A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> In this paper, an on-line dispatching algorithm is proposed for the FMS scheduling problem. The algorithm uses various priority schemes and relevant information concerning the load of the system and the status of jobs in the scheduling process. This information is organized into hierarchical levels. The scheduling decision process is hierarchical in the sense that different decision criteria are applied sequentially to identify the most appropriate part and the machine to be served. The algorithm schedules the jobs on a machine or an automated guided vehicle (AGV) one at a time as the scheduling decision is needed (or as the status of the system changes). Performance of the proposed algorithm is compared with several machine and AGV scheduling rules by using the mean flow-time and the mean tardiness criteria. Simulation results indicate that the proposed algorithm produces significant mean flow-time and mean tardiness improvements over existing scheduling rules for a variety of experimental conditions. <s> BIB001 </s> A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> Abstract Automated guided vehicle (AGV) systems are widely used in flexible manufacturing systems (FMSs) for material handling purposes. Although the AGV systems have provided high flexibility, the design issues on AGV dispatching rules are still to be resolved. The AGV-dispatching rules in an FMS are generally based either on a push or a pull concept. A Petri-net-based simulation was used to investigate the effect of different dispatching rules on the FMS performance. The developed simulation model consists of two modules: a Petri net model and an AGV dispatcher. The two modules are integrated so that the AGV dispatcher controls AGV tokens in the Petri net model. It was shown that there was no significant difference in output rate between push- and pull-based AGV-dispatching rules for a busy FMS. <s> BIB002 </s> A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> In this paper we first deal with the design and operational control of Automated Guided Vehicle (AGV) systems, starting from the literature on these topics. Three main issues emerge: track layout, the number of AGVs required and operational transportation control. An hierarchical queueing network approach to determine the number of AGVs is decribed. Also basic concepts are presented for the transportation control of both a job-shop and a flow-shop. Next we report on the results of a case study, in which track layout and transportation control are the main issues. Finally we suggest some topics for further research. <s> BIB003 </s> A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> The literature shows that the manner in which empty devices in a trip-based material handling system are dispatched affects the overall productivity of the handling system. Existing dispatching rules: (1) do not assign a move request until a device deposits a load and becomes empty; (2) with the exception of one rule do not allow an empty device to be reassigned to another move request; (3) do not consider the status and current assignment(s) of other devices before assigning a move request; and (4) may lead to sizable empty travel (also known in industry as “dead-heading”), which reduces system efficiency. This paper presents two new dispatching rules, modified shortest travel time first (MOD STTF) and bidding-based dynamic dispatching (B2D2), which address the above shortcomings and take advantage of information that should be readily available in centrally controlled systems. The latter rule also uses a new concept for device dispatching in that all the devices (empty or otherwise) are allowed to “bid” on incoming move requests. Simulation results indicate that, compared to existing rules, both MOD STTF and B2D2 improve system performance considerably. <s> BIB004 </s> A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> Abstract Efficient material flow is essential in achieving high productivity levels. Design and control of the material handling system directly affect the efficiency of the material flow. In the case of automated guided vehicle (AGV) systems, this translates to guidepath design, AGV dispatching, scheduling, and routing. The focus of this paper is on vehicle dispatching, or vehicle task assignment, which is defined as the selection of the next load for pickup and delivery. The algorithm developed in this research is inherently a demand-driven strategy (that is, pull), which further prioritizes the loads requiring an AGV according to the value added to them as they go through the manufacturing processes. Benchmarking for several performance factors, such as throughput and flow time, through simulation experiments shows this algorithm to outperform some of the best dispatching rules reported in the literature. <s> BIB005 </s> A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> Abstract This paper presents the development of a dispatching system for a fleet of automated guided vehicles in a flexible manufacturing environment which is based on a hybrid Fuzzy–Taguchi approach. A fuzzy decision-making system emulates the human behavior necessary for multi-objective directed decision making in a dynamically evolving environment. A statistical approach based on the Taguchi method tunes the fuzzy rules to achieve near optimal performance. Simulation results demonstrate the effectiveness of this marriage of computational tools in dealing with the well-known NP-complete scheduling problem. <s> BIB006 </s> A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> We consider a manufacturing job shop with automated guided vehicles. Such a system is characterized by several complex features such as the random pattern of material flows between machines, the limited capacity of machine buffers, shop deadlock phenomena, floating bottlenecks, etc. We will describe a heuristic procedure for