# An Action Classification Method for Forklift Monitoring in Industry 4.0 Scenarios
## 4 Experimental Analysis  
### Experimental Results  
Figure 12a,b shows an example of a successful and unsuccessful _IN_ classification, respectively. As apparent in Figure 12b, the unwrapping fails by causing a wrong sign estimation of the slope coefficient \(m\). Similarly, Figure 12c,d shows an example of a  
Figure 11: Time behavior of the normalized unwrapped phase in the asymmetrical antenna deployment for _IN_ and _OUT_ actions by varying the speed \(v\) when the parameters are the following: \(f=865.7\) MHz, \(l_{1}=1\) m, \(l_{2}=4\) m, \(\Delta h=2.5\) m, and \(T=50\) ms.  
successful and unsuccessful _OUT_ classification. In such a case, the slope coefficient \(m\) of Figure 12d is wrongly estimated as negative. Table 1 resumes the principal features of the showed curves in terms of the number of samples \(N_{R}\), time duration of the crossing action \(T_{d}\), forklift speed \(v\), average sampling time \(\Delta_{T}\), average spatial sampling \(\Delta_{S}\) and measured slope \(m\). There are multiple causes of unsuccessful classification, mainly related to a failed phase unwrapping in industrial scenarios. First of all, the multipath phenomena can introduce strong and unpredictable contributions on the phase variation \(\phi^{m}[n]\). Second, the presence of multiple tags close to the gate demanding for the communication channel may slow the forklift-tag reading rate. Finally, the speed of the forklifts, which can move up to 3.5 m/s, may cause a poor sampling of the phase curve and consequently a wrong phase unwrapping. To better understand the effect of the forklift speed \(v\) and evaluate the classification accuracy, an experimental campaign was conducted. We analyzed a total of \(N_{T}=264\) trials acquired from the gate placed at the production line end during the regular forklift operations. The number of recorded \(IN\) and \(OUT\) actions is \(N_{IN}=164\) and \(N_{OUT}=100\), respectively. The reason for such difference is due to the exclusion from the experimental analysis of all the cases where the optical barrier sensors failed, so it was not possible to determine the forklift speed and recognize the ground truth of the forklift passage. In 100% of the cases, at least one of the two tags placed on the forklift was detected by the CP antenna at least once. The classification accuracy computed for different ranges of the forklift speed \(v\) is shown in Figure 13. The overall action classification accuracy of the method is 92% but reaches a maximum value of 97-98% when the forklift travels at a speed between 0.5 m/s, and 1.5 m/s. It is apparent that, when the forklift overpasses the speed of 1.5 m/s, the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Trial** & \(N_{R}\) & \(T_{d}\) (s) & \(v\) (m/s) & \(\Delta_{T}\) (ms) & \(\Delta_{S}\) (cm) & \(m\) (rad/s) \\ \hline Successful \(IN\) & 85 & 3.7 & 1.57 & 44 & 7 & \(-13.28\) \\ \hline Wrong \(IN\) & 72 & 3.3 & 0.98 & 46 & 4.5 & 0.1063 \\ \hline Successful \(OUT\) & 114 & 9.1 & 0.49 & 80 & 3.3 & 0.62 \\ \hline Wrong \(OUT\) & 76 & 5.7 & 0.77 & 74 & 5.7 & \(-0.97\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main features of the trials represented in Figure 12.  
Figure 12: Examples of measured normalized unwrapped phase \(\phi^{m}[n]\). (**a**) Successful \(IN\) classification, (**b**) wrong \(IN\) classification, (**c**) successful \(OUT\) classification, (**d**) wrong \(OUT\) classification. accuracy of the action classification method decreases as the phase unwrapping fails. On the other hand, a low forklift speed can be detrimental too, since the phase slope could be too close to zero, making the action classification less reliable. This effect is apparent in Figure 13 for \(v<0.5\) m/s. The number of tag readings is a crucial parameter for the success of the classification algorithm. Therefore, the average number of samples with respect to the forklift speed is also reported (Figure 14). As expected, the number of available readings decreases with the increase of the forklift speed.