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ABSTRACT: Virtually all manufacturing processes are subject to variability, an inherent characteristic of most production processes. No two parts can ever be exactly the same in terms of their dimensions. For machining processes such as drilling, milling, and lathing, overall variability is caused in part by machine tools, tooling, fixtures and workpiece material. Since variability, which can be accumulated from tolerance stacking, can result in defective parts the number of parts produced in a batch is limited. When there are too many parts in a batch, the likelihood of producing all acceptable parts in a batch decreases due to the increased tolerances. On the other hand, too small a batch size incurs an increase of manufacturing costs due to frequent setups and tool replacements, whereas the likelihood of acceptable parts increases. To address this challenge, we present a stochastic model for determining the optimal batch size where we consider part-to-part variation in terms of tool wear, which tends to be proportional to batch size. In this paper, a mathematical model is constructed based on the assumption that the process used for producing preceding parts affects the state of subsequent parts in a probabilistic manner.
International Journal of Production Research 07/2009; 47(NO. 14):3919-3936. · 1.12 Impact Factor
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ABSTRACT: L.(2010) 'Using finite state automata (FSA) for formal modelling of affordances in human-machine cooperative manufacturing systems', Modelling complex systems poses significant challenges on how one represents the system components and interactions among them. In order to provide a systematic approach to represent human participation as a part of a dynamic system, this paper presents a formal automata model of human-machine cooperative systems that incorporates human capabilities with respect to system conditions. Specifically, we propose a control model for human-involved shop floor systems based on discrete event-based systems (DES) and an environmental concept known as an affordance. When modelling human-involved systems where a human operator is considered a crucial system component, it is necessary to analyse the model complexity that increases significantly due to a human's behavioural patterns. From the perspective of the temporal and physical state domains a human operator's behaviour is usually limited by attention and resource constraints. We investigate these limitations and map them into constrained system affordances, and then construct a formal human-machine cooperative model based on the finite state automaton (FSA) model. The proposed model can provide a framework to combine human activities into systems operations in consideration of human's effectivities and system affordances. A detailed application example is provided to illustrate that the proposed model can effectively be applied to manufacturing control systems.
International Journal of Production Research 01/2009; 48(5):1303-1320. · 1.12 Impact Factor
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ABSTRACT: Formal modelling techniques that employ principles of finite state automata have been increasingly used for shop-floor control. These approaches have focused primarily on unmanned systems where the self-contained logic for control of the systems is embedded within the model(s). To date, human operators are seldom configured within the modelling detail required for these systems. The result has been that the interface of humans within automatic systems has been limited. In general, humans performing physical activities in an automatic system must mimic the detailed responses normally transmitted via control computers and equipment, making the human interaction painful and prone to error. This paper discusses (1) the formal modelling specifics required for the control of such an automated shop floor system; and (2) the role and interactions of the human primarily in automatic systems. The human in the automatic manufacturing system addressed here serves primarily as a material handler and material transporter, providing almost unlimited possibilities regarding the accessibility of product flow from any system resource to any other system resource—a major constraint in many operational systems. The impact on the complexity of the accessibility graph, issues related to automatically controlling such a system and the human interactions within such a system are also developed. The paper introduces many complex issues and interactions, and concludes with an example of how a human handler can be effectively included in a finite state automaton.
International Journal of Production Research 04/2007; 45:1953-1971. · 1.12 Impact Factor
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ABSTRACT: Formal modelling techniques that employ principles of finite state automata have been increasingly used for shop-floor control. These approaches have focused primarily on unmanned systems where the self-contained logic for control of the systems is embedded within the model(s). To date, human operators are seldom configured within the modelling detail required for these systems. The result has been that the interface of humans within automatic systems has been limited. In general, humans performing physical activities in an automatic system must mimic the detailed responses normally transmitted via control computers and equipment, making the human interaction painful and prone to error. This paper discusses (1) the formal modelling specifics required for the control of such an automated shop floor system; and (2) the role and interactions of the human primarily in automatic systems. The human in the automatic manufacturing system addressed here serves primarily as a material handler and material transporter, providing almost unlimited possibilities regarding the accessibility of product flow from any system resource to any other system resource—a major constraint in many operational systems. The impact on the complexity of the accessibility graph, issues related to automatically controlling such a system and the human interactions within such a system are also developed. The paper introduces many complex issues and interactions, and concludes with an example of how a human handler can be effectively included in a finite state automaton.
