A Web-based Kanban system for job dispatching, tracking, and performance monitoring

ORIGINAL ARTICLEA Web-based Kanban system for job dispatching,tracking, and performance monitoringHung-da Wan&F.Frank ChenReceived:4January2007/Accepted:25June2007/Published online:20July2007#Springer-Verlag London Limited2007Abstract A Kanban system facilitates lean principles in a simple and effective way.While reportedly successful in many manufacturing firms,the conventional Kanban systems using physical cards suffer from human errors, limited tracking capability,and so on.To make the information flow leaner,software providers add new features to their existing programs for manufacturing systems to computerize Kanban activities.As Web-based technologies advance rapidly,developing an entirely Web-based Kanban system appears to be feasible and promising. This paper presents the advantages,limitations,and challenges of Web-based Kanban systems.An experimental program has been developed based on PHP+MySQL,a popular Web programming platform.The server-executed program features cross-platform compatibility,real-time tracking and performance monitoring,and greatly enhanced information contents compared to physical Kanbans. Human errors are minimized by the automated transactions; nevertheless,the interfacing and data maintenance require further research efforts.Keywords Web-based Kanban.E-Kanban.Pull system. PHP+MySQL1IntroductionThe Kanban system,one of the most important components of Toyota Production System(TPS),is a simple and effective tool to accomplish the pull concept of lean manufacturing [1].A Kanban system typically performs efficiently and effectively in shop floor control when the demand is repetitive and stable.It also applies to adjacent suppliers, customers,and even within a global supply chain to facilitate just-in-time(JIT)delivery[2,3].Under the global competition,however,the increased product variety and physical distance among suppliers in supply chains amplify the complexity of Kanban applications and result in more mistakes,such as delays,lost cards,and incorrect Kanban deliveries[4–7].Furthermore,conventional Kanban sys-tems lack the ability to track and monitor the physical cards.As a result,the conventional Kanban system,which was created to improve manufacturing efficiency,needs improvement in itself.The dramatic improvement of information technologies in the past decades provides a natural solution for improving the Kanban system.An electronic Kanban(e-Kanban) system can minimize human mistakes and facilitate tracking, monitoring and performance measurement.As a result, software providers for manufacturing information systems start to support Kanban activities by adding new features to their existing products.On the other hand,the rapid advancement of Web-based technologies has demonstrated a significant impact on almost all aspects of our lives.Web pages are no longer merely computerized yellow pages. They are now functional tools in various applications.With the improved capability,building fully Web-based Kanban systems is not only feasible but also a promising direction for lean practitioners.This paper investigates the advantages,limitations,and challenges of developing a fully Web-based Kanban system for a manufacturing environment.A functional Web-based program is developed with PHP+MySQL,a widely used Web platform.The economic platform is commonly used for database-related tasks,such as news archives and photoInt J Adv Manuf Technol(2008)38:995–1005 DOI10.1007/s00170-007-1145-2H.-d.Wan(*):F.F.ChenDepartment of Mechanical Engineering, University of Texas at San Antonio,San Antonio,TX78249,USAe-mail:hungda.wan@albums.The experimental Web-based Kanban system developed in this research has not yet been applied to real manufacturing systems.Instead,several hypothetical cases were employed to test the system,including the examples in Sect.4for demonstrating the workflows.The server-executed Web-based Kanban program provides excellent cross-platform compatibility and facilitates the advanced functionality of Kanban systems,including job dispatching and processing,real-time tracking,and performance pared to the conventional Kanban system,the Web-based system delivers various advantages,including minimized human errors,real-time tracking,performance monitoring,greatly enhanced information contents of Kanbans,instant delivery of Kanbans,etc.Nevertheless, interfacing with equipment and software programs,system configuration and maintenance,and some other challenges need to be addressed while implementing the system as a lean information system in a large-scale application.In the next section,the evolvement of Kanban systems is reviewed.Following that,the platform,framework,func-tionalities and examples of the experimental Web-based Kanban system are presented.Finally,the advantages, limitations,challenges,and future research directions are discussed and summarized in the last two sections.2From Kanban cards to E-KanbanIn the past decade,lean manufacturing concepts have shown significant influence in the way jobs are performed. Eliminating non-value added activities in response to customer’s true demand,i.e.,value,makes manufacturers leaner and eventually stronger in the marketplace.Womack and Jones[8]summarize lean thinking into value,value stream,flow,pull,and perfection.Among them,the pull concept is the key to carrying out a smooth flow of value stream.It aligns production targets throughout the system with end customer’s demand and hence minimizes inven-tory and work-in-process(WIP).Kanban,a Japanese word,means a board or card