Making CNC machine tools more open,interoperable and
intelligent—a review of the technologies
X.W.Xu a,*,S.T.Newman b
a Department of Mechanical Engineering,School of Engineering,The University of Auckland,
Private Bag92019,Auckland,New Zealand
b Department of Mechanical Engineering,University of Bath,Bath,BA27AY,UK一群鸭子开会
Received30August2004;accepted7June2005
Available online10October2005
Abstract
The aim of the next generation of computer numerically controlled(CNC)machines is to be portable,interoperable and adaptable.Over the years,G-codes(ISO6983)have been extensively ud by the CNC machine tools for part programming and are now considered as a bottleneck for developing next generation of CNC machines.A new standard known as STEP-NC is being develope
d as the data model for a new breed of CNC machine tools.The data model reprents a common standard specifically aimed at the intelligent CNC manufacturing workstation,making the goal of a standardid CNC controller and NC code generation facility a reality.It is believed that CNC machines implementing STEP-NC will be the basis for a more open and adaptable architecture.This paper outlines a futuristic view of STEP-NC to support distributed interoperable intelligent manufacturing through global networking with autonomous manufacturing workstations with STEP compliant data interpretation, intelligent part program generation,diagnostics and maintenance,monitoring and job production scheduling.
#2005Elvier B.V.All rights rerved.
Keywords:CNC;Interoperability;STEP;STEP-NC
1.Introduction
From the start of craft production in the1800s to the pioneering mass production of the early1900s there have been a number of revolutionary changes to manufacturing system’s configurations.The most recognid traditional configuration of manufacturing systems was the dedicated transfer(machine) line,which enabled mass production at high efficiency and low cost.With the need
of the1970s and1980s to produce a wider range of parts,‘‘flexible’’manufacturing was developed to meet the needs for the production of smaller batches of different parts.The systems ud groups of computer numerically controlled(CNC)machines that could be reprogrammed to make different parts combined with automated transport systems and storage.The CNC machines became the central elements in the systems such asflexible transfer lines,flexible manufacturing systems(FMS)andflexible manufacturing cells(FMC).
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However,the amount offlexibility existing in the systems was still believed to be limited.In order to prepare manufacturing companies to face increasingly frequent and unpredictable market changes with confidence,interoperable and more open manufacturing systems are needed.In the process of designing and operating interoperable and open manufacturing systems there is a need to distinguish from among system-level issues,achine and control)issues,and ramp-up time reduction issues[1,2]. Most of the rearch effort has been spared on the issues at the system level,some at the component level and little on the ramp-up time reduction issues.At the component level, rearch work has primarily centred around the control issues concerning machine tools,with the aim to provide enabling CNC technologies for modular and open-architecture control [3,4].
CNC machine tools are the main components in any manufacturing system.There are demands and new opportu-nities to empower the current CNC machines with the much-needed features such as interoperability,adaptability,agility and reconfigurability.To this end,there are two major issues that need to be addresd namely product data compatibility/
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Computers in Industry57(2006)141–152
常可*Corresponding author.Tel.:+6493737599x84527;fax:+6493737479.
E-mail address:x.xu@(X.W.Xu).
0166-3615/$–e front matter#2005Elvier B.V.All rights rerved.
doi:10.pind.2005.06.002
interoperability and adaptable CNC machines.Up till now little rearch has been carried out in thisfield,but due to the developments of the new CNC data model known as STEP-NC, there has been a surge of rearch activities in trying to address the above-mentioned issues.This paper reports on the rearch activities and tries to address the issues of interoper-ability and adaptability for CNC machine tools.
2.Impediments of current CNC technologies
Today’s CNC machine designs are well developed with capabilities such as multi-axis control,error compensation and multi-process bined mill/turn/lar and grinding machines).In the mean time,the capabilities have made the programming task increasingly more difficult and machine tools themlves less adaptable.Some effort has been made to alleviate this problem,in particularly the trend towards open architecture control,bad on OSACA[5]and open modular architecture controller(OMAC)[6],where third party software can be ud at the controller working within a standard windows operating system.One further recognisable industrial development is the application of software controllers,where PLC logic is captured in software rather than in hardware.
Although the developments have improved software tools and the architecture of CNC systems,vendors and urs are still eking a common language for CAD,CAPP,CAM,and CNC, which integrates and translates the knowledge of each stage with no information loss.Though there are many CAM tools supporting NC manufacture,the problem of adaptability and interoperability from system to system was and is still en as one of the key issues in limiting the wider u of the tools.
