# Design and Performance of a XBee 900 MHz Acquisition System Aimed at Industrial Applications
## 6 Conclusions  
This work presents a novel wireless acquisition system aimed at industrial applications with QoS requirements. This system eases the tasks of monitoring mobile devices/parts and difficult-to-reach sensors. It also improves configuration flexibility and eliminates costly cabling. These benefits improve productivity and efficiency in industrial processes. The use of the 2.4 GHz frequency band was avoided for critical communications, in order to reduce interferences caused by other communication technologies, which typically operate in this band. Thus, the 2.4 GHz channel may also be used for other tasks related to noncritical operations, such as connecting industrial devices with services available at the factory or in the cloud. The authors also designed a logical protocol for critical operations aimed at avoiding congestions and collisions and a device to ease the connection to the process sensors. The details of the acquisition system were discussed, including: the topology and configuration of the selected communication technology, XBee at 900 MHz; the nodes involved, the so-called Coordinator Node and Smart Measuring Nodes; and the design of the logical protocol and frames. The proposed system 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, a module, the so-called Smart Measuring Node, was specifically designed for easing the connection of the sensors. This module also provides the communication as well as edge processing capabilities. XBee 900 MHz modules were used for establishing the communications between the nodes, Coordinator Node and Smart Measuring Nodes, due to the fact that it is a low-cost, well-proven and easily available technology. However, this is a highly configurable  
Figure 15: Comparison values acquired with sensors located at the bar-knot structure when a force is applied. Frequency: 40 Hz; Measuring/acquisition separation: 12.30 m. Test duration 3 min. LVDT sensor. Wireless values shown in blue and wired values in red. technology and must be carefully configured in order to achieve the QoS requirements of industrial monitoring applications. However, this technology is based on CSMA/CA MAC policies which do not ensure determinism. For this reason, 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. Since the 900 MHz band is not so crowded at industrial environments as the 2.4 GHz, a higher degree of determinism may be achieved. This approach leaves free the use of WiFi 2.4 GHz for other operations related to integration of the information or connecting to services available at the factory or in the cloud. The proposed monitoring system extends the communication capabilities of the Compact RIO series by adding wireless connectivity to the processes and may be used in combination with the wired capabilities that this platform already offers. 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 at distances of up to 12-15 m. 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 in industrial facilities. Conceptualization, I.C., O.B. and J.V. ; methodology, I.C. and J.M.G.-G.; software, I.C., E.V. and A.F. ; validation, E.V., A.F. and J.V. ; formal analysis, I.C. and J.M.G.-G.; investigation, I.C., E.V. and A.F. ; writing--original draft preparation, I.C. and J.M.G.-G.; writing--review and editing, P.F.-B., O.B., C.N. and J.V. ; visualization, C.N.