# Time Management in Wireless Sensor Networks for Industrial Process Control
## 1 Introduction  
Wireless communication is one of the emerging technologies that have been aggressively adopted in the last decade, and which allows us to transfer huge amounts of data, faster and over much greater distances in real time. Almost every device on the market incorporates at least one piece of wireless technology, whether we are talking about Wi-Fi, Bluetooth, or others. Moreover, cost optimization and the emergence of new technologies meant the availability of smart and cheap devices known as the Internet of Things or the Industrial Internet of Things for industry. The industrial community has been more reluctant to adopt wireless technologies, especially in process control [1]. Wireless technology is seen as unstable and unpredictable for data transfers in an industrial process as it brings additional communication latencies [2]. In the past two decades, this has gone through a major change with the emergence of the IEEE 802.15.4 standard [3] and its amendments, that allow the secure and efficient transfer of data in an environment where this is highly critical, such as in an industrial environment [4]. This standard has led to the development of wireless communication protocols like ISA100 Wireless (mostly adopted in the USA) [5]and WirelessHART (mostly adopted in Europe) [6] which mainly allow the monitoring of industrial processes. Industrial process control, although mentioned in the technical specifications of these two protocols, refers to the transmission over wireless of the set-point rather than the command. In Figure 1, we have explained the evolution of using wireless communication in process control starting from a classic control loop (a), current implementations described in recent papers (b), and our proposal (c). Our proposal is not new, as there have been previous attempts to use wireless communication in process control by using those protocols designed to be used in industrial applications. In [7], the authors have used WirelessHART to transmit the feedback from the process to the controller and observed that the wireless device connected to the process sends this feedback randomly, causing system instability. They proposed using a Kalman filter to reduce noise and a stat estimator to estimate the process output when it is not available. By adding these improvements, the differences between a classic wired control loop and a wireless one are minor. A similar approach to what we propose has been described in [8], where the authors have used WirelessHART to transmit the command to the process and the feedback to the controller using wireless communication. In this study, the wireless node connected to the process sends the feedback to the controller once every 15 s, where the controller has a 1 s sampling time. The authors address two possible solutions to solve this problem: use the last received measurement in the control computation or employ a state estimator that calculates the measurement when it is not updated over the wireless network. The results show that the control system stabilizes at different setpoint values when the measurement is unavailable. Furthermore, the authors conclude that the WirelessHART protocol is not recommended for fast process control, as it has data transmission periods in the order of seconds. The use of state estimators and the last received command ensures system stability in situations where the measurement is not received and/or the wireless network is inactive.