2.1.Product data compatibility and interoperability
CNC machine tools complete the product design and manufacturing lifecycle,and more often than not they have to communicate with upstream sub-systems,such as CAD,CAPP and CAM.In the ca when neutral data exchange protocols, such as SET,VDA,and initial graphics exchange specification (IGES)are ud,information exchange can happen between heterogeneous CAD and/or CAM systems.This is however only partially successful since the protocols are mainly designed to exchange geometrical information and not totally suitable to all the needs of the CAD/CAPP/CAM industry. Thus,the international community developed the ISO10303[7] t of standards,well known as STEP.
By implementing STEP AP-203[8]and STEP AP-214[9] within CAD systems,the data exchange barrier is removed.Yet, data exchange problems between CAD/CAM and CNC systems remain unsolved.CAD systems are designed to describe the geometry of a part precily,whereas CAM systems focus on using computer systems to generate plans and control the manufacturing operations according to the geometrical information prent in a CAD model and the existing resources on the shop-floor.Thefinal result from a CAM system is a t of CNC programs that can be executed on a CNC machine.STEP AP-203and STEP AP-214only unify the input data for a CAM system.On the output side of a CAM system,a50-year-old international standard ISO6983(known as G-Code or RS274D)[10]still dominates the control systems of most CNC machines.Outdated yet still widely ud,ISO6983only supports one-way informationflow from design to manufactur-ing.The CAD data are not utilid at a machine tool.Instead, they are procesd by a post-processor only to obtain a t of low-level,incomplete data that makes modification,verifica-tions and simulation difficult.The changes made at the shop-floor cannot be directly fed back to the designer.Hence, invaluable experiences on the shop-floor cannot be prerved and re-utilid.
2.2.Inflexible CNC control regime
The ISO6983standard focus on programming the path of the cutter centre location(CL)with respec
t to the machine axes, rather than the machining tasks with respect to the part.Thus, ISO6983defines the syntax of program statements,but in most cas leaves the mantics ambiguous,together with low-level limited control over program execution.The programs,when procesd in a CAM system by a machine-specific post-processor,become machine-dependent.In order to enhance the capability of a CNC machine,CNC controller vendors have also developed their own tailored control command ts to add more features to their CNC controllers to extend ISO6983. The command ts once again vary from vendor to vendor resulting in further incompatible data among the machine tools.
The current inflexible CNC control regime means that the output from a CAM system has no adaptability,which in turn denies the CNC machine tools of having any interoperability. The main reason is that a G-code bad part program only contains low-level information that can be described as‘‘how-to-do’’information.The CNC machine tools,no matter how capable they are,can do nothing but‘‘faithfully’’follow the G-code program.It is impossible to perform intelligent control nor machining optimization.
3.The STEP-NC standard
Today a new standard namely ISO14649[11–16] recognid informally as STEP-NC is being developed by vendors,urs and academic institutes world wide to provide a data model for a new breed of intelligent CNCs.The data model reprents a common standard specifically aimed at NC programming,making the goal of a standardid CNC controller and NC code generation facility a reality.Currently two versions of STEP-NC are being developed by ISO.Thefirst is the Application Reference Model(ARM)(i.e.ISO14649) and the other Application Interpreted Model(AIM)of ISO ISO10303AP-238[17]).For more information on the u and differences between them readers are referred to [18,19].
Contrary to the current NC programming standard(ISO 6983),ISO14649is not a method for part programming and does not normally describe the tool movements for a CNC machine.Instead,it provides an object oriented data model for
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CNCs with a detailed and structured data interface that incorporates feature-bad programming where a range of information is reprented such as the features to be machined,tool types ud,the operations to perform,and the quence of operations to follow.Though it is possible to cloly define
the machine tool trajectory using STEP-NC,the aim of the standard is to allow the decisions to be made at a latter stage by a new breed of intelligent controller—STEP-NC controller.It is the aim that STEP-NC part programs may be written once and ud on many different types of machine tool controller providing the machine has the required process capabilities.In doing this,both CNC machine tools and their control programs are made adaptable and interoperable.Fig.1 illustrates that both geometric and machining information can now be bi-directionally transferred between a CAD/CAM system and a STEP-NC controller[20].One critical issue is that the tool path movement information is optional and ideally should be generated at the machine by the STEP-NC controller.
