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Integrated Manufacturing
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On the futuristic machine control in a STEP-compliant
manufacturing scenario
C. Zhang a; R. Liu a; T. Hu a
a
School of Mechanical Engineering, Shandong University, Jinan, China

Online Publication Date: 01 September 2006

To cite this Article: Zhang, C., Liu, R. and Hu, T. (2006) 'On the futuristic machine
control in a STEP-compliant manufacturing scenario', International Journal of
Computer Integrated Manufacturing, 19:6, 508 — 515
To link to this article: DOI: 10.1080/09511920600623682
URL: http://dx.doi.org/10.1080/09511920600623682

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 International Journal of Computer Integrated Manufacturing, Vol. 19, No. 6, September 2006, 508 – 515
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On the futuristic machine control in a STEP-compliant

manufacturing scenario
C. ZHANG*, R. LIU and T. HU

School of Mechanical Engineering, Shandong University, Jinan, 250061, China

STEP-NC (ISO 14649) provides a new data interface for computer numerical control
(CNC) machine tools. However, replacing the current ISO 6983 with STEP-NC is not
only a matter of change in NC programming language but it also has a far-reaching and
profound effect on the future CNCs and even on manufacturing systems. The CNCs are
destined to evolve from pure controllers into integrated systems with both decision-
making and control abilities. In this paper, strategies for implementing the STEP-
compliant CNC systems are discussed and a framework for the autonomous STEP-NC
controller is proposed. The controller is organized into two agents: the planning agent
and the machining agent. This paper also provides a general discussion on issues about
the part program interpretation, the on-line process planning. In order to support
dynamic planning as well as meet the real-time requirements in NC machining, it is
proposed to solve the on-line planning issues in two stages, i.e. the shop-floor planning
and the real-time planning. It is also proposed to extend the feature-based algorithm to
the interpolation stage.

Keywords: STEP-NC; Agent; Intelligent manufacturing; CNC; Process planning

modern manufacturing, it is evident that it has become a

1. Introduction
bottleneck, because of data model noncompliance through
Since the early 1950s, numerical control (NC) machine the CAD, computer aided process planning (CAPP), CAM
tools have been revolutionizing the metal-working practices and CNC (Xu and He 2004). STEP-NC (Weck et al. 2001)
by incorporating the latest advances in electronics, pro- is a new data interface that extends STEP (ISO 10303) into
gramming techniques and information technology. In the the NC domain (Hardwick and Loffredo 2001). However,
1970s, computers were incorporated into the NC system replacing the current ISO 6983 with STEP-NC is not only a
and thus NC evolved into CNC (computer numerical matter of change in NC programming language but it also
control). The advent of CNC brought a massive improve- facilitates the current CNC to become more capable.
ment in the capacities of the NC/CNC machines tools. Basically CNC is a part program executor. However, a
Meanwhile the NC programming task has become so number of schemes have been proposed for the CNC
burdensome that, most of the time, off-line software tools architecture in recent years. The open architecture CNC
have to be applied in modern workshops. From the has been one of the main objectives since the 1980s
automated programming tool (APT) to modern computer (Pritschow 1993). The holonic numerical controller (HNC)
aided manufacturing (CAM) systems, the off-line solutions is a rather intelligent CNC featuring autonomy and
for NC programming have improved efficiency greatly; cooperation (Bongaerts et al. 1995). The TRUE-CNC
however, the CNC programming interface, known as G & (Yamazaki 2001), VIVID-CNC and then OMNI-CNC
M codes (ISO 6983), essentially stayed the same. Although extend the conventional CNC into integrated manufac-
the international standard ISO 6983 is still widely used in turing information system to enhance the ability of

