(A) Guide-path layout including horizontal, vertical, diago- nal, and curving paths.
(B) Similar as Figure I.1a but now with wider vertical paths in between the segments.
FIGURE5.5: Guide-path base layout. Illustration of the cross aisle linking two segments within the same potroom together.
which thefijoccurs,V is the average vehicle travel speed,tlandtuare respectively the mean time to
load and unload a vehicle,tis the expected lost time by each vehicle during a time period ofT due to battery change (Le-Anh and Koster, 2006).
This analytical approach provides an initial estimate of the required number of AGVs in the sys- tem. In the scenario analysis (Chapter 7), we use the result from this formula to determine an initial number of the required AGVs. We use simulation to further assess the impact of a varying number of AGVs in the system.
5.4
Vehicle Scheduling
The flexible characteristic of AGV systems makes the task of controlling AGVs challenging. In the design of an AGV system, issues regarding dispatching, vehicle routing, and vehicle scheduling have to be addressed. By using MAS, we can design communication schemes and protocols that can be used in controlling AGVs. In this subsection, we discuss our approach regarding vehicle scheduling and dispatching.
The transport orders are classified as:
1. A fresh pallet transport containing from rodding shoprto segmentscellcaccording to orien- tationo.
2. A butt pallet transport from segmentscellcaccording to orientationoto rodding shopr. 3. An empty transport from rodding shoprto segmentscellcaccording to orientationo. 4. An empty transport from segmentscellcaccording to orientationoto rodding shopr.
EachAGV Control Managermaintains its own schedule which consists of a sequence of actions to be executed. The actions considered in such a schedule are: (1) travel with a load from positionito
j, (2) travel empty from positionitoj or (3) wait at nodejuntil timet(Mes, Heijden, and Harten, 2007). The latter one also includes vehicle idling or charging.
In the AGV system design, we consider applying dispatching rules for scheduling AGVs. The use of dispatching rules is a reactive scheduling technique in which decisions are based on triggers within the potroom and requires information exchanges among the defined agents. We decide to in- vestigate the allocation of two integrated dispatching functionalities for the system: vehicle-initiated
and workcenter-initiated. Below we first address the motivation for using this approach, after which we discuss the considered approach in our AGV system.
5.4.1
Motivation For Using Dispatching Rules
As addressed in the literature review chapter, scheduling vehicles (among other vehicle control ac- tivities) can be decided upon simultaneously or separately, and offline or online. We decide to use relatively simplistic dispatch rules for handling vehicle scheduling. An advantage of using these kind of rules is that it is easy to understand approach. Also, the computational effort in this strategy is rela- tively limited in comparison to, for example, more centralized hierarchical approaches. As we aim to hold a satisfactory degree of scalability and genericness under various stochastic circumstances (e.g., job arrivals, job density, pathway blockades, etc.), we favor using dispatching strategies.
A drawback of using these rules is that the collection of defined rules may not yield or guaran- tee an optimal result in every aluminium manufacturing facility and, therefore, lack in providing an integral approach that takes into account arising demand over an extended time-horizon. Despite these drawbacks, basic dispatch rules can easily be extended by, for example, considering dynamic variants, look-ahead periods, vehicle reassignments or other modifications. For the aforementioned reasons, a clear explanation to clients, and re-usability of the model for various manufacturing lay- outs, the use of dispatching rules seems to provide an appropriate solution.
5.4.2
Vehicle Scheduling Solution Approach
An approach solely based on workcenter- or vehicle-initiated dispatch rules is not sufficient. A workcenter-initiated dispatching strategy does namely not check whether there is a new transport job when an AGV has dropped-off a pallet. It may occur that orders are waiting to be transported, but that the workcenter-initiated dispatching rule will not take these orders into account because AGAPTVs could become available in the system one at a time. In particular, if the system load is high and the number of AGAPTVs is relatively low, the AGAPTVs then can form the bottleneck in the system. Likewise, a pure vehicle-initiated dispatching approach is not appropriate because if all AGAPTVs are idling at the moment a transport job becomes available, there is no initiator of the job assignment. This would often occur in systems where the load on the system is low and multiple AGAPTVs are available to transport orders.
For these reasons, the incorporated vehicle scheduling approach includes triggers and rules from both type of strategies. A consequence is that the load on the system may influence the system per- formance because different dispatch rules are used. The following triggers are identified:
• Workcenter-initiated: Pallet transport job initiated; • Vehicle-initiated: AGAPTV dropped-off a pallet;
• Vehicle-initiated: AGAPTV that is charging its battery reached the plateau levelbplat(see Sub-
section 5.6.1 for the definition).
