## 4 Discussion  
The health monitoring of the rolling mill includes a careful measurement of vibration and temperature in service, despite the difficulty of access to the mill cage and to bearings, in particular. Therefore, in a preliminary step, this investigation analyzed the layout of the rolling mill, and some possibilities have been found:  
* In the cold rolling mill, the fixed pins of backup bearings are suitable to host the fiber optics sensors to monitor the temperature of the inner rings of bearings;
* In the hot rolling mill, the system constraining the outer rings of bearings allows access to the fiber optics sensors. A second step has been focusing on the sensors' technology:
* The FBG sensors look suitable for this application, although the assembling system may be critical for measurement. Particularly, the FBG sensor suffers the effect of pressure and/or of structural deformation. A third level of details includes some recommendations for the assembly:
* Creating a cavity where the sensor measures temperature allows uncoupling of the effect of pressure and of temperature, respectively;
* Thermal measurement may benefit from filling the cavity with either a liquid or gas. In principle, this material might consist of air bubbles, but the performance in terms of promptness and uniformity of temperature is poorly evident. Demineralized water allows surrounding the fiber and transferring heat quite fast and uniformly, although the use of water must be verified for all industrial layouts currently used. Materials selection might consider some properties of the monitoring system as detectability of temperature:
* If the system must never be operated above 70 \({}^{\circ}\)C, a sacrificial layer made of thermoplastic resin, whose melting point is around 75 \({}^{\circ}\)C, deposed on the surface of the cavity, might help in warning the operators when the maximum temperature is reached. In principle, above 100 \({}^{\circ}\)C boiling could add an intuitive alarm, provided that no other problems are foreseen. The operation of the FBG sensors looks feasible and practical after a preliminary calibration of the monitoring system. As it was demonstrated in the previous section, by experiment, reliable identification of the main parameters useful for the measurement activity is assured, despite the number of steps that require an accurate implementation. The sensitivity of the FBG sensor looks even better than in other technologies.  
Figure 8: Characterization of the FBG sensor and comparison between mathematical approximation and experimental results.  
The harsh environment significantly affects the monitoring system behavior, in general, but the FBG sensors allow reaching a high temperature, are remotely controlled, look suitable to access the inner parts of the mill cage, and do not suffer the interaction with electric and magnetic fields.  
The cost of FBG sensors is never negligible because of the number of elements composing the monitoring system. Nevertheless, the technological benefits of fiber optics and the availability of a reliable monitoring system combined with the overall cost of the mill may allow considering this solution feasible for a real smart manufacturing implementation. It is true that only a systematic approach, such as the model-based systems engineering used in the case of aircrafts, for instance [70], will allow the development of the whole monitoring system, and the heterogeneous simulation tools [71] nowadays available will allow for integrating the network of distributed sensors within the mill cage elements.