## 6 Results and Discussion
### Assembly Stations  
The simulation results obtained in the first experimental environment show that it is not possible to identify the best control architecture (Table 5).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Variables**} & \multicolumn{4}{c}{**Centralized and Heterarchical Control Architecture**} \\ \cline{3-5}  & & **1** & **2** & **3** \\ \hline \multirow{5}{*}{\% non-compliant products} & 0 & \(\bullet\) & \(\bullet\) & \(\bullet\) \\  & 25 & \(\bullet\) & \(\bullet\) & \(\bullet\) \\  & 50 & \(\bullet\) & \(\bullet\) & \(\bullet\) \\  & 75 & \(\bullet\) & \(\bullet\) & \(\bullet\) \\  & 100 & \(\bullet\) & \(\bullet\) & \(\bullet\) \\ \hline \multirow{3}{*}{Random AGV failures} & Absent & \(\bullet\) & & \\  & MTBF = 3 min and MTTR = 1 min & & \(\bullet\) & \\ \cline{1-1}  & MTBF = 9 min and MTTR = 1 min & & \(\bullet\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: EDF simulation model parameters.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Component**} & **Description** & **Parameter** & **Value** \\ \hline \multirow{4}{*}{Electric monoral system (EHB)} & \multirow{4}{*}{Transporter} & Conveyor speed & 0.4 m/s \\  & & Total length of route & Approx. 70 mt \\  & & Position of the vertical storage & At 0 mt \\  & & Position of the transfer carriage (3 VW03) & At 32 mt \\  & & Number of vehicles & 2 \\ \hline \multirow{2}{*}{Vertical warehouse} & \multirow{2}{*}{Space for pallets storage} & Storage capacity & 49 pallets \\  & & Storage dimension & 7\(\times\) 7 storing positions \\ \cline{1-1}  & & Storage system & Chaotic \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont_.  
The results of the first experimental case show that the best result was obtained by applying centralized control, where the mean throughput time with centralized control was lower than the mean throughput time with heterarchical control.  
For the second experimental case, the mean throughput time was almost the same for both control architectures, therefore centralized and heterarchical controls were comparable.  
In the third experimental condition, the heterarchical control results were slightly better as compared to centralized control throughput time.  
The fourth experimental case was different from the third one for the position where control was applied near the assembly stations. Also, in this case, the mean throughput time resulting from heterarchical control was lower than the mean throughput time resulting from centralized control. This means that the position where control was applied did not influence the results.  
As for the last two experimental conditions, even if the throughput time resulting from centralized control was lower than that of heterarchical control, the time saved was not enough to compensate for the loss coming from the non-manufactured units. Indeed, under centralized control, in case of failure the production system was blocked and there were almost 80 units that could not be manufactured. Instead, under heterarchical control, it was possible to switch assembly stations in real time and process the materials without stopping the production process.  
The results obtained from the six experimental cases demonstrate that for deterministic and predictable systems, centralized control is the best solution. However, this is an ideal condition, very far from the manufacturing reality. As soon as a failure occurs, a more flexible control is preferred because the time saved is not enough to cover the cost of non-manufactured units.