1,429 research outputs found

    Development of a completely decentralized control system for modular continuous conveyors

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    To increase the flexibility of continuous conveyor systems, a completely decentralized control system for a modular conveyor system was developed. The system is able to carry conveyor units without any centralized infrastructure. Based on existing methods of data transfer in IT networks, single modules operate autonomously and, after being positioned into the required topology, independently connect together to become a functioning conveyor system

    Development of a completely decentralized control system for modular continuous conveyors

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    To increase the flexibility of application of continuous conveyor systems, a completely decentralized control system for a modular conveyor system is introduced in the paper. This system is able to carry conveyor units without any centralized infrastructure. Based on existing methods of decentralized data transfer in IT networks, single modules operate autonomously and, after being positioned into the required topology, independently connect together to become a functioning conveyor system

    Using Logical Time to Ensure Liveness in Material Handling Systems With Decentralized Control

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    We describe a method for decentralized control of route-based material handling systems in which devices have no central controller (by definition), no common source of information, and no synchronized or common clocks with which to plan and execute their activities. The control scheme is based on the concept of \emph{logical time}, which is a means of partially ordering events in computer operating systems. We modify the concept to the domain of material handling systems and prove system liveness. We conclude by describing GridSorter, a conveyance-based sorter that uses decentralized control and logical time to sort packages. A prototype has been successfully built and tested at the Institute for Material Handling and Logistics at Karlsruhe Institute of Technology

    Flexible high-density puzzle storage systems

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    In a traditional storage system, when one of the mechanical or electrical parts or the software fails to operate correctly, it may lead to the failure of the entire system imposing huge costs to the system (Gue et al., 2014). However, in a grid storage network, if a grid fails to operate, the system continues to operate. Therefore, failure of a system component will not jeopardize the whole system functionality. As such, reliability is one of the key advantages of grid storage networks. Another benefit of grid storage systems is flexibility. Various systems such as GridFlow, GridPick, GridStore, are developed that consist of multiple autonomous conveyors to store and move items in different directions. A unit module is capable to work independently. To retrieve an item, empty modules open a temporary aisle to move the item to the retrieval point. This study developed a tote retrieval algorithm for high-density storage systems (HDSS). The algorithm is capable of retrieving totes from all sides of a storage puzzle. This is novel since methodologies of prior studies were confined to retrieving items from a single grid or a single side of a puzzle. The algorithm was decentralized and agent-based where each grid acts as an agent. The study undertook developing algorithms that could minimize the number of movements in the network for tote retrievals and to prevent deadlocks. Deadlock prevention algorithms are capable to resolve a diverse range of situations that cause network deadlocks. An object-oriented software program was developed that implemented the algorithms. The software tool simulated puzzles of various sizes with different number of escorts available for retrieving requested totes. Thousands of iterations of the puzzle configurations were resolved for the analysis. It was found that incremental increase in the number of escorts in the puzzle reduces the number of movements for retrieving totes. However, depending on the puzzle size, there were increments that additional escorts had minimal impact. It was also found that average retrieval movements for 3 totes increase logarithmically with the number of cells in a puzzle. To validate the methodology and its software implementation, results from the simulation were compared with the results from other studies with mathematical solutions. The program replicated the optimum number of movements for a puzzle that contained one escort and retrieved one tote

    Development of a completely decentralized control system for modular continuous conveyors

    Get PDF
    To increase the flexibility of continuous conveyor systems, a completely decentralized control system for a modular conveyor system was developed. The system is able to carry conveyor units without any centralized infrastructure. Based on existing methods of data transfer in IT networks, single modules operate autonomously and, after being positioned into the required topology, independently connect together to become a functioning conveyor system

    On Minimum-time Control of Continuous Petri nets: Centralized and Decentralized Perspectives

