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Making CNC machine tools more open, interoperable and intelligent—a review of the technologies
X.W. Xu *, S.T. Newman
Department of Mechanical Engineering, School of Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand
Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK Received 30 August 2004; accepted 7 June 2005 Available online 10 October 2005
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 (ISO 6983) have been extensively used 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 developed as the data model for a new breed of CNC machine tools. The data model represents a common standard specifically aimed at the intelligent CNC manufacturing workstation, making the goal of a standardised 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.
# 2005 Elsevier B.V. All rights reserved. Keywords: CNC; Interoperability; STEP; STEP-NC
1. Introduction
From the start of craft production in the 1800s to the pioneering mass production of the early 1900s there have been a number of revolutionary changes to manufacturing system’s configurations. The most recognised 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 the 1970s and 1980s to produce a wider range of parts, ‘‘flexible’’ manufacturing was developed to meet these needs for the production of smaller batches of different parts. These systems used groups of computer numerically controlled (CNC) machines that could be reprogrammed to make different parts combined with automated transport systems and storage. These CNC machines became the central elements in the systems such as flexible transfer lines, flexible manufacturing systems (FMS) and flexible manufacturing cells (FMC).
However, the amount of flexibility existing in these 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, component-level (i.e. machine and control) issues, and ramp-up time reduction issues [1,2]. Most of the research 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, research 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 opportunities 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 addressed namely product data compatibility interoperability and adaptable CNC machines. Up till now little research has been carried out in this field, but due to the developments of the new CNC data model known as STEP-NC, there has been a surge of research activities in trying to address the above-mentioned issues. This paper reports on these research activities and tries to address the issues of interoperability 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 manufacture (e.g. combined mill/turn/laser and grinding machines). In the mean time, these capabilities have made the programming task increasingly more difficult and machine tools themselves less adaptable. Some effort has been made to alleviate this problem, in particularly the trend towards open architecture control, based on OSACA [5] and open modular architecture controller (OMAC) [6], where third party software can be used 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 these developments have improved software tools and the architecture of CNC systems, vendors and users are still seeking 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 seen as one of the key issues in limiting the wider use of these 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 case when neutral data exchange protocols, such as SET, VDA, and initial graphics exchange specification
(IGES) are used, information exchange can happen between heterogeneous CAD and/or CAM systems. This is however only partially successful since these 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] set 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 precisely, whereas CAM systems focus on using computer systems to generate plans and control the manufacturing operations according to the geometrical information present in a CAD model and the existing resources on the shop-floor. The final result from a CAM system is a set of CNC programs that can be executed on a CNC machine. STEP AP-203 and STEP AP-214 only unify the input data for a CAM system. On the output side of a CAM system, a 50-year-old international standard ISO 6983 (known as G-Code or RS274D) [10] still dominates the control systems of most CNC machines. Outdated yet still widely used, ISO 6983 only supports one-way information flow from design to manufacturing. The CAD data are not utilised at a machine tool. Instead, they are processed by a post-processor only to obtain a set of low-level, incomplete data that makes modification, verifications and simulation difficult. The changes made at the shopfloor cannot be directly fed back to the designer. Hence, invaluable experiences on the shop-floor cannot be preserved and re-utilised.
2.2. Inflexible CNC control regime
The ISO 6983 standard focuses on programming the path of the cutter centre location (CL) with respect to the machine axes, rather than the machining tasks with respect to the part. Thus, ISO 6983 defines the syntax of program statements, but in most cases leaves the semantics ambiguous, together with low-level limited control over program execution. These programs, when processed in a CAM system by a machine-specific postprocessor, become machine-dependent. In order to enhance the capability of a CNC machine, CNC controller vendors have also developed their own tailored control command sets to add more features to their CNC controllers to extend ISO 6983. These command sets 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 based 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 Gcode program. It is impossible to perform intelligent control nor machining optimization.
3. The STEP-NC standard
Today a new standard namely ISO 14649 [11–16] recognised informally as STEP-NC is being developed by vendors, users and academic institutes world wide to provide a data model for a new breed of intelligent CNCs. The data model represents a common standard specifically aimed at NC programming, making the goal of a standardised CNC controller and NC code generation facility a reality. Currently two versions of STEP-NC are being developed by ISO. The first is the Application Reference Model (ARM) (i.e. ISO 14649) and the other Application Interpreted Model (AIM) of ISO 14649 (i.e. ISO 10303 AP-238 [17]). For more information on the use and differences between them readers are referred to [18,19].
Contrary to the current NC programming standard (ISO 6983), ISO 14649 is 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 CNCs with a detailed and structured data interface that incorporates feature-based programming where a range of information is represented such as the features to be machined, tool types used, the operations to perform, and the sequence of operations to follow. Though it is possible to closely define the machine tool trajectory using STEP-NC, the aim of the standard is to allow these decisions to be made at a latter stage by a new breed of intelligent controller—STEPNC controller. It is the aim that STEP-NC part programs may be written once and used 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 ‘‘Working steps’’ performed on one or more features. These Working steps provide the basis of a ‘‘Workplan’’ to manufacture the component. Fig. 2 illustrates an actual extract of such data for a part with aWorkplan consisting of Working steps 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. This file would be interpreted by the controller, enabling CNC operators to interact at a Working step (i.e. 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].
