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A new hybrid dynamic modelling approach for process planning
Journal of Materials Processing Technology 167 (2005) 22–32
A new hybrid dynamic modelling approach for process planning T.S. Mujber∗ , T. Szecsi, M.S.J. Hashmi School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin 9, Ireland Received 23 August 2004; accepted 17 September 2004
Abstract Today’s highly competitive market places a great emphasis on improving efficiency and reducing costs. Simulation modelling is being widely used as a successful tool to design and analyze manufacturing systems. Enhancing the product design is an important issue. In addition, generating efficient simulation models with shorter lead times has become equally important to winning customer satisfaction. To meet these criteria, a new hybrid modelling approach for generating a simulation model and virtual environment from a process plan is presented. The proposed approach integrates simulation and virtual reality in order to render a dynamic shop floor model automatically using the basic process configurations. This is accomplished using an input database. The model interface is user-friendly so that the new users of simulation and administration can use the model easily once the input database requirement is fulfilled. Moreover, the model allows the user to trace the performance criteria of any process on the shop floor level at any instant as well as to experiment any production scenario. The virtual environment is used to verify through visualization the simulation model and also to provide a better understanding of the activities of the shop floor. © 2004 Elsevier B.V. All rights reserved. Keywords: Simulation; Virtual reality; Process plan; Shop floor; Head mounted display
1. Introduction Simulation modelling is being widely used in areas, such as manufacturing, health, network communications, training, education, and military. Such popularity of simulation has resulted in a large number of simulation software tools available on the market [1], some of them are general-purpose simulators and some are designed specifically to model and analyze manufacturing systems. Manufacturing simulation has been one of the primary application areas of simulation technology. It has been widely used to improve and validate the designs of a wide range of manufacturing systems [2]. Virtual reality (VR) may play very significant rule to support the simulation tools to understand the results and the dynamic behaviour of the model [3]. Virtual reality offers the opportunity to visualize, explore, manipulate, and interact with objects within a computer generated environment, and is a useful tool to enhance process planning involved in design, development, ∗
Corresponding author. Tel.: +353 85 1420534. E-mail address: [email protected] (T.S. Mujber).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.09.086
evaluation, and validation [4]. Layout planning is not a new problem in the manufacturing environment. Virtual factories help in evaluating plant layout before actually building them and it assists in avoiding the costs involved in doing a physical re-layout. It also allows the user to get a better perspective than what could be achieved in 2D solutions [5]. Lee et al. [6] have presented the techniques, which are used in applying virtual reality in manufacturing applications. The problem with the simulation packages available on the market today, it requires good knowledge about programming and modelling techniques. Also, it is very time and money consuming to develop a simulation model for a manufacturing system [1]. This paper is devoted to the development of software with Graphic User Interface (GUI) using Visual Basic programming language that provides non-expert users with a flexible tool to develop simulation model dynamically of real shop floor activities. Virtual environment of the shop floor is generated automatically based on the input data, which is given by the user. The Graphic User Interface has three main options, which include input, process, and output. The input
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options are used to construct the process model of the manufacturing system that generates the process plan of the shop floor. The process options are used to generate the simulation model of the shop floor as well as the virtual model. The process options are also used to run the simulation model. Finally, the output option is used to display the statistics of the shop floor.
2. Description of the manufacturing system The Graphic User Interface is developed for an existing manufacturing system that does treatment operations. The manufacturing system consists of a number of machines named Ovens, which does coat sheets with a special material for heat resistance. The sheets can be coated as much as required. Different processes can be associated with each Oven, these processes include: Oven setup, coating sheets, cleaning the Oven, maintenance the Oven, changeover the sheets to be recoated. The software that has been developed will allow the users who do not have any knowledge about simulation to build simulation and virtual model of the described manufacturing system. The GUI of the software can be used to as a tool to simulate the different activities of the shop floor to measure the performance of the system. The following section describes the main components of the developed software that integrates the simulator, the process model, and the virtual reality model.
3. Graphic User Interface Graphic User-friendly Interface is developed using Visual Basic 6.0 to allow non-expert users to develop simulation model of the shop floor dynamically using simulation software called Witness, and it can be used also to create and implement virtual model of the shop floor automatically using virtual reality software called Superscape VRT. Fig. 1 shows
Fig. 1. Main menu of the Graphic User Interface.
Fig. 2. Input Menu Options.
Fig. 3. Create New Process Plan.
a snapshot of the main menu of the Graphic User Interface. The GUI has three main options to build and simulate the manufacturing system. These options include: Input Menu Options [to input data], Process Menu Options [to process the data], and Output Menu Options [to output the results]. 3.1. Input Menu Options The Input Menu Options can be used to construct the process model of the shop floor. The Input Menu Options provide the user with two options to enter the process plan of the shop floor. These options are: CreateNewProcessPlan and LoadExistingProcessPlan. Both of these options can be accessed by clicking on the Input option from the user interface as shown in Fig. 2. 3.1.1. CreateNewProcessPlan Once the user selects the CreateNewProcessPlan option from the Input menu, a window appears with two options as shown in Fig. 3. The first option is used to define all the resources and the processes that need to be analyzed using the resources details form. Fig. 4 shows the resources details form that needs to be used to define the machines and the parts by giving them
Fig. 4. Resources details.