*Corresponding author. Email: crzhang@sdu.edu.cn

International Journal of Computer Integrated Manufacturing

ISSN 0951-192X print/ISSN 1362-3052 online ª 2006 Taylor & Francis
http://www.tandf.co.uk/journals
DOI: 10.1080/09511920600623682
 Futuristic machine control in STEP-compliant manufacturing 509

autonomous information generation and acquisition. With operator. In this paper this new breed of controller is called
the progress in STEP-NC, a few testing and implementa- STEP-NC controller.
tion works have been reported (Lee and Bang 2003), and Apparently the STEP-NC controller will overcome the
an intelligent STEP-NC controller that takes advantage limit of the traditional CNC concept. First, it will be a
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of the new data model becomes one of the main concerns computer application system with the abilities of decision
in academia and industries. The autonomous STEP- making as well as control of the machine tool. It should be
compliant CNC (ASNC) (Suh et al. 2002) is a conceptual able to understand the STEP-NC part program, which is
framework that consists of basic modules, functional comprehensive but somewhat like ‘uncooked raw materials
modules, interface modules and control modules. with a rough recipe’ (Suh et al. 2002), and decides and
prepares the machine-specific details for the machining
process. Second, with STEP-NC having set up a bi-
2. STEP-NC controller and STEP0-compliant manufacture
directional information highway between the machine and
Generally speaking, a mechanical part has to go through at the CAD/CAM system, STEP-NC controller will take a
least two stages in order to be actually machined in a CNC more active role in manufacturing. Undoubtedly the
machine tool after being designed in a computer aided controller will act more intelligently than ever before
design (CAD) system. The first is the programming stage, because it is ready to be informed of the available resources
which is usually done in aid of a CAM system. The second and the operating condition of the machine tool. That is to
is the actual machining stage, which is carried out in a CNC say, decisions made by the STEP-NC controller are more
machine tool. The part program bridges the gap between reliable than the estimations of an upstream system. So it is
the two stages. So the paradigm for programming is foreseeable that the STEP-NC controller will probably
decisive to the functional requirements of both CAM and become a decision maker in the manufacturing system.
CNC. In a conventional process chain (figure 1), decisions
have to be made in almost every detail in the programming
3. Implementation strategies for STEP-NC controller
stage. The part programs include detailed motion and
switch instructions so CNC does not need any more There are a number of options for interpretation and
intelligence as long as it dutifully follows the commands implementation of STEP-NC in CNC systems. Therefore,
and controls the machine tool. On the contrary, in a STEP- STEP-NC controllers may be implemented at different
compliant manufacturing scenario the part program is a levels in the future. The different implementations are given
range of information such as the feature to be machined, in this paper as interfaced STEP-NC controller, CAM-
tool types used, the operations to perform, etc. while the embedded STEP-NC controller and intelligent STEP-NC
very machine-specific decisions are left to the CNC and its controller.

Figure 1. Information flow in process chain.

