ADAM: R&D Article
Ho, K.L. and Ranky, P.G.: An Object Oriented Approach to Flexible Conveyor System Design, Programming and Operation Control
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Addresses: John K. L. Ho, PhD, Associate Professor, Department of Manufacturing Engineering, City University of Hong Kong, Hong Kong. (Please note, that Dr. Ho can be reached via ADAM by email: firstname.lastname@example.org)
Paul G. Ranky, Dr. Techn/PhD, Research Professor, The Department of Industrial and Manufacturing Engineering, New Jersey Institute of Technology, Newark, NJ 07102, USA, Email: email@example.com
This work is aimed at exploring new methods, a novel object-oriented system design and operation control concept, and new system software and hardware design concepts of computer (PLC) controlled flexible conveyors.
In order to to cope with changes imposed by today's dynamic market the validated and implemented methods explained in this paper could be used to design and build open, flexible and reconfigurable material handling systems such as the "Open & Reconfigurable Conveyor System, ORCS" (as referred to in this paper), in a flexible Computer Integrated Manufacturing/ Assembly (CIM) environment.
The proposed generic system architecture, as a result of our research, offers an integrated and flexible platform for the development and/or modification of new and/or existing conveyor systems, as well as other types of lean and agile material handling systems.
Open & Reconfigurable Conveyor System (ORCS), Computer Integrated Manufacturing/ Assembly (CIM), system modeling and design, Object oriented design of conveyor systems, PLC, Programmable Logic Controller
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In response to the radical changes that occurred in recent years in world-wide industrial markets, many manufacturing enterprises have begun to adopt new, innovative ways to manufacture. Compared to the past, customers demand an ever wider range of relatively low-cost, reliable and high quality products with increasingly shorter and more reliable delivery times [Bielge, U et. al., 1977, CASA/SME 1993, Redmond, 1997, Lawrence, 1997].
In order to deal with the continuous production of varying mixes of products and the quick changes of production requirements in response to the changing demands in the market, the manufacturing / assembly system must have a high degree of dynamically adjustable level of flexibility [Kickert 1985, Gupta & Buzacott 1989, Sethi et al. 1990, Pyoun & Choi 1994].
A flexible assembly system generally encompasses a series of different operations including assembly, material handling and storage, etc. with the aid of human operators and robots, and different types of programmable devices under computer supervision. However, any further improvements of the assembly system's reliability, efficiency and flexibility still depend upon the advancement of the design, configuration, software control and implementation of the material handling system [Ho & Law 1990, Ho & Ranky 1994a].
As examples, Figure 1 and Figure 2 represent conceptual system designs of flexible systems, and Figure 3 is a photo of a real-world conveyor system, implemented as part of the FALS project (Flexible Assembly Line System), at the first author's university laboratory in Hong Kong.
Most material handling devices, both hardware and software, are highly specialized and are inflexible and costly to be configured, installed and maintained [Andreasen & Ahm 1986, Ho 1993, Redmond, 1997]. Any changes to the material handling system's configuration such as the change of the conveyor system's layout, or the replacement of the conveyor that provides the continuous flow of work-parts by an Automated Guided Vehicle (AGV) in a flexible assembly system is almost out of question due to the fact that AGVs can only handle high-cycle time production and often their cost is high [Ho & Ranky 1994b, Bielge, U et. al., 1977].
In view of the radical changes in customers' demand, classical material handling systems with low degree of flexibility are delaying and often blocking the development of highly flexible assembly systems [Eastman 1987, Ranky 1990a, 1996, Redmond, 1997, Bielge, U et. al., 1977].
Conveyors are fixed in terms of their locations and the conveyor belts are running according to their synchronized speeds, making any changeover of the conveyor system very difficult and expensive [Materials Handling News 1993+, Modern Materials Handling 1986+, Ho 1991].
Over the past several years many researchers as well as the assembly industry itself have called for the development of a Flexible Assembly Line System (FALS) integrated into a CIM environment [Koff & Boldrin 1985, Harris & Charnley 1922, Ranky 1990a, 1996]. The manufacturing / assembly industry is looking for a new generation of material handling systems with an open CIM system architecture to meet the need of modularity, flexibility, speed, accuracy, reliability, cost and object-oriented software control system design.
Modularity and flexibility allows the material handling system to be selected, configured, integrated and upgraded in a simple and feasible way. An open CIM system architecture is, thus, required to allow the adoption of different components to form multi-vendors. Furthermore, to achieve flexible integration, the hardware and software architecture requires of its objects to give details of their interface boundaries, their functionality and inter-relationships [Gupta & Buzacott 1989, Pyoun & Choi 1994, Lawrence, 1997].
In terms of production flexibility the material handling system should have the capability to enable the assembly line to handle work-parts in an effective, programmed way, complying with the following general requirements [Nof & Woo 1991, Ho 1992a, Ranky 1992, Ho & Ranky 1994a]:
Research on material handling systems is focused on the flexibility and intelligence of the manufacturing system. The main research effort is concentrating on AGV systems rather than on the conveyor which can provide continuous flow production [Koff & Boldrin 1985, Harris & Charnley 1992].
