# Classification of Systems and Maintenance Models
## 2 Literature Review
In [29], the authors considered corrective maintenance policies for scheduled imperfect inspections. The maintenance effectiveness indicators are the average availability and long-run average cost per unit time. A study in [30] suggested a new CBM model that uses an energy efficiency indicator. The suggested approach encourages CBM optimization to consider both useful output performance and maintenance costs. The study in [31] focused on the analytical modeling of a condition-based inspection and replacement policy for a stochastically and continuously degrading single-unit system. The developed mathematical model allows for the evaluation of the effectiveness of the maintenance policy and minimizes the expected long-term maintenance cost per unit time. A CBM policy was developed in the study [32] for a two-component system with stochastic and economic dependencies. The long-term expected maintenance cost per unit time is an indicator of maintenance effectiveness. In [33], a mathematical model of preventive maintenance with imperfect continuous condition monitoring of wind turbine components was presented. The derived mathematical equations allow the calculation of both the average lifetime maintenance cost and expected maintenance cost per unit time. A study [34] proposed a comprehensive approach for the reliability modeling and maintenance planning of parallel repairable systems that suffer from hidden failures. This study aimed to reduce the expected maintenance cost per unit time by jointly determining the optimal inspection interval and maintenance thresholds. A study [35] established maintenance policies for a system under periodic inspections. Maintenance actions were performed for any failures that were found. Repair is imperfect. The expected variable cost rate is an indicator of maintenance effectiveness. A study [36] examined the problem of determining the optimal aircraft equipment maintenance intervals while minimizing maintenance costs and assuming an Erlang distribution of time between failures. The study in [37] focused on a technique for calculating the optimal threshold during the implementation of the condition-based maintenance of radio equipment. The minimum average operational costs served as the optimization criterion. The study in [38] examined the cost-effectiveness of applying CBM in manufacturing companies. The findings indicate that two potentially important benefits of CBM include lowering the probability of maximum damage to manufacturing machinery and of production losses, particularly in high production volumes. The study in [39] considered a sensory-updated predictive maintenance policy that forecasts and updates the residual life distribution of a simple manufacturing cell using degradation models and sensor information. The overall maintenance costs are then computed. By considering imperfect remaining useful life (RUL) prognostics based on condition monitoring data, [40] proposed a dynamic, predictive maintenance scheduling framework for a fleet of aircraft. This framework reduced the maintenance costs associated with engine failures to only 7.4% of the overall maintenance costs. A study [41] examined a production system that was regularly inspected. Manufacturing, inventory, lost sales, repair, inspection, and maintenance costs are all included in the expected cost function. The study [42] considered a strategic queuing model to examine how a maintenance service provider should allocate capacity and set prices in the face of imperfect IoT-based diagnostics, which continuously monitor various pieces of equipment using sensors. The study in [43] investigated two maintenance strategies for wind-turbine gearboxes using continuous temperature monitoring. The maintenance effectiveness indicator is the total expected cost per unit time. According to a numerical analysis of the presented models, the optimal imperfect preventive maintenance strategy was 46% more effective than the optimal renewal strategy.