## 2 Control Architectures for WSN: A State of the Art
### Towards Hybrid Control Architectures  
Control architectures have been applied across various domains and extensively developed within the manufacturing sector, where the adaptability of control systems is crucial for responding to environmental shifts. These architectures delve into the amalgamation of distributed and centralized organizational structures, employing diverse decision-making frameworks. Grounded in a holonic model, these architectures designate individual units as \"holons\" [22]. Holons sharing common interests or features can aggregate into higher-level holons, while those with a common parent can engage in communication. Predominantly rooted in the holonic paradigm, manufacturing control architectures are commonly known as Holonic Control Architectures (HCA). Additionally, they are recognized as heterarchic architectures, as they amalgamate both distributed and centralized (or hierarchical) elements. Such architectures are frequently deployed using multi-agent systems.  
One of the pioneering HCAs, PROSA (product-resource-order-staff), was introduced by [PERSON] [23]. This framework comprises three fundamental holon types: order holons, product holons, and resource holons, as depicted in Figure 5. The authors also introduced staff holons, which can be incorporated to support basic holons in their tasks. Collaboration among holons enables the fulfillment of adaptation and flexibility requirements.  
Holons are structured using object-oriented concepts, linking a physical entity with an informational one. The resource holon comprises a physical component, which is the production resource, and an informational component that governs the resource. The product holon houses both process and product knowledge, while the order holon assigns tasks to other holons to ensure production. Within this architecture, each holon can be broken down into sub-holons. The staff holon provides an overview of all resources and orders, generating schedules as guidance for the order and resource holons. Other holons may follow this guidance in the absence of disturbances. However, when disruptions or changes arise, the basic holons may disregard the advice from the staff holon and make autonomous decisions.  
ADACOR (Adaptive Holonic Control Architecture) is a bio-inspired holonic architecture introduced by [24]. It employs a pheromone-like propagation mechanism to disseminate information during disturbances. In this architecture, four holons are defined, as follows: product holon, task holon, operational holon, and supervisor holon. These  
Figure 5: PROSA architecture [23].  
holons are organized within a decentralized control architecture to adapt to environmental changes (see Figure 6).  
In this architecture, holons are arranged in a hierarchical structure comprising four levels, as follows: planning, coordination, and operational levels. Supervisor holons propose optimized execution plans to task holons and oversee the execution of operational holons. When disturbances occur, operational holons communicate the need for reorganization through the supervisor holon. Adjacent operational holons detect the pheromone signal and restructure themselves into a heterarchical arrangement. Subsequently, holons can directly interact with task holons to devise an alternative schedule plan that minimizes the impact of unexpected disturbances.  
Holonic Manufacturing Systems take a step further by introducing dynamic HCAs, where the architecture can dynamically adjust to environmental changes. In such cases, the entire decision tree structure may undergo modification. Various dynamic architectures have been proposed in the literature, with the latest being POLLUX [25], which is discussed later. POLLUX incorporates an adaptation mechanism for the architecture, utilizing governance parameters to either expand or constrain the behavior of low-level holons in response to disturbances observed at higher levels.