KnowRob supports the Resource Description Framework (RDF) which is similar to classical modelling approaches such as entity-relationship and class diagrams. In the RDF model, statements about resources can be represented in form of subject-predicate-object triples that express a relationship between subject and object of the triple. Models in KnowRob usually build ontop of RDF. One category of KnowRob Models are extensions of the modelling language that introduce new resources such as XML Schema Data Types (XSD), RDF Schema (RDFS), and Web Ontology Language (OWL). The other category of models define RDF ontologies that conceptualize entities, and allow some form of reasoning.
RDFS is an ontology language that can be used to represent taxonomies
through subClassOf and subPropertyOf relationships.
It further allows to define the domain and range of properties --
i.e. what type of subject can be the host the property,
and what type of object can be the value of it.
RDFS is integrated into the KnowRob Language through a set
of language terms including has_type/2,
has_range/2, and has_domain/2.
OWL is another ontology language, but more expressive then RDFS. Please refer to the OWL2 overview for complete information about it. The OWL language supports some more advanced class relationships such as intersectionOf and complementOf, property cardinality restrictions, and additional property characteristics such as inverse and functional properties. KnowRob integrates with the OWL language by declaring a set of language terms, and how they map into the RDF model. An example of an OWL language term is shown below:
has_inverse_property(Property,Inverse) ?+>
triple(Property,owl:inverseOf,Inverse).
This querying+projection rule declares that the term has_inverse_property/2 is mapped
to RDF by using owl:inverseOf as predicate in the triple.
KnowRob's language further supports OWL class expressions in arguments of
language terms. Such class descriptions can used with the core terms
instance_of/2, subclass_of/2, and holds/3, for example:
:- instance_of(Subject,some(Predicate,Object)).
Where the term some/2 maps to an existential restriction class
in the OWL language.
Please refer to source code documentation for a full list of OWL
language terms.
KnowRob ships with a set of OWL ontologies organized into upper-level, domain and application ontologies.
A upper-level ontology (also called foundational ontology) defines concepts that are useful across disciplines such as event and object, and general relations such as parthood and participation. KnowRob uses the DOLCE+DnS Ultralite (DUL) upper-level ontology which is a "lightweight, easy-to-apply foundational ontology for modeling either physical or social contexts".
A domain ontology is used to fix general terms in some domain. The domain KnowRob was designed for is everyday activities. KnowRob ships with the Socio-physical Model of Activities (SOMA) ontology which derives its notions from the DUL upper-level. However, it is possible to configure KnowRob for other discipines by using different domain ontologies. Another domain ontology in KnowRob has the scope to capture categories underying robot control. This includes the concepts sensor, actor, and capability.
Finally, an application ontology contains all application-specific definitions. As KnowRob is a general framework, no such ontology will be auto-loaded by default. However, some application ontologies are available as part of public repositories, and hosted by the openEASE webservice.
In KnowRob, an Action is defined as an Event where at least one agent that participates in the event executes a Task which is typically defined in a Plan. Tasks are used to classify actions, similar to how roles are used to classify objects within some situational context. There may be multiple plans defining the same task which is useful to capture different ways to achieve the same goal. The distinction between Action and Task is further important as it enables us to put individual tasks into discourse without referring to a particular execution of them (i.e. an Action). This is needed because a Plan is a generalization of action executions, abstracting away from individual objects that were involved by only referring to the roles they have played.
Plans are used to structure tasks, asserting how they are composed of steps and in which order they should be executed. KnowRob supports relations from Allen's Interval Algebra to assert ordering constraints between steps, and also allows to only specify partial ordering. Each step of a plan is a task itself, and may also be defined by some plan(s). However, the action model of KnowRob allows to go deeper by decomposing a task into phases. A phase is a Process or State that occurs during task execution which includes force dynamic events, and motions. Processes are classified by one of the ProcessType concepts, and states are classified by one of the Gestallt concepts defined in the model.
Roles are used to classify objects that participate in some event. This includes the agent that performed the action, tools that were used, objects that were affected, as well as locations of interest. KnowRob defines a taxonomy of roles with the most general concepts being Patient, Instrument, and Location.