Logistic control for fully automated large scalefreight transport systems; Event based control forthe Underground Logistic System SchipholGuide Words:Logistic control; Evaluation of the logisticAbstract: Freight transport systems of tomorrow will differ largely from the systems that are usedtoday. They will be large, and they might be fully automated. These new freight transport systems askfor a new logistic control approach. This paper provides new concepts for logistic control of highlyautomated transport systems.Further-more, some novel characteristics of logistic control aredescribed.The concepts are illustrated by examples from a large research project on a highlyautomated transport system, the Underground Logistic System Schiphol (OLS).The most importantaspects of the OLs logistic control are decentralization, distribution, and an event-based informationexchange between hierarchical control layers. The logistic control developed for the OLS was testedand evaluated by using a combination of simulation and real (scale) models and prototypes of theequipment that will be used within the OLS. It proves to work very well. In current studies the controlconcepts of the OLS are introduced for other complex transport systems as well. The first results areverypositiveI.INTRODUCTIONFreight transport systems of the early decades of the 21st century will differ largely from the onesused in the past decades (Rijsenbrij 1999). The freight flows will have larger scales, since the freightflows between‘rich'countries continue to grow strongly.The customers are becoming increasinglymore demanding. Customers demand more reliability,faster throughput times, higher service levels.and more flexibility (Versteegt 1999; Evers 1999; Evers 2000). These changes put a heavy burden onthetransport systems.On the other hand new possibilities have arisen. The equipment that will be used in futuretransport systems is highly automated, like Automatic Guided Vehicles (AGV) and automatedtransshipment facilities. These high levels of automation makefaster throughput times possible, whilesimultaneously reducing labor costs.Furthermore, the use of new information and communicationtechnology (ICT), like wireless networking, improve mobile communication. This makes it possible to
Logistic control for fully automated large scale freight transport systems; Event based control for the Underground Logistic System Schiphol Guide Words：Logistic control; Evaluation of the logistic. Abstract：Freight transport systems of tomorrow will differ largely from the systems that are used today. They will be large, and they might be fully automated. These new freight transport systems ask for a new logistic control approach. This paper provides new concepts for logistic control of highly automated transport systems. Further-more, some novel characteristics of logistic control are described. The concepts are illustrated by examples from a large research project on a highly automated transport system, the Underground Logistic System Schiphol (OLS). The most important aspects of the OLS logistic control are decentralization, distribution. and an event-based information exchange between hierarchical control layers. The logistic control developed for the OLS was tested and evaluated by using a combination of simulation and real (scale) models and prototypes of the equipment that will be used within the OLS. It proves to work very well. In current studies the control concepts of the OLS are introduced for other complex transport systems as well. The first results are very positive. I. INTRODUCTION Freight transport systems of the early decades of the 21st century will differ largely from the ones used in the past decades (Rijsenbrij 1999). The freight flows will have larger scales, since the freight flows between ‘rich’ countries continue to grow strongly. The customers are becoming increasingly more demanding. Customers demand more reliability, faster throughput times, higher service levels, and more flexibility (Versteegt 1999; Evers 1999; Evers 2000). These changes put a heavy burden on the transport systems. On the other hand new possibilities have arisen. The equipment that will be used in future transport systems is highly automated, like Automatic Guided Vehicles (AGV) and automated transshipment facilities. These high levels of automation make faster throughput times possible, while simultaneously reducing labor costs. Furthermore, the use of new information and communication technology (ICT), like wireless networking, improve mobile communication. This makes it possible to
control automated and moving systems on a real-time basis. These possibilities are the enablers ofnew transport systems.These increased demands on transport systems and new possibilities ask for a different approachfor the logistical control of these transport systems. The research described in this paper focuses onthe development of logistic control for a new highly automated transport system, the UndergroundLogistic System Schiphol(OLS).Afterthisintroduction,thenext sectionwill providea shortoverviewof logisticcontrol.Thethirdsection will introduce the case study for which the logistic control was designed, the UndergroundLogistic System Schiphol.Section 4discusses the implementation of new types of control concepts inthe case study. The evaluation and tests of the logistic control are covered in section 5. This paperends with conclusions and some thoughts on future research on logistic control.II.LOGISTICCONTROLControl is defined here as a set of mechanisms used to regulate or guide the operation of amachine, apparatus or system.A logistic control system is responsible for controlling the flow ofentities (goods, passengers, or even information) through the entire system, fulfilling demands of the(final)customersas muchas possible."A logistic control system has two kind of control activities (Euwe 1999). A logistic control actionis a request for an activity in a logistic system on a certain moment in time. A logistic coordinationaction is a specification of a dependency at a certain moment