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Fig. 5. Input schedule form.
names and X, Y positions. By defining all the resources and the processes of the shop floor, the second option in Fig. 3 will be enabled to allow users to enter the schedule of the shop floor using the input schedule form (see Fig. 5). In the schedule input form, the lists of the part names, processes, machine names will be generated automatically based on the data that have been given by the user using the resource details form (see Fig. 4). Before the user can enter the schedule of the shop floor, a file needs to be created to hold resources information and the schedule of the shop floor within four sheets named: “Machines”, “Parts”, “Processes”, and “Schedule”. By clicking on “Create New File” button, an open window will appear that allows the user to specify the name of the excel file that should be created to be processed later using the simulator. The data that need to be entered by the user in the schedule form includes: selecting the part name, process, machine name, start time in day; time format, and the finish time. The cycle time will be calculated automatically based on the finish time and the start time. Instead of specifying the start time and finish time, the user can enter only the cycle time. By clicking on “Next” button, all the data that have been specified by the user will be sent to the excel sheets, and the user selections will be cleared to enable the user to enter a new operation. Once the user finishes entering the schedule, the “Save Data and Closed File” button needs to be clicked to save the resources details, processes, and the schedule within the four
Fig. 6. Machines sheet.
Fig. 7. Parts sheet.
sheets in the excel file that has been specified by the user. The resources details will be written within the “Machines Sheet” and the “Parts Sheet” as shown in Figs. 6 and 7. The “Processes Sheet” will include the names of the processes that need to be analyzed by the simulation software (see Fig. 8). The “Schedule Sheet” as shown in Fig. 9 will include the part name, the name of process that will be used as a term that describes the operation of processing a part, the name of the machine that will process the part, the start time of when the machine will start processing a part, and the cycle time of how long does the machine needs to process a part. 3.1.2. LoadExistingProcessPlan This option can be accessed form the Input Menu Options, which allows the user to load an existing process plan saved
Fig. 8. Processes sheet.
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Fig. 9. Schedule sheet.
in an excel file. Once the process plan has been loaded, the user can edit it and change resources details as well as the schedule using the form below (see Fig. 10). 3.2. Process Menu Options The Process Menu Options are used to generate the simulation model as well as the virtual model based on the given process plan. The Process Menu Options provide the user with the four options to process the process plan of the shop floor, which include: Create New Model, Generate Simulation Model, Generate Virtual Model, and Model Run. 3.2.1. Create New Model This option erases the current Witness model from memory, and also it deletes all the objects within the virtual model to enable the user to develop a new simulation model and virtual environment of the model. 3.2.2. Generate Simulation Model Witness simulation software is chosen as simulation tool to generate the simulation model of the shop floor. Based on
Fig. 10. Load existing process plan.
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a survey has been conducted by Vlatka Hlupic [1], Witness package is the most used simulation software by industrial users and the second most used simulation software by educational users. Witness is a Windows application from the Lanner Group. The Witness simulation package was developed specifically to model material flow in manufacturing systems. It is a hybrid of a simulator and a general simulation language. The interface provides the user with a set of menu options, which are used to develop a generic model of a manufacturing system. The Witness simulation package is an “OLE Automation Server”. Therefore, it can be controlled by an “OLE Automation controller”, such as Visual Basic. Through this mechanism it is possible to control, edit or construct an entire model [11]. Building simulation models in Witness involves three steps: “define”, “display”, and “detail”. In the define phase, all the elements of simulation model should be defined. Witness provides the modelling elements used by manufacturing systems, e.g. machines, parts, labours, and transportation systems. In the display phase, all the elements need to be presented by icons in order to build up a pictorial representation of the manufacturing system. Finally, the detail phase, in this phase the logic of the simulation model is specified. The developed software comes with a tool that permits the users who does not have any knowledge about Witness as will as simulation modelling to build simulation model of a manufacturing system. The phases of building the simulation model are done automatically using two mechanisms: Witness OLE Control and Witness Command Language (WCL). Witness OLE control is used to link Visual Basic with Witness. WCL is used to build and display the simulation model. So, once the process plan is loaded using the LoadingExistingProcessPlan form (see Fig. 10), the Generate Simulation Model option in Fig. 11 can be used to generate a simulation model dynamically. If the process plan has not been loaded yet, an open window appears to allow the user to select the file that includes the process plan. The non-detailed flowchart (Fig. 12) shows how the simu-
Fig. 11. Process Menu Options.
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Fig. 12. An algorithm of generating simulation model dynamically from process plan.
lation model is generated form process plan saved in excel sheets. The following command is an OLE Automation command, which has been used to link a running version of Witness with the developed software: Set WitObj = GetObject (,“Witness.WCL”) As the flowchart in Fig. 12 shows, once the user clicks on the Generate Simulation Model in the Process Menu Options (see Fig. 11), an algorithm developed using Visual Basic reads the data within the Machines Sheet, Parts Sheet, and the Processes Sheet. Then, these data will be sent to the simulation model in Witness using Witness OLE control and Witness command language to generate the parts and the machines. In the define phase of the simulation model, the names in machines sheet and the parts sheet are used to define the machines and the parts. The x and y positions are used in the display phase to display the machines and the parts on the main window of the simulation model as shown below (see Fig. 13). In the detail phase, a number of variables are generated to be attached to the parts and the machines to hold the data of the schedule later. Fig. 14 shows the detail window of a machine. The name variable “Oven1L” and the cycle time vari-
able “Oven1L CT” are generated and attached to the machine detail form automatically. WCL commands will be written automatically within the input rule editor of the machine to define the input rule of the machine. Fig. 15 shows the input rule editor for the machine “Oven1L”. The explanation of the written code within the editor can be described as follows: once the time of the sim-
Fig. 13. Display the simulation model in Witness.