International Journal of Production Research 04/2007; 45(9):1953-1971. · 1.12 Impact Factor
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ABSTRACT: To achieve an effective integration framework for manufacturing systems, a formal model of a system is highly desired. In spite of significant work on automated manufacturing systems, human operators still play a critical role in virtually every system, especially for material-handling processes. To build a model for control and analysis of a system where a human operator is integrated, a formal functional specification of a human material handler (MH) is presented in a hierarchical framework. Two types of human operational errors associated with material-handling tasks are also classified and discussed. A shop floor control example is provided to illustrate the proposed modeling framework
IEEE Transactions on Systems Man and Cybernetics - Part A Systems and Humans 08/2006; · 2.12 Impact Factor
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ABSTRACT: This paper presents a formal approach to resolve an important question concerning changes in the control of computerized manufacturing systems when a human operator is involved as a task-performing agent. It requires building a model of human functional specifications used in executing tasks and integrating it into a control scheme for the model. More importantly, analysis of control complexity needs to be conducted to build an effective control mechanism. In this paper, a human material handler is considered, and an assessment of part flow complexity affected by human tasks in a highly automated manufacturing system is presented. For this purpose, a formal model of human task-performing processes is proposed in terms of a part and location(s) of a task. A classification for human material handling tasks is presented based on the proposed model. Furthermore, human errors and the impact of human errors on part flow are considered. Part flow complexity of a manufacturing system from the control perspective is then investigated in terms of the human tasks and errors. A shop floor control example where a human operator performs material handling tasks is provided to illustrate the proposed model.
IEEE Transactions on Systems Man and Cybernetics - Part A Systems and Humans 02/2006; · 2.12 Impact Factor
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ABSTRACT: This paper presents a top-down mechanism for coordinating distributed discrete event simulation (DDES) models using an MRP/ERP system as the federation coordinator. The same MRP/ERP system, which is typically used as a coordination tool for interactions between complex highly variable manufacturing systems, serves to coordinate and synchronize complex highly variable simulation models of these same systems. This research focuses on enabling each system entity modeled by DDES models to constantly correct its performance with respect to reference trajectories which consist of planned orders and the size of a time bucket generated by an MRP/ERP system, and trigger a global coordinator which consists of the MRP/ERP system and adapter if necessitated by any discrepancies observed by the entity through simulation models. A global coordinator can synchronize timing of DDES models and provide adaptive time buckets using the cost-based mathematical model and corrected plans using the updated time bucket.
Simulation Conference, 2004. Proceedings of the 2004 Winter; 01/2005
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ABSTRACT: Increased complexity of simulation models and the related modeling needs for global supply chains have necessitated the execution of simulations on multiple processors. While distributed simulation promises reduced complexity (as the result of decomposition), increased parallelism and convenient analysis of geographically distributed systems, it poses a challenging problem: synchronizing the distributed simulation federates. This paper discusses a new discrete event distributed simulation framework, which is designed with two goals in mind: (1) easy and fast development of distributed simulations and (2) efficient adaptive synchronization of simulation processes. This research uses state machine models for the automated synthesis of so called 'local synchronization agents' and an adaptive synchronization algorithm has been developed based on pacing of simulation processes using real-time. Upon completion, this scalable framework is expected to shorten the lead-time to develop distributed simulation systems with reasonable performance characteristics.
Simulation Conference, 2004. Proceedings of the 2004 Winter; 01/2005
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ABSTRACT: The past two decades have seen significant improvements in optimization modeling and software solvers for large-scale optimization problems, especially discrete problems. We feel that a critical feature of many of these systems is being overlooked. That is, the process control engineer adjusts process parameters while only considering the local efficiency or not considering process efficiency at all. Production control engineers, while optimizing the global system performance, consider process parameters as given and fixed, i.e., unchangeable. Combining the optimization of the process parameters with a global system view can significantly improve the overall system performance. In practice, "hot jobs" are treated in this ad hoc manner, making sure that all resources are available and operate at peak efficiency (minimum production time) for these critical products. This phenomenon occurs not only in manufacturing but also in many other industries. This modeling part of the optimization problem can be even more important than "optimal versus heuristic"-based solution decisions made. In this paper, we present an aggregative high-fidelity modeling approach and illustrate the formulation of parameter variability in three different domains: manufacturing, air travel, and food processing.