in time between different controlsystems.The design of logistic control has become an important part within the field of logistics. In thenear future, when the transport systems become more complex and higher performance is needed, itwill play an even more important role. Several authors support this development (Roderique 1999,Kulick and Sawyer 1999, and Ryan 1998).III.CASE:UNDERGROUNDLOGISTICSYSTEM SCHIPHOLIn the Netherlands around Amsterdam Airport Schiphol and the Flower Auction Aalsmeer theroads are heavily congested. This leads to long throughput times and unreliable delivery rates of thetransport of time-critical and expensive airfreight (e.g. flowers, computer parts, newspapers) betweenAmsterdam Airport Schiphol, logistics centers near Schiphol, the Flower Auction Aalsmeer, and a(future)RailTerminalnearSchiphol.Tosolvethisproblemanundergroundlogisticsystemhasbeendesigned for the transport of the expensive and time-critical air-cargo. Because of the separation of
control automated and moving systems on a real-time basis. These possibilities are the enablers of new transport systems. These increased demands on transport systems and new possibilities ask for a different approach for the logistical control of these transport systems. The research described in this paper focuses on the development of logistic control for a new highly automated transport system, the Underground Logistic System Schiphol(OLS). After this introduction, the next section will provide a short overview of logistic control. The third section will introduce the case study for which the logistic control was designed, the Underground Logistic System Schiphol. Section 4 discusses the implementation of new types of control concepts in the case study. The evaluation and tests of the logistic control are covered in section 5. This paper ends with conclusions and some thoughts on future research on logistic control. II. LOGISTIC CONTROL Control is defined here as a set of mechanisms used to regulate or guide the operation of a machine, apparatus or system. A logistic control system is responsible for controlling the flow of entities (goods, passengers, or even information) through the entire system, fulfilling demands of the (final) customers as much as possible.” A logistic control system has two kind of control activities (Euwe 1999). A logistic control action is a request for an activity in a logistic system on a certain moment in time. A logistic coordination action is a specification of a dependency at a certain moment in time between different control systems. The design of logistic control has become an important part within the field of logistics. In the near future, when the transport systems become more complex and higher performance is needed, it will play an even more important role. Several authors support this development (Roderique 1999, Kulick and Sawyer 1999, and Ryan 1998). III. CASE: UNDERGROUND LOGISTIC SYSTEM SCHIPHOL In the Netherlands around Amsterdam Airport Schiphol and the Flower Auction Aalsmeer the roads are heavily congested. This leads to long throughput times and unreliable delivery rates of the transport of time-critical and expensive airfreight (e. g. flowers, computer parts, newspapers) between Amsterdam Airport Schiphol, logistics centers near Schiphol, the Flower Auction Aalsmeer, and a (future) Rail Terminal near Schiphol. To solve this problem an underground logistic system has been designed for the transport of the expensive and time-critical air-cargo. Because of the separation of
other traffic the transport can be carried out congestion-freetnal (amerailtmin=mini-terminalatSchipholJouble one-waytube (one tubeforeach direction)singleone-waytubewo-way tube (one tubeforbothdirections)Figure I: Overview of the OLS Schiphol.The Underground Logistic System Schiphol (OLS) is highly automated, as it will use AutomaticGuided Vehicles (AGVs) and fully automated transshipment facilities. The OLS will use around 400eight meter long AGVs weighing 10 ton each when it is fully operational.The AGVs are autonomousi. e. they have control responsibility of their own activities. In the early design stages, it was decidedbecauseof scalabilityto maketheAGVsunableto communicate witheachother.Theycanonlycommunicatewithlocal controllersusingawirelessTCP/IPnetwork.Thecontrolofthetransshipmentequipment and management of the facilities are also fully automated.Although there clearly is an economical and environmental need for such an underground logisticsystem, the implementation phase has not yet been started. One of the main obstacles is the largeamountofuncertaintiessurroundingtheimplementationofsuchasystem.Thetechnologythatwillbeused is new, there is almost no experience with automated transport systems of such a large scale, andlittle is known about fully automated control of transport systems of this size.The OLs research project, of which this paper is a result, aims at solving some of theuncertainties for the logistic control of fully automated, large scale transport systems.IV.LOGISTICCONTROLFORTHEUNDERGROUNDThis section discusses some of the choices that were made in the design of the logistic control forthe Underground Logistic System Schiphol. These choices were derived from the demands that thepotential future customers and operators put on the system. Some of the most important choices thathavebeenmadeforthelogistic controlare:ODe-central anddistributedapproachFunctional decomposition