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Fig. 14. Detail machine.
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3.2.3. Generate Virtual Model This option is used to generate a virtual model of the shop floor dynamically using virtual reality software called Superscape VRT. Superscape VRT is a complete three-dimensional (3D) authoring studio for personal computers that lets users create interactive 3D worlds that can be published on the Internet using Superscape’s Viscape, or displayed on the standalone Visualiser platform. The package consists of an integrated suite of editors, which are used to create worlds, with two browsers, Visualiser and Viscape, which are used to view the worlds. Textures and sounds can be added to objects in the worlds to make them more realistic, and different lighting setups can also be introduced. Using Superscape Control Language (SCL), a control language based on the popular “C”
Fig. 15. Input rule editor.
ulator matches the indexed start time of the machine named Oven1L and the name of this machine is Oven1L, a part is pulled out of the world. Otherwise, the machine waits until the previous statement is executed. The name of the part depends on the value of the variable PartName Oven1L(i Oven1L). Dummy machines and parts are generated automatically to be used to collect the statistics of the different processes of the shop floor.
Fig. 16. Shape editor of Superscape VRT.
Fig. 17. An algorithm of generating the layout of the virtual shop floor automatically.
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Fig. 18. Virtual shop floor generated automatically.
language, behaviour to objects in the world can be assigned, and complex actions can also be performed [10]. Simple three-dimensional drawing of the shapes of an Oven is done using the shape editor of Superscape VRT (see Fig. 16). Then, the world editor is used to assemble the shapes to create the Oven. Some attributes are applied to the Oven object such as, colours, sound and dynamics. Properties are added to the Oven object to hold the values of x, y positions of the Ovens within the virtual environment to construct the layout of the shop floor. A string property is added to the Oven to hold the name of the process. Some attributes and behaviours are applied to the Oven using Superscape Control Language to simulate processes of the Ovens. For example, the following SCL code is added to the Oven:
if (property (me, “ProcessName”) == “Pass”) xrot (‘roll’) = xrot (‘roll’) + 1; The previous code checks if the value of the ProcessName variable matches the process name “Pass” of the Oven, the Oven will start process the Roll by rotating the Roll ten degrees in the X-axis to simulate coating the roll. Some other behaviour also has been applied to the Ovens, such as the sound to give an indication once the Oven is working. After all the properties, attributes, and behaviours have been assigned to the Oven machine, the Oven is saved in the Warehouse library of Superscape to be called later to build the virtual shop floor (Fig. 17). Superscape VRT comes with an ActiveX control called “Superscape 3D Control”. This control is used to link the developed software with the virtual environment. So, once the user clicks on Generate Virtual Model option in the Process Menu Options (see Fig. 11), the layout of the shop floor will be generated automatically depends on the resources details that has been read from the excel sheets (see Fig. 6). The following flowchart shows how the virtual environment of the shop floor is generated. Fig. 18 shows a virtual shop floor generated automatically. The shop floor consists from four Ovens. The main processes are simulated as follows: Pass is simulated by rotating a roll within the Oven to simulate the coating process. Idle is simulated by showing the Oven without any roll to show that the Oven is waiting for a product. Changeover is simulated by showing an operator carrying a roll and standing beside the Oven. Maintenance and Cleaning are simulated by showing an operator dressed differently standing besides the Oven. Any undefined process is simulated by an operator standing next to the Oven. Once the user clicks on the operator, a message appears with the name of the process.
Fig. 19. Transferring the schedule data from excel sheet to the simulation model in Witness.
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3.2.4. Load schedule This option is used to read the schedule data from an excel sheet that has been specified by the user (see Fig. 9) and then the schedule data are sent to arrays variables generated automatically using WCL commands in the simulation model in Witness. The following flowchart shows what data are read and sent to the simulation model elements. First, an algorithm written using VB reads the data from the excel sheet, and then Witness OLE Automation Control is used as mechanism to transfer the data from VB to the arrays in the simulation model in Witness through Witness Command Language. The part names array, machine names array, start times array, and cycle times array will be associated with the Ovens. The same arrays will be associated with the dummy machines as well as the processes array and a dummy machine. The schedule is considered as the process plan of the shop floor, where all the parts have to follow predefined sequence to convert a part from an initial form to a final form (Fig. 19).
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Fig. 20. Model Run interface.
3.2.5. Model Run The Model Run options can be accessed from the main user interface by clicking on the run option or by using the developed interface that allows the user to run the simulation model and the virtual model simultaneously. There are three options in the model run interface as shown in Fig. 20.