Geometric information is defined by machining features (similar to AP-224[22])with machining operations termed ‘‘Workingsteps’’performed on one or more features.The Workingsteps provide the basis of a‘‘Workplan’’to manu-facture the component.Fig.2illustrates an actual extract of such data for a part with a Workplan consisting of Workingsteps for slotting,drilling and pocketing.One important point to note is that this code is the STEP-NC transfer(physical)file,which is imported/exported into and out of a STEP-NC intelligent controller.Thisfile would be interpreted by the controller, enabling CNC operators to interact at a machining operation)level via an intelligent manual data interface(MDI)or CAD/CAM system at the controller.Some of the benefits
with using STEP-NC are as follows[23]. STEP-NC provides a complete and structured data model, linked with geometrical and technological information,so that no information is lost between the different stages of the product development process.
Its data elements are adequate enough to describe task oriented NC data.
The data model is extendable to further technologies and scalable(with conformance class)to match the abilities of a specific CAM,SFP or NC.
Machining time for small to medium sized job lots can be reduced becau intelligent optimisation can be built into the STEP-NC controllers.
Post-processor mechanism will be eliminated,as the inter-face does not require machine-specific information.
Machine tools are safer and more adaptable becau STEP-NC is independent from machine tool vendors.
Modification at the shop-floor can be saved and fed back to the design department hence bi-directional informationflow from CAD/CAM to CNC machines can be achieved.
XMLfiles can be ud as an information carrier hence enable Web-bad distributed manufacturing.
A detailed discussion on value proposition for STEP-NC can be found in a report produced by the OMAC STEP-NC Working Group[24]and other publications[20,23,25].
4.STEP-NC international community
In the cond half of the1990s,an effort from the international community backed by ISO started the major change in the concept of NC programming,through an international intelligent manufacturing systems(IMS)pro-gramme[26].The programme was co-ordinated across four
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143
Fig.1.Bi-directional informationflow with STEP-NC[21].
worldwide regions each with individual projects namely Europe,Korea,Switzerland and the USA.The major co-ordinators of the programme are Siemens (EU),CADCAMa-tion (Switzerland),STEP Tools (USA)and ERC-ACI (Korea).STEP-NC Europe is responsible for milling,turning and inspection of the ISO 14649standard.It has 15partners,led by Siemens,with urs such as Daimler Chrysler,V olvo,and the support of rearch institutes such as WZL RWTH-Aachen and ISW Stuttgart University.The Swiss are leading the develop-ment of the standard for wire-cut and die-sink EDM in collaboration with vendors such as Agie,Starrag and CAM manufacturer CADCAMation.The work in Korea has been carried out by both Pohang University of Science &Technology (PosTECH)and the Seoul National University in the areas of milling and turning architectures for ISO 14649compliant controllers.Other rearch teams working in the area include tho in the UK and New Zealand.In the United Kingdom,an Agent-Bad,STEP-compliant CAM (AB-CAM)system has been developed in Wolfson School of Mechanical and Manufacturing Engineering,Loughborough University
[27,28].In New Zealand,the Manufacturing Systems Laboratory at the University of Auckland has been using the AIM of STEP-NC [17]to develop a STEP-compliant CAPP system for collaborative manufacturing [29,30].
The STEP-NC programme in the USA called SuperModel led by STEP Tools Inc.and sponsored by National Institute of Standards and Technology (NIST)has made major advances to fully automate the CAD to CNC manufacturing process through the u of STEP or rather AP-238.This project involved a strong group of industrial partners including Boeing,Lockhead Martin,General Electric and General Motors,together with recognid CAM vendors such as Gibbs Associates and MasterCAM.
5.STEP-NC for more open and interoperable machine tools
There are four types of rearch work related to STEP-NC:(1)conventional CNC control using STEP-NC;(2)new STEP-NC enabled control;(3)STEP-NC enabled intelligent control;
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144Fig.2.Example STEP-NC physical file [20].
and(4)collaborative STEP-NC enabled machining.The degree of adaptability increas from Type1to Type4.It is to be noted that STEP-NC together with STEP is now forming a common data model for reprenting complete product information.Its far-reaching effect lies in a total integration of CAD,CAPP, CAM and CNC with desired interoperability and adaptability across the complete design to manufacturing chain.Due to the limited scope of this paper,only the rearch work directly related to STEP-NC enabled CAM/CNC is discusd.
