tions where the tapping process is ongoing.
Pot Tending
Pot tending activities are initiated by cell characteristics and are not always necessary. Pot tend- ing activities include activities like bath leveling and beam raising (e.g., see Table B.1). Recall that bath leveling compensates a difference in bath material occurred due to processes going on in the electrolytic-cell (see Subsection 2.1.3). A commonly used approach in practice is to perform bath lev- eling activities a few hours after the cell is being tapped. During bath leveling, no other vehicles, machines, and equipment can pass the corresponding aisle in a similar manner as during the tapping activities.
Beam raising involves repositioning the anode in a cell. Typically, smelters are arranged such that one anode should be realigned in a cell on a daily basis (see Figure 2.3a). The same blocking properties hold for beam raising as for bath leveling.
2.2.3.3 Continuous-based Schedule
Although smelters typically use the shift-based planning approach, there is a growing tendency in achieving a planning approach that is not bounded to the shift and section way of working. For example, in the shift-based planning approach, it is possible that a shift ends earlier than expected. Consequently, the corresponding workforce has to wait until the next shift starts while it may be more efficient to already start with other cells in the next section. A better understanding of aluminium production processes and technological advances, such as the establishment of AGVs in aluminium smelters, contribute to the development of a production environment that enables a 24/7 operation.
One step further towards this development, is providing insights into the impact different plan- ning approaches have on the aluminium smelter’s performance. Modifications to section and shift- based working schemes would already be valuable. In the model we develop, we could examine the impact of various section, shift and anode demand distributions.
In this study, we mainly cover the shift-based scheduling approach because this approach is gen- erally used in practice. However, by means of models parametrizations concerning the section- and shift-based working approach, we aim to provide insights into different ways of working.
2.3
Internal Anode Transportation
In this section, we successively describe the internal logistic processes of anode transportation and the specification of transportation jobs that should be carried out by the anode transport vehicles.
2.3.1
Anode Transportation Processes
The anode hauling process starts with a transport request generated by the MES. Each transport re- quest is restricted by an earliest- and latest delivery time of an anode pallet. Also, the transport request has a pick-up and delivery place. Furthermore, the front of the anode pallet must be placed according to a certain orientation (North, West, South or East), which depends on the destination lo- cation. This subsection commences with presenting some typology and a base layout of the potrooms. Next, pallet orientation and placement restrictions with respect to the cells are addressed. After that, driving blockades and safety measures of the AGAPTV are discussed.
2.3.1.1 Typology and Base Layout
In the aluminium smelters we consider, electrolytic-cells are positioned following the end-to-end lay- out. That is, the shortest sides of the cells are placed next to each other (e.g., see Figure 2.6a). Fig- ure 2.16 illustrates a layout including the AGV guide-paths. In general, the width of the center aisle is sufficient large to cover two parallel paths in which the AGAPTV can travel. In the center aisle, the cells are positioned in series facing the front ends to the same side side (see Figure 2.7a). The back side of the cell (see Figure 2.7b) is reachable via the back aisle. The back aisle is typically smaller than the center aisle and therefore at most one AGAPTV can travel on that side. As shown in the figure, the aisles intersect on crossings.
FIGURE2.16: Smelter layout base typology within a segment.
As there are often multiple potrooms, the crossings may lead towards another potroom or to another segment. Cells are clustered in a segment and these segments are physically separated from other segments by means of cross aisles as well. Figure 2.17 shows the used typology to classify potrooms and segments throughout this thesis.
A modern smelter roughly contains around 300 to 700 pots covering a total length of approxi- mately 1 kilometer. The length of a potroom and the physical placement of the different support systems play a crucial role in the time duration of a trip carried out by the AGAPTV. Imagine that the AGV must travel 1 kilometer with a driving speed of 5 kilometer per hour, then a round trip would already results in a duration of 40 minutes solely by driving.
