# Multi-AGV Scheduling under Limited Buffer Capacity and Battery Charging Using Simulation Techniques
## 4 Multi-AGV Scheduling Rules
### Simulation Model for a Multi-AGV System
This paper proposes a multi-AGV scheduling procedure summarized in Figure 4. The following three kinds of rules must be determined by the analyzer before the simulation experiment: the job selection rule \(r_{job}\), the AGV selection rule \(r_{AGV}\), and the charging station selection rule \(r_{CS}\). At first, a job is chosen from \(S_{job}\) by applying \(r_{job}\). If \(n(S_{job})=0\), we have to wait until a new transportation batch is generated. After a job is chosen, an AGV is chosen from \(S_{AGV}\) by applying \(r_{AGV}\). If there is no available AGV, we have to wait for an available one. Once an AGV is chosen, the chosen job is assigned to the AGV. When the AGV unloads the associated transportation batch at the drop-off point, we have to check the start-charge condition before handling the next transportation job. If the condition is met, a charging station is chosen from \(S_{CS}\) by applying \(r_{CS}\), and the  
Figure 3: Layout of shopfloor. AGV travels to the chosen station. An AGV staying at a charging station can perform a transportation task if and only if it satisfies the stop-charge condition. Note that the performance of the proposed multi-AGV scheduling procedure is affected by the following three kinds of rules: \(r_{job}\), \(r_{AGV}\), and \(r_{CS}\). In addition, the following model parameters and constraints are applied to the simulation model:  
1. Each production line produces five types of products, and the processing time of each type can be found in Table 4. Each type accounts for 20% of the entire product. 2. The raw materials arrive at the input buffer of each production line. The capacity of the input buffer is unlimited, and the inter-arrival times of raw materials are uniformly distributed between 100 and 200 (s). 3. The finished products are forwarded to the output buffer with limited capacity \(W_{max}(L_{i})\). \(L_{i}\) stops operation when \(W(L_{i})=W_{max}(L_{i})\). 4. An AGV can transport only one product at a time. That is, a product is a transportation batch in the simulation model. 5. The detailed parameters of AGV are as follows: * Speed on the straight section of a path line: 1.8 m/s; * Speed on the curved section of a path line: 1.5 m/s; * Acceleration/deceleration: 1 m/s2;  
Figure 4: Overall multi-AGV scheduling procedure. \begin{tabular}{l p{142.3 pt}} \(\bullet\) & Battery capacity: 100 (Ah); \\ \(\bullet\) & Battery C-rate when busy (discharging): 0.25 C; \\ \(\bullet\) & Battery C-rate when idle (discharging): 0.005 C; \\ \(\bullet\) & Battery C-rate when charging: 0.5 C; \\ \(\bullet\) & Pickup and drop-off time: 5 (s); \\ \(6\) & Two or more AGVs cannot occupy a single charging station at a time. \\ \(7\) & Overtaking of the AGV is not allowed. \\ \(8\) & The shopfloor is operating 24 h a day with regular break periods (12:00-13:00, 18:00-19:00). During the break period, the production lines do not operate, but AGVs can perform transportation tasks if \(n(S_{job})>0\). Consequently, \(W(L_{i})\)s tends to be small at the end of a break period. \\ \(9\) & The simulation end time for a single experiment is 604,800 s (7 days). \\ \end{tabular}