IEEE Transactions on Robotics and Automation 09/2003;
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ABSTRACT: Techniques based on discrete-event simulation have been widely used for network analysis and policy optimization in the domain of supply chain management. Previous researchers have developed and implemented architectures for simulation-based control for shop floor. A more detailed and high-fidelity simulation model is used for control purposes as opposed to that used for analytical purposes alone. This paper discusses the issues related to implementing a simulation based control architecture for actively controlling supply chain interactions.
Simulation Conference, 2002. Proceedings of the Winter; 01/2003
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ABSTRACT: This paper presents an architecture and a design for a federation object coordinator (FOC) for simulation based control and analysis. This research focuses on developing a methodology for implementing a distributed simulation control mechanism which can be adopted to virtual manufacturing or virtual enterprises. In this method, distributed fast or real time simulation models interact with low level controllers and among themselves to actively control a system. The timing and coordination requirements of the simulation models to interact with the MRP systems and control systems as well as the interaction among the distributed simulation models are discussed in this paper.
Simulation Conference, 2002. Proceedings of the Winter; 01/2003
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ABSTRACT: Flexibility has become an important characteristic of today's
manufacturing industry. Unfortunately, today's standard of using a fixed
(linear) process plan is not flexible enough to adapt to different
manufacturing system conditions aiming at increasing the system's
efficiency. One means to overcome such a limitation is to prepare
pre-planned alternatives in the process plan. If this can be easily done
and implemented, the most appropriate plan can be selected in real time
from all possible alternatives according to the conditions of the shop
floor. The objective of the paper is to demonstrate this concept in
detail and to discuss the influence of process plan alternatives in a
dynamic shop floor environment at equipment level. In order to verify
whether the presence of alternatives in process plans increases the
efficiency of the manufacturing system, a simulation will be carried out
using some example parts currently being manufactured in Penn State's
Factory for Advanced Manufacturing Education (FAME)
Systems, Man, and Cybernetics, 2001 IEEE International Conference on; 02/2001
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ABSTRACT: Various analytical and empirical methods assuming the existence of
steady state and requiring homogenous properties of the product have
been used with limited success in estimating freezing times in the food
processing industry. Irrespective of the method adopted for estimating
freezing time requirements, a critical process issue that needs to be
considered is that of system control. Simulation models suggest that a
feedforward control strategy, as discussed in this paper, can be used to
control a freezing tunnel and obtain considerable energy savings while
ensuring 'appropriate' freezing of all products. The control strategy
discussed in this paper, involves the continuous monitoring of product
input and controlling either or both of the refrigerant flow and
conveyor speed. The primary objective of this paper is to demonstrate
the use of simulation to predict process parameters for 'intelligent
control' of freezing tunnels, and provide an estimate of potential
energy savings
Simulation Conference, 2001. Proceedings of the Winter; 02/2001
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ABSTRACT: Simulation models are usually developed as a one-time use analytical models by a systems analyst (usually from an external firm) rather than for routine and interactive use by a shop floor engineer. This is because it usually takes a longer time to generate a result from the simulation, and the simulation model of a manufacturing system is usually too sophisticated and time-consuming to use as an interactive tool by the manufacturing/production engineer. A CAMS reduces this complication by encapsulating the 'complicated-logic' and automating the 'tedious data-acquisition' with a more user-friendly interface like a spreadsheet or database input form. The paper describes how CAMS can automatically generate a simulation model; specifically, techniques and issues to structure the model to hide those tasks, so that it is a user-friendly interactive decision support with minimal amount of automation code. The paper concludes with a capacity analysis example from the real industry
Simulation Conference, 2001. Proceedings of the Winter; 02/2001
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ABSTRACT: Researchers at the National Institute of Standards and Technology
have proposed the development of neutral libraries of simulation
components. The availability of such libraries would simplify the
generation of simulation models, enable component-based modeling, and
speed Internet-based simulation services. The result would be a
reduction in the complexity of simulation modeling and analysis. We
consider a discrete-event simulation of the flow of jobs through a job
shop. We describe the information requirements for the components in
that simulation and provide formal models based on those requirements.
