## 2 Materials and Methods
### The Crawler Unit: Overview  
Referring to Figure 2, each Crawler Unit consists of three main bodies plus the docking module. Each module is connected with the subsequent through an active kinematic chain with 3 degrees of freedom (DoFs). The Crawler Unit equips a combination of active and passive tracks for locomotion. In particular, the tracks on the two longest modules are active, while the others are passive. The single DC motor concurrently drives the tracks on both sides of active modules. For turning, the Crawler can adopt two configurations: the C-configuration and the S-configuration. In the former, the Crawler rotates the modules in the same direction using the yaw joints, and the system assumes a configuration similar to the letter C. When the active tracks spin in the same direction, the Crawler begins to turn with a given curvature radius. In the S-configuration, the Crawler rotates the modules in opposite directions using the yaw  
Figure 1: Representation of the Hybrid Platform robot executing different tasks. In **(a)**, the Crawler Units are deployed/recovered near/from an inspection site; in **(b)**, the Main Base operates the Crawler Units as robotic arms to inspect a target on sidewalls; in **(c)**, the Crawler Units perform inspection in narrow areas, moving around some obstacles; in **(d)**, the Crawler Units adopt the snake configuration to reach a manometer placed above the ground; in **(e)**, the Crawler Units adopt the snake configuration to inspect the internal surface of a tank. Figure 2: CAD drawing of the Crawler Unit version two. Each module is 0.11 m high and 0.22 m wide. In total, the robot is 1.54 m long. joints; the robot assumes a configuration similar to the letter S. When the active tracks spin in opposite directions, the Crawler starts to turn on the spot. The system is redundant either in kinematics and actuation. Kinematic redundancy confers great adaptability to different terrains and obstacles. Kinematic redundancy is crucial when the Crawler Unit operates as a ground robot in unknown environments, and when the Crawler Unit attaches to the Main Base and reconfigures into a robotic arm. Actuation redundancy ensures an additional degree of fault-tolerance and reliability to the Crawler Unit. Two prototypes of the Crawler Unit have been developed, see Figure 3. Both the models have similar macroscopic features, e.g., number of modules, kinematics, tracks. However, the two versions present some differences on specific components. Such differences will be discussed in next subsections. The modules are called from the left to the right: rear module, central module, front module and docking module. In both versions, each module has a hollow chassis to store the actuators and electronic boards that drive the system. In particular, the rear module houses the electronics, the drive system and the yaw joint actuator. In version one, the central module encloses two servomotors that act as pitch joints and it has some free space for possible payloads. In version two, however, the free space is reduced to give room to the two pitch joints and their new transmissions. The front module contains another drive system and two yaw joints, one in the front and the other in the back. Lastly, the docking module accommodates another pitch joint, the docking mechanism and its driver circuit. Referring to Figure 3, the chassis of the modules deeply differ between versions. In particular, the Crawler frames of version two present several external ribs, and such a feature is not merely aesthetic. As a matter of fact, version one of the Crawler Unit is quite heavy as a result of the over-sizing of the mechanical parts. In developing version two, a preliminary structural analysis guaranteed that the most sensible components, such  
Figure 3: Snapshots of the Crawler Unit prototypes. On top, version one of the Crawler. Due to the conservative design, each module has an average weight of 2 Kg. On bottom, version two of the Crawler. The new prototype includes thinner parts and design improvements.