the control of materials flows and automated guided vehicles in such a system. The procedure is based on the idea of workload balancing; it tries to balance the workload among machines as well as the workload between the machines and the transporters. A deadlock prevention algorithm is embedded in the procedure. We then present the results of simulation experiments to evaluate the performance of the proposed procedure. <s> BIB007 </s> A review of design and control of automated guided vehicle systems <s> Hierarchical dispatching rules <s> Guided vehicles (GVs) are commonly used for the internal transportation of loads in warehouses, production plants and terminals. These guided vehicles can be routed with a variety of vehicle dispatching rules in an attempt to meet performance criteria such as minimizing the average load waiting times. In this research, we use simulation models of three companies to evaluate the performance of several real-time vehicle dispatching rules, in part described in the literature. It appears that there is a clear difference in average load waiting time between the different dispatching rules in the different environments. Simple rules, based on load and vehicle proximity (distance-based) perform best for all cases. The penalty for this is a relatively high maximum load waiting time. A distance-based rule with time truncation, giving more priority to loads that have to wait longer than a time threshold, appears to yield the best possible overall performance. A rule that particularly considers load-waiting time performs poor overall. We also show that using little pre-arrival information of loads leads to a significant improvement in the performance of the dispatching rules without changing their performance ranking. <s> BIB008
This type of dispatching rules is typical for manufacturing systems where the added value of a part during the manufacturing process is taken into account when the dispatching decision has to be made. BIB001 use a dynamic dispatching algorithm for scheduling machines and AGVs in a flexible manufacturing system (FMS). In their algorithm, different decision criteria are applied sequentially to identify the most appropriate part and the machine to be served. They identify four hierarchical logic levels: push logic, buffer logic, pull logic and push-pull logic. At each logic level, some priority rules are applied to select the part and the machine. Their algorithm performs quite well compared with simpler dispatching rules. Similar approaches have been proposed for scheduling AGVs in FMS by BIB002 , BIB005 and BIB006 . BIB007 introduce a hierarchical rule based on workload balancing. At the first level, the jobs are prioritized and at the second level a vehicle is assigned to the job with the highest priority. A complex priority index based on workload balancing among machines (dominant factor) and the urgency of jobs is defined. Dispatching rules using a look-ahead period, or vehicle reassignment (pre-emption) BIB004 introduce two dispatching rules that consider reassignment of moving vehicles. These are modified shortest-travel-distance-first (MOD STTF) and bidding-based device dispatching (B 2 D 2 ). The MOD STTF rule is similar to the STTF rule in the sense that it assigns empty vehicles to move requests based on the proximity of the vehicle and the load location, and each vehicle has only one request at a time. The difference is that an empty vehicle may be reassigned to another move request or an empty vehicle may "release" another empty vehicle. If a vehicle travels "uncommitted" to its assigned destination, it may be reassigned to a new arrival request according to some specific conditions BIB004 . To some extent, the B 2 D 2 rule is similar to the MOD STTF rule, but it is much more complicated. Using a quite extensive simulation study (four layouts and a large set of experimental conditions) BIB004 show that MOD STTF and B 2 D 2 perform much better than STTF. Look-ahead dispatching rules use some advance information about loads to be available shortly to dispatch vehicles BIB003 . BIB008 improve the AGV system performance using dispatching rules with prior information on the availability of loads. They experiment with several widely used dispatching rules (NWF, MODFCFS etc.) with and without pre-arrival information. According to their simulation experiments, a very short look-ahead period has significant positive effects on the system performance. Table 4 presents a guideline for designers to choose a suitable vehicle scheduling system for implementation. Vehicles in simple guide-path systems (single-loop, tandem) can be dispatched using simple dispatching rules without reducing the system performance. In practice, the available SFC system may not have the capability to deal with a complicated controller. In this case, a scheduling system that requires more information and advanced monitoring systems may not be applicable. In highly stochastic environments, it is impossible to schedule vehicles over a long horizon, so dispatching rules might be a better option in this case. In case of a high job density, vehicles are busy most of the time so implementing a complicated scheduling system will not be very helpful. Because of their simplicity, vehicle dispatching rules are easy to implement. However, as indicated by Yang et al. (1999) and  , dynamic vehicle scheduling is often more efficient.  also indicates that dynamic vehicle scheduling has the capability of taking other factors such as co-ordination between different transportation means in facilities into account. Another issue, which has to be considered in the scheduling, is vehicle parking. Most scheduling problems suppose that vehicles can stay at the load's pick-up/drop-off locations, however this is not true in some AGV systems. Hence, vehicle parking problem should also be included in the vehicle scheduling problem. This is considered in the next section.