Fig. 21. Run execution algorithm.
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model (see Fig. 23). The simulation model will mimic the virtual model through two mechanisms: Witness OLE Control and Superscape 3D Control. Witness OLE control is used to link the developed software with Witness to read the data on the shop floor using Witness command language, then these data will be sent to the virtual model using an ActiveX control called Superscape 3D control. Superscape control language is used to give commands to the Ovens to simulate the activities of the shop floor in real time that matches what happening in the simulation model. 3.2.7. Interacting with the virtual factory Virtual reality technology increases the interaction between the users and the virtual environment of the manufacturing system. Interaction techniques allow users to see, hear, touch, and interact with the product model. By being immersed in the virtual design environment, engineers can create and modify their designs in real time, seeing the effects of their modifications immediately [7]. The user can interact with the virtual shop floor in real time either by a mouse, keyboard, and joystick. Voice commands also can be used to do some interactions with the virtual shop floor, where the user can breakdown or repair the Ovens. Head Mounted Display (HMD) was used as an immersive output device to view the virtual shop floor in 3D mode and to give the user the impression of being in the virtual world with the ability to do some navigation by moving the user’s head. 3.3. Output
Fig. 22. An algorithm of collecting statistics from the dummy machines.
• Run: run command runs the simulation model with an animated display in which parts appears to move instantly between Ovens on the screen. The virtual model is used to simulate the activities of the shop floor in 3D presentation once the user runs the model. • Stop: stop command halts the current simulation run. • Reset: reset command resets the simulation clock to the start of the run. All the simulation elements are set to the idle state and all the statistics are cleared. The algorithm in Fig. 21 is executed once the user runs the model. Another algorithm is also executed to collect the statistics of the processes of the shop floor that have been specified by the user (see Fig. 22). 3.2.6. Linking the simulation model with the virtual model Once the user runs the model, the activities on the simulation model will be simulated in three-dimensional presentation in real time mode. An algorithm is developed using Visual Basic to link the simulation model with the virtual
The output option is used to display the statistics of the activities of the shop floor to measure its performance. The output option is called a pie chart (Fig. 24). 3.3.1. Pie charts A pie chart is a graphical element display that presents the simulation results in graphic format. The main purpose of the pie chart is to measure the utilization of the Ovens. Fig. 24 shows the utilization of Oven1L. Each colour in the pie chart represents different process. So, by watching the current time of the simulation model and displaying the utilization of the Ovens, the user can be able to find out how long a process is taken for a period of time.
4. Validation and Verification There are a number of techniques that can be used to verify a simulation model. Robinson [8] states that both the model logic and real-world behaviour can be verified by watching the model. The technique that has been used to verify the model is viewing the animation and simulation clock simultaneously while running the model in slow speed. This technique is used to point out any gross discrepancies in sending the rolls to the right Ovens and processing times. The virtual environment has been used also to verify the simulation
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Fig. 23. Linking the simulation model with the virtual model.
model by providing a visual trace of events as they happen. Validation is the process of determining whether the model reflects reality. However, there are no validation techniques that will give 100% certainty in the results of a model. The model is validated by trying to make sure the model performance measures match the real shop floor performance measures. VR in this system is used for verification purposes.
Fig. 24. Utilization of Oven1L.
5. Conclusion Simulation is a key technology to support manufacturing in the 21st century, and no other technology offers more potential than simulation and modelling for improving products, perfecting processes, reducing design-to-manufacturing cycle time, and reducing product realization costs [9]. 9VR is becoming an important method to solve engineering problems because of its special simulation features, such as immersion, real-time interaction, and 3D graphics. Its further application will not only depend on the development of VR technology, but more on its capability to provide cost effective solutions to real industrial problems [4]. This paper has discussed the development of software that integrates simulation and virtual reality technology to allow non-expert users to build simulation model and virtual model automatically through Graphic User Interface. The developed software has been tested with a real case study to build simulation model and virtual model of shop floor that consists of a number of Ovens, which does treatments operations. The software is developed to be user-friendly that allows users who does not have good knowledge about simulation modelling techniques and programming to develop dynamically simulation model of a manufacturing system
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and create virtual environment to simulate the activities of the manufacturing system in three-dimensional presentation in real time. The developed software can be used as tool to use some of the capabilities of Witness simulation software without a need to learn the modelling techniques of this software.
References [1] V. Hlupic, Simulation software: An Operational Research Society Survey of Academic and Industrial Users, in: Proceedings of the 2000 Winter Simulation Conference, 2000. [2] S. Miller, D. Pegden, Introduction to manufacturing simulation, in: Proceedings of the 2000 Winter Simulation Conference, 2000. [3] T.S. Mujber, T. Szecsi, M.S.J. Hashmi, Virtual Reality Applications In Manufacturing Process Simulation, in: AMPT 2003 Conference, Dublin, Ireland, 2003.
