TriplestarKB

TriplestarKB is a ROS2-enabled knowledge base, backed by the oxigraph SPARQL graph database.

ROS to RDF conversions

ROS(2) defines its own set of interfaces (messages and services) for representing data like timestamps, points, integers, floats, polygons etc. To facilitate the integration of these datatypes into the kb, which is based on RDF, these types need to be converted to suitable RDF types.

ROS msg type	Python / Shapely type	RDF literal type
geometry_msgs/Point, Point32, PointStamped	shapely.geometry.Point	geo:wktLiteral
geometry_msgs/Pose	shapely.geometry.Point	geo:wktLiteral
geometry_msgs/Vector3, Vector3Stamped	shapely.geometry.Point	geo:wktLiteral
geometry_msgs/Polygon, PolygonStamped, PolygonInstance, PolygonInstanceStamped	shapely.geometry.Polygon	geo:wktLiteral
builtin_interfaces/Time	datetime.datetime	xsd:dateTime
std_msgs/Float32, Float64	float	xsd:float
std_msgs/Int8, Int16, Int32, Int64	int	xsd:integer
std_msgs/UInt8, UInt16, UInt32, UInt64	int	xsd:integer
std_msgs/Char, Byte	int	xsd:integer
std_msgs/Bool	bool	xsd:boolean
std_msgs/String	str	xsd:string

Query-time subscribers

Some data, such as room geometries or class hirarchies, will be quite static in your KB, while other data, such as the robots own location or battery level, will change frequently. For this requently-chaning information it is possible to add query_time_subscribers to the kb node, which keep track of the messages published on a certain topic and expose them to oxigraph's underlying SPARQL evaluator to be run at query time. As an example:

PREFIX ex: <http://example.org/>
SELECT ?bl WHERE {
  BIND(ex:robotBatteryLevel() AS ?bl) .
  FILTER(?bl > 0.2)
}

This query contains the special function robotBatteryLevel, which accesses the latest message on a certain topic at query time. These query time subscribers can be added by modifying the config file.

Query-time TF subscribers

ROS(2) makes use of the tf2 library to publish transforms between coordinate frames

• Pellissier Tanon, T. (n.d.). Oxigraph.