 510 C. Zhang et al.

3.1. Interfaced STEP-NC controller 3.3. Intelligent STEP-NC controller

As shown in figure 2, the simplest way to interface the 3.3.1. Functional requirements. The above discussion
STEP-NC codes with CNC is probably to add an concentrates on a CNC that can receive a file with STEP-
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interpreting module to CNC. The interpreting module NC data and proceed milling accordingly. There is still
converts the STEP-NC based program into G&M codes much room for improvement. For example, they are too
and then feed them into the NC kernel. In this case, the dependent on the operator’s experience. Considering that
interpreter acts as a comprehensive post processor trans- STEP-NC provides an information highway as well as
planted from CAM to CNC, and a conventional CNC can facilitates the improvement on modern CNC, it is highly
be used with little modification. It should be noted that part desirable to develop an intelligent STEP-NC controller
program must be well organized in advance on a CAM with an efficient mechanism to take full advantage of the
system by a programmer who is familiar with the machine comprehensive information of the data model.
resource, for this kind of STEP-NC controller has no more Some functional requirements of a STEP-NC controller
intelligence than a traditional CNC except for the capacity may be outlined.
of interpreting the STEP-NC codes. Since this kind of
STEP-NC controllers depends highly on more capable (a) Real-time control. As a machine controller, the basic
CAD/CAM systems, Newman et al. (2003) presented a function of the STEP-NC controller is to control the
number of CAD/CAM solutions and implemented a system machine tool by sending control signals to the
(AB–CAM) for the STEP-compliant manufacture in the actuators and receiving feedback information for
light of this kind of STEP-NC controllers. verification. It is also assumed that the controller can
execute its program as quickly as necessary to meet
the real-time constraints of the machining process.
3.2. CAM-embedded STEP-NC controller
(b) Integrated active quality control. The system is
As shown in figure 3, the CAM-embedded STEP-NC intelligent sensor based and can dynamically
controller is basically a combination of an STEP-NC code acquire the information of machining process and
interpreter, a basic CAM and a conventional CNC. The machine tool and perform the diagnostics and
interpreter translates the physical file into the internal data monitoring to assure the product quality.
format, the CAM makes decisions on the machine-specific (c) STEP-NC interfaced. The controller must be able
details and generates G&M codes, and finally the CNC to read, understand and convert the information
executes the codes. In this case, the STEP-NC controller contained in a STEP-NC file. In addition, a graphic
can be implemented with certain intelligences; however the user interface should be provided for visualizing of
problems between CAM and CNC are not solved but editing, operation and simulation of the machining
hidden. process.

Figure 2. Interfaced STEP-NC controller.

Figure 3. CAM-embedded STEP-NC controller.

 Futuristic machine control in STEP-compliant manufacturing 511

(d) Micro-process planning and inspection. The con- controller with cooperative ability. It is capable of
troller should be able to make decisions on the e-consultation with outside including job-cooperative and
process-related details, including generating tool information service. A holonic STEP-NC controller is
trajectory on a feature/working-step basis, optimiz- suitable for web-based manufacturing. It can be an agent
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ing the technological parameters, sequencing the that can not only perform the task of process planning and
working-steps etc. As on-machine inspection is vital control well, but can also evaluate the received task and
to the time reduction and quality control in the part negotiate with other agents on internet.
manufacture, the controller should also support the
part inspection. Nowadays ISO 14649 part 16 is 3.3.3. Other considerations. As to the implementation of
under development for the STEP-compliant inspec- STEP-NC, the following aspects should also be taken into
tion (Ali et al. 2005). consideration.
(e) Resources management. To manage and control
the tool magazine, loading/unloading robots etc. (a) Current approaches and tendencies of CNC tech-
The controller can only make the best decision nology. Open architecture is a trend of CNC
when it knows well about the available resources. technology. PC-based vendor-neutral hardware
(f) Autonomy. The controller should be able to plan and software result in a control system that is
and control the machining process autonomously, economical, maintainable, open, modular and
including the ability to deal with unexpected scalable. It also provides the possibility for user
change/failure. and developer to integrate STEP-NC functions into
(g) Cooperativeness. As one system of the manufactur- system. Therefore STEP-NC controller should be
ing chain, the controller should be able to implemented as open as possible.
communicate with other systems. (b) As compatible with STEP application as possible.
(h) Knowledge acquisition. The adoption of the STEP- Although STEP-NC just defines an STEP compli-
NC paradigm brings forth the opportunity to ant interface for CNC, the compatibility of the
consider not just the information but also the CNC system with STEP compliant CAD/CAM
knowledge requirements for process planning and systems will be essential for the seamless connection
machining discrete parts (Newman 2004). To of these systems in an e-manufacturing scenario.
support the intelligent manufacturing, the control- (c) More capable mechanism. STEP-NC controller is a
ler needs to have a lot of manufacturing knowledge, decision-making and control system. It cannot be
and more importantly it should be able to learn and regarded as an extension in functionalities of the
acquire new knowledge. conventional CNC. The decision-making tasks and
the control tasks should not be separated.
3.3.2. Types of the intelligent STEP-NC controller.
Although the approach of using intelligent systems is not
new, and every feature mentioned above is not a new topic 4. An agent-based framework
in manufacturing, there are some major challenges to
4.1. Architecture of ADACOR agent
incorporate so many features to the machine controller.
Maybe the STEP-NC controllers can be grouped into three The performance of a STEP-NC controller depends on the
different functional levels, as shown in figure 4. design goal as well as on its architecture. In this paper, the
Autonomous-type controller is of an ability of process architecture of ADACOR (adaptive and cooperative
and operation planning, analysis and control. It should control architecture for distributed manufacturing systems)
be implemented with most of the above-mentioned func- agent (Paulo and Restivo 2001) is considered in our
tional requirements from (a) to (f). Autonomous- and approach.
cooperative-type controller is a special autonomous As shown in figure 5, in the agent there are three basic
modules and a database, which stores all relevant informa-
tion about its behaviour. The communication module deals
with the need of interaction between distributed agents.
The local control and supervision module intends to
control and supervise the operational execution of the
agent. Only this module interacts with the physical devices.
The decision-making module is supposed to be in charge of
all the intelligent activities as a whole. To make a decision it
Figure 4. Different levels of the intelligent STEP-NC uses the knowledge stored in the local database, and when
controller. necessary it starts a cooperation process with other agents,
 512 C. Zhang et al.
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Figure 5. Architecture of the ADACOR agent.