[4] Q. Peng, F.R. Hall, P.M. Lister, Application and evaluation of VR-based CAPP system, J. Mater. Process. Technol. 107 (2000) 153–159. [5] M. Iqbal, M.S.J. Hashmi, Design and analysis of a virtual factory layout, J. Mater. Process. Technol. 118 (2001) 403–410. [6] W.B. Lee, C.F. Cheung, J.G. Li, Appl. Virtual Manuf. Mater. Process. 113 (2001) 416–423. [7] M.A. Bossak, Simulation based design, J. Mater. Process. Technol. 76 (1998) 8–11. [8] R. Stewert, Simulation model verification and validation: increasing the users confidence, in: Proceedings of the Winter Simulation Conference, 1997. [9] Integrated Manufacturing Technology Roadmap (IMTR) Modeling and Simulation Workshop Group and IMTR Roadmapping Project Team, IMTR Roadmap for Modeling and Simulation, Integrated Manufacturing Technology Roadmapping Project Office, Oak Ridge Centers for Manufacturing Technology, Oak Ridge, TN, 1998. [10] Superscape Inc. Superscape VRT 5.6 User’s Guide, Superscape VR plc, third ed., U.K., 1998. [11] Lanner Group Inc. Witness 2000, User Manuals, 1998.
A new hybrid dynamic modelling approach for process planning T.S. Mujber∗ , T. Szecsi, M.S.J. Hashmi School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin 9, Ireland Received 23 August 2004; accepted 17 September 2004
Abstract Today’s highly competitive market places a great emphasis on improving efficiency and reducing costs. Simulation modelling is being widely used as a successful tool to design and analyze manufacturing systems. Enhancing the product design is an important issue. In addition, generating efficient simulation models with shorter lead times has become equally important to winning customer satisfaction. To meet these criteria, a new hybrid modelling approach for generating a simulation model and virtual environment from a process plan is presented. The proposed approach integrates simulation and virtual reality in order to render a dynamic shop floor model automatically using the basic process configurations. This is accomplished using an input database. The model interface is user-friendly so that the new users of simulation and administration can use the model easily once the input database requirement is fulfilled. Moreover, the model allows the user to trace the performance criteria of any process on the shop floor level at any instant as well as to experiment any production scenario. The virtual environment is used to verify through visualization the simulation model and also to provide a better understanding of the activities of the shop floor. © 2004 Elsevier B.V. All rights reserved. Keywords: Simulation; Virtual reality; Process plan; Shop floor; Head mounted display
1. Introduction Simulation modelling is being widely used in areas, such as manufacturing, health, network communications, training, education, and military. Such popularity of simulation has resulted in a large number of simulation software tools available on the market [1], some of them are general-purpose simulators and some are designed specifically to model and analyze manufacturing systems. Manufacturing simulation has been one of the primary application areas of simulation technology. It has been widely used to improve and validate the designs of a wide range of manufacturing systems [2]. Virtual reality (VR) may play very significant rule to support the simulation tools to understand the results and the dynamic behaviour of the model [3]. Virtual reality offers the opportunity to visualize, explore, manipulate, and interact with objects within a computer generated environment, and is a useful tool to enhance process planning involved in design, development, ∗
Corresponding author. Tel.: +353 85 1420534. E-mail address: [email protected] (T.S. Mujber).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.09.086
evaluation, and validation [4]. Layout planning is not a new problem in the manufacturing environment. Virtual factories help in evaluating plant layout before actually building them and it assists in avoiding the costs involved in doing a physical re-layout. It also allows the user to get a better perspective than what could be achieved in 2D solutions [5]. Lee et al. [6] have presented the techniques, which are used in applying virtual reality in manufacturing applications. The problem with the simulation packages available on the market today, it requires good knowledge about programming and modelling techniques. Also, it is very time and money consuming to develop a simulation model for a manufacturing system [1]. This paper is devoted to the development of software with Graphic User Interface (GUI) using Visual Basic programming language that provides non-expert users with a flexible tool to develop simulation model dynamically of real shop floor activities. Virtual environment of the shop floor is generated automatically based on the input data, which is given by the user. The Graphic User Interface has three main options, which include input, process, and output. The input
T.S. Mujber et al. / Journal of Materials Processing Technology 167 (2005) 22–32
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options are used to construct the process model of the manufacturing system that generates the process plan of the shop floor. The process options are used to generate the simulation model of the shop floor as well as the virtual model. The process options are also used to run the simulation model. Finally, the output option is used to display the statistics of the shop floor.
2. Description of the manufacturing system The Graphic User Interface is developed for an existing manufacturing system that does treatment operations. The manufacturing system consists of a number of machines named Ovens, which does coat sheets with a special material for heat resistance. The sheets can be coated as much as required. Different processes can be associated with each Oven, these processes include: Oven setup, coating sheets, cleaning the Oven, maintenance the Oven, changeover the sheets to be recoated. The software that has been developed will allow the users who do not have any knowledge about simulation to build simulation and virtual model of the described manufacturing system. The GUI of the software can be used to as a tool to simulate the different activities of the shop floor to measure the performance of the system. The following section describes the main components of the developed software that integrates the simulator, the process model, and the virtual reality model.
3. Graphic User Interface Graphic User-friendly Interface is developed using Visual Basic 6.0 to allow non-expert users to develop simulation model of the shop floor dynamically using simulation software called Witness, and it can be used also to create and implement virtual model of the shop floor automatically using virtual reality software called Superscape VRT. Fig. 1 shows
Fig. 1. Main menu of the Graphic User Interface.
