# A Review on Vibration-Based Condition Monitoring of Rotating Machinery
## 4 Analytical Review
### Category p1: Signal Type, Sensors, Pre-Processing
In fact, despite the fact that they describe the shaft transmitting torque function, they are free from the extra AM effect due to time-varying transmission paths and have simpler frequency contents. [PERSON] et al. in 2020 [56] presented a study on the torsional vibrations analysis of the shaft of the centrifugal pump in a nuclear research reactor. They found pump shaft torsional vibrations close to rotor's first torsional mode and concluded that torsional vibrations reveal important signs related to the operating condition, not highlighted by lateral vibrations. [PERSON] and [PERSON]'s lab experimental tests, based on torsional vibrations, diagnosed sun, planet and ring gears local faults, despite the running conditions time-variability [96]. In [54], instead, amplitude modulation and frequency modulation (AM-FM) processes were used to model induced faults by torsional vibrations in resonance region. An investigation on the points of measurement is the last significant aspect that emerged from the analytical review at signal level [97,98]. [PERSON] in 2020 [58] studied the solution to measure vibrations at the tower instead of at the gearbox in a wind turbine and achieved positive outcomes in the bearing diagnostics without impacting the wind turbine operation. [PERSON] in 2009 [57] treated the distortions suppression for the remote measurements issue and it emerged that the results of the different signal processing techniques were highly influenced by the signal attenuation and interference. Among  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Investigated Aspect** & **Sel-Class** & **Bearing** & **Green/Captness** & **Motion** & **Wind Turbions** & **Pumps** & **General** & **Non  
the tested SP methods, time synchronous average (TSA) proved to be less sensitive to the problem. For the sensor type aspect, various unconventional vibration sensors are the objects of investigation. In [59], [PERSON] et al. consider a fiber-optic based vibration sensor characterized by low-cost, compact size, easy-to-fabricate, and excellent anti-interference ability. The adopted Sagnac interferometer and fiber ring laser (FRL) obtained an accurate frequency of the vibration signal within a relative error of 1.0%. [PERSON] et al. in [60] developed and tested a non-contact laser based vibration sensor for bearing condition monitoring and concluded that such a methodology may be effectively used for machine CM. [PERSON] et al., in [62], propose a microwave sensor (handheld ultra-wide band (UWB) radar) to non-invasively achieve vibrations for the CM of bearings in an induction motor, by projecting the microwave on the squirrel cage induction motor (SCIM) and by capturing the reflected signal. A new conception of piezoelectric accelerometer (the most frequently used type in industry) is presented by [PERSON] et al. in [63], with the aim of obtaining more accurate results. The use of vibrations signals obtained with sensors conceived with the microelectromechanical system (MEMS) technology is discussed by [PERSON] et al. in [61] and [PERSON] et al. in [64], due to their size, cost, portability, and flexibility. Low-cost solution is another sub-class of the sensor type [99]. [PERSON] et al. in [66] use successfully low-cost piezoelectric sensors for fault diagnosis in brushless dc motor drives, concluding that these smart IoT sensors could be used for an inexpensive CM of electric motors. A low-cost data acquisition system based on Raspberry-Pi is moreover presented by [PERSON] et al. in [65]. Compared to other commercial units, the proposed solution has a double recording capacity, any connection to an external computer is required, since storage is carried out directly in its memory, remote capture control is enabled, has a compact size that allows the positioning in difficult access areas and costs less than half the price of comparable devices. Furthermore, [PERSON] et al.