## 3 Results
### Experimental Set-up for Thermal Monitoring  
After selecting the smart bearing technology and the most accessible path within the mill cage, a preliminary design of the monitoring system is performed. A preliminary technological assessment on a test rig helps in checking the suitability of the proposed approach. Validation on a real plant is then required, despite the cost and some practical issues related to the implementation of a full-scale system. Practically speaking, the main issues of this design include the first characterization of FBG sensors when monitoring pressure and load applied to bearings, as well as vibration, and then when measuring temperature. In the latter case, a current discussion in the literature is how the sensor must be located upon the monitored structure, resorting to a dedicated cavity. Particularly, shaping and filling of the cavity are additional design issues, as the literature proposes some solutions of operation in either oil or air. Moreover, the design of FBG sensors for the rolling mill has to take into account several targets, such as failure detection, predictive maintenance of elements, and prevention of combustion, with associated automatic alarming action. Considering the above-mentioned tasks, the literature already demonstrated the possibility of equipping bearings with FBG sensors for measuring the pressure applied to its raceways in operation. Particularly, this application is not so far from the case analyzed in [62], providing a complete demonstration of operation, despite the different sizes and applications considered. Those demonstrations fit the requirements of the rolling mill. Therefore, a further discussion about that task seems poorly attractive in this paper. Many approaches have been inherently developed based on soft computing techniques [64], multiwavelet denoising [65], and isolation of bearings' signals in drivers [66]. Figure 5: Example of location of sensors for pressure (**a**) and temperature (**b**) measurement in the backup bearing of a cluster cold rolling mill drawn upon rendering of bearing [61] and assembly [63]. Figure 6: Example of location of sensors for pressure and temperature measurement upon back bearings of a hot rolling mill drawn upon rendering of bearing (**a**) and assembly (**b**). In contrast, the measurement of temperature through the FBG sensors looks intrinsically a current topic of the literature when applied to steelmaking [67]. Temperature detection is required to uncouple mechanical from thermal excitation, respectively, as has been previously discussed. In both the hot and cold rolling mills, the detection of a temperature higher than 70 \({}^{\circ}\)C at the two locations selected, i.e., close to the inner ring in the cluster mill and to the outer ring in the hot rolling mill, may reveal an incipient condition of failure, and surely an undesired working condition for lubricant. It is sufficient to need warning operators while a temperature above 100 \({}^{\circ}\)C activates an alarm. Moreover, the detection of temperature in warning and alarm conditions, respectively, must be fast, evident and intuitive for operators, as well as safe for the overall system. Those motivations lead to exploiting water to fill the cavity where the FBG sensor is recovered, more than lubricant. A sacrificial layer can even be used to cover the inner surface of the cavity, being made with a material whose melting point is close to 70 \({}^{\circ}\)C. It could emphasize the alarm signal by coloring the water flowing back from that cavity, where the FBG sensors are monitoring temperature. Therefore, a key issue for the technological assessment of the monitoring system looks at the design of the cavity and the selection of material surrounding the fiber. The FBG sensors assure appreciable linearity of their response in operation. They are operated in multiplexing mode by inscribing several sensors on the same fiber. A precise calibration allows exploiting all those benefits.