# Comparison of Multi-Tracked Running Gears in Terms of Obstacle Negotiation Capabilities
## 2 Methodology
### Obstacle Profiles and Assessment Criteria  
As a direct measure of the specific UGV solution's capabilities, obstacle clearance is considered, e.g., the height of a curb [37] or step [34], a log diameter [16], the ditch width [5; 27; 46]. These will not have repeatable values in different weather, on different ground, or in different conditions. A change in ground moisture, for instance, may cause  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Module mass & 200 kg \\ Track mass & 6.4 kg \\ Track length & 2445 mm \\ Track weight & 160 mm \\ Track contact length on the ground & 700 mm \\ Number of road wheels & 4 \\ Road wheel diameter & 75 mm \\ Drive wheel/idler diameter & 300 mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Track module parameters.  
Figure 2: Schemes of solutions of multi-tracked systems selected for simulation.  
significant variations in the ability to overcome obstacles [7,39]. Therefore, to assess the ability to overcome obstacles in the compared solutions, the minimum ground traction required for obstacle negotiation was proposed.  
Three characteristic terrain obstacles were selected: a roadside ditch of standard dimensions, a city curb, and a log with a round cross section (Figure 3). The drive wheel of each module was driven at a constant peripheral speed of 3 m/s, which corresponds to the actual operating speed of currently used light UGV solutions [5,36] during teleoperation, as well as when accompanying a walking person [47,48].  
The running gear solutions assessment methodology involved two tests:  
* First test.  
Each of the solutions overcame the obstacles multiple times. With each successful attempt to overcome the log, its diameter was increased by 10 mm. In the case of overcoming a curb and a ditch, after a successful attempt, the ground traction was reduced by decreasing the ground shear strength \(\tau_{max}\), affecting the \(F_{max}\)--the maximum possible platform traction (Equation (1)) from [49]. The change in this characteristic was introduced by modifying the tangent of the ground shearing resistant angle (\(tan\varnothing\)) by 0.05 in each subsequent test. The main criterion was the maximum log diameter \(\mathcal{O}\)log that could be overcome and the lowest ground shearing resistant angle (\(tan\varnothing\)) for negotiating the ditch and curb.  
\[F_{max}=A\cdot\tau_{max}=A\cdot(C+\sigma\cdot tan\varnothing) \tag{1}\]  
where \(A\) is the tracks-ground contact area, \(C\) is the ground cohesion, \(\sigma\) is the ground normal stress.  
* Second test.  
All solutions overcame an obstacle of the same dimensions and under the same ground conditions. According to Test 1, we determined a log diameter and ground shear strength  
Figure 3: Profiles of overcoming obstacles: (a) roadside ditch, (b) curb, (c) log with variable diameter.  
appropriate for all solutions to overcome an obstacle. During the negotiation, the following parameters were assessed:  
1. Obstacle overcoming time [s]--measured at the same distance before and after the obstacle, so that the length of the solution does not affect this parameter;
2. Track slip of the module at maximum load of the system's driving force;
3. Maximum pitching angle of the platform frame [degrees]--the frame was assumed to be the longitudinal axis of the first module for solutions A, B, C, and D and the common frame axis in solution E;
4. Maximum pitching angle speed of the platform frame [degrees/s]--calculated as a derivative of the pitching angle.