with visual information.In TPS,Kanbans are used to conduct the information flows in the manufacturing system in order to pull the material flows from upstream to downstream. The operations of various types of Kanban systems were documented thoroughly by Monden[9]and published even before the TPS has caught the attention of the majority of U.S.manufacturers.In the late1980s and early1990s, researchers started to intensively investigate the mechanism and benefits of Kanban systems.Various studies were carried out,such as simulation analysis[10],analytical modeling[11],system re-design[12],and so on.As lean manufacturing gains popularity globally,the impact of implementing pull or Kanban system becomes clearer.Sipper and Bulfin[13]summarize the major advantages of implementing pull system,including:(1) shorter lead time,and hence,higher flexibility to demand changes;(2)reduced levels of inventory and other wastes: (3)capacity considerations that are restricted by the system design,and(4)inexpensive to implement.Hopp and Spearman[14]compare the pull-and-push systems in terms of production planning and control and conclude that pull system is more efficient,easier to control,more robust,and more supportive of improving quality.In general,imple-menting Kanban system for pull concept contributes to a higher level of customer satisfaction by providing products with lower cost,shorter lead time,and more stable quality, while the supplier enjoys a more manageable production environment with lower WIP and inventory.From the viewpoint of implementing Kanban system in practice,the“paper card”system is known to be simple and effective,and requires little investment.Various types of Kanban systems and related techniques have been devel-oped at Toyota[1].The system can be applied internally on a shop floor and also externally between distant facilities. Fax and e-mail are commonly used as the means of dispatching Kanbans among distant sites,when delivering physical Kanban is no more considered efficient.For production control,the number of Kanban can be adjusted within a range to meet the capacity ing demand leveling,the pull system remains stable when demand fluctuates in a certain range.When the product variety,demand fluctuation,capacity requirement,or dis-tance between facilities drops out of the acceptable range,the Kanban system becomes too complicated and difficult to manage.As a result,mistakes arise,and significant work-force can be wasted on managing and maintaining the Kanban system.The most common problem with the paper-based Kanban system is lost cards,which leads to material outages,waiting,extra cost,and eventually,a lower service level[6,7].The cards can be lost due to mistakes of handling even within a facility.For long-distance Kanban applications,the chances of human mistakes are even higher due to increased transportation and handling. Furthermore,Kanbans sent by e-mail or fax can be missing due to unreliable e-mail services or simply running out of fax paper.In addition to missing Kanbans,the conventional Kanban system also suffers from other human errors such as delivering the wrong information or sending the cards to wrong destinations.Cutler[5]summarized several false operations of Kanban applications resulting in lost or incorrect information in the transactions.Beside the effectiveness of information delivery,visibility is another critical issue of the paper-based Kanban system. Within a workstation or production cell,the conventional Kanban system enhances visibility of workflows by thepaper cards.However,the system lacks the capability of tracking and monitoring the cards in plant-wide applications, and the transactions of Kanban do not automatically provide data to performance measurement systems[15].Further-more,the visibility is completely lost when the Kanbans go to distant suppliers[6].Yet another issue with the paper-based Kanban system is the limited scalability of the system due to the difficulty on adjusting Kanban quantities responsively based on demand variation[5].The above-mentioned issues and weaknesses of conventional Kanban systems are summarized in Table1.Due to the weaknesses of conventional Kanban systems,improvements are urgent-ly needed for practitioners to effectively compete in the global marketplace.With the advancement of information technologies,the computerized Kanban system provides an opportunity to address these issues.“Seeing”the flow of value stream is the key to building a lean rmation technologies provide the tools that can greatly enhance the visibility of a Kanban system. Therefore,e-Kanban system with real-time data transaction and monitoring becomes the natural solution for eliminating the weaknesses of the Kanban systems.E-Kanban systems have been developed based on existing ERP systems, electronic data interchange(EDI)connections,and Web-based technology[6].In order to support the Kanban activities,providers of enterprise resources planning(ERP) systems started to extend their products with pull or Kanban modules[16].A few other e-Kanban systems have also been developed recently by software providers, including Datacraft Solutions,eBots,SupplyWorks,etc. Even Toyota,the creator of the Kanban system,has adapted an e-Kanban system for sending external pulling signals to distant suppliers[17].Among the different technology bases of the e-Kanban system,ERP and EDI require larger amount of investment which may be a burden for smaller enterprises.On the other hand,Web-based programming has several features facili-tating the development of an e-Kanban system to be simple,effective,inexpensive,and