# Peer-to-Peer Ultra-Wideband Localization for Hands-Free Control of a Human-Guided Smart Stroller
## 3 Prototype Overview  
To test the performance of the proposed system, we implemented a prototype on the wheelchair WHILL (Model C2, WHILL Inc., Tokyo, Japan), which is a non-holonomic mobile platform. The details of the components of the prototype are summarized in Table 1. The UWB sensor we selected is LinkTrack AOA (SZ Nooploop Technology Co., Ltd., Shenzhen, China). We devised a preliminary test to measure the performance of the UWB sensor in a short-distance (3 m) indoor environment. In the preliminary test, we collected data from the measurement points in a semi-circular pattern with angular positions at \(-\)75, \(-\)60, \(-\)30, and 0 degrees relative to the anchor. These points are located at radial distances of 0.5, 1.0, 1.5, and 2 m from the anchor. Additionally, there are two measurement points positioned on the vertical axis (0 degrees) at distances of 2.5 m and 3 m. The sampling rate of the UWB sensor we used was set as 100 HZ, the measurement time was 5 s at each point. Ten trials were conducted for each point to obtain a reliable measurement result. The outcome revealed that distance measurement and angle measurement achieved stable results with smaller errors (distance error = 15 cm, angle error = 5 degrees) at the angle of 0 degrees and distance of 2 m. Based on the outcome of the preferred relative positions and the UWB measurement errors, we mounted the UWB anchor at the front of the robot and set the following distance \(l\) to 0.85 m so that the distance between the anchor and tag was close to 2 m. We fixed the UWB tag on the target human's waist as shown in Figure 4. A visual odometry sensor (Realsense T265, Intel Corp., Santa Clara, California, USA) was mounted on the front of the WHILL. In addition to functional safety, such as the robust perception method, a rope physically connects the user and the stroller as mechanical insurance. A pull switch is mounted on the stroller and can be operated through the rope, the user could pull the switch to interrupt the automatic control of the stroller when intervention is needed. The stroller could resume moving when the user pulls the switch again. The pull switch is located on the backside of the wheelchair. A Jetson Nano (Nvidia Corp., Santa Clara, California, USA) was used to implement the control logic. The modular design of the robot operation system (ROS) enhances system development flexibility and scalability by enabling independent development, testing, and deployment of components. Supported by a vast community, ROS offers access to extensive libraries and essential tools like RViz and Gazebo, which accelerates development and addresses complex challenges. Additionally, ROS facilitates interoperability by easily integrating diverse hardware and software components through robust inter-node communication capabilities, accommodating a wide range of sensors and actuators. Therefore, we imple  
Figure 3: (**a**) Top view illustration of the human-guided system. (**b**) The proposed human-guided system: the human adjusts the relative distance to virtually push the robot moving forward and virtually rotates the robot direction with distance-shift. mented software using ROS running on Ubuntu 20.04. The overall control architecture is presented in Figure 5. \begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Details** \\ \hline Platform & Model C2 (WHILL) \\ UWB sensor & LinkTrack AOA (Nooploop) \\ Visual odometry & T265 (Realsense) \\ Microcomputer & Jetson Nano (Nvidia Corporation) \\ Canopy switch & WSS201 HP (Panasonic) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Components of the prototype. Figure 4: System Overview. Comprehensive design and components of the proposed system. The system is implemented on a commercial electric wheelchair, equipped with an ultra-wideband (UWB) anchor positioned at the front. The human guides the wheelchair while wearing a UWB tag on the waist.