5.1.Conventional CNC control using STEP-NC
This type of rearch marked the beginning of STEP-NC related rearch endeavour.The main purpo is to answer two questions:‘‘Does a STEP-NCfile contain enough and just enough information for CNC machining?’’and if it does,‘‘Can it be ud on a traditional CNC machine tool without making changes to the system hardware?’’.The main rearch is to do with the development of‘‘translators’’which can read in a STEP AP-203or AP-224file and convert it into G-code format that the targeted CNC machine tool can understand.The translator is somewhat similar to the‘‘post-processor’’ud in many CAD/CAM or CAM systems.The only difference is that the CAD/CAM,CAM
and CNC systems are now made interoperable in a n that the STEP compliant information can be ud across the board.Also,the design information that can be embedded in a STEP-NCfile is made available to the CNC systems.This scenario reprents conventional solid-bad manufacturing as enabled by STEP AP-203.
The work carried out in thefirst two stages of the three-stage SuperModel Project falls into this type of work.In stage one,a range of software ST-Plan,ST-Machine,and STIX [31])were developed involving GibbsCAM and various pieces of third-party software.The GibbsCAM STEP translator can read in the demonstration part in STEPAP-203format.The part is then programmed using GibbsCAM’s graphical interface, and visually verified using its cut part rendering capability[32]. In the cond stage,the AP-238file was read using a GibbsCAM STEP-NC Adaptor plug-in,developed by STEP Tools Inc.An MDSI Open CNC controller(software-bad CNC)[33]retrofitted to a Bridgeport vertical machining centre was ud as the platform for the GibbsCAM and STEP-NC software.Using the tooling and operation parameters specified in the AP-238file,the STEP-NC Adaptor created GibbsCAM tooling,process and geometry elements and executed GibbsCAM functions to generate tool–paths corresponding to the AP-238machining features.Once again,the cut part rendering was ud for visual verification prior to post-processing the data to generate conventi
onal G-code output. This work has demonstrated the ability of STEP-NC to completely automate CAM processing and tool–path genera-tion.It has also significantly reduced the lead-time in the CAD/ CAM to CNC programming time by up to85%[32].
More recently,at the Jet Propulsion Lab(JPL)in Pasadena, California,in January2003,STEP Tools Inc.demonstrated the conversion of AP-203design data into he AIM version of STEP-NC),feature by feature,with the u of ST-Plan AP-238data.AP-238data was then transferred to GibbsCAM with the assistance of ST-Machine,and then to a five-axis Fadal machining centre.In June2003at NIST,a similar t-up saw MasterCAM interface with anotherfive-axis machine tool.
5.2.New STEP-NC enabled control
Working cloly with some of the popular CNC controllers or Open Modular Architecture Controller[6],veral rearch teams around the world have been able to process STEP-NC information internally is a CNC controller.This is made possible by developing for,and integrating a STEP-NC Interpreter into,the controllers that can faithfully performs the machining tasks as specified in ISO14649.
The third stage work of the US SuperModel Project saw GibbsCAM integrated with an OMAC machi
ne tool.An AP-238datafile provided all the manufacturing information to allow GibbsCAM to generate the tool–path data.The tool–path data was then nt to a horizontal machining centre in so-called’’stroke-level inter-process communication’rather than con-ventional G-codes,demonstrating a higher level of CAM/CNC integration than is normally realid through ISO6983.
Most of the work carried out in EU falls into this category of rearch.The main focus has been on the development of the STEP-NC enabled CNC control using Siemens840D controller [34].This enables the STEP-NC physicalfiles to be integrated directly with the controller,with visualisation of the machining features and associated Workingsteps in a STEP-NC compliant version of their ShopMill CAM system.Programming developments in parallel with this work have been undertaken at WZL,University of Aachen,Germany,with the WZL Shopfloor Programming System incorporating WZL Mill,a STEP-NC compliant programming system and WZL-WOP (workshop-oriented programming).Commercial applications in Europe with CATIA and OpenMind systems have been prented by V olvo and Daimler Chrysler[34,15]illustrating the capability of incorporating the standard within the CAD/ CAM products and exporting the STEP-NC output to the Siemens840D controller.
