The design of a guide-path is an important element in an AGV system. The guide-path depends on the allocation of shop-floor space, layout of storage zones, and the arrangement of handling stations (Le-Anh and Koster, 2006).
5.2. Guide-path Design 75
FIGURE5.4: AGAPTV pathways blocked during anode changing process at the back aisle. During the anode changing process at the back aisle, the driving path near the corresponding cell is blocked as well as the center aisle path near that cell (depicted
red). Notice that it is not permitted to place pallets on the back aisles.
In our case, the shop-floor space and the P/D locations are given. In addition, we face several restrictions regarding the guide-path design because of limitations imposed by physical factory prop- erties. Therefore, we focus on considering the layout of the facility and the P/D-locations as input factors, and design the guide-path.
The road network can be described as the collection of paths and junctions on which the AGVs have to travel. A guide-path can be schematically represented by a graph consisting of a set of nodes (intersections and P/D-locations) and arcs (paths). Guide-paths can roughly be classified by the char- acteristics as shown in Table 5.1. A conventional flow topology consists of a network of roads and crossings. The conventional guide-path can be unidirectional or bidirectional (Le-Anh and Koster, 2006). Unidirectional paths allow vehicles to travel in only one direction, while bidirectional paths allow vehicles to travel in both directions. Unidirectional paths are easier to control (Vis, 2006) but using bidirectional paths can reduce the travel distance because AGVs can take shortcuts. In a sin- gle loop configuration, vehicles travel in only one loop without any shortcut or alternative routes (Le-Anh and Koster, 2006). Usually, these layouts are unidirectional, because in bidirectional trav- eling it is likely that vehicle interfere. In a tandem guide-path system, guide-paths are divided into several non-overlapping closed loops. Only one vehicle may travel in a zone and transfer stations are used to interface between zones. In a segmented guide-path configuration, the system has one or more zones, separated into non-overlapping segments which are served by a single vehicle. The guide-path topologies can be further specified in road segments that contain a single lane or multiple parallel lanes. Multiple parallel lanes require more space but may increase the throughput. Also, the combination of a mixed uni-bidirectional guide-path is possible.
TABLE5.1: Characteristics of guide-paths (Le-Anh and Koster, 2006).
Flow topology Number of parallel lanes Flow direction
Conventional Single lane Unidirectional flow Single loop Multiple lanes Bidirectional flow Tandem
Segmented
In our case, the AGAPTV can travel both forwards and backwards, which enhances the flexibility of maneuvering efficiently through guide-paths. As the production facilities are typically long build- ings with not that many crossings, it is not desirable to consider a single loop flow topology. Another reason to not consider this topology is that the cell sections may be blocked unexpectedly and that
other vehicles may interfere with the AGV such that a single loop flow does not yield an efficient per- formance. A counter argument to include this configuration would be that the anode demand usually follows a predictable cyclic pattern and with the single loop configuration, the AGAPTV’s driving route could then be arranged such that AGAPTVs perform their tasks appropriately. However, for the purpose of our generic model (applicability in many layouts), providing a robust solution for ex- tensions to our model (e.g., including crucible transporters), and flexibility in driving courses, such a single loop approach is not desirable. A tandem configuration requires (intermediate) storage buffers at the end of each segment and additional time to transfer loads at buffers. Although a tandem and a segmented system seem promising as it can easily be expanded and is often used in manufacturing environments where workstations are grouped, we do not prefer this approach because of defining buffer areas, not all clients have additional storage space, and less tolerance to system failures. It would be interesting to investigate this approach in a further research. A conventional approach, on the other hand, provides flexibility in control and tolerance to system failures, which is of importance in capital intensive industries like the aluminium industry. Disadvantages of a conventional topology are that it is complicated to control, lead to congestion and interference problems, and face difficul- ties in expanding (Le-Anh and Koster, 2006). We argue that we could tackle or at least limit these potential deficiencies by our further AGV system design and incorporated MAS control strategy.
Discussions with the management have led to a base for the guide-path layout as shown in Fig- ure 5.5. Limitations due to physical properties are taken into account and the P/D-points were con- sidered as a guideline in this design. These physical properties include vehicle characteristics like AGAPTV width, length, and maneuvering capabilities, as well as general potroom characteristics like the typical center aisle width and cross aisle length. The management considered two cross- road designs. One design is using single bidirectional lanes that connect sections with each other. However, in general, there is sufficient space to let two AGAPTVs pass each other on the cross aisle. Therefore, we decide to include two parallel lanes in the cross aisle that are connected to the center and back aisles in the way as depicted in Figure 5.5. This would potentially increase the flexibility (for example, when possible collisions are detected). Lastly, as the AGAPTV can drive forwards and backwards, the direction of the path is by definition bidirectional. Interviews with the management of Hencon resulted in the following guide-path construction rules:
1. The back aisles consist of one single aisle that can be traversed by one AGAPTV. 2. The center aisle consists of two parallel center aisles in between the cells.
3. AGAPTVs can make a turn from the back aisle to the center aisle or the other back aisle. 4. Segments are linked together by means of horizontal paths and zigzag paths as shown in Fig-
ure 5.5. These diagonal paths allow AGAPTVs to change driving lanes.
5. Potrooms are linked together by means of vertical paths. Zigzag paths allow the AGAPTVs to change driving aisles.
6. Although zigzagging within the cell segment paths is generally feasible, the management de- cides to not incorporate this feature yet because the current guide-path provides plenty of pos- sibilities to change directions.
7. All paths are bidirectional.