Fig. 1. Bi-directional information flow with STEP-NC [21].
_ 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 classes) to match the abilities of a specific CAM, SFP or NC.
_ Machining time for small to medium sized job lots can be reduced because intelligent optimisation can be built into the STEP-NC controllers.
_ Post-processor mechanism will be eliminated, as the interface does not require machine-specific information.
_ Machine tools are safer and more adaptable because STEPNC is independent from machine tool vendors.
_ Modification at the shop-floor can be saved and fed back to the design department hence bi-directional information flow from CAD/CAM to CNC machines can be achieved.
_ XML files can be used as an information carrier hence enable Web-based 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].
Fig. 2. Example STEP-NC physical file [20].
5. STEP-NC for more open and interoperable
machine tools There are four types of research work related to STEP-NC: (1) conventional CNC control using STEP-NC; (2) new STEPNC enabled control; (3) STEP-NC enabled intelligent control; and (4) collaborative STEP-NC enabled machining. The degree of adaptability increases from Type 1 to Type 4. It is to be noted that STEP-NC together with STEP is now forming a common data model for representing 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 research work directly related to STEP-NC enabled CAM/CNC is discussed.
5.1. Conventional CNC control using STEP-NC
This type of research marked the beginning of STEP-NC related research endeavour. The main purpose is to answer two questions: ‘‘Does a STEP-NC file contain enough and just enough information for CNC machining?’’ and if it does, ‘‘Can it be used on a traditional CNC machine tool without making changes to the system hardware?’’. The main research is to do with the development of ‘‘translators’’ which can read in a STEP AP-203 or AP-224 file and convert it into G-code format that the targeted CNC machine tool can understand. The translator is somewhat similar to the ‘‘post-processor’’ used 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 sense that the STEP compliant information can be used across the board. Also, the design information that can be embedded in a STEP-NC file is made available to the CNC systems. This scenario represents conventional solid based manufacturing as enabled by STEP AP-203.
The work carried out in the first two stages of the three-stage Super Model Project falls into this type of work. In stage one, a range of software tools (i.e. ST-Plan, ST-Machine, and STIX [31]) were developed involving Gibbs CAM and various pieces of third-party software. The Gibbs CAM STEP translator can read in the demonstration part in STEPAP-203 format. The part is then programmed using Gibbs CAM’s graphical interface, and visually verified using its cut part rendering capability [32]. In the second stage, the AP-238 file was read using a Gibbs CAM STEP-NC Adaptor plug-in, developed by STEP Tools Inc. An MDSI Open CNC controller (software-based CNC) [33] retrofitted to a Bridgeport vertical machining centre was used as the platform for the Gibbs CAM and STEP-NC software. Using the tooling and operation parameters specified in the AP-238 file, the STEP-NC Adaptor created Gibbs CAM tooling, process and geometry elements and executed Gibbs CAM functions to generate tool–paths corresponding to the AP-238 machining features. Once again, the cut part rendering was used for visual verification prior to post processing the data to generate conventional G-code output. This work has demonstrated the ability of STEP-NC to completely automate CAM processing and tool–path generation. It has also significantly reduced the lead-time in the CAD/CAM to CNC programming time by up to 85% [32].
More recently, at the Jet Propulsion Lab (JPL) in Pasadena, California, in January 2003, STEP Tools Inc. Demonstrated the conversion of AP-203 design data into AP-238 (i.e. The AIM version of STEP-NC), feature by feature, with the use of ST-Plan AP-238 data. AP-238 data was then transferred to Gibbs CAM with the assistance of ST-Machine, and then to a five-axis Fadal machining centre. In June 2003 at NIST, a similar set-up saw MasterCAM interface with another five-axis machine tool.
5.2. New STEP-NC enabled control
Working closely with some of the popular CNC controllers or Open Modular Architecture Controller [6], several research 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, these controllers that can faithfully performs the machining tasks as specified in ISO 14649.
The third stage work of the US SuperModel Project saw Gibbs CAM integrated with an OMAC machine tool. An AP-238 data file provided all the manufacturing information to allow Gibbs CAM to generate the tool–path data. The tool–path data was then sent to a horizontal machining centre in so-called’stroke-level inter-process communication’ rather than conventional G-codes, demonstrating a higher level of CAM/CNC integration than is normally realised through ISO 6983.
Most of the work carried out in EU falls into this category of research. The main focus has been on the development of the STEP-NC enabled CNC control using Siemens 840D controller[34]. This enables the STEP-NC physical files 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 presented by Volvo 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 Siemens 840D controller.
In addition to STEP-NC milling developments the technology has also been extended to CNC turning. The prototype STEPTurn software module has been developed by ISW Stuttgart, working with a Siemens 840 control on a Boehringer NG200 lathe [34], STEPTurn software can importCAD 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 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 setup; automatic and optimal tool path generation; accurate machining status and result feedback; complete collision avoidance check (taking into account of fixture and in-process geometry); optimal Workingstep sequence; adaptive control and on-machine inspection.
The researchers at the N
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