easy to use.With the ubiquitous network connections nowadays,the visibility of Kanbans and the efficiency of deliveries are inherently embedded whether in one facility or over a long distance.The automated processes can minimize human errors.More importantly,performance of the Kanban-enabled manufac-turing system can be analyzed and reported through the Internet in real time.The driving forces for the integration of Kanban systems and Web-based technology are summa-rized as follows[15].1.Standard user interface:Various Web browsers(e.g.,Explorer by Microsoft,Firefox by Netscape,etc.) interpret the standard HyperText Markup Language (HTML)in the same way even with different operating systems,such as Windows,Apple,or UNIX.In addition,more and more handheld devices,such as the personal digital assistant(PDA)and cell phones, can browse Web pages.The cross-platform compatibil-ity makes Web-based programs accessible to users connected to the server without program downloads or extra installations.The standard interface also saves lots of programmers’efforts on managing graphical interfaces,devices,memory allocations,etc.2.Ubiquitous networks:In the past decade,a Webconnection has become readily available in most of the work environment,which provides Internet and/or Intranet services.Web services can be accessed by computers or handheld devices through wired or wireless network globally or locally.Implementing a Web-based Kanban system requires little or no addi-tional investment if Internet or Intranet connection is already available at workstations on the shop floor. 3.Improved Web-based program capability:In contrast tothe conventional static Web pages,the dynamic Web pages facilitate interactive Web browsing and compu-tational ing Web-based databases,the servers can provide functional Web pages for various purposes.For Web-Kanban systems,the expected functions such as real-time tracking,performance measurement,interactive input/output,dynamic dis-play,etc.,can all be achieved.With these features of Web-based programs,developing a Kanban system using Web-based technology becomes a promising direction of the e-Kanban systems.Therefore,by migrating from conventional paper-based system to a Web-based system,the following improvements are expected:1.Automate the data transactions and eliminate human errors.2.Deliver and monitor Kanbans in real time,regardless ofdistance.3.Adjust Kanban quantities to quickly adapt to demandchanges.Table1Problems and weaknesses of conventional Kanban systems Common mistakes in practices-Lost Kanbans-Incorrect delivery of Kanbans-Delivering Kanbans with inaccurate informationWeakness in system infrastructure-Less efficiency in distant delivery-No visibility in distant delivery-Limited tracking and monitoring capability-Limited support for performance measurement-Limited scalability to handle spikes in demand-Consuming workforce on managing the cards4.Analyze performance and suggest performance targetsin real time.In order to carry out the expected features,appropriate Web-based technologies need to be identified.In this research,the PHP+MySQL platform is selected to form the infrastructure. Details of the server-executed Web-based program with Internet database are discussed in the next section.3Web-based programming with PHP+MySQLWeb-based programs executed through dynamic Web pages have profoundly changed the role of the Internet.Web-service providers have been aggressively seeking opportu-nities of offering online services in workplaces as well as in daily life.For manufacturing,research on Web-based applications has been conducted in a wide range of areas, including process planning[18],production planning[19], production monitoring and control[20–22],and so on. However,the development of a fully Web-based Kanban system attracts more attention from industry than from academic research.A dynamic Web page is enabled by two major components, i.e.,Web-based programming language and Web database. Web-based programming languages enable the computational power of a Web monly used Web-based program-ming languages include ASP(Microsoft Active Server Pages),JSP(Java Server Pages),PERL(Practical Extraction and Report Language),ColdFusion(Adobe),and PHP (PHP:Hypertext Preprocessor).Among the languages,some of them send scripts to the viewer’s browser to be executed locally(e.g.,JSP);the others run server-executed programs (e.g.,PHP),which send only the standard HTML to the Web browsers.Both ways provide the computational power for Web applications.Another enabler of dynamic pages is Web database,such as IBM DB2,MySQL,Oracle9i,Microsoft SQL Server,and Sybase Adaptive Server.The databases provide online data storage and interact with scripting languages tightly to enable the dynamic Web pages.Beside the relational databases,the Extensible Markup Language (XML)is an alternative for data transfer and storage.The structured data representation of XML developed by the World Wide Web Consortium(W3C)facilitates cross-platform applications with greater flexibility than the relational databases.Yet,efficiency of handling XML docu-ments is compromised due to the extensible structure. Conventional Web databases outperform XML applications when variety of data structure is not an issue.As a result,the latest trend is to integrate XML with relational databases to take advantage of both systems[23].The goal of the research presented in this paper is to investigate the development a fully Web-based Kanban system with commonly used programming platform that is mature and economical.Therefore,the open source script-ing language PHP and Web database MySQL are selected for the development of the experimental system.The combination,supported by the Linux development group, is economically advantageous,and the reputation of the Linux group helps the wide acceptance of this combination [24].