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

Abstract

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 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, Karlsruhe Institute of Technology. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —Decentralized control is a means of distributing the control of a complex system away from a central computing source toward individual devices and subsystems. In material handling, complex systems can malfunction due to deadlock, livelock, or starvation. This article presents a new method for controlling decentralized material handling systems based on logical time, a method from computer operating systems describing a sequence of activities at each resource. We modify logical time for material handling and show that the system is deadlock-free, thus giving engineers an easy-to-implement method of controlling route-based material handling systems such as automated guided vehicles (AGVs) or conveyors.