## 2 Mechanical Design and Structural Implementation  
For the mechanical and structural design, the main challenge was to obtain a robotic platform capable of driving with enough stability over rough terrain, according to the requirements set out in Table 1. As the platform is designed for rough terrain, its structure must take into account the different parameters that affect the drivetrain power requirements and the general motion system design of the platform (terrain, platform weight, tyre pressure, wheel diameter, and suspension type [15]). In a previous study [16], it was concluded that the most important determining factor for calculating the rolling resistance coefficient (RRC) is the terrain. Table 2 compiles the general values of RRC depending on the terrain used, as shown in Figure 1.  
Once the possible ground conditions have been determined, it is necessary to define the design of the drivetrain to be used. Some of the possible options are front two-wheel drive (FWD), rear two-wheel drive (RWD), or all-wheel drive (AWD). In our case, the AWD option has been chosen to increase the traction of the robotic platform, using an arrangement with six independent drive wheels. This will guarantee maximum contact with the ground, providing greater traction to the platform for use on rough and difficult-to-navigate terrain. Figure 2 shows a 3D image of the design proposed by the authors. The proposed design has dimensions of 800 mm \(\times\) 500 mm \(\times\) 630 mm (length--X axis, width--Y axis, and height--Z axis) and weighs approximately 20 kg (empty platform). The use of two pivot joints ((1) in green, Figure 2) on each side of the platform chassis allows the wheels to move freely in the ZX plane. The distance from point (1) to the ground is 27 cm; this allows Hygrobot to cross puddles of water without incurring risk to its operation. The differential bar ((2) in red) located at the top of the platform ensures that the effort exerted on these wheels during movement is compensated by the rest of the wheels. This differential bar is formed by a 6-millimetre-thick rhomboid-shaped plate, which is attached to two vertical arms fixed on each side of the moving parts of the chassis. Since \begin{table}
\begin{tabular}{c c c} \hline \hline
**Surface** & **Mean** & **Standard Deviation** \\ \hline Bituminous Material (1) & 0.02 & 0.01 \\ Grass (2) & 0.07 & 0.02 \\ Gravel (3) & 0.06 & 0.02 \\ Sand/Mud (4) & 0.30 & 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Rolling resistance coefficient (RRC) values for different terrains. Figure 1: Different operating terrains. From left to right: **(1)** bituminous material, **(2)** grass, **(3)** gravel, and **(4)** mud. the pendulum motion of the vertical arms not only generates movement along the X-axis but also along the Y-axis, a joint-based connection system ((3) in blue) has been included. To achieve the pivoting connection needed in the YX plane between the differential bar and the upper chassis, a new piece ((4) in yellow) has been designed that is fixed to the aluminium modular profiles and houses 2 bearings and an M8 bolt, minimizing friction and ensuring a secure connection with the structure. The central part of the structure is designed to house the power system components (described in Section 3). It is built in a modular way, using 20 mm \(\times\) 20 mm aluminium profiles and \"V\" threaded connections. Finally, the chassis that connects the wheels to the central structure consists of two pieces of 6.0 mm thick aluminium sheet ((5) in pink), joined together by a system of axial and radial bearings that allow movement along the XZ axis. The central structure of the platform, which houses the propulsion and control systems, was built from modular aluminium profiles with a section of 20 mm \(\times\) 20 mm, as Figure 3a shows. The moving parts were machined using a 6-millimeter-thick 5754 H11 aluminium sheet as the raw material (see Figure 3b). Figure 3: (**a**) Central structure of the platform. (**b**) Moving part of the platform.