in Figure 8. PosSFP first interprets the AP203 file based on the various schema of STEP (SDAI of Part 22 and 23, Geometry of Part 42, and AP224). Then, by a built-in feature recognition system (developed by our research team), PosSFP automatically recognizes the geometric features in terms of the machining-features defined in Part 10 of ISO 14649. For each feature, the user is guided to enter attributes of process plans in terms of the workingstep described in Part 11 of ISO 14649. Upon finishing the input procedure, PosSFP generates the ISO 14649 part program in physical file format. As illustrated in Figure 9, PosSFP is designed for either an external CAM system or a built-in SFP system. Current implementation of PosSFP is a built-in type within CNC. Example screens showing the procedures carried out with PosSFP are displayed in Figure 10.
In PosSFP also, the generated part program can be graphically verified by Code Viewer. For the part program shown on the left side of Figure 11, the geometric features to be machined are graphically displayed on the right side of Figure 11. Note that it does not show any tool path for machining, but only the geometric features in wire frame. (This is because the tool path is not available at this stage.) Using this, the user can verify the contents of the generated ISO 14649 part program before proceeding further.
PosTPG and PosTPV
As mentioned before, the tool path in the ISO 14649 part program is optional. In most cases, it is not given. Thus, tool path generation capability needs to be built into the CNC. Taking the ISO 14649 part program as an input, which is interpreted by Interpreter, PosTPG generates the tool path for machining each workingstep (Figure 12). The tool path to be generated includes operations for machining and inspection. It is composed of four segments: approach, machining, retract, and departure. PosTPG generates the tool path for each segment based on the machining strategy (e.g., `zig-zag’ tool approach, `contour parallel’ machining path, and so on) specified by the ISO 14649 part program. Collision avoidance between tool paths is crucial. For such a purpose, the tool is retracted up to a security plane as specified by the part program. The currently developed PosTPG generates the tool path in accordance with the information given by the ISO 14649 part program. Furthermore, for intelligent and autonomous machining, an adaptive tool path algorithm having modifying capability is under implementation. ‘Adaptive’ means that the tool path stops the operation, upon recognizing unexpected situations such as unavailability of a specified tool due to tool breakage that may occur during machining of a certain feature, and resumes the operation (for example, after tool change) for the remainder of the removal job. The generated tool path can be graphically visualized by PosTPV (Figure 13), after which the visual verification is stored in the tool path DB, as shown in Figure 12. PosCNC and Implementation To realize the STEP-NC data model into machine tool motion, execution mechanism (NCK/PLC) is required. For this end, the existing CNC kernel may be used as in the U.S. Super Model demonstration (where Bridgeport CNC is used) and in the EU STEP-NC prototype (where Siemens 840D is used). However, a new prototype CNC, PosCNC, is developed because the existing CNC does not allow access to its internal structure, and in an effort to take full advantage of the STEP-compliant CNC. Although the main function of PosCNC is NCK and PLC, its architecture is different from the conventional architecture. As illustrated in Figure 14, it has an open and modular architecture communicating with various application modules (ISO 14649 Parser, Task Scheduler, PosSFP, and PosTPG, and so on) and databases (such as tool path DB, Machine DB, and Controller DB) via CORBA. The NCK/PLC itself is a soft-CNC (written in C++) capable of NURBS interpolation, look-ahead control, position/velocity interpolation, and PID control. The software-based NCK/PLC modules are interfaced with machine tool hardware (drivers and motors) via an I/O board. The NCK/PLC algorithms developed were explained in detail together with a pseudo code in a book recently published in Korean (Kang and Suh 2002).
The Korea STEP-NC is currently implemented in two computers, as shown in Figures 15a and b. The first PC includes modules that can run in non real time under Windows NT OS, like PosSFP, PosTPG, PosTPV, and PosMMI. In the second PC, modules dealing with such time-critical tasks as NCK/PLC of PosCNC running on real time OS and I/O board interfacing machine tools are implemented. All the modules in the two PCs communicate via CORBA through LAN. Demonstration Scenario
To show how the developed system works, a demonstration scenario was set up from the perspective of PosMMI, as illustrated in Figure 16. To create or upload the STEP-NC part program, the operator pushes a button in PosMMI by which PosSFP is invoked via CORBA communication. Then, the control is shifted to PosSFP, by which the operator can edit or create a new part program followed by code verification, as illustrated in Figures 10 and 11.
