## 1 Introduction  
One of the most significant problems in underground tunnels is the survey of the proper performance of the security sensors available all along the whole corridor. Inspection of large tunnels can be laborious when performed by operators. However, robots may help with performing 4D tasks (dirty, dangerous, difficult, and dull), reducing risks for the personnel. Inspection in underground tunnels use to be complicated because of (a) the long time to access the facilities, (b) the long time to escape the facilities in case of evacuation, (c) the strong safety protocols, and (d) the lack of GPS signal to locate where you are. Usually used tunnels have other ways to communicate with the outside, such as 4G and a WiFi network. To enhance the process of inspection in long tunnels, a robotic platform is presented in this paper. This mobile robot has been focused in the surveillance of the SPS where a large amount of restriction are present due to its actual state. However, the platform could be used in any other underground tunnel, where the only requirement is found in the floor material, which should guarantee that the omnidirectional wheels work properly. The SPS is the second largest machine in CERN's accelerator complex, with a circumference of approximately 7 km. Like the rest of tunnels of these characteristics, the corridor of the tunnel becomes completely monotonous, making navigation and SLAM an authentic challenge. Environmental sensors, which measure the radiation, temperature and oxygen concentration among others, are essential to guarantee material and personal safety. Due to the need of periodic check-ups of the sensors, inspections have to be carried out every month. Thus, in this paper we propose the design of an omnidirectional wheeled robot. The robot should be stored in a charging station, located in one of the entrances of the tunnel, where the batteries are charged. When required, it has to go around the ring and record data about the environmental variables. During its journey, the robot brings the needed sensors to record these variables and locate itself in the environment. One of the most challenging features of the SPS is the fact that it has 19 partition doors which separate sections of the tunnel. These doors have an aperture of 400 \(\times\) 200 mm, allowing the robot to cross them. Furthermore, the charging station is located in an area of difficult access. It means that the robot has to do several maneuvers to enter and go out of the station. In conclusion, the wheeled robot has the following requirements:  
* The survey should take around 80 min. To achieve that, the robot should drive at a speed of 5.5 km/h, or 1.5 m/s, approximately. * The robot should be able to deal with small spaces, such as doors of 200 \(\times\) 400 mm. * The motors should guarantee the nominal velocity of the robot of 5.5 km/h. * The robot's autonomy should be of 3 h at least, guaranteeing that the robot completes the surveillance even with the presence of unforeseen circumstances. * The robot should be equipped with the required devices to perform the surveillance autonomously, or through tele-operation if the situation requires it. Autonomous inspection is understood as the fact that the robot travels through the tunnel without direct control by an operator. In such a way, that the robot is able to leave the charging station until the start of the journey, navigate from door to door, cross the door to go from one section to another, and go back to the charging station to charge the batteries. In this line, it is expected that the robot will need external sensors, such as cameras, a 3D LIDAR, and distance sensors to be placed in the middle of the tunnel. The paper is organized as follows: In Section 2, an overview of wheeled robots is presented. In Section 3, we present the reasons which justify the selected locomotion arrangement.