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    Muchos sistemas artificiales, como los sistemas de manufactura, de logística, de telecomunicaciones o de tráfico, pueden ser vistos "de manera natural" como Sistemas Dinámicos de Eventos Discretos (DEDS). Desafortunadamente, cuando tienen grandes poblaciones, estos sistemas pueden sufrir del clásico problema de la explosión de estados. Con la intención de evitar este problema, se pueden aplicar técnicas de fluidificación, obteniendo una relajación fluida del modelo original discreto. Las redes de Petri continuas (CPNs) son una aproximación fluida de las redes de Petri discretas, un conocido formalismo para los DEDS. Una ventaja clave del empleo de las CPNs es que, a menudo, llevan a una substancial reducción del coste computacional. Esta tesis se centra en el control de Redes de Petri continuas temporizadas (TCPNs), donde las transiciones tienen una interpretación temporal asociada. Se asume que los sistemas siguen una semántica de servidores infinitos (velocidad variable) y que las acciones de control aplicables son la disminución de la velocidad del disparo de las transiciones. Se consideran dos interesantes problemas de control en esta tesis: 1) control del marcado objetivo, donde el objetivo es conducir el sistema (tan rápido como sea posible) desde un estado inicial a un estado final deseado, y es similar al problema de control set-point para cualquier sistema de estado continuo; 2) control del flujo óptimo, donde el objetivo es conducir el sistema a un flujo óptimo sin conocimiento a priori del estado final. En particular, estamos interesados en alcanzar el flujo máximo tan rápido como sea posible, lo cual suele ser deseable en la mayoría de sistemas prácticos. El problema de control del marcado objetivo se considera desde las perspectivas centralizada y descentralizada. Proponemos varios controladores centralizados en tiempo mínimo, y todos ellos están basados en una estrategia ON/OFF. Para algunas subclases, como las redes Choice-Free (CF), se garantiza la evolución en tiempo mínimo; mientras que para redes generales, los controladores propuestos son heurísticos. Respecto del problema de control descentralizado, proponemos en primer lugar un controlador descentralizado en tiempo mínimo para redes CF. Para redes generales, proponemos una aproximación distribuida del método Model Predictive Control (MPC); sin embargo en este método no se considera evolución en tiempo mínimo. El problema de control de flujo óptimo (en nuestro caso, flujo máximo) en tiempo mínimo se considera para redes CF. Proponemos un algoritmo heurístico en el que calculamos los "mejores" firing count vectors que llevan al sistema al flujo máximo, y aplicamos una estrategia de disparo ON/OFF. También demostramos que, debido a que las redes CF son persistentes, podemos reducir el tiempo que tarda en alcanzar el flujo máximo con algunos disparos adicionales. Los métodos de control propuestos se han implementado e integrado en una herramienta para Redes de Petri híbridas basada en Matlab, llamada SimHPN

    Deadlock Prevention Policy with Behavioral Optimality or Suboptimality Achieved by the Redundancy Identification of Constraints and the Rearrangement of Monitors

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    This work develops an iterative deadlock prevention method for a special class of Petri nets that can well model a variety of flexible manufacturing systems. A deadlock detection technique, called mixed integer programming (MIP), is used to find a strict minimal siphon (SMS) in a plant model without a complete enumeration of siphons. The policy consists of two phases. At the first phase, SMSs are obtained by MIP technique iteratively and monitors are added to the complementary sets of the SMSs. For the possible existence of new siphons generated after the first phase, we add monitors with their output arcs first pointed to source transitions at the second phase to avoid new siphons generating and then rearrange the output arcs step by step on condition that liveness is preserved. In addition, an algorithm is proposed to remove the redundant constraints of the MIP problem in this paper. The policy improves the behavioral permissiveness of the resulting net and greatly enhances the structural simplicity of the supervisor. Theoretical analysis and experimental results verify the effectiveness of the proposed method

    Logical Time for Decentralized Control of Material Handling Systems

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    The fourth industrial revolution aims to transform production systems. In this work, Logical Time which is a control principle for distributed systems is transferred to material handling systems with decentralized control. The GridSorter, a modular sorter with grid-like structure, is chosen as showcase system. The system is proven to be deadlock-free and is robust against varying transport times. The time-window-based route reservation process is described as Iterative Deepening A*
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