# Robot Localisation Using UHF-RFID Tags: A Kalman Smoother Approach +
## 3 Materials and Methods
### Experimental Setup  
For the experimental campaign, we used a self-made unicycle wheeled mobile robot with a differential front-drive kinematics as the one reported in (2), depicted in Figure 2a and built by the Italian Company AITRONIK srl.  
The robot axle length is equal to \(l_{a}=0.53\) m and the height is \(h=0.68\) m. The robot is equipped with two commercial rotary encoders belonging to the Parallax Arlo Robot kit. The encoders operate with 144 pulses for a full tire revolution and 100 Hz acquisition frequency. Since the robot tires have a diameter of 152 mm, the induced quantisation step on the linear displacement is around 3.3 mm. The robot is also equipped with an Impinj Speedway R420 UHF-RFID reader configured to transmit a signal with a frequency \(f_{0}=865.7\) MHz (ETSI channel #4). It is connected with four Time-7 Compact Outdoor RAIN RFID circularly polarised (CP) antennas with HPBW = \(115^{\circ}\) and \(Gain=5\) dBiC, where \"dBiC\" stands for the decibel above the gain of a CP isotropic antenna. Each antenna faces a different area as shown from the top view in Figure 2b. All of them are placed at a height of 0.58 m from the ground, except antenna #4 which is placed at a height 0.68 m. The robot adopted in the experiments can be driven manually through a remote control, i.e., a joystick. Since we are interested in actual industrial applications, where the estimate of the set of trajectories followed by the available moving robots is of major relevance either if they are autonomous or actually controlled by human workers, and just focused on the trajectory estimation (how the localisation uncertainty acts on the robot control law can be found, e.g., in [43; 44]), we decided to steer the robot in the environment using the remote controller.  
To validate the method performance, the ground truth data of the vehicle trajectory are computed through a state-of-the-art SLAM algorithm by acquiring data from a Slantec RP LIDAR A34 Laser Range Finder (LRF). All the payload-sensors are power supplied with two batteries and properly synchronised in time, and a small on-board PC is employed to gather and transmit all the data to an external laptop PC via a Wi-Fi connection.  
The test area consists of a single-room office-like environment with a total size of around 26 m, divided in two areas of \(3.85\times 4.10\) m\({}^{2}\) and \(3.90\times 2.60\) m\({}^{2}\), respectively, as shown in Figure 3a.  
47 EasyRFID Bone reference tags with Monza R6 chip are placed according to a not regular spacing along the room walls (Figure 3b) at a height of 0.70 m from the floor. The average tag spacing on the \(x\)-direction is 0.53 m, whereas the tag spacing on the \(y\)-direction is 0.35 m (see Figure 3b). The tags were placed both with vertical and horizontal orientations with respect to the ground plane to reduce the mutual electromagnetic coupling among  
Figure 2: (**a**) Robot used for the experimental campaign with its different components. (**b**) Schematic representation of the robot (top view). The robot antennas are denoted with the symbols #1, #2, #3, and #4.  
them (tags are represented with triangular and squared markers in Figure 3b). During the experiments only antenna #1 (indicating in Figure 2a) is on. With respect to the robot local reference frame, the antenna #1 points the negative direction of the \(y\)-axis (the right side, see Figure 2b), and its displacement with respect to the robot rotation centre is \(\Delta_{\texttt{F1}}=[-0.31,-0.11]^{T}\) m.