# A Semantic Model in the Context of Maintenance: A Predictive Maintenance Case Study
## Maintenance Ontology and Implementation  
### Entities of the Maintenance Ontology  
To ontologically represent the structure of a target system and its failure mechanism in a standard way, the maintenance ontology refers to the Failure Mode, Effects and Criticality Analysis (FMECA) methodology, a widely recognized tool to study and analyze the reliability of a process [46]. The International Electrotechnical Commission published IEC 60812 to explain how the FMECA variant is planned, performed, documented, and maintained for general use [47]. In IEC 60812, the system structure is represented as follows:  
* System breakdown to major subsystems that include functional relationships
* Properly tagged inputs and outputs and identification numbers to consistently reference each subsystem
* Redundancies, alternative signal paths, and key engineering features that facilitate protection against system failures  
Referring to IEC 60812, a logical diagram (Figure 3) was created as part of the implementation of our Z-BRE4K project [48]. This logical diagram includes failure mechanism entities, physical entities, and information entities. Failure mechanism entities describe how a component's failure affects system failure and related failure symptoms. Physical entities depict the physical structure of maintainable items and embedded data-source components. Maintainable items are the physical entities that form a part or an assembly of parts--such as a part, module, subsystem, or system--that are at the lowest level in the equipment hierarchy during maintenance. ISO 14224:2006 [49] Appendix A provides examples of maintainable items for a variety of equipment. Information entities represent information and data generated from advanced enabling technologies that can be used in management. The maintenance ontology was created through iterative cycles of the methodology we proposed. This task was accomplished as part of the H2020 Z-BRE4K project. The Z-BRE4K project [48] provides a platform for PdM to eliminate unplanned breakdowns and hence extend the useful life of production systems. The Z-Bre4k solution is comprised of the introduction of eight scalable strategies at component, machine, and system levels: (i) the prediction of failure based on evidence (Z-PREDICT), (ii) the early detection of current or emerging failure (Z-DIAGNOSE), (iii) the prevention of failure occurrence, accumulation, or propagation within the production system (Z-PREVENT), (iv) the estimation of the remaining useful life (RUL) of assets (Z-ESTIMATE), (v) the management of these strategies through event modeling, key performance indicator (KPI) monitoring, and real-time decision support (Z-MANAGE), (vi) the replacement, reconfiguration, re-use, retirement, and recycling of components or assets (Z-REMEDIATE), (vii) the synchronization of remedial actions, production planning, and logistics (Z-SYNCHRONISE), (viii) the ongoing preservation of the safety, health, and comfort of human workers (Z-SAFETY) [12]. The Z-BRE4K ontology serves as a backbone of the Z-BRE4K platform since it enables semantic interoperability between system components. The standardization in Z-BRE4K is based on the design of the semantic model of the maintenance entities. It is also based on the reference ontology, which provides standards-based access to all the data in the domain of interest. The Z-BRE4K ontology manages the design and implementation of the semantic model and the mechanisms for intelligent filtering. It provides a unified and all-spanning semantic model covering the multi-domain knowledge of all Z-BRE4K use cases. Therefore, Z-BRE4K achieves semantic enrichment by receiving maintenance knowledge that results from cross-sectoral knowledge exploration and filtering. Thus, it requires semantic interoperability: the ability of information systems to exchange data unambiguously with shared meanings as a standard. This section depicts the form of the Z-BRE4k ontology and its implementation within the PdM context.