Fig. 2. Input Menu Options.
Fig. 3. Create New Process Plan.
a snapshot of the main menu of the Graphic User Interface. The GUI has three main options to build and simulate the manufacturing system. These options include: Input Menu Options [to input data], Process Menu Options [to process the data], and Output Menu Options [to output the results]. 3.1. Input Menu Options The Input Menu Options can be used to construct the process model of the shop floor. The Input Menu Options provide the user with two options to enter the process plan of the shop floor. These options are: CreateNewProcessPlan and LoadExistingProcessPlan. Both of these options can be accessed by clicking on the Input option from the user interface as shown in Fig. 2. 3.1.1. CreateNewProcessPlan Once the user selects the CreateNewProcessPlan option from the Input menu, a window appears with two options as shown in Fig. 3. The first option is used to define all the resources and the processes that need to be analyzed using the resources details form. Fig. 4 shows the resources details form that needs to be used to define the machines and the parts by giving them
Fig. 4. Resources details.
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Fig. 5. Input schedule form.
names and X, Y positions. By defining all the resources and the processes of the shop floor, the second option in Fig. 3 will be enabled to allow users to enter the schedule of the shop floor using the input schedule form (see Fig. 5). In the schedule input form, the lists of the part names, processes, machine names will be generated automatically based on the data that have been given by the user using the resource details form (see Fig. 4). Before the user can enter the schedule of the shop floor, a file needs to be created to hold resources information and the schedule of the shop floor within four sheets named: “Machines”, “Parts”, “Processes”, and “Schedule”. By clicking on “Create New File” button, an open window will appear that allows the user to specify the name of the excel file that should be created to be processed later using the simulator. The data that need to be entered by the user in the schedule form includes: selecting the part name, process, machine name, start time in day; time format, and the finish time. The cycle time will be calculated automatically based on the finish time and the start time. Instead of specifying the start time and finish time, the user can enter only the cycle time. By clicking on “Next” button, all the data that have been specified by the user will be sent to the excel sheets, and the user selections will be cleared to enable the user to enter a new operation. Once the user finishes entering the schedule, the “Save Data and Closed File” button needs to be clicked to save the resources details, processes, and the schedule within the four
Fig. 6. Machines sheet.
Fig. 7. Parts sheet.
sheets in the excel file that has been specified by the user. The resources details will be written within the “Machines Sheet” and the “Parts Sheet” as shown in Figs. 6 and 7. The “Processes Sheet” will include the names of the processes that need to be analyzed by the simulation software (see Fig. 8). The “Schedule Sheet” as shown in Fig. 9 will include the part name, the name of process that will be used as a term that describes the operation of processing a part, the name of the machine that will process the part, the start time of when the machine will start processing a part, and the cycle time of how long does the machine needs to process a part. 3.1.2. LoadExistingProcessPlan This option can be accessed form the Input Menu Options, which allows the user to load an existing process plan saved
Fig. 8. Processes sheet.
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Fig. 9. Schedule sheet.
in an excel file. Once the process plan has been loaded, the user can edit it and change resources details as well as the schedule using the form below (see Fig. 10). 3.2. Process Menu Options The Process Menu Options are used to generate the simulation model as well as the virtual model based on the given process plan. The Process Menu Options provide the user with the four options to process the process plan of the shop floor, which include: Create New Model, Generate Simulation Model, Generate Virtual Model, and Model Run. 3.2.1. Create New Model This option erases the current Witness model from memory, and also it deletes all the objects within the virtual model to enable the user to develop a new simulation model and virtual environment of the model. 3.2.2. Generate Simulation Model Witness simulation software is chosen as simulation tool to generate the simulation model of the shop floor. Based on
Fig. 10. Load existing process plan.
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a survey has been conducted by Vlatka Hlupic [1], Witness package is the most used simulation software by industrial users and the second most used simulation software by educational users. Witness is a Windows application from the Lanner Group. The Witness simulation package was developed specifically to model material flow in manufacturing systems. It is a hybrid of a simulator and a general simulation language. The interface provides the user with a set of menu options, which are used to develop a generic model of a manufacturing system. The Witness simulation package is an “OLE Automation Server”. Therefore, it can be controlled by an “OLE Automation controller”, such as Visual Basic. Through this mechanism it is possible to control, edit or construct an entire model [11]. Building simulation models in Witness involves three steps: “define”, “display”, and “detail”. In the define phase, all the elements of simulation model should be defined. Witness provides the modelling elements used by manufacturing systems, e.g. machines, parts, labours, and transportation systems. In the display phase, all the elements need to be presented by icons in order to build up a pictorial representation of the manufacturing system. Finally, the detail phase, in this phase the logic of the simulation model is specified. The developed software comes with a tool that permits the users who does not have any knowledge about Witness as will as simulation modelling to build simulation model of a manufacturing system. The phases of building the simulation model are done automatically using two mechanisms: Witness OLE Control and Witness Command Language (WCL). Witness OLE control is used to link Visual Basic with Witness. WCL is used to build and display the simulation model. So, once the process plan is loaded using the LoadingExistingProcessPlan form (see Fig. 10), the Generate Simulation Model option in Fig. 11 can be used to generate a simulation model dynamically. If the process plan has not been loaded yet, an open window appears to allow the user to select the file that includes the process plan. The non-detailed flowchart (Fig. 12) shows how the simu-
Fig. 11. Process Menu Options.