trying to find out the necessary information or negotiate

about the task. Apparently this architecture and mecha-
nism conform to the idea of developing an autonomous and Figure 6. Structure of the planning and machining agents.
cooperative controller in many aspects.

The machining agent controls the physical process of the

4.2. Framework for the controller
machining according to the tasks given by the planning
Considering the comprehensive functionality and require- agent and collects the status of the machine tool resources.
ment for real-time motion control, some modifications have When the machining cycle starts, it reads the unit tasks and
to be made for a STEP-NC controller. From the perspective converts them into specific instructions in turn. When a
of planning, many issues do not need to be solved in real- unit task fails owing to unexpected occurrence such as a
time. From the control point, motion control, machine tool breakage, the real-time planning module will check if
Input/Output device control, robot control etc. have different there is another tool available in the magazine. If ‘yes’, an
time-critical level requirements, and among them, motion additional unit task will be inserted automatically. Other-
control is of top priority. Therefore as shown in figure 6, the wise, it reports to the planning agent for help. The NC
task of the motion control and a little decision-making work kernel module in figure 7 stands for something such as the
are separated from the whole and organized into an agent software-based motion control as a whole, which interfaces
called machining agent, while others (each task may be directly with the machine tool. The adaptive control means
organized as an agent in the future) are assigned into another, that the cutting parameters are dynamically determined
called planning agent. Both agents are expected to deal with during operation. The communication module in this part
certain amount of both decision-making and control issues. is for internal interaction (reporting, supervising etc) with
Figure 7 shows the two agents are integrated. Actually the planning agent, thus can be implemented in a simple
they are organized in a hierarchical way. The planning and direct way.
agent decides on the operation strategies and controls all
the preparatory activities before the real-time machining,
including understanding the program, checking out the 5. Major issues in implementation
available resources, analysing the given recipe, planning as
5.1. Part interpretation
a whole, resource management, etc. When a task is
received, the agent will interpret it into the internal data As for the implementation of the STEP-NC controller, the
model, check the available resources and check whether the first thing we are confronted with is the interpretation or
work can be accomplished by the machine agent, and then recognition of the part program based on STEP-NC.
decides whether to take the job or not. Generally speaking, Actually the STEP-NC program is a text file based on ISO
if ‘no’, it should form a report and stop; if only just for a 10303-21. Owing to the complex data model and mapping
little problem, it may negotiate through its communication procedure from ARM through AIM to the file structure,
module using the communication mechanism as discussed the STEP-NC file is usually complicated and beyond
in the reference (Paulo and Restivo 2001) with the upstream readability. In addition to keep compatible or share data
systems in order to make some changes; if ‘yes’, it will find with other STEP applications, the STEP methodology
out the best way and decompose it into sequenced unit should be adopted.
tasks that can be easily completed on the machine. The set- In fact, STEP-NC controller can be regarded as a special
up module, in charge of the fixture and tooling tasks, acts STEP application. The interpretation module can be
as the local control module of the planning agent. Note that implemented referring to the STEP methodology. As
the physical activities of the machine tool are separated in shown in figure 8, the interpretation usually takes two
light of the real-time requirement of motion control. steps. First, each entity or data type defined in the
 Futuristic machine control in STEP-compliant manufacturing 513
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Figure 7. A framework for the STEP-NC controller.

