# Protocol for Energy-Efficiency in Networked Control Systems Based on WSN
## 1 Introduction  
Currently, the implementation of control techniques for dynamic systems through digital communication networks assumes an ideal communication channel, where the existence of data loss or delays is neglected. This assumption is possible thanks to high-performance state-of-the-art industrial networks such as ControlNet or DeviceNet [1]. However, the use of networking technologies with lower Quality of Service (QoS) in systems driven by classical controllers may bring significant drawbacks to ensure performance and stability [2; 3; 4; 5]. Besides, other issues like coverage range, energetic autonomy and channel use [6; 7; 8; 9; 10; 11] also must be taken into account by the designer. Despite all these difficulties, in recent years there has been increasing interest from the industrial sector as the technology can act as an alternative to wireless connect sensors, actuators and controllers; this provides benefits such as flexibility, monetary savings, ease of installation and mobility to connect wireless sensors and actuators (most unreachable with wired networks). In accordance with industrial requirements for wireless communication technologies, which involve several economical and practical constraints, many applications adopt Wireless Sensor Networks (WSN) technologies [5] for connecting sensors and controllers, instead of using (relatively) high QoS wireless infrastructures such as those based on the IEEE 802.11 standards. WSNs are most based on the IEEE 802.15.4 standard, which was conceived for Low-Rate Wireless Personal Area Networks (LR-WPANs). Devices implementing those standards are known for being very resource-constrained (in processing/memorization capabilities, radio power output, etc. ), in order to get as much energy savings as possible. Clearly, the economy of resources brings a set of problems that must be solved to assure high QoS levels, as required by classical control techniques. To address the problems associated to regular WSN used as communication channel in closed-loop control, networked control systems (NCS) theory arises as an appealing control framework, allowing to obtain theoretical performance certificates even in presence of phenomena inherent to this kind of application, as packet loss and delay, as discussed in [2; 3; 12; 13]. However, the available researches do not incorporate heuristics at the logical level aiming energy savings requirements, assuming that both channel use and the packet loss process are time-invariant and are not conditioned by external events. The search for mechanisms for improving energy efficiency in the transmission of data in WSN is a transversal issue, but heuristics for minimizing energy costs in NCS are not abound. The work in [14] proposes a heuristic intending to provide energy-savings, but it assumes full-reliability in the communication protocol (property, which in practice, is not simple to guarantee). Motivated by the aforementioned facts, the main contribution of this paper is the proposition of a communication protocol for Multi-Hop WSN suitable to deal with control applications, where two conflicting criteria must be fulfilled simultaneously: the minimization of the energy consumption and the stability and performance of the closed-loop system. Note that the latter can only be assured in full-reliable communication networks (no packet loss, implying in an unlimited number of retransmissions) and, clearly, is an unrealistic assumption in WSN scenario (low QoS). The particular performance criterion to be investigated is the \(\mathcal{H}_{\infty}\) norm, that is a well known robustness criterion associated to the attenuation of external disturbances, but other criteria could be considered as well. The proposed protocol is based on a Hop-by-Hop transport scheme and can be implemented by means of three distinct heuristics.