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Fig. 12. An algorithm of generating simulation model dynamically from process plan.
lation model is generated form process plan saved in excel sheets. The following command is an OLE Automation command, which has been used to link a running version of Witness with the developed software: Set WitObj = GetObject (,“Witness.WCL”) As the flowchart in Fig. 12 shows, once the user clicks on the Generate Simulation Model in the Process Menu Options (see Fig. 11), an algorithm developed using Visual Basic reads the data within the Machines Sheet, Parts Sheet, and the Processes Sheet. Then, these data will be sent to the simulation model in Witness using Witness OLE control and Witness command language to generate the parts and the machines. In the define phase of the simulation model, the names in machines sheet and the parts sheet are used to define the machines and the parts. The x and y positions are used in the display phase to display the machines and the parts on the main window of the simulation model as shown below (see Fig. 13). In the detail phase, a number of variables are generated to be attached to the parts and the machines to hold the data of the schedule later. Fig. 14 shows the detail window of a machine. The name variable “Oven1L” and the cycle time vari-
able “Oven1L CT” are generated and attached to the machine detail form automatically. WCL commands will be written automatically within the input rule editor of the machine to define the input rule of the machine. Fig. 15 shows the input rule editor for the machine “Oven1L”. The explanation of the written code within the editor can be described as follows: once the time of the sim-
Fig. 13. Display the simulation model in Witness.
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Fig. 14. Detail machine.
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3.2.3. Generate Virtual Model This option is used to generate a virtual model of the shop floor dynamically using virtual reality software called Superscape VRT. Superscape VRT is a complete three-dimensional (3D) authoring studio for personal computers that lets users create interactive 3D worlds that can be published on the Internet using Superscape’s Viscape, or displayed on the standalone Visualiser platform. The package consists of an integrated suite of editors, which are used to create worlds, with two browsers, Visualiser and Viscape, which are used to view the worlds. Textures and sounds can be added to objects in the worlds to make them more realistic, and different lighting setups can also be introduced. Using Superscape Control Language (SCL), a control language based on the popular “C”
Fig. 15. Input rule editor.
ulator matches the indexed start time of the machine named Oven1L and the name of this machine is Oven1L, a part is pulled out of the world. Otherwise, the machine waits until the previous statement is executed. The name of the part depends on the value of the variable PartName Oven1L(i Oven1L). Dummy machines and parts are generated automatically to be used to collect the statistics of the different processes of the shop floor.
Fig. 16. Shape editor of Superscape VRT.
Fig. 17. An algorithm of generating the layout of the virtual shop floor automatically.
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Fig. 18. Virtual shop floor generated automatically.
language, behaviour to objects in the world can be assigned, and complex actions can also be performed [10]. Simple three-dimensional drawing of the shapes of an Oven is done using the shape editor of Superscape VRT (see Fig. 16). Then, the world editor is used to assemble the shapes to create the Oven. Some attributes are applied to the Oven object such as, colours, sound and dynamics. Properties are added to the Oven object to hold the values of x, y positions of the Ovens within the virtual environment to construct the layout of the shop floor. A string property is added to the Oven to hold the name of the process. Some attributes and behaviours are applied to the Oven using Superscape Control Language to simulate processes of the Ovens. For example, the following SCL code is added to the Oven:
if (property (me, “ProcessName”) == “Pass”) xrot (‘roll’) = xrot (‘roll’) + 1; The previous code checks if the value of the ProcessName variable matches the process name “Pass” of the Oven, the Oven will start process the Roll by rotating the Roll ten degrees in the X-axis to simulate coating the roll. Some other behaviour also has been applied to the Ovens, such as the sound to give an indication once the Oven is working. After all the properties, attributes, and behaviours have been assigned to the Oven machine, the Oven is saved in the Warehouse library of Superscape to be called later to build the virtual shop floor (Fig. 17). Superscape VRT comes with an ActiveX control called “Superscape 3D Control”. This control is used to link the developed software with the virtual environment. So, once the user clicks on Generate Virtual Model option in the Process Menu Options (see Fig. 11), the layout of the shop floor will be generated automatically depends on the resources details that has been read from the excel sheets (see Fig. 6). The following flowchart shows how the virtual environment of the shop floor is generated. Fig. 18 shows a virtual shop floor generated automatically. The shop floor consists from four Ovens. The main processes are simulated as follows: Pass is simulated by rotating a roll within the Oven to simulate the coating process. Idle is simulated by showing the Oven without any roll to show that the Oven is waiting for a product. Changeover is simulated by showing an operator carrying a roll and standing beside the Oven. Maintenance and Cleaning are simulated by showing an operator dressed differently standing besides the Oven. Any undefined process is simulated by an operator standing next to the Oven. Once the user clicks on the operator, a message appears with the name of the process.
Fig. 19. Transferring the schedule data from excel sheet to the simulation model in Witness.
