Hierarchical Hypergraphs for Knowledge-centric Robot Systems: a Composable Structural Meta Model and its Domain Specific Language NPC4

Enea Scioni, Nico Huebel, Sebastian Blumenthal, Azamat Shakhimardanov, Markus Klotzbuecher, Hugo Garcia, Herman Bruyninckx


Many robotics applications rely on graph models in one form or another: perception via probabilistic graphical models such as Bayesian Networks or Factor Graphs; control diagrams and other computational ``function block'' models; software component architectures; Finite State Machines; kinematics and dynamics of actuated mechanical structures; world models and maps; knowledge relationships as ``RDF triples''; etc.In traditional graphs, each edge connects just two nodes, and graphs are ``flat'', that is, a node does not contain other nodes.
This paper advocates the research hypothesis that hierarchical hypergraphs are a better structural meta model: (i) an edge can connect more than two nodes, (ii) the attachment between nodes and edges is made explicit in the form of ``ports'' to provide a uniquely identifiable view on a node's internal behaviour, and (iii) every node can in itself be another hierarchical hypergraph.These properties are encoded formally in a Domain Specific Language (or ``meta meta model''), called ``NPC4'', built with node, port, connector, and container as primitives, and contains and connects as relationships. The formal model of NPC4 is designed to maximally support its composability as a meta modelling language, for both the structural and behavioural parts of more concrete DSLs that can be built on top of it, each in a specific domain context.
NPC4 introduces a particular primitive, the container, to support  overlapping contexts. It targets the following major targets in knowledge-centric robotics systems: (i) various levels of abstraction in domain models, (ii) ``multiple inheritance'' from (or rather ``conformance to'') different knowledge domains, and (iii) connecting one or more domain DSLs to the same software infrastructure in which they all have to be ``activated''.

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