# Comparison of Multi-Tracked Running Gears in Terms of Obstacle Negotiation Capabilities
## 1 Introduction  
Unmanned ground vehicles (UGVs) are widely used to perform tasks in terrain that is inaccessible to or dangerous for humans [1, 2, 3]. The UGV operating area is often diverse: urban areas, which have a structured character [4] with roadside curbs and ditches [5], as well as rough rural environments with objects such as logs, stones, and clumps of grass [6, 7, 8]. The nature of the tasks performed, including the need for agility in maneuvering, economy, and transportability, determines the use of light-class UGVs (up to 1000 kg). Such robots may have difficulty overcoming open terrain and obstacles. Small running gear dimensions make the roughness greater and obstacles relatively higher [9, 10]. As a result, when climbing obstacles, there are high deviation angles, the uphill resistance increases, and the stability of the platform is reduced [11]. In addition, this exposes the sensors, which the platform transports, to constant changes in position [12, 13]. An example would be cameras whose images build situational awareness and whose vibrations lead to incorrect operator reactions that threaten smooth driving [14]. To improve mobility, tracked systems are used [15], but when entering an obstacle, they still have a limited track-ground contact surface [16]. This causes higher slip, which may result in a loss of traction and immobilization or extend the time needed to overcome the obstacle, leading to errors in the platform trajectory [17] and higher energy consumption, limiting the range of platform use [18, 19]. To increase the contact surface by adjusting the running gear shape to the obstacle profile, more complex tracked structures have been designed [6, 20, 21], such as multi-track running gears. These show good mobility properties on very soft deformable ground such as snow [22] or the seabed [23], but above all, theyare used for work on rough irregular terrain [24]. Due to the body architecture of tracked systems (Figure 1), the tracks can be passive articulated, active/passive articulated, or passive articulated [25]. Passive/active systems (Figure 1a) with pitch-controlled flippers are used to actively change the track approach angle [26; 27] and increase the contact length and, as a result, increase stability. In this way, a robot equipped with flippers could overcome a 12.5% wider ditch than a robot without them [5]. In active systems (Figure 1b), the position of all tracks can be controlled. This provides greater possibilities of changing the shape of the drive system, providing high obstacle clearances [28; 29], even higher than the whole tracked system itself [30; 31]. Studies confirmed that this also improved the stability and traction when moving across slopes of loose material [32]. A significant limitation of this UGV type is the complicated remote control system, which increases the cognitive load for the operator, especially while performing tasks with limited visual feedback. In this case, complex autonomous systems may be necessary, because manual control excessively reduces the operating speed and increases the trajectory errors [33]. Passive solutions (Figure 1c) are only capable of the passive pitch rotation of track systems when adjusting to the terrain [24]. Their obstacle clearance is lower than in active systems (Figure 1), but they do not require special control procedures when overcoming obstacles, which reduces costs and the operator's workload [34]. They are better suited for use in heavier structures up to 1000 kg [35] and allow higher operational speeds--up to 20 km/h [36]. This makes them better adapted to replace human work in the field. For all these reasons, these systems will be considered in further research. A simple example of such a passive articulated solution is the connection of several tracked chassis with passive joints. One study [37] indicated that such a system of three tracked modules allowed over 30% higher steps to be overcome than by a single module.