# A Semantic Model in the Context of Maintenance: A Predictive Maintenance Case Study
## Maintenance Ontology and Implementation
### Implementation of the Maintenance Ontology  
The maintenance ontology describes the context based on the competency questions described in Section 4.1. It comprises physical entities, realizable entities, and information entities. Physical entities are existing three-dimensional objects, realizable entities are objects or events that can be achieved or made to happen, and information entities are knowledge collected through measurement records. The maintenance ontology has _System_, _System_Component_, and _Embedded_Data_Source_ as physical entities to describe the structure of a maintainable item. The _System_ is defined as a _Material_Entity_: a universal set of a target system that can be a single or multiple machine or product. _System_Component_ is a material part of the _System_. The subcomponent also can be a _System_Component_. The _Embedded_Data_Source_ represents a _System_Component_ that is intended to generate data: it is a type of sensor or digital device. The _Stakeholder_ is also a physical entity: the human (or being) that manages a maintenance scenario. As realizable entities, there are _Failure_Cause_ and _Quality_ entities. _Failure_Cause_ is a defect in a process that is the underlying cause of a failure or that initiates a process leading to failure. _Quality_ is a characteristic of physical entities. Failure can be a sort of _Quality_. The information entities are _Indicator_, _Process_definition_, and _Failure_Symptom_. The _Indicator_ is an observed value that indicates the level or state of an entity. _Process_definition_ is the structured set of activities that are designed to accomplish a particular objective. These three types of entities, as a foundation of classes for a maintenance ontology, were extended in the maintenance ontology (Figure 4). We present the list of classes together with their definitions in Table A1 in Appendix A.  
Figure 4: \(Z\)-BRE4K semantic model hierarchy. To enhance the ontology of maintenance, this study deals with information and data from the packaging industry. The packaging company provides service to its end users for compression molding machines concerning PdM. The ontology was specialized for this industry after it was populated since this ontology is a generalization of the business scenarios, a reference semantic model. The populated ontology codifies and encapsulates explicit knowledge of the machine operating in the packaging company. In this study, we present the case of the compression molding equipment as a target system. It consists of pumps, screws, bypass valves, cooling fans, mixers, heaters, cylinders, and polymer as system components (Figure 5). Their failure modes and symptoms are described in Table 1 in Appendix 0.A. According to the Z-BRE4K ontology, a structure of a target system can be presented within the class system and _system_parts_. The physical objects of a system and its parts should be instances under these classes. The notions of physical objects are described as roles, failure modes, and symptoms associated with these notions. A physical object can have a sensor as an _embedded_data_ source that generates the data properties _embedded_data_, which has a measured value, and date and time. _Embedded_data_ is interpreted as _Performance_Parameters_, which are indications of quality measurement. If a PdM service detects a potential failure, it is described as a _predicted_quality_ with an expected date and time. Figure 6 depicts how the Z-BRE4K ontology describes an example scenario. This example from the packaging industry serves as a guide for users of this ontology. It provides a basic idea of how we designed the maintenance ontology so that users can understand the correct position of data in the ontology. Since this ontology was created as part of the H2020 Z-BRE4K project, the maintenance ontology for the packaging industry was validated and tested with a semantic component on the Z-BRE4K platform named the knowledge base system (KBS).