# Forklift Tracking: Industry 4.0 Implementation in Large-Scale Warehouses through UWB Sensor Fusion
## 4 Experimental Analysis  
### The RFID Smart Forklift  
This section aims at describing the sensors installed on the forklift. Forklifts are electrically driven vehicles and exist with either three wheels or four. The employed forklift is a four-wheel OM Still RX 20-18P/Li-Ion. It has a front wheel drive, so traction is carried out by the two front wheels, which can also rotate for the vehicle steering.  
The odometry data were not available and, therefore, we could not adopt the unicycle or bicycle vehicle model. However, thanks to the presence of the IMU and the OFS, it is still possible to reconstruct the dynamic state model of the vehicle.  
The sizes of the vehicle are \(L=2.78\) m, \(W=1.14\) m, \(H=2.08\) m. The onboard hardware is the following:  
* Decawave DWM1001 UWB module [61] configured as tag;
* A box containing the kinematic proprioceptive sensors: one Agilent ADNS3080 Optical Flow Sensor [62], four STMicroelectronics VL53L0X Time-of-Flight laser  
Figure 8: Mean distance error \(\tilde{\epsilon}_{d}\) as a function of the initial uncertainty matrix \(\mathbf{P_{0}}\). For each configuration described by Table 1, 100 test cases with similar features as the one represented in Figure 3 were conducted.  
ranging distance sensors [63], STMicroelectronics LSMDS3 inertial measurement unit platform [64];
* a RENESAS SK-S7G2 microcontroller [65]; The forklift also has, installed, onboard, the RFID hardware for the detection of the loaded pallets:
* UHF-RFID reader Impini Speedway Revolution R420 [66];
* two UHF-RFID Keonn Advantenna\(-\)p11 antennas for loaded pallet recognition [67];
* two UHF-RFID Alien A0501 antennas for stocked pallet detection and localization [68];  
A picture of the RFID Smart Forklift is shown in Figure 9. The box containing the OFS, the four Time-of-Flight laser-ranging distance sensors, and the IMU was installed close to the right-front wheel of the forklift at a height of 15.5 cm, in such a way that the OFS camera pointed towards the floor. The box is assumed as the origin of the body reference frame \(O_{b}^{(k)}\). The OFS and the IMU were jointly used for the sensor fusion localization algorithm together with the UWB data, whereas the Time-of-Flight laser-ranging distance sensors were used to accurately measure the height of the OFS from the floor and keep track of possible height variations of the sensor. Such measurements are necessary to calibrate the sensor measurements, which turns out to have a spatial resolution of \(r=2.3\) cm when the sensor is placed at a 15.5 cm height. According to the datasheet of the LSMDS3 IMU, the acceleration noise has a standard deviation \(\sigma_{a}=16.7\) mm/s\({}^{2}\), whereas the angular speed noise exhibits a standard deviation \(\sigma_{\omega}=2.4\) mrad/s.  
Considering that the UWB anchors will be installed at the room ceiling, the UWB tag is instead positioned on the roof of the forklift, to ensure its visibility. The smart forklift is equipped with a Decawave DWM1001 UWB module of the MDEK1001 development kit placed on the forklift roof with a displacement of \(\Delta\mathbf{p_{UWB}}=[\Delta x_{UIWB},\Delta y_{UIWB},\Delta z_{UIWB}]^{T}=[-0. 61,0.56,2.04]^{T}\) m, with respect to the box containing the IMU and the OFS. Due to forklift structure reasons, it was not possible to place the UWB tag directly above the IMU and OFS, so it also necessary to foresee that the 2D position measured by the UWB system does not correspond to that of the other two sensors. All onboard sensors are properly time-synchronized together through the RENESAS SK-S7G2 microcontroller, which sets the sampling time at \(\Delta t=10\) ms.