In addition to STEP-NC milling developments the technology has also been extended to CNC turnin
g.The prototype STEPTurn software module has been developed by ISW Stuttgart,working with a Siemens840control on a Boehringer NG200lathe[34],STEPTurn software can import CAD geometry and machining features,define machining strategies and technologies and generate STEP-NC output.The Siemens controller receives this output and converts it into the Siemens ShopTurn system via a STEP-NC import facility. 5.3.STEP-NC enabled intelligent control
The dream of performing intelligent control on a CNC machine has never been truly realized.The main reason is that the information(G-code)available to a CNC machine is too low-level information,with which only minimum amount of optimization work can be carried out in real time or near real
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time.With STEP-NC,both design and process planning information is available to a CNC machine.It is possible for the CNC machines,or their controllers,to perform high-level, intelligent activities,such as automatic part tup;automatic and optimal tool path generation;accurate machining status and result feedback;complete collision avoidance check(taking into account offixture and in-process geometry);optimal Workingstep quence;adaptive control and on-machine inspection.
The rearchers at the NRL-SNT(National Rearch Laboratory for STEP-NC Technology)in PosTECH,Korea have developed a Feature-Bad STEP-NC autonomous control
system bad on an Open Architectural Virtual Manufacturing System[35–37].The information in an ISO14649part program is converted through an Interpreter into the internal data format, i.e.process quence in form of‘‘Process Sequence Graph (PSG)’’.The EXPRESS compiler in the Interpreter converts the physicalfile,in form of‘‘task description’’into a PSG,bad on the information such as geometry,technology and tool description.PSG reprents a non-linear quence of Work-ingsteps described in terms of machining_feature and machining_operation using the‘‘AND–OR’’relationship (Table1and Fig.3).As prented in the PSG,the part can be machined in a number of ways,making CNC execution flexible,optimal,intelligent and autonomous.The non-linear process quence schema enables a STEP-compliant CNC autonomous system.In preparation for executing a STEP-NC program,a Tool Path Generator(PosTPG),Tool Path Simulator (PosTPS)and a soft-CNC called NCK/PLC have been developed[35].NCK/PLC can convert the STEP-NC data model into machine tool motion,and is capable of NURBS interpolation,look-ahead control,position/velocity interpola-tion and PID(Proportional,Integral,Derivative)control.It interfaces with machine tool hardware(drivers and motors)via an I/O board.
A STEP-compliant CNC machine tool that demonstrated a G-code free machining scenario has been developed at the Manufacturing Systems Laboratory,University of Auckland [38].This rearch work consists of two parts:retrofitting an existing CNC machine and the development of a STEPcNC (STEP-compliant NC)Converter.The CompuCam’s motion control system[39]is ud to replace the existing CNC controller,which is programmable using its own motion control language—6K Motion Control language and capable of interfacing with other CAPP/CAM programs through lan-guages such as Visual Basic,Visual C++and Delphi.The STEPcNC Converter can understand and process STEP-NC codes,and interface with the CNC controller through a Human–Machine Interface.It makes u of STEP-NC information such as Workplan,Workingstep,machining strategy,machining features and cutting tools that is prent in a STEP-NC AIM file.
5.4.Collaborative STEP-NC enabled machining
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It can be said that the ultimate goal for the STEP-NC enabled machining is to support Web-bad,distributed and collaborative manufacturing(Fig.4),a scenario of‘‘design anywhere/build anywhere’’.This is possible as a STEP-NC program can parate the‘‘generic’’manufacturing information(what-to-do),from the manufacturing information(how-to-do)that is native to a specific machine tool.Therefore,a generic STEP-NC program can be made machine-independent and has an
advantage over the conventional,G-code bad NC program which is always generated for a particular CNC machine.For this type of STEP-NC program to be implemented on a native CNC system,the native manufacturing knowledge has to be incorporated.To fulfil this function,a native STEP-NC mapping system called‘‘Native STEP-NC Adaptor’’has been developed[40].The adaptor is built with three parts:a native CNC system knowledge databa, a Translator and a Human–Computer Interface.The native CNC system knowledge databa has a proprietary data structure so that the work in developing the Translator is made simpler and coherent programming of NC components across the enterpri is enabled.
Recently,there has been a trend of using XML(or rather ISO 10303Part28)instead of EXPRESS language(or ISO10303 Part21[41])to reprent the STEP-NC information.The reason for this is obvious.The XML processing ability can
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Table1
Workingstep list[37]
Workingstep
ID Feature Operation
1Planar_face Plane_rough_milling
2Clod_pocket Bottom_and_side_rough_milling 3Round_hole Drilling
4Round_hole Drilling
5Slot Bottom_and_side_rough_milling 6Slot Bottom_and_side_rough_milling 7Round_hole Drilling
8Slot
Bottom_and_side_rough_milling
Fig.3.Process quence graph[37]
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Fig.4.Distributed,STEP complaint NC machining[20].