2.3.1.2 Pallet Orientation and Pallet Placement Restrictions
Each cell is reachable from two opposite sides, however, the pallets may not be placed on the back side of the potrooms because of the narrowness of the path. As addressed before, pallets are not symmetrical because the anodes require a certain orientation in the cell. Anodes have a cove on the width side of the block (see Figure 2.12) and these need to be pointed to the electrolytic-cell. The pallet’s orientation is essential during delivery of full buckets to the potroom. As shown in Figure 2.18, the southern parts of the cell require a similar orientation. The same holds for the northern parts of the cell. So, arising demand from either the northern or southern of the cell could potentially be combined in an anode pallet.
The pallet should not be stored in the section too far away from the cells where the anodes are actually needed. Disobedience of the latter could undermine an efficient workflow because the trans- portation time within a section then increases. So, before the AGAPTV delivers the pallet, the route,
2.3. Internal Anode Transportation 31
FIGURE2.17: Smelter layout base typology within a smelter. A potroom contains two potlines with parallel series of electrolytic-cells. The cells are connected in series and lined-up in an end-to-end position. Cross aisles designate a physical separation be- tween a series of cells. The area covering two parallel series of cells in one potrooms
separated by cross aisles, is defined as a segment.
FIGURE2.18: Anode pallet orientation in the potroom. Anodes positioned near the seg- ments’ northern back aisle and southern center aisle should be fulfilled through pallets located at the northern center aisle (indicated blue). Pallets at the southern center aisle
fulfill anode demand from the southern parts of the cells (indicated red).
driving direction, and storage location should be determined. Likewise, the sequence in which an- odes are replaced by the workforce should be taken into consideration as this influences the pick-up
and delivery scheme.
The anode placement time plays an important role in the aluminium production process and re- quires on-time availability of anodes on nearby pallets that are reachable by anode changing equip- ment. New anode pallets are picked-up from the rodding shop or anode bake plant. On the one hand, when new anodes arrive too early at their destination, the anode placing equipment does not need them which could consequently lead to unnecessary stacks. On the other hand, a too late delivery affects the electrolytic process in the cell and could disturb an efficient aluminium production process. Pallet orientation is less important when pallets are dropped-off at the rodding shop. Empty pallets and pallets with spent anodes can be dropped-off and picked-up from both sides. Hence, the pick-up orientation is then not an issue. Likewise, dropping pallets at the rodding shop requires no special pallet orientation.
2.3.1.3 Driving Blockades and Safety Measures
Besides that (parts of) driving lanes are blocked for the AGAPTV when smelter or pot tending opera- tions are in progress (see Subsection 2.3), there are some other limitations that affect driving through by this vehicle as well. The AGAPTV is not only equipped with a safety scanner that detects possible physical obstacles within an observable range, but also with a control system that communicates with other vehicles and machines. This technology enables AGAPTVs to avoid obstacles and taking into account driving restrictions when determining safe routes. Possible blockades, driving restrictions, and safety measurements which are generally applicable within smelters we consider include:
• Possible vehicle collisions should be avoided by restricting a minimum distance between AGAPTVs (see Appendix C);
• For safety reasons (to avoid electrical short cut) and to enable other trucks to drive through, a minimum distance between pallets placed on different sides of the center aisle must be main- tained (see Figure 2.19a and Figure 2.19b);
• It is allowed to drop pallets or park vehicles close near each others if they are placed on the same side of the center aisle (see Figure 2.19c). Other vehicles and equipment can then traverse the lane via the other lane of the center aisle;
• Limited space within the main aisle(s), restrictions concerning properties of the AGAPTV (e.g., size, length, turning angle, etc.), and securities measures to prevent possible accidents and dead- locks (e.g., tipped pallets, deployed maneuvers which are intermediately blocked, etc.) impose a few restrictions that should be considered. Firstly, the AGAPTV cannot turn to change driving directions once an aisle has been entered (see Figure 2.19d). Thus, it is not possible to change driving directions from, for example, heading north to heading south when driving in a lane. However, the AGAPTV could change its driving direction from forwards to backwards or visa versa on each path. Also, at crossings the AGAPTV may change driving directions. At these locations, the vehicle can change aisle directions by performing, for example, two 90 degree curves. Secondly, the AGAPTV may not easily make a turn in case of pallets standing in the outer parts of the segments. As illustrated in Figure 2.19e and Figure 2.19f it is desirable to not place pallets directly in the outer parts of the segment. This to avoid possible collisions by turning vehicles and preclude complex driving maneuvers for the AGAPTV.
