# Forklift Tracking: Industry 4.0 Implementation in Large-Scale Warehouses through UWB Sensor Fusion
## 4 Experimental Analysis
### Results  
To test the effectiveness of the UKF localization for the RFID Smart Forklift, tests were carried out in the warehouse. An adhesive tape was placed on the ground to create a reference path for the forklift driver during the trials. By analyzing the estimated trajectories and comparing them with the reference path, it was possible to evaluate the localization performance of the UWB system alone, of the dead-reckoning process (IMU + OFS), and of the UKF. It must be highlighted that, the method used by the Decawave UWB system to estimate the tag location is not known to users, but it is known that no more than four UWB anchors at the same time are employed (this is why we performed the simulations in Section 3 by considering only four anchors per timestep when computing the multilateration). Figure 11 shows a photo of a part of the reference path indicated by the adhesive tape stuck on the ground. To make sure that the forklift was following the reference path accurately, the OFS mounted on the forklift was used. In fact, this sensor projects on the ground a red-light beam footprint, which can be used to check if the reference points marked with the tape are correctly crossed during the forklift motion. Figure 11: Part of the reference path for the forklift indicated by the adhesive tape (**left**) and red light beam (**right**) projected by the OFS to make sure the forklift was actually following the path. Figure 10: (**a**) A UWB anchor installed at a metallic bar. (**b**) Position of UWB anchors in the warehouse area (blue asterisks). Nine forklift trajectories were performed with different speeds. The main features of the performed test case trajectories were resumed in Table 2 in terms of forklift forward speed \(v\), path-length \(L\), path shape, and starting location and orientation. In Figure 12, we represent two test-case trajectories about the cases IV (Figure 12a) and IX (Figure 12b), respectively, which have comparable forklift speeds (1.2 m/s and 1.6 m/s, respectively), but different path-lengths (\(\sim\)100 m and \(\sim\)289 m, respectively). The ground truth trajectory (continuous gray line) is the pre-determined reference path built by using the adhesive tape. The measured path with the UWB is depicted as continuous blue line, and it is the output of the Decawave commercial system after a mobile-median filter that filters out the outliers. The measured trajectory with IMU and OFS only is the dashed red line, and the estimated trajectory by using the UKF is the pointed green line. As it can be seen, the dead-reckoning and the UKF localization methods, thanks to the use of onboard kinematic sensors, outputs very smooth trajectories, which are therefore realistic due the high inertia of heavy industrial vehicles, such as forklifts. On the contrary, the UWB localization system exploiting multilateration can have some large oscillations around the reference path, as, for example, in the \"straights\" portions of the trajectory represented in Figure 12b. However, for long trajectories, and consequently during the regular running of forklift operations in the warehouse where the localization system must never be interrupted, the dead-reckoning suffers from drifts. An example of this phenomenon can be seen in Figure 12b, when the forklift performs a closed-loop path, but the dashed red line does not close itself to its starting point.