Figure 8. EXPRESS to Cþþ.

EXPRESS schema should be mapped to internal data

format (Cþþ is only considered in this paper) that can be
used by the programming language, for example, defining
the class CSlot for Slot. Then in the second step, each
instance of the entity Slot will be transformed into a CSlot
object. At the same time, the logical link of the entities is
also expressed as relationship between Cþþ classes, e.g.
inheritance, membership etc.

5.2. Process planning

Figure 9. A three-stage process planning mode.
As well as the interpretation of the part program, the
process planning is also essential in implementation of
STEP-NC controller, and a feature-based approach is
needed. However it is not necessary to incorporate a the shop-floor planning include manufacturability analysis,
complete CAM system into the controller. In general, workingstep-level planning, feature-level planning and
process-related issues can be solved in two stages, i.e. off- workpiece-level planning (Liu et al. 2004a).
line planning and on-line planning. Considering the special Tasks for real-time planning include (i) real-time
requirements in STEP-NC machining, the on-line planning generation of the tool trajectory and control codes just
is divided into a shop-floor planning stage and a real-time before the execution of the workingstep; and (ii) real-time
planning stage, thus a three-stage process planning mode adjusting of the technological parameters in machining
is outlined in this paper (figure 9). Issues involved in process.
 514 C. Zhang et al.

feature-based algorithm into the interpolation stage

5.3. Interface between the two agents
(figure 11). It is expected to achieve the following goals:
To generate and control the tool path precisely is vital to
NC machining. Traditionally path generation and control (i) to eliminate the G codes both outside and inside
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is actually carried out in different systems. As shown in the controller, which will save time spent in coding
figure 10, the machining of the part has to go through a lot and encoding;
of procedures in both the CAM and CNC systems. (ii) to provide the control module with more detailed
Although a CAM system may apply the feature-based information for real-time decision making to some
approach in planning, the results have to be decomposed extent;
into low-level messages to be readable to CNC. (iii) to make the information flow smooth by adopting
However, STEP-NC breaks this rule, and the machine an internal data interface;
controller has to deal with both. This indicates the current (iv) to support dynamical planning by working-step
situation, in which the low-level geometrical elements must based tool path description.
be used in path planning and interpolation might be
changed. Based on the STEP-NC data model, a feature- To date this has not been implemented, but the idea of
based approach is thus proposed in this paper to extend the using feature-based approach is not new. For example,

Figure 10. Traditional process from planning to control.

Figure 11. Proposed process from planning to control.

 Futuristic machine control in STEP-compliant manufacturing 515

Mazak’s CAMWARE already successfully integrated their

Acknowledgements
offline CAMWARE system with Mazak CNC machines,
with Mazak defined CNC features. This shows a feature The authors wish to thank National Natural Science
based machining approach for STEP-NC is definitely Foundation of China (grant No. 50445004) and Natural
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feasible. Science Foundation of Shandong (grant No. Y2003F10)

for their support of this research.
6. Conclusions
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