## 5 Classification of Competing Technical Systems  
The information presented in this section explains the rationale behind the subdivision of harvesting machines into comparable technical systems. All of them can be consequently considered as representative of a single value curve.  
An introduction to technical aspects of machines for harvesting shell fruits is presented in [56]. Analogously, the evolution of the harvesting techniques for hazelnuts and chestnuts is described in [57] and [58], respectively. All the scholars agree in classifying the harvesting machines based on the functioning principle exploited for removing the shell fruits from the orchard ground (belonging to the first of the three main functions above illustrated), i.e., (1) \"_vacuum_\" or (2) \"_mechanical_\" [56; 57; 58; 59]. The vacuum machines create airflows capable of sucking the shell fruits into the machine thanks to the thus-created drag forces on each shell fruit, directed toward the suction inlet. Instead, mechanical machines move shell fruits inside them by exploiting a direct contact of the shell fruits with properly shaped solid bodies (e.g., brushes or rakes) that materially gather, grasp, or embed the shell fruits. It is worth noting that mechanical machines can exploit air flows, and vacuum machines can benefit from mechanical movements (as long as these do not perform the main function of removing the fruits from the ground). Indeed, it is common that mechanical solutions help the preparation of the shell fruits to be collected by vacuum tubes and that air flows facilitate the separation of the lighter elements collected mechanically.  
A further classification is based on the way the machine is moved within the orchard [56; 57; 58; 59] (belonging again to the first of the three main functions above illustrated), and four categories can be individuated.  
1. When such a machine is towed by a tractor, it is referred to as \"_trailed_\" (or \"_pulled_\").
2. When a harvesting machine is instead \"rigidly connected\" to a tractor (e.g., through the front/rear three-point hitch), it is usually called (tractor-) \"_mounted_\". In both the described cases (i.e., 1 and 2), the harvesting machine is configured as a farm implement, i.e., a machine that is not independent because it has to be necessarily interfaced with an agricultural motor machine to be moved and powered.  
Additionally, when the machine is moved directly (and exclusively) by the operator, e.g., if it is shaped like a backpack, a cart, or a rake, it can be called \"_portable_\". This category is mostly neglected by the scientific literature, probably because this type of machines achieves the lowest productivity; nevertheless, as illustrated in the following sections, it is mentioned in many patents and, therefore, it was included in this study.
4. Finally, when the moving system is integrated in the harvesting machine, this is called \"_self-propelled_\".  
Table 1 shows an explanatory solution (picked up from patents) for each of the eight categories (\(2\times 4\)) that emerged from the factorial combination of the two illustrated functioning principles. According to this classification, each category is indicated in this document using a label. The first letter (X/\(\bullet\)) indicates the functioning principle exploited for removing the shell fruits from the orchard ground (V for vacuum, M for mechanical). The second letter or group of two letters after the slash symbol (\(\bullet\)/XX) indicates the way the machine is moved in the orchard (P for portable, T for trailed, M for mounted, SP for self-propelled).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Movement of Machines** & \multicolumn{2}{c}{**Removal the Shell Fruits from the Orchard Ground**} \\ \cline{2-3}
**in the Orchards** & _Vacuum (V/\(\bullet\))_ & _Mechanical (M/\(\bullet\))_ \\ \hline \hline \end{tabular}
\end{table}
Table 1: Visual illustration of the eight categories of harvesting machines for shell fruits based on the two indicated functioning principles as classification keys.