Information Model

Information Models use a strict object-oriented paradigm with Object Types and Variable Types that support inheritance hierarchies. A BaseObjectType defines common attributes, while specialized subtypes extend it with domain-specific properties.

• Subtyping: A 'Motor' type inherits from 'Equipment', adding motor-specific variables like RPM and Torque.
• Instantiation: Each physical motor on the factory floor becomes an Object instance of the Motor type in the Address Space.
• Encapsulation: Types bundle data (Variables), behavior (Methods), and state (Alarms) into single, reusable definitions.

Nodes are connected via References, which are themselves typed and carry explicit semantic meaning. Unlike simple pointers, a Reference defines why two Nodes are related.

• HasComponent: Indicates a part-of relationship (e.g., a Motor HasComponent a Drive).
• HasProperty: Links an Object to a characteristic that cannot have its own children (e.g., a Motor HasProperty SerialNumber).
• Organizes: Defines a hierarchical grouping for browsing, not a structural dependency.
• HasCause: Links an Alarm to the Variable that triggered it, enabling root cause analysis.

Every OPC UA Server starts with the OPC UA Base Information Model, a mandatory, pre-defined set of types in namespace 0 (http://opcfoundation.org/UA/). This provides the foundational vocabulary for all other models.

• Defines core types: BaseObjectType, BaseDataVariableType, PropertyType.
• Establishes the Address Space structure itself, including the Root, Objects, Types, and Views folders.
• Provides the base Event and Alarm model for stateful condition monitoring.
• All vendor or industry-specific models extend from this common root, ensuring baseline interoperability.

Industry working groups publish Companion Specifications—standardized, domain-specific Information Models that enable plug-and-produce interoperability across vendors.

• OPC UA for Robotics: Defines types for RobotController, MotionDeviceSystem, and TaskProgram so any compliant robot exposes the same interface.
• OPC UA for Machinery: Standardizes machine tool identification, job management, and condition monitoring.
• Weihenstephan Standards: Models for packaging and food & beverage equipment.
• These specifications are stored and versioned in the OPC UA Cloud Library for global access and consistency.

When defining a type, you specify Instance Declarations—placeholders for child Nodes that will exist on every instance. Modelling Rules dictate whether these declarations are mandatory or optional.

• Mandatory: Every instance of the type must have this child Node (e.g., every Motor must have a CurrentSpeed Variable).
• Optional: An instance may include this child Node (e.g., a Motor may have a CoolantTemperature Variable if equipped with a sensor).
• This mechanism allows Clients to discover the guaranteed structure of any Object by examining its type definition, enabling robust, generic client applications.

Information Models are partitioned into Namespaces identified by unique URIs, preventing naming collisions between different standards and vendors.

• Namespace 0: Reserved for the OPC UA Base Model.
• Namespace 1: Typically used for a server's local, vendor-specific extensions.
• Custom Namespaces: Each Companion Specification or vendor model gets its own URI (e.g., http://yourcompany.com/UA/Machines).
• A Node's NodeId combines a Namespace Index and an Identifier, ensuring global uniqueness across the entire Address Space.

The fundamental atomic unit within an OPC UA Address Space. Every Node is uniquely identified by a NodeId and possesses a set of Attributes that describe its characteristics. Nodes are categorized into classes such as Objects, Variables, and Methods, which collectively represent physical assets, sensor values, and executable functions. The Information Model defines the valid types and constraints for these Nodes.

A directed relationship between two Nodes that provides context and topology. Unlike simple data points, References define how components are connected—for example, a HasComponent Reference links a Motor Object to its Speed Variable. The Information Model uses References to create hierarchies, expose type definitions, and define the semantic meaning of connections, enabling Clients to browse and interpret the structure.

A standardized Information Model developed by industry working groups to define domain-specific semantics. These specifications provide pre-built type systems for verticals like robotics, machine vision, or machinery. By adopting a Companion Specification, engineers ensure that a 'Temperature' Variable from Vendor A has the same semantic meaning and engineering units as one from Vendor B, enabling true plug-and-produce interoperability.

The modeling paradigm that underpins the entire Address Space. It supports inheritance, abstraction, and polymorphism. A generic 'MotorType' can be defined once and instantiated multiple times for specific physical motors. Each instance inherits the base structure but can be extended with unique properties. This approach ensures consistency and reusability across complex automation systems.