Deadlock prevention in a completely decentralized controlled materials flow systems

In this paper, we develop policies to prevent deadlocks in a decentralized modular materials flow. The system consists of locally controlled conveyor units (modules) that can be plugged together in order to flexibly interconnect materials flows within a given production area. The controller of each module is a local agent that follows its own specific control rules. The behavior of the overall system is generated as the result of the interaction of the modules. The findings in this paper are based on the dissertation research by Dr. Stephan Mayer (Development of a completely decentralized control system for modular continuous conveyors. Dissertation, Universitätsverlag Karlsruhe, 2009), which provides a comprehensive description of the development of decentralized control policies for in-plant continuous materials flow systems. 1 The limits of centrally controlled materials flow systems The investment for the installation of conventional in-plant conveyor technology is very high. Especially for smaller companies, it severely limits their ability to automate materials flow processes. That is why the in-plant transportation of unit loads in industry still is carried out by discontinuous conveyors like fork lifts or manually operated lift trucks. Even where the initial investment is not prohibitive, the option of conventional fixed installations of continuous conveyor systems and the related necessity of a centrally, individually programmed control are not cost effective in many cases, because increasingly shorter product life cycles in industry cause frequent changes in material flow patterns and volumes which, in turn, cause high changeover cost and are time-consuming. The alternative use of driverless transport systems as an automated solution features higher flexibility, but typically features significantly lower throughput and relatively high investment and operating costs. Even though manufacturers of continuous conveyors offer systems that are modularly constructed and easily connectable, the expenditures for changing their configuration are still high, when conventional systems of central control are being applied. Reconfigurations and customizing of those systems require extensive rewiring or, at the least, rewriting the control programs. Each installation and putting into operation adds very high cost. The example of modern IT networks shows that significant cost savings can be realized with systems that are modular and self-controlled: A large number of independent nodes create a network that is capable of moving large amounts of data without the necessity of an expensive, central computer. Individual computers independently find new connections after being linked. In very short time, they create their own map of the real topology in form of a routing table. When a message is received, the destination is identified and forwarded to the next node on the route. Mechanisms are applied here that ensure that collisions and time losses are avoided. The following discussion shows how the idea of a decentralized control concept in IT networks can be transferred to the design of innovative materials flow S. Mayer (&) Institute of Conveyor Technology and Logistics (IFL), University of Karlsruhe (TH), Karlsruhe, Germany e-mail: [email protected] K. Furmans Head of the Institute of Conveyor Technology and Logistics (IFL), University of Karlsruhe (TH), Karlsruhe, Germany 123 Logist. Res. (2010) 2:147–158 DOI 10.1007/s12159-010-0035-4