Additionally, two approaches for rescheduling routes based on the occurrence of blocking re- strictions are commonly used in practice. The first approach is acting in a reactive manner once a blocking occurs. once a job is dispatched to a vehicle, the route is determined only in the beginning (with considering blocking restrictions). As soon as a blocking restriction occurs, the vehicle could not drive further and the route will then be rescheduled. The other considered approach is a proactive rescheduling approach in which affected routes are adjusted immediately based on information about blocking restrictions. We decide to incorporate both approaches such that we can expose differences in achieved performance. For an brief explanation about differences in the rescheduling approaches, we refer to the literature review in Chapter 3.
2.3.2
Types of Transportation Jobs for the Automated Guided Anode Pallet Trans-
port Vehicles
The pick-up and delivery of anode pallets should be thoroughly planned. Considering the driving blockades and safety measurements addressed in the previous section, the AGAPTV transport orders
2.3. Internal Anode Transportation 33
(A) A minimum distance between placed obstacles on each side of the center aisle must be maintained to enable vehicles and equipment to pass through. For example, one could con- sider a minimum distance of at least 2 times the pot length.
(B) Large vehicles should be able to pass anode pallets or vehi- cles in a safe way.
(C) Equipment may be parked with less than one pot length between them if they are placed on the same side of the center
aisle.
(D) Performing a U-turn within section aisles is not possible. Simply a change in the driving direction of the AGAPTV is possible because the vehicle can drive both forwards and back-
wards.
(E) Anode pallets should be placed with an appropriate dis- tance from the cross aisle, otherwise it may cause dangerous
situations.
(F) Anode pallets should be placed with an appropriate dis- tance from the cross aisle, otherwise it may cause dangerous
situations
FIGURE2.19: Various driving blockades, driving restrictions, and safety measurements concerning anode pallets and AGAPTVs. The rules are general applicable to the pri-
mary aluminium smelters with end-to-end positioned electrolytic-cells.
should be carried out in an adequate manner. A wrong transport decision made may disturb the aluminium production process significantly. Therefore, usually, upfront of the change of anodes, the pick-up and delivery sequence should be known. Otherwise, the transport may result in deadlocks and aluminium process disturbances. Thus, the sequence of completing transport orders should be tailored to the production process and visa versa.
As customers may use different anode changing schemes, which often deviates from day-to-day, it is important to cover a variety of approaches that are realistic in practice. Typically, an order list is received from the MES with transportation jobs to be carried out by the AGAPTV. MES should provide information concerning the type of transport, the pick-up and delivery location, and the job release time. A goal of MES is to provide the required anode transports appropriately such that the workforce can start working immediately once the anode changing shift starts. The following transportation jobs can be specified:
• Pick-up an anode pallet with fresh anodes from a storage location such as a rodding shop or conveyor belt, and transport it to an electrolytic-cell;
• Pick-up an anode pallet without anodes from a storage location and transport it to an electrolytic- cell;
• Pick-up a pallet full with spent anodes from an electrolytic-cell and transport it to a storage location;
• Pick-up an empty pallet from an electrolytic-cell and transport it to a storage location;
• Pick-up an anode pallet with some fresh anodes left from an electrolytic-cell and transport it to an intermediate storage location or other cell.
MES should not only consider the release of transport jobs based on the required fresh anodes, but also the imposed pallet placement restrictions and the sequence in which a workforce is replacing the anodes. One should consider the transition from anode demand to anode pallet demand. From thereon, the pick-up and delivery sequence should be determined in close collaboration with the team responsible for changing the anodes.