# Design and Performance of a XBee 900 MHz Acquisition System Aimed at Industrial Applications
## 1 Introduction
Although the use of the 5 GHz band is increasingly used at WiFi technology, it suffers from shorter coverage and more difficulties for avoiding obstacles. In addition, it is less common in industrial environments. In this scenario, the only mainstream wireless low-power technology which operates outside the 2.4 GHz frequency band is ZigBee, at 900 MHz [14; 18]. This is the technology chosen for our approach, since it provides a new communication channel which avoids interferences with WiFi technology. More specifically, XBee 900 MHz modules were employed. These are low-cost modules which implement the IEEE 802.15.4 protocol. These modules implement the CSMA/CA medium-access control method, which is not suitable for determinist applications since transmission times are not guaranteed [19]. In this research, the authors present a novel wireless acquisition system aimed at measuring through distributed sensors in industrial applications with QoS requirements. By using this system, it is possible to monitor mobile devices or difficult-to-reach sensors. It also improves configuration flexibility, eliminates costly cabling and allows for easier network expansion. In general, these advantages improve productivity and efficiency of the industrial processes [10]. The presented system avoids using the 2.4 GHz for critical communications. Thus, the interferences caused by most communication technologies, which typically operate in this band [14], may be avoided. Thus, WiFi 2.4 GHz provides a different channel for other noncritical operations related with integrating the process information or connecting to services available at the factory or in the cloud [4; 17]. In addition, the authors designed a logical protocol based on time-slotted media-access policies (TDMA), which permit a central node to coordinate all transactions of the acquisition system according to a static schedule [20]. The use of this protocol for critical operations avoids congestions and collisions achieving a higher degree of determinism in order to guarantee the QoS requirements of the applications. The TDMA protocol was implemented over a low-cost wireless technology, such as XBee 900 MHz modules, not frequently considered for deterministic applications. An architecture, based on a star topology, with two types of nodes is also presented: the so-called Coordinator Node and Smart Measuring Nodes. The Coordinator Node was implemented over a real-time embedded platform, commonly accepted in industrial applications, such as the Compact RIO series, provided by National Instruments. In addition, the Smart Measuring Node device was specifically designed for easing the connection of the sensors. This module provides the XBee communication to the distributed sensors as well as edge processing capabilities. Finally, the proposed monitoring system extends the communication capabilities by adding wireless connectivity to the processes and may be used in combination with the wired capabilities of a commonly accepted platform for industrial scenarios, the Compact RIO series. Experimental tests were carried out to validate the design of the system and evaluate its performance. The obtained experimental results proved that it may be used for a number of sensors in the range of teens of sensors, reaching sampling frequencies of up to 40 Hz, going beyond the frequency rate of 10 Hz, which is commonly accepted in [13]. The performance of the system was tested in industrial facilities. More particularly, it was used for acquiring the position of an aeronautical reconfigurable tooling prototype. The obtained results proved that the followed approach may be valid for monitoring diverse variables in big machinery at industrial facilities. The layout of the article is as follows: Section 2 discusses some related studies. Section 3 presents the design of the monitoring system. The following section presents some experimental tests of the acquisition system. Section 5 discusses the results of the application in industrial facilities to the prototype aeronautical tooling.