## 3 Results
### Uneven Terrains  
The experiments on version one of the prototype were performed on a mock-up scenario with climbs, descents and steps, as shown in Figure 15. Be aware that, in this experimental scenario, version one of the Crawler Unit faced only descending steps and ascending slopes. The pitch joints in this version are not able to lift the modules. Therefore, all the pitch joints were disengaged with the exception of those connecting the front module to the docking module. The back, central and front modules adapted to the terrain, while the docking module weight slightly increased the friction force of the front module. Combining the forward and turning motions, and rotating the modules allowed the operator to recover the Crawler Unit from stuck conditions. As shown in the sequence of Figure 15, version one of the Crawler Unit successfully traversed the 2 m long terrain in a time \(t=30\) s. Even if the passivity of pitch joints was very useful in descending transitions of terrain, it reduced the overall maneuverability.  
Figure 14: Climbing ramp sequence. The Crawler Unit version two climbs a 0.8 m ramp with a slope up to \(20^{\circ}\) in \(t=15\) s.  
Figure 13: Climbing ramp sequence. The Crawler Unit version one climbs a 0.6 m ramp with a slope up to \(20^{\circ}\) in \(t=11\) s.  
This limitation represented one of the reasons why version two of the Crawler Unit has been developed. As already discussed, the new version equips pitch joints with transmissions that increase the output torques and an enhanced joypad, which provides the operator with the ability to actively control the vertical motion of the Crawler Unit modules. The experiments over the uneven terrain were repeated using the newer version. As displayed in Figure 16, version two traversed a scenario even more complex. Here, the Crawler Unit started with the front and docking modules on a platform placed \(0.10\,\mathrm{m}\) above the central and back modules. Therefore, these last two segments had to climb a step, which was comparable with the height of tracks. Referring to Figure 16, the Crawler Unit successfully overcame that transition height by moving the modules vertically and traversed the \(1.5\,\mathrm{m}\) long terrain in a time \(t=$30\,\mathrm{s}$\). Such results were encouraging and suggested that version two of the Crawler Unit produced better performance over uneven terrains.  
Another possible challenge the Crawler Unit may deal with is the crossing of ditches. Both the versions of the robot were tested in this kind of scenario. As shown in Figure 17, version one succeeded in traversing a gap of length \(d=$0.25\,\mathrm{m}$\). During this experiment, all the pitch joints were active and maintained zero pitch angles between modules. Beyond the threshold \(d=$0.25\,\mathrm{m}$\), the modules got stuck in the ditch due to undesired deflections of pitch joints.  
The new Crawler Unit exhibited better performance than the previous version, as shown in Figure 18. As a matter of fact, version two successfully traversed larger gaps, whose length is up to \(d=$0.4\,\mathrm{m}$\). During these tests, the operator drove the robot, choosing how to move the active joints in order to overcome the ditch and reach the top. Here, the  
Figure 16: Uneven terrain sequence. The Crawler Unit version two traverses a \(1.5\,\mathrm{m}\) long uneven terrain, which includes ascending and descending steps and ascending ramp.  
Figure 15: Uneven terrain sequence. The Crawler Unit version one traverses a \(2\,\mathrm{m}\) long uneven terrain which includes ascending ramps and descending steps.  
threshold \(d=0.4\,\)m was not due to the weakness of the pitch joints, but it was determined by the low friction with the ground. In particular, the Crawler became stuck when one of the active modules was not in contact with the ground. In this case, the other active module started to slip on the ground without producing any motion.