“PHP”is a recursive acronym for“PHP:Hypertext Preprocessor”[25].It is a scripting language that can be inserted into HTML contents and executed by the Web server.Consequently,no download or installation is needed at the Web viewers’end,and only pertinent information is delivered through the Internet.The load of the network stays stable with good security,but the server may carry a heavier load on executing the Web pages.For the programmers,the standard interface eliminates the need to manage graphical interfaces,devices,memory allocations, etc.Popular applications of PHP include online photo albums,news archives,surveys,library systems,etc.The Web-based Kanban system cannot be completed with only the PHP programs.The online database is another critical part for the dynamic Web page.MySQL is an open-source Web database that supports PHP programs.It is a relational database managed by Structured Query Language (SQL)[26].In the Web-based Kanban system,all types of data are stored in the MySQL database,including system configuration,operations,and performance data.The SQL commands are sent by the PHP Web server to the MySQL server to fulfill every request from the clients.Thus,the Web-Kanban runs on the PHP+MySQL platform can record and provide data in real time.The infrastructure of PHP+MySQL enabled Kanban system is illustrated in Fig.1.UsersSystemAdministratorShop FloorOperatorsManager Fig.1PHP+MySQL infrastructure for Web-based Kanban system4A Web-based Kanban system enabledby PHP+MySQLIn order to address the weaknesses of the conventional Kanban system,a Web-based Kanban system is developed in this research.As shown in Fig.1,the framework of the proposed Web-based Kanban system contains the following major components.1.Web service platform:The PHP+MySQL infrastructurethat supports dynamic Web-pages with computational power.2.System configuration module:The program and inter-face for administrators to configure the Web-based Kanban system.3.Kanban operation module:The program and interfacefor operators to receive,process,and fulfill the Kanbans through a Web browser.4.Performance monitoring module:The program andinterface for managers to analyze and monitor the time-based performance of the manufacturing system.The three functional modules enabled by PHP+MySQL provide the desired functionality of the Web-based Kanban system.Different categories of users access the system through Web pages that provide appropriate functions for authorized users only.Figure2depicts the roles of various personnel and the desired functions of the Web-based Kanban system when applied to a manufacturing system. Compared to the conventional Kanban system,the role of operators remains similar.Jobs are still triggered by Kanbans,but it is now done electronically.The major differences are the roles of system administrator andmanagement.In order to run the computerized system,the physical manufacturing system needs to be mapped virtually into the Web server.An administrator is required to configure the Web-based system and update for changes. For the management personnel,the Web-based system provides real-time performance information that has been lacking on the conventional paper-based system.The enhanced visibility offers better supporting information for decision-making.4.1Functionality of the Web-based Kanban systemThe three functional modules of the proposed Web-based Kanban system provide the desired functionality for applica-tions in manufacturing systems.The mechanisms and func-tions of the three modules are further introduced as follows.4.1.1System configurationBefore the Web-based Kanban system can operate,the system needs to be configured in order to virtually map the procedures of the actual manufacturing system into the Web server.This mapping is achieved by defining the system configurations through the system configuration interface, which requests the following parameters and settings from system administrator.1.Data of materials:including material names anddescriptions,start-up inventory level,etc.2.Data of operations:including the input/output materialswith required quantity,work instructions,standard operations procedures,performance standards,etc. 3.Relationships between materials and operations:toconstruct the bill of materials(BOM).4.Relationships between operations and workstations:toselect available workstations and construct the routings of materials.5.Buffer size:to limit the WIP and inventory level.6.Reordering point:to pull material from upstreamoperations.er groups:to assign and display appropriate tasksand functions for individual users when logged in.Fig.2Applying Web-based Kanban to a manufacturing systemThe Web-based system is configured by a system administrator based on the actual manufacturing system and updated in response to any changes.As shown in Fig.3,the system configuration interface uses text-based input boxes that allow users to enter or select the appropriate information.After all the data is configured properly,the system administrator must review the config-uration of the virtual system.Instead of viewing the text-based data in detail,a vertical value stream map is proposed to graphically demonstrate the configuration of each material or product type.As shown in Fig.4,the map combines bill of material (BOM)with the processes and the workstations performing the tasks,and