Upon finishing PosSFP, the control is back to PosMMI via CORBA communication. By pressing a button in PosMMI, PosTPG is invoked to generate tool path. The generated tool path is stored in the PosMMI database. By pressing a button in PosMMI, the generated tool path can be visualized as shown in Figure 13. After verifying the tool path, the part program is executed by pressing the cycle start button in PosMMI. Then, PosCNC starts the machining operation. During the operation, the machining status is displayed on the screen of PosMMI. Finally, the machined part of the geometry given in AP203 is shown in Figure 17. Concluding Remarks
This paper presented a prototype STEP-compliant CNC system called Korea STEP-NC. Although it is a prototype, the core technology required for the STEP-compliant CNC is fully incorporated in it. The architecture and functional details were derived from the user requirement analysis and the paradigm of the STEP-NC data model. Compared with other prototype systems based on an external interface with an existing CAD/CAM system or CNC system, the Korea STEP-NC is distinguished in that it is a comprehensive and integrated STEP– NC incorporating all the required component technologies to realize the benefit of STEP-NC as much as possible.
As far as functionality is concerned, the present Korea STEP-NC lies between Type 2 and Type 3, meaning that some intelligent modules have yet to be developed. Because it is newly developed under open architecture without using commercial package or CNC kernel, the details of each module and functionality can be easily modified and expanded. We are currently developing Type 3 STEP-NC incorporating the intelligent functions based on the architecture shown in Figure 5.
Acknowledgment
This research was in part supported by the grant (No. 1999-2-315-002-3) for the multidisciplinary research program by KOSEF (Korea Science and Engineering Foundation).
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S.H. Suh (shsCpostech.ac.kr), D.H. Chung, B.E. Lee, J.H. Cho, S.U. Cheon, H.D. Hong, and H.S. Lee, National Research Lab for STEP-NC Technology (http://stepnc.postech.ac.kr), School of Mechanical and Industrial Engineering, Pohang University of Science & Technology (POSTECH), Pohang, Korea
Authors’ Biographies
S.H. Suh is a professor in the School of Mechanical & Industrial Engineering and the Director of the National Research Laboratory for STEP-NC Technology (NRL-SNT) at the Pohang University of Science & Technology (POSTECH) in Korea. He obtained his BS and MS in industrial engineering from Korea University and KAIST in 1976 and 1978, respectively, and his PhD in manufacturing engineering from Ohio State University in 1986. Before joining POSTECH in 1987, he was with the Center for Research on Integrated Manufacturing (CRIM) at the University of Michigan. Since he established the E-Manufacturing Lab (formerly CAM Lab) in 1987, Professor Suh and his research team have been engaged in various researches in the area of CAD/CAM/CNC. His current research interests include manufacturing information technology, e-manufacturing, and intelligent CNC. He recently authored two books, Principles and Design of CNC Systems and Open Architecture CNC and Development. In 2000, his laboratory was designated as the National Research Laboratory for STEP-NC Technology by the Ministry of Science & Technology in Korea, where he and his research staff are researching theories and implementation technology for realizing an “intelligent” STEP-NC paradigm. He is the chairman of Korea TC184/SCI and is an active member of ISO TC184/SCI and SC4.
Jung-Hoon Cho is a postdoctoral researcher in the National Research Laboratory on STEP-NC Technology at POSTECH. He received his BS in industrial automation from Inha University (Incheon, Korea) in 1992 and his MS and PhD degrees in mechanical and industrial engineering from POSTECH in 1994 and 2001, respectively. He was involved in developing the GearCAM system (a comprehensive CAx system for covering up to spiral bevel gears). Dr. Cho has published in the area of CAD/CAM and STEP-NC. His research interests include STEP manufacturing, CAD/CAM, and STEP-NC.
Dae-Hyuck Chung received his BS in industrial engineering from KAIST in 1997 and his MS in mechanical and industrial engineering from POSTECH in 1999. He is now a PhD candidate in the School of Mechanical & Industrial Engineering at POSTECH. He was involved in developing the GearCAM system. Mr. Chung’s research interests include open architecture CNC, STEP-NC, and virtual machining.
Byeong-Eon Lee received his BS and MS in mechanical and industrial engineering from POSTECH in 1999 and 2001, respectively. He is now a PhD candidate in the School of Mechanical & Industrial Engineering at POSTECH. His research interests include feature technology, STEP manufacturing, and STEP-NC.
Sang-Uk Cheon received his BS in industrial engineering from KAIST in 1994. From 1994 to 2000, he was with Cubic Technology (Seoul) as a software engineer. He is in the master’s program in the School of Mechanical & Industrial Engineering at POSTECH. Mr. Cheon’s research interests include geometric modeling, STEP-NC, and feature-based process planning.
Hee-Dong Hong received his BS in industrial engineering from KAIST in 1996 and MS in mechanical and industrial engineering from POSTECH in 1998. He is now a PhD candidate in the School of Mechanical & Industrial Engineering at POSTECH. He involved in developing the GearCAM system. Mr. Hong’s research interests include holonic manufacturing systems, multi-axis machining, and STEP-NC.
Hyun-Soo Lee received his BS in industrial engineering from Sung Kyun Kwan University (Suwon) in 2000. He is now in the master’s program at the School of Mechanical & Industrial Engineering, POSTECH. Mr. Lee’s research interests include data modeling and Internet interface technology, including XML, Java, and CORBA.
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