other traffic the transport can be carried out congestion-free. Figure 1: Overview of the OLS Schiphol. The Underground Logistic System Schiphol (OLS) is highly automated, as it will use Automatic Guided Vehicles (AGVs) and fully automated transshipment facilities. The OLS will use around 400 eight meter long AGVs weighing 10 ton each when it is fully operational. The AGVs are autonomous, i. e. they have control responsibility of their own activities. In the early design stages, it was decided because of scalability to make the AGVs unable to communicate with each other. They can only communicate with local controllers using a wireless TCP/IP network. The control of the transshipment equipment and management of the facilities are also fully automated. Although there clearly is an economical and environmental need for such an underground logistic system, the implementation phase has not yet been started. One of the main obstacles is the large amount of uncertainties surrounding the implementation of such a system. The technology that will be used is new, there is almost no experience with automated transport systems of such a large scale, and little is known about fully automated control of transport systems of this size. The OLS research project, of which this paper is a result, aims at solving some of the uncertainties for the logistic control of fully automated, large scale transport systems. IV. LOGISTIC CONTROL FOR THE UNDERGROUND This section discusses some of the choices that were made in the design of the logistic control for the Underground Logistic System Schiphol. These choices were derived from the demands that the potential future customers and operators put on the system. Some of the most important choices that have been made for the logistic control are: ●De-central and distributed approach. ●Functional decomposition
OLayeredhierarchical approachInformationaspects.EachofthechoiceswillbediscussedbelowinmoredetailThe control responsibilities are decentralized and distributed.There is no central controller thatcontrols the entire system. Each component of the system has its own local control responsibilities.Each terminal, AGV, and dock is responsible for the control of its own activities, i.e. there is a highlevelofautonomyorself-control(VanAken1978).Whenthereisapossibleconflictbetweenmoreresources, a higher level control may be needed to avoid dead-locks or conflicts. The higher levelcontrol is responsibleforcoordinatingthecriticalactivitiesof a number of decentralizedcontrollersAnexampleisabidirectional tunnel,wherevehiclesshould notbeallowedtoenterfrombothsidesatonce. Each vehicle has its own vehicle control, but the bi-directional tunnel control can intervenewhen entering the tunnel would lead to deadlocks.The decentralized approach was needed because a number of reasons. The airport authorities donot allow any other party to control vehicles on the airport terrain for safety reasons.The airportauthorities want to control the part of the OLS that is on the airport terrain themselves. Furthermore, adecentralized approach makes the control system very scalable.The system can be scaled-up (or down)to any size, without a substantial loss of efficiency of the logistic control. When more centralizedcontrol systems are scaled up the complexity of the logistic control follows a high-order-polynomialor even an exponential curve (Evers et al 2000). Distributed control is concerned with the execution ofcontrol on geographically distributed computers interconnected via a local area and/or wide areanetwork (Fujimoto 1998; Fujimoto 1999). Each control component has its own hardware and software.Distributed control has a number of advantages. By subdividing the control activities intosub-activities that can be executed concurrently, the execution time of the control can be reduced up toa factor equal to the number of the processors that are used. Furthermore, a lot of information withinlogistic and transport systems is only locally available and thus of a distributed nature. A distributedcontrol system closely represents this and it can take local decisions based on the information that islocally available. If the protocols for information exchange are chosen carefully (e.g. CORBA), thecomputers and software that are used the sub-activities can be made by different manufacturers. Adistributed setting allows the systems to easily replace, remove or add individual controllers, ratherthan creating a new central system after each change that occurs in the overall systemReduction of communication bandwidth was another reason to strive for a high level of autonomy
●Layered hierarchical approach. ●Information aspects. Each of the choices will be discussed below in more detail. The control responsibilities are decentralized and distributed. There is no central controller that controls the entire system. Each component of the system has its own local control responsibilities. Each terminal, AGV, and dock is responsible for the control of its own activities, i.e. there is a high level of autonomy or self-control (Van Aken 1978). When there is a possible conflict between more resources, a higher level control may be needed to avoid dead-locks or conflicts. The higher level control is responsible for coordinating the critical activities of a number of decentralized controllers. An example is a bidirectional tunnel, where vehicles should not be allowed to enter from both sides at once. Each vehicle has its own vehicle control, but the bi-directional tunnel control can intervene when entering the tunnel would lead to deadlocks. The decentralized approach was needed because a number of reasons. The airport authorities do not allow any other party to control vehicles on the airport terrain for safety reasons. The airport authorities want to control the part of the OLS that is on the airport terrain themselves. Furthermore, a decentralized approach makes the control system very scalable. The system can be scaled-up (or down) to any size, without a substantial loss of efficiency of the logistic control. When more centralized control systems are scaled up the complexity of the logistic control follows a high-order-polynomial or even an exponential curve (Evers et al 2000). Distributed control is concerned with the execution of control on geographically distributed computers interconnected via a local area and/or wide area network (Fujimoto 1998; Fujimoto 1999). Each control component has its own hardware and software. Distributed control has a number of advantages. By subdividing the control activities into sub-activities that can be executed concurrently, the execution time of the control can be reduced up to a factor equal to the number of the processors that are used. Furthermore, a lot of information within logistic and transport systems is only locally available and thus of a distributed nature. A distributed control system closely represents this and it can take local decisions based on the information that is locally available. If the protocols for information exchange are chosen carefully (e.g. CORBA), the computers and software that are used the sub-activities can be made by different manufacturers. A distributed setting allows the systems to easily replace, remove or add individual controllers, rather than creating a new central system after each change that occurs in the overall system. Reduction of communication bandwidth was another reason to strive for a high level of autonomy