thus,the routing of materials can be graphically displayed.The WIP level of each material can also be shown in the map for the monitoring purpose.4.1.2Kanban operationsThe Web-based Kanban system interacts with the actual manufacturing system through the operators.Therefore,the Kanban operation module provides interfaces for operators at the workstations to receive,process,and fulfill the Kanbans.Operators log on to the system to view the designated Kanbans to that particular workstation.Based on the predefined user groups and authority level,only the authorized users can see the designated Kanbans and information through the operating pages.The main window for each operator is a job list,including new jobs,in-process jobs,pending jobs,and completed jobs.With certain criteria,the task of highest priority is prompted in the Web page so that the operator can follow and execute jobs in the order of urgency.When a Kanban (job)is selected,details of the job are brought up,including materialrequirement,work instructions,and so on.Therefore,operators receive all pertinent information about the job,which ensures standardized work and prevents confusion while changing tasks.In addition to the conventional text and graphical information,audio or video instructions can easily be linked from the page.After finishing the job,Kanbans are fulfilled through the Web page.Every action leaves a time stamp in the Web server for tracking and performance analysis.Details of the workflow of Kanban operations are demonstrated in Sect.4.2.4.1.3Performance monitoringEnabling the capability of real-time tracking and perfor-mance monitoring is a significant improvement from a conventional Kanban system to the Web-based system.All transactions of Kanbans leave time stamps in the server that provide the data for various performance analyses.Time stamps of each activity are automatically recorded in real time.Therefore,the performance information is always up-to-date.Exporting the time-based data as text files allows other software programs to perform further analysis.WithFig.3Text-based data input for systemconfigurationFig.4Vertical value stream map combines bill of material with material routingthe availability of performance data,more performance measures can be defined and programmed as needed while implementing the system.The performance monitoring module of the experimental system offers the functions of Kanban tracking,work-in-process (WIP)level monitoring,and time-based perfor-mance analysis.The authorized users can track the quantity,status,and timing of all dispatched Kanbans through the Kanban tracking interface.The WIP level of each material is demonstrated in the vertical value stream map as demonstrated in Fig.4.Time-based performance of each Kanban is graphically compared by displaying accumulated value-added (V A)time and non-value-added (NV A)time.Example screenshots of the performance-monitoring pages are illustrated in Sect.4.2.4.2Workflow of Web-based Kanban operationsThe system has been tested with several hypothetical cases.In order to demonstrate the workflow of the system,some screenshots of the hypothetical cases are included in this section.In a Kanban system,jobs are initiated when a Kanban is received from the downstream workstation or from the management unit who places the initial job requests.Figure 5depicts the pull production system enabled by the Web-based Kanban program.Each workstation receives pull signals,i.e.,Kanbans,from its downstream workstation or management unit.The workstations perform the desig-nated tasks and meanwhile upload the performance data automatically to the server.After finishing the tasks,the products are moved to the online buffers,inventory,or the next workstation as needed.The cycle of Kanban operations starts from the order placement.Authorized users place orders on selected materials through the order placement page shown in Fig.6.With specified quantity and due date,the order is automatically transformed into Kanbans to trigger the required jobs.Orders can be placed in batches for pulling multiple units all at once,or an automatic leveled pulling mechanism is developed in this system to evenly spread the requested quantity over a specified period of time.Thus,when leveled pulling of a demand is specified,the Kanbans will be dispatched intermittently by the Web server,and the chance of human errors on timing the releasing of Kanbans can be minimized.Upon the receipt of a Kanban,the workstation operator follows the instructions to start,process,and fulfill the jobs.The process map in Fig.5shows the procedures of operations within a workstation.Kanbans are first checked-in to determine if there is a need to place an order to pull more material from upstream.As shown in Fig.7,a job list tells the operator what to do next.Upon clicking on the links to pending jobs,the job processing page shows the detailed information of the job requirements and instruc-tions (Fig.8).After completing the specified tasks,the Kanban is fulfilled through the processing page,and the job is then labeled as “finished.”When a Kanban is checked in without available material or when the inventory runs low after retrieval,a pulling signal (i.e.,Kanban )is sent to the upstream workstation to request for more material.While operating the Kanbans at the workstations,each activity of the operations leaves a record in the Web database.The managerial personnel can track the Kanbans in real time.Figure 9demonstrates the Kanban tracking page that shows active and inactive Kanbans with links toFig.5Workflow of the Web-based Kanban system operations。