A review of the literature has indicated that no significant new research work has been done in recent years on the system design and the operational concepts to improve of the continuous flow production system such as conveyor. However, some improvement have been carried out by individual conveyor manufacturers in the production of their wide range of proprietary, standard-building components of conveyor so that the conveyor can be customized [Materials Handling News 1993+, Modern materials handling 1986+, Lawrence, 1997].
In actuality this "customized" approach only enhances the degree of flexibility of the physical conveyor's layout, but does not really enable the flexible integration and production flexibility that most manufacturing enterprises need. Strictly speaking, this type of "customized" approach currently adopted by some conveyor vendors are reducing the openness of the conveyor system in that customers have to rely on the same supplier to obtain the required physical compatibility [Ho 1992b].
At present, the operation of a conveyor system is still largely based on the conventional, rather inflexible mass production philosophy [Hitomi 1979, Merchant 1982, Sweeney 1990, Ranky 1983, 1986, 1990a,b, 1995, 1996]. In today's radically changing industrial markets, there is a need to implement a new system design methodology, a new manufacturing system operation planning concept and a new system control software and hardware development concept that can be applied to the design of a new generation of open, flexible and reconfigurable material handling systems [Sethi et al. 1990, Ho & Ranky 1994a,b, Lawrence, 1997].
The model of the flexible and reconfigurable conveyor system
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Most layouts of assembly factories are set-up according to the product-flow. Work-parts are moved by the conventional conveyor(s) from one station to the other typically at a pre-defined speed. The product is progressively fabricated as it flows through the pre-defined sequence of assembly heads and workstations.
Nevertheless the conventional conveyor system has some serious problems, summarized as follows:
In order to resolve this problem we have proposed and implemented various prototypes of a new conveyor system, the "Open & Reconfigurable Conveyor System", or ORCS.
Furthermore we have extended the CIM-OSA concept using object-oriented principles and partially implemented a new 3D reference model for factory automation and in particular for the software control of the ORCS [Ranky 1990a,b,c, Ho 1992a,b, Ranky 1993 & 1996].
The expansion of the CIM-OSA model, "Hierarchical Objects' Structure" (HOS), is shown in Figures 4 and Figure 5 for the design and development of a new open, reconfigurable material handling system. It contains decomposition flow and object growth flow through the following system generation processes: requirements definition, design of system & it's building objects, development and implementation of system & it's building objects.
In order to create a highly flexible, 3D conveyor system using object-oriented control software techniques that could be part of an integrated CIM solution, we have extended the flat "CIM wheel" (i.e. the horizontal slice of our 3D CIM reference model) with the factory hierarchy as the vertical dimension (Figure 6).
This activity led to the creation of the HO'S system-building object (Figure 7 and Figure 8), the core of our system design.
Furthermore, as discussed in detail below, we have implemented various prototypes of our new 3D reconfigurable and flexible modular conveyor system (ORCS) to experiment with the proposed 3D CIM infrastructure as well as to validate the concept using real-world working models [Ho 1991 & 1992a, Ranky 1994, Beckman 1989].
HO'S model organization and notation
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The design framework and the notation of HO'S model is based on the construction of HO'S system-building object and the model's graphical structure based on the well known diagrammatic representations of IDEF0, the data flow diagram and the object-oriented concepts of Ranky's CIMpgr. The advantage of using these familiar graphical representations is to make the model easy to be understood by everyone.
For each model, HO'S diagrams are organized following a topdown structure, gradually showing the decomposition of the system blocks. The top of the model is a block which represents the most general characteristics of the given system. In this research the top of the model was the OFCS at the shop floor control area .
As suggested by HO'S model, this block may be broken down into a number of blocks, each of which represents a sub-block of the original function. These sub-blocks may be further decomposed until the system is described at the required level. At any of the levels, the higher level diagram is said to be the "parent" of a lower diagram or the "child" diagram.
The indexing and the naming conventionality of HO'S model is similar to that of the IDEF, or CIMpgr. Each object or block in the model is identified by it's node, which is derived from the block number, with a level number in the hierarchical factory control of the 3D CIM reference model, as shown in Figure 9. As it can be seen in this figure, in the research, HO'S diagrams were created following a node number order bound with the level number(s) of the 3D CIM reference model.
HO'S model is specially designed for handling the static and the dynamic function and information for the development of portable, reconfigurable system-building object. Therefore, the model does not consider such resources as organization and human resources, like IDEF, CIM-OSA, Purdue or as the CIMpgr models do. [Beeckman 1989, Doumeingts et al. 1987, Williams 1992, Ranky 1993].
In HO'S model, there are two types of blocks: "function" and "system-building object". The "function" block is represented by a box which in software terms means a procedure or subroutine. The system-building objects are represented by notations and are indicated by blue, rounded boxes as shown in Figure 10.
Apart from the block notation, there are other notations for the representation of databases and files as shown in Figure 11.