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3.2.4. Load schedule This option is used to read the schedule data from an excel sheet that has been specified by the user (see Fig. 9) and then the schedule data are sent to arrays variables generated automatically using WCL commands in the simulation model in Witness. The following flowchart shows what data are read and sent to the simulation model elements. First, an algorithm written using VB reads the data from the excel sheet, and then Witness OLE Automation Control is used as mechanism to transfer the data from VB to the arrays in the simulation model in Witness through Witness Command Language. The part names array, machine names array, start times array, and cycle times array will be associated with the Ovens. The same arrays will be associated with the dummy machines as well as the processes array and a dummy machine. The schedule is considered as the process plan of the shop floor, where all the parts have to follow predefined sequence to convert a part from an initial form to a final form (Fig. 19).
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Fig. 20. Model Run interface.
3.2.5. Model Run The Model Run options can be accessed from the main user interface by clicking on the run option or by using the developed interface that allows the user to run the simulation model and the virtual model simultaneously. There are three options in the model run interface as shown in Fig. 20.
Fig. 21. Run execution algorithm.
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model (see Fig. 23). The simulation model will mimic the virtual model through two mechanisms: Witness OLE Control and Superscape 3D Control. Witness OLE control is used to link the developed software with Witness to read the data on the shop floor using Witness command language, then these data will be sent to the virtual model using an ActiveX control called Superscape 3D control. Superscape control language is used to give commands to the Ovens to simulate the activities of the shop floor in real time that matches what happening in the simulation model. 3.2.7. Interacting with the virtual factory Virtual reality technology increases the interaction between the users and the virtual environment of the manufacturing system. Interaction techniques allow users to see, hear, touch, and interact with the product model. By being immersed in the virtual design environment, engineers can create and modify their designs in real time, seeing the effects of their modifications immediately [7]. The user can interact with the virtual shop floor in real time either by a mouse, keyboard, and joystick. Voice commands also can be used to do some interactions with the virtual shop floor, where the user can breakdown or repair the Ovens. Head Mounted Display (HMD) was used as an immersive output device to view the virtual shop floor in 3D mode and to give the user the impression of being in the virtual world with the ability to do some navigation by moving the user’s head. 3.3. Output
Fig. 22. An algorithm of collecting statistics from the dummy machines.
• Run: run command runs the simulation model with an animated display in which parts appears to move instantly between Ovens on the screen. The virtual model is used to simulate the activities of the shop floor in 3D presentation once the user runs the model. • Stop: stop command halts the current simulation run. • Reset: reset command resets the simulation clock to the start of the run. All the simulation elements are set to the idle state and all the statistics are cleared. The algorithm in Fig. 21 is executed once the user runs the model. Another algorithm is also executed to collect the statistics of the processes of the shop floor that have been specified by the user (see Fig. 22). 3.2.6. Linking the simulation model with the virtual model Once the user runs the model, the activities on the simulation model will be simulated in three-dimensional presentation in real time mode. An algorithm is developed using Visual Basic to link the simulation model with the virtual
The output option is used to display the statistics of the activities of the shop floor to measure its performance. The output option is called a pie chart (Fig. 24). 3.3.1. Pie charts A pie chart is a graphical element display that presents the simulation results in graphic format. The main purpose of the pie chart is to measure the utilization of the Ovens. Fig. 24 shows the utilization of Oven1L. Each colour in the pie chart represents different process. So, by watching the current time of the simulation model and displaying the utilization of the Ovens, the user can be able to find out how long a process is taken for a period of time.
4. Validation and Verification There are a number of techniques that can be used to verify a simulation model. Robinson [8] states that both the model logic and real-world behaviour can be verified by watching the model. The technique that has been used to verify the model is viewing the animation and simulation clock simultaneously while running the model in slow speed. This technique is used to point out any gross discrepancies in sending the rolls to the right Ovens and processing times. The virtual environment has been used also to verify the simulation
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Fig. 23. Linking the simulation model with the virtual model.
model by providing a visual trace of events as they happen. Validation is the process of determining whether the model reflects reality. However, there are no validation techniques that will give 100% certainty in the results of a model. The model is validated by trying to make sure the model performance measures match the real shop floor performance measures. VR in this system is used for verification purposes.
Fig. 24. Utilization of Oven1L.
5. Conclusion Simulation is a key technology to support manufacturing in the 21st century, and no other technology offers more potential than simulation and modelling for improving products, perfecting processes, reducing design-to-manufacturing cycle time, and reducing product realization costs [9]. 9VR is becoming an important method to solve engineering problems because of its special simulation features, such as immersion, real-time interaction, and 3D graphics. Its further application will not only depend on the development of VR technology, but more on its capability to provide cost effective solutions to real industrial problems [4]. This paper has discussed the development of software that integrates simulation and virtual reality technology to allow non-expert users to build simulation model and virtual model automatically through Graphic User Interface. The developed software has been tested with a real case study to build simulation model and virtual model of shop floor that consists of a number of Ovens, which does treatments operations. The software is developed to be user-friendly that allows users who does not have good knowledge about simulation modelling techniques and programming to develop dynamically simulation model of a manufacturing system
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and create virtual environment to simulate the activities of the manufacturing system in three-dimensional presentation in real time. The developed software can be used as tool to use some of the capabilities of Witness simulation software without a need to learn the modelling techniques of this software.
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