## 3 Using a Knowledge Graph for a Home Service Robot
### Construction of Concept Layer of the Home Service Robot Knowledge Graph  
Domain ontology refers to the concepts, entities, and interrelationships in a specific domain and describes the rules and characteristics of knowledge in this domain. A domain ontology of the home environment is constructed by classifying the domain data of the home environment. This approach enables the description of people, objects, and robot service information at the semantic level to enable the sharing of domain knowledge for different pieces of equipment and different types of information. Two basic methods are typically used for ontology construction. The top-down approach is sufficient to meet the application requirements in fields with relatively mature and complete knowledge systems. The broadest concepts in the field are defined first and are subsequently refined. However, in some fields, the data are not systematic, and the knowledge is incomplete. Thus, a bottom-up and data-driven approach is required. The most specific concepts are defined first and summarized next. The top-down and bottom-up approaches are combined in real-life applications of developing complex domain ontology. A concept layer is first established using the top-down approach. Then, the classes and attributes are supplemented by the bottom-up approach using various data. The following two steps are used in this study to construct the domain ontology model of the home environment:  
* Determine the classes in the home environment and their hierarchical relationship to establish an empty ontology model with a class structure. This involves analyzing the home environment to categorize relevant entities such as objects and locations. This initial step lays the foundation for the ontology by creating an organized framework of classes that represent the primary components of the home environment. * Determine the object attributes that describe the relationships between classes and the data attributes that describe the class (or instance). The domain and range for each property are established to ensure clear relationships between different classes and their instances. These two steps are used to construct the concept layer of the knowledge graph, which has the following three parts:  
1. Classification of knowledge in the home environment. The environment class represents the environment domain, including the subclasses object class and location class. The object class contains all objects in the home environment,including furnishings, household items, and household appliances. The location class contains location information, and the user class represents the use characteristics, including the subclasses people class and behavior class. The people class contains information on the family, and the behavior class describes the behavior of the family. The robot service class represents robot services, including the operation and service subclasses. The operation class contains information on the robot's operations of various objects and equipment. The service class contains details on specific service tasks of the robot. 2. Define the relevant object attributes, use the class as the definition domain and the value domain, and establish the connection between classes. \"isLocation\" describes the location of objects in the home environment. It uses the location class as the value domain to establish a connection between the object class and the people class \"toDo\" determines the actions involved in the robot service operation. Its definition domain is the service class, and its value domain is the operation class. \"actOn\" determines the objects involved in the robot service operation; its definition domain is the operation class, and its value domain is the object class. \"hasBehavior\" describes the behavior of the family. Its definition domain is the people class, and its value domain is the behavior class. 3. Define the data attributes used to describe objects in the home environment, i.e., object knowledge representation.