## 3 Methodology  
A comparison of centralized and heterarchical control architectures is presented in this paper to identify their pros and cons. The starting point of the paper was the identification of the manufacturing system on which the simulations could be carried out. To test the control principles, a physical model of a manufacturing system was chosen [42].  
Two scenarios were considered in this paper. In the first scenario, centralized and heterarchical control were applied to assembly stations, while in the second scenario the control architectures were tested for materials handling systems. In the first scenario, the final product is obtained assembling 2\(\times\)2 yellow Lego(r) brick on a green one. In the second scenario the final product is obtained assembling three metallic parts with a black plastic cover and different materials handling systems were used to transport the elements in the manufacturing system.  
Figure 1 shows the two scenarios and the control methods applied. As indicated in the conceptual framework, the field of application of centralized and heterarchical control is different in both scenarios even if the control principles remain the same.  
According to [PERSON] (2011) and [PERSON] et al. (2006), centralized control consists in centralized decision-making system [43; 44]. In the first scenario, the assembly stations were fixed and there were no possibilities to change the production schedule. In the second scenario, the materials handling system was defined a priori. Conversely, heterarchical control consists in applying the agent technology concept [30] to create flexible elements able to communicate and cooperate autonomously [35; 41]. In this latter case, the assembly stations (first scenario) and the material handling systems (second scenario) were not defined a priori.  
Basically, the theoretical centralized and heterarchical control concepts were translated as:  
* Selection principle of the processing assembly station (Scenario 1);
* Selection principle of the material handling system (Scenario 2).  
Thus, in the first scenario, centralized control consisted in predefining the assembly stations (AS) (i.e., the first product was assigned to the Assembly Station 1 (AS1), the second product to Assembly Station 2 (AS2), the third product to AS1, and so on). Conversely, heterarchical control consisted in testing a more flexible control since the assembly stations were chosen considering their availability real time. Whenever a raw material was released, AS1 was questioned about its availability, if it was free the product was assembled in AS1, if not AS2 was questioned to check its availability. In cases where both assembly stations were unavailable, a non-manufactured unit occurred. Under heterarchical control, failures can be managed in a proper way: in the case of failure of AS1, the finished product can be assembled in the AS2.  
In the second scenario, the same principles of centralized and heterarchical control were applied. Under centralized control, the material handling systems were predefined, whereas under heterarchical control, the selection of the material handling system was based on its availability.  
These principles were applied to the physical models for both scenarios. Afterwards, the physical systems were used to collect data and create simulation models to widen the scope of the research work.