## 2 Materials and Methods
### The Mill Accessibility and Analysis of Monitoring Paths and Technologies  
Some previous analyses based on the model-based system engineering [41] identified the fiber optic sensors [42] and piezoelectric microsensors [43] as the preferred choice for designing the monitoring system of the rolling mill. Piezoelectric sensors, associated with a telemetric unit, an energy storage system, and a microelectronic unit for the analysis and processing of data [29], need to be located in the proximity of monitored elements. This solution looks suitable for back bearings because of their size, while their application to work and support rolls is more difficult. They include some electronic circuits exposed to the risk of short-circuiting, especially in a harsh environment. The presence of flammable lubricant makes the effects of short circuits uncontrolled. The reliability of wireless communication even needs to be assessed [24; 32]. Since the microelectronic units need to be very close to the monitored elements, in the cold rolling mill, interference, and noise, due to saddles of back bearings (see Figure 2, part 28) affect the communication. Figure 1: (**a**) Layout of the Sendzimir cluster mill; (**b**) layout of the 4-high hot rolling mill. Overcoming those limitations is possible by resorting to fiber optic sensors. The fiber Bragg gratings (FBG) sensors offer many advantages since they are intrinsically passive elements and can be installed even in hazardous environments. They are inensible to electromagnetic interference and provide a smooth operation, even at high temperatures. In addition, the interrogation instrumentation connected to FBG sensors can be installed even far from sensors, and thus the obstacles to the access of inner elements of the mill are less effective than in the case of wired MEMS. It is true that FBG is never autonomous in terms of power supply, and they need a main system emitting and receiving light, but the length of connection is less critical than in wired devices. According to current industrial practice, in the rolling mill, the spin speed, lubrication condition, acoustic emission, load, and vibration amplitude of bearings are often monitored, as well as vibration and temperature [45]. Those measurements are seldom performed all simultaneously, i.e., a set of data allowing monitoring of the system in operation is collected during a duty cycle by measurements discretized in time. The location of the measuring device is usually quite far from the area of material processing, and some data can only indirectly monitor the machine condition, as in working rolls and bearings. Frequency and vibration amplitude detected in the mill bearings reveal the presence of damages in either the inner or outer raceways and thus allow performing the \"in-monitoring\" [46]. Measuring the strip speed, in addition to its damages [47] or the motors' rotational speed [48], allows for identifying the mean speed of rolls, although the cluster layout is quite different between rolls. Due to their location, back bearings are suitable to measure the system dynamics, which allows for predicting some surface defects in the rolled strip (\"out-monitoring\"). Temperature monitoring may detect a severe anomaly within the system and helps in the prognosis of rolls undergoing thermomechanical fatigue [36]. Load measurement is beneficial in both the cold and hot rolling mills to control the pressure distribution on the rolled product. To provide a synthesis, and according to the above-mentioned needs and practices, respectively, in the cluster mill, the monitoring activity can be performed at back bearings. Since they hold thin inner rings, poorly suitable for the installation of sensors along their radial direction, and external rings rotate, the FBG sensors can be installed just by resorting to their fixed pins. In contrast, in the hot rolling mill, the FBG sensors can be installed into the outer ring of either supporting or backup bearings when present.