So, both allocation strategies are considered in the model. Below we subsequently address the vehicle- and workcenter-initiated dispatching rules.
5.4.2.1 Vehicle-initiated Dispatching
Vehicle-initiated dispatching includes the assignment of jobs based on triggers from theAGV Control Agent. Whenever an AGAPTV becomes idle, theAGV Control Managerinforms theVehicle Scheduler
about its position (see Figure 5.6a). We decide to let theVehicle Schedulersend a request to allSection Managersto submit their transport job characteristics (i.e, load type, earliest and latest delivery and release time, P/D-locations). Based on these characteristics, theVehicle Scheduler selects the most suitable transport job candidate and informs theAGV Control Managerabout the course of actions (see Figure 5.6b). The considered dispatching rules are discussed below.
In this vehicle-initiated dispatching approach, theSection Managerskeep track of a list of transport jobs that should be carried out within their group of cells. Transport orders containing fresh anode pallet requests are generated by theDemand Managementand send to theSection Management. The butt pallet requests are generated by theSection Management. TheSection Managementretains information about both fresh anode pallet transport requests and butt pallet transport requests. The considered vehicle-initiated dispatch rules are shown in Table 5.2.
5.4. Vehicle Scheduling 79
(A) Information flow from theAGV Control Agentto theSection Agent.
(B) Information flow from theSection Agentto the
AGV Control Agent.
FIGURE5.6: Vehicle-initiated communication scheme. TheAGV Control Agentstarts by sending AGV information to theVehicle Scheduling Agent. TheVehicle Scheduling Agent
then request a list of transport jobs from theSection Agent. Based on this list and the dispatch rule, theVehicle Scheduling Agentselects an appropriate AGV.
For efficiency reasons, we also include dispatch rules that try to combine fresh pallet dispatch orders with butt pallet dispatch orders. For example, if a fresh anode pallet is required from rodding shoprto segmentscellc, we check whether there is a butt pallet dispatch order to rodding shopr
that is nearby the current position of the AGAPTV or on the AGAPTV’s route towards the rodding shop that can be picked-up intermediately and dropped-off at the directed rodding shop. To this end, we include a modified First-Come-First-Served (FCFS) dispatch rule that checks whether there is a butt transport request in the drop-off segment of the fresh anode pallet (and then possibly takes a random one). When there is no such a job, we check if a butt pallet transport request exists on the AGAPTVs route to the rodding shop, and if so, this job is picked-up and dropped-of intermediately. In the case of multiple jobs, the first available butt pallet is chosen.
Of course, one could consider alternative vehicle-initiated dispatching concepts. An alternative design would be, for example, to eliminate the Section Manager and let theAGV Control Managers
communicate directly with theDemand Manager. However, as we use an approach that involves making multiple decisions on the section level of a smelter (see Section 6.1) with the possibility to deviate demand generation procedures, we prefer using the previously discussed vehicle dispatching approach including theSection Management. Furthermore, our approach provides the flexibility to adjust handling procedures in sections easily. In comparison to fully centralized approaches, our approach provides a flexible and fast schedule that can adjust rapidly to changing potroom events.
TABLE5.2: Overview of vehicle-initiated dispatching rules. Adopted from Egbelu and Tanchoco (1984).
Rule Description
First-Come-First-Served (FCFS) Select the job that entered the system as first Random Job (RJ) Select a random transport job
Closest to Latest Release Time (CLRT)
Select job closest to its latest release time Shortest Travel Distance (STD) Select the nearest transport job
Longest Travel Distance (LTD) Select the farthest transport job
5.4.2.2 Workcenter-initiated Dispatching
Workcenter-initiated dispatching covers the selection of an AGV based on triggers from theDemand ManagementorSection Management. TheDemand Managementinitiates transport jobs from the rodding shop to the cells and theSection Managementinitiates transport jobs from the cells to the rodding shop. As soon as a transport job from the rodding shop to a cell arises, theDemand Managementinforms theSection Managementabout the job characteristics. The dedicatedSection Managementthen sends a transportation request to the Vehicle Scheduler. Consequently, the Vehicle Schedulerobtains AGV characteristics (e.g., current position, idling status, utilization, etc.) for all the AGVs from theAGV Control Managers. On its turn, theVehicle Schedulerthen selects the most suitable candidate, if any, and informs the selectedAGV Control Managerabout the transport task. The candidate is chosen based on the heuristic rules as shown in Table 5.3. An overview of the hierarchical communication scheme is depicted in Figure 5.7.
FIGURE5.7: Workcenter-initiated communication scheme. Dispatch jobs are initiated in theSection Agentand pushed to theVehicle Scheduling Agent, which on its turn informs
5.5. Vehicle Parking 81