## 3 Findings
### Diagnostics for Unit Systems
#### 3.2.2. Predictive Maintenance  
Nowadays, predictive maintenance is considered as the key point for many manufacturing industries because of a major part of the operational cost and system failure impacts on product quality and equipment availability. [PERSON] et al. [55] explained that predictive maintenance considers close past information for predicting future tendencies, biases, behaviors, etc. through correlation. [PERSON] et al. [53] introduced that predictive maintenance is an analytic technique to eliminate prospective failures and improve the mission dependability of production systems. Consequently, a coordinated predictive maintenance procedure considering item degree and mission dependability state was proposed from reasoning of prediction and manufacturing. [PERSON] et al. [52] proposal focused on predictive maintenance of manufacturing systems to improve the production process quality.  
[PERSON] et al. [19] have attempted to work on a flexible structure of a versatile manufacturing system to satisfy different needs and item varieties and to build up a PHM structure for assembling with different online sensors and flexible structures utilizing different sensors-based degradation data for registering and predicting each machine's time to failure. For example, [PERSON] et al. [49] discussed the execution of predictive maintenance of  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**Literature Review on Predictive Maintenance in the Context of Flexible Unit Systems**} \\ \hline
**Sl. No.** & **References** & **Findings** \\ \hline
1 & [49] & A general framework has been developed and that has been applied to manufacturing tools by using predictive maintenance. \\
2 & [50] & Conducts a study of the predictive maintenance on industrial equipment. \\
3 & [19] & Presents a prognostic method that uses sensor degradation data for calculating the time to failure of machines, with maintenance policy. \\
4 & [51] & Develops a cutting tool wears monitoring and predictive maintenance system. \\
5 & [20] & Proposes a multisensor prognostic method, that uses multistream signs to anticipate the Remaining \\
6 & [52] & Useful Life of partially degraded systems. \\
7 & [53] & The proposal focuses on predictive maintenance of manufacturing systems and tools. \\
8 & [54] & Introduces the predictive maintenance system, and joints product quality as well as mission \\  & & reliability imperatives. \\
9 & [55] & An extended model with a system that connects a low-level execution condition \\
10 & [56] & Presents the design and implementation of a conductance sensor for micromachining processes. \\ \hline \hline \end{tabular}
\end{table}
Table 10. Literature review on predictive maintenance related to flexible unit systems.  
milling cutting tool information, and the collection as validation of a structure, and [56; 57] presented a model-driven approach using embedded artificial intelligence strategies by the development and implementation of a quality monitoring framework, and also presented a sensory system for high precision monitoring, applicable to all machining and milling operations on conductive materials. [PERSON] et al. [58] proposed a sensory updated degradation based predictive maintenance strategy. Their proposed maintenance strategy used degradation methods that combine part-specific continuous degradation data obtained during activity to predict the remaining useful life distribution. [PERSON] et al. [54] expanded an adaptive predictive generator maintenance model that has been presented. From the different literature, on predictive maintenance, it can be concluded that the predictive maintenance of the machines allows extending of the machine's life and the lowering of maintenance costs by addressing the problems before they cause machine failures.