We then derive a database structure from these formal models and discuss
the population of that database with the data entries for a sample job
shop. Finally, we examine the translators we developed to go from the
neutral representation of the simulation components to the
representation required by a commercial simulation package
Simulation Conference, 2000. Proceedings. Winter; 02/2000
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ABSTRACT: Most control implementations of flexible manufacturing cells have
been developed specific to a particular facility, and no generic format
or tools exist for the systematic planning and creation of control. This
paper presents the first phase of research in automatic generation of
control software. It focuses on the development of theoretical
foundations and generic issues necessary to understand and implement
control. Specific formal models are developed for the physical
activities, system actions, and individual machines comprising the
manufacturing cell. In a subsequent paper, the formal models presented
here are used to provide the basis for creating context free control
grammars which are used to automatically generate software for
controlling flexible manufacturing cells
IEEE Transactions on Robotics and Automation 09/1995;
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ABSTRACT: Flexible manufacturing systems are capable of producing a broad
variety of products and changing their characteristics quickly and
frequently. This flexibility provides for more efficient use of
resources, but makes control of these systems more difficult. Control
problems previously unstudied now require practical resolution, like
system deadlock. A system deadlock is a situation that arises due to
resource sharing in manufacturing systems, when the flow of parts is
permanently inhibited and/or operations on parts cannot be performed.
This problem has been ignored by most scheduling and control studies,
which usually assume infinite machine queue capacity and unlimited
tooling resources. FMS's, however, have little or no queue capacity and
limited tooling resources. In this paper, graph-theoretic deadlock
detection and resolution procedures are presented which are suitable for
real-time control of manufacturing systems. These procedures determine
whether part movement in the system causes system deadlock or not. To
this end, a system status graph representing part routings is virtually
updated for every part movement before parts move physically to the next
destination. Two types of system deadlocks, part flow deadlock and
impending part flow deadlock, are detected using the updated system
status graph. If a deadlock detection and recovery method is used to
recover from a deadlock using a storage buffer, only part flow deadlocks
need to be detected. On the other hand, if no buffer is available, both
types of existing as well as impending system deadlocks need to be
detected to avoid a deadlock situation
IEEE Transactions on Robotics and Automation 07/1995;
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ABSTRACT: The paper describes an application of discrete event simulation for shop floor control for a flexible manufacturing system. In this application, the simulation is used not only as an analysis and evaluation tool, but also as a "task generator" for the specification of shop floor control tasks. Using this approach, the effort applied to the development of the simulation is not duplicated in the development of the control system. Instead, the same control logic is used for the control system as was used for the simulation. Additionally, since the simulation implements the control, it provides very high fidelity performance predictions. Implementation experience in two flexible manufacturing laboratories is described. These implementations use a special feature of the Arena/SIMAN simulation language which allows Arena/SIMAN to interact directly with the shop floor control system through an interprocess communication mechanism. This feature is described in detail.
Simulation Conference Proceedings, 1994. Winter; 01/1995
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ABSTRACT: One specific problem of control, namely, system deadlock that can
arise in an unmanned flexible manufacturing system (FMS) is addressed,
and some related work in the deadlocking of computer systems is
described. A formal model for manufacturing system deadlock detection is
presented. Necessary and sufficient conditions for manufacturing system
deadlock based on actual manufacturing-system characteristics are
defined along with a set of bounds for searching for deadlocks. An
implementation approach is also presented
IEEE Transactions on Robotics and Automation 01/1992;
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ABSTRACT: Presents a systematic approach to automate the development of
control software for flexible manufacturing cells at the cell,
workstation and equipment controller level. A functional architecture
and context-free grammars are proposed as the formal basis, and the FMS
system is controlled as a by-product of recognizing context-free
grammars. Sematic actions associated with the productions in the
grammars provide the required control actions. The underlying system
architecture, along with the physical, system and functional models are
presented for the various components of the FMS. Implementation issues
of the proposed approach are also discussed
System Sciences, 1991. Proceedings of the Twenty-Fourth Annual Hawaii International Conference on; 02/1991