or self-control, especially in underground transport systems of a large scale. The high level ofautonomy reduces the amount of communication between equipment and controllers. The AGVs onlyhave to communicate with local controllers, like controllers for a safe passing of crossings and dockcontrollers. Within a tunnel communication with the AGVs is difficult. Reducing the need forcommunication in the tunnels reduces the need for special and expensive communication equipment.The high level of autonomy makes the logistic control also very leap.Most activities are notcontrolled by separate logistic controllers, but by the equipment that carries out the activity. This isimportant when in future the OLs is extended to a larger area or even an (underground) logisticsystem for the entire Netherlands. This means that in future several thousand vehicles can drivedistances of 1: unereds of kilometers without major changes in the control systems used. The highlevel of autonomy makes it possible that the vehicles drive these distances entirely by themselves.Aminimum of external control is needed. A further advantage is that when a part of the control systemcommunication system, or physical system brakes down, the other parts of the system can continue tooperate in a normal manner and the autonomous equipment can finish its current activities. Acentralized system will break down (almost) immediately after the controller fails (Rogers andBrennan 1997).The logistic control for the OLS was decomposed in a functional way.Different parts of thesystem that have to be controlled all have their own representation in the logistic control. Some of thecontrol components are: AGV control, transshipment equipment control, parking control, and orderhandling control. This makes the control transparent and extendable. New components can be addedwithoutchangingtheexistingparts.Of course it is necessary to exchange information between different sub-systems now and thenWhen an AGV wants to transship a load, the dock needs to be available at the right time. Instead ofhaving different subsystems communicate directly with each other, a layered hierarchical approachwas used for the logistic control (see figure 2). The decisions that have to be taken by the controlsystem are divided into a number of sub-decisions that are less complex that the 'overal' decision.Furthermore, by using several layers of control it is possible to make changes to single layers of thecontrol system without changing the other layers of the system. Hierarchy divides the power ofdecision among the layers. The top layers have the highest level of power, the lower lavers have lesspower
or self-control, especially in underground transport systems of a large scale. The high level of autonomy reduces the amount of communication between equipment and controllers. The AGVs only have to communicate with local controllers, like controllers for a safe passing of crossings and dock controllers. Within a tunnel communication with the AGVs is difficult. Reducing the need for communication in the tunnels reduces the need for special and expensive communication equipment. The high level of autonomy makes the logistic control also very leap. Most activities are not controlled by separate logistic controllers, but by the equipment that carries out the activity. This is important when in future the OLS is extended to a larger area or even an (underground) logistic system for the entire Netherlands. This means that in future several thousand vehicles can drive distances of 1: unereds of kilometers without major changes in the control systems used. The high level of autonomy makes it possible that the vehicles drive these distances entirely by themselves. A minimum of external control is needed. A further advantage is that when a part of the control system, communication system, or physical system brakes down, the other parts of the system can continue to operate in a normal manner and the autonomous equipment can finish its current activities. A centralized system will break down (almost) immediately after the controller fails (Rogers and Brennan 1997). The logistic control for the OLS was decomposed in a functional way. Different parts of the system that have to be controlled all have their own representation in the logistic control. Some of the control components are: AGV control, transshipment equipment control, parking control, and order handling control. This makes the control transparent and extendable. New components can be added without changing the existing parts. Of course it is necessary to exchange information between different sub-systems now and then. When an AGV wants to transship a load, the dock needs to be available at the right time. Instead of having different subsystems communicate directly with each other, a layered hierarchical approach was used for the logistic control (see figure 2). The decisions that have to be taken by the control system are divided into a number of sub-decisions that are less complex that the 'overall' decision. Furthermore, by using several layers of control it is possible to make changes to single layers of the control system without changing the other layers of the system. Hierarchy divides the power of decision among the layers. The top layers have the highest level of power, the lower lavers have less power