Input information, output information, database link and activation condition
Four types of arrows can be associated with a block: Input Information, Output Information, Database Link and Activation Condition.
The Input Information is on the left side of the block whereas the Output Information on the right; the Activation Condition at the top of the block whereas the Database Link at the bottom.
Input and output information arrows
Arrows indicate the input and output information from or to a block. These information are usually using the "direct data transfer" method to transmit data between blocks. The information are short data such as task command and process status.
Data link arrow
Arrow indicates the relationship between blocks and databases. A block reads or writes information from or to a database using the "file transfer" method. The information can contain large amount of data.
Activation condition arrow
The Activation Condition Arrow indicates the required conditions to activate the block. The conditions are set by the software or hardware interrupts during the system in operation.
An example of a connection diagram of HO'S model is shown in Figure 12.
The extraction of the 3D CIM Reference Model
The 3D CIM reference model of HO'S methodology has generated 105 possible items for the development of a manufacturing system such as OFCS, or Open Flexible Conveyor System), [Ho 1993, Ho & Ranky 1994a,b]. The items are a function of the fundamental disciplines of the CIM enterprise wheel [Daniel 1985], and the extended seven-level hierarchical factory model [CAM-I 1983, Albus et al. 1981, ISO/TR 10314-Part:1, 2, 1990].
In order to validate HO'S concept, as well as to satisfy other research and educational goals, in the first stage, as recommended by HO'S methodology, an overall conceptual architecture was established. (The types of system-building block (Figure 13) identified for this activity identify the 105 possible items for the development of a generic CIM system. However, not all the items had a significant effect on the FALS project, thus some were discarded).
Those items marked with "x" in Figure 13 were considered in this research affecting the design and the operation of the novel OFCS, or Open Flexible Conveyor System, in a FALS (Flexible Assembly Line System, a real-world validation case in the Department in Hong Kong) and were explored in detail.
As a result of all above activities, an overall conceptual architecture was established for the creation of an OFCS, or Open, Flexible Conveyor System (Figure 14).
Based on this generic model, an actual customized model (Figure 15) was created for the FALS implementation.
As can be seen, this "make-to-order" model considers the customer to be at the top, effectively forcing the flexible assembly system to produce goods that are needed, and only as and when they are needed by the customer.
The beauty of this approach is that from the lowest level of sensors, up to the customer, all important objects are identified and dealt with, (or can be dealt with) in this system architecture.
The benefits of the proposed 3D CIM reference model
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The benefits of using the proposed 3D CIM reference model of the expansion of the CIM-OSA model, "Hierarchical Objects' structure", when developing CIM hardware and software control, such as the conveyor system, can be summarized as follows:
The design and the application/integration criteria
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In terms of overall requirements we have set the following design and application/integration criteria:
The bottom three layer requirements
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We have defined the bottom three layer requirements (i.e. the Sensor/actuator, Module/equipment and Subsystem/workstation layers) of our system as follows:
Conclusions and Summary
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Due to the increasing competition in the manufacturing/ assembly industry assembly systems require rapid and flexible response to meet the challenges set by changing customer requirements. In order to resolve this problem we have extended the CIM-OSA concept using object-oriented principles and researched, developed, validated and implemented a new approach to the design and configuration of flexible material handling systems.
In the first part of this series of articles we have dealt with the HO'S methodology and we have shown some examples for its application possibilities. In the forthcoming article we deal with the creation and programming of conveyor objects, as well as with their integration into larger systems of such objects.
To summarize, the objectives achieved in this research project were as follows:
1. Investigate the problems encountered with existing material handling systems in the face of a customer-driven manufacturing / flexible assembly philosophy
2. Investigate and establish a new, generic methodology and new modeling concepts to be used in the design and implementation of open, modular and flexible material handling systems
3. Design and develop some of the most important corresponding software tools for the new, generic methodology and modeling concepts, using an object-oriented approach
4. Based on the new methodology and modeling concepts, establish a set of generic models to validate the open, flexible and reconfigurable material handling system
5. Design and develop some hardware components and build prototypes to validate the methods
6. Design and develop system operation software modules to validate the new object-oriented methodology and integration concepts in a research laboratory-based flexible, robotized assembly system
The proposed "Open & Reconfigurable Conveyor System", ORCS encompasses a modular mechanical and electronic hardware design and an open, reconfigurable and flexible software architecture based on object-oriented analysis and design concepts and some advanced software control and simulation tools.
The new 3D conveyor model as well as the object-oriented software control tools have been successfully implemented and validated using experimental conveyor modules, or objects. The generic system architecture created offers an integrated and flexible platform for the development and/or modification of new and/or existing conveyor systems.
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The authors would like to express their appreciation to The Department of Manufacturing and Engineering Management, City University of Hong Kong, Hong Kong, The Hong Kong University Grant Commission, NJIT, The Department of Industrial and Manufacturing Engineering, Newark, New Jersey, USA, the University of East London, the Department of Manufacturing Engineering & Design, London, FESTO, ALLEN-BRADLEY, Bosch and Digital for their continuous support of this research project.
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