OPC UA (Open Platform Communications Unified Architecture) uses information modeling to create digital representations of physical factory equipment and processes. This tutorial explains the OPC UA base information model, how nodes work as building blocks, and how companion specifications enable standardized interoperability across industries.

Understanding these concepts is essential for developing OPC UA applications that communicate consistently with other systems and devices.

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What is OPC UA Information Modeling

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Information modeling in OPC UA provides a framework for digitally describing physical objects and processes on the factory floor. This digital description enables machines and software systems to understand equipment structure, data points, and relationships without human interpretation.

The basic unit of information in OPC UA is called a node. Developers use nodes to create variables, methods, and objects, then interconnect them to build a complete information structure representing a machine, device, or sensor.

Since these information structures are consumed by other machines and applications, OPC UA defines strict rules about how information models are built and presented. This ensures consistency in how all OPC UA-enabled machines and applications interpret information.

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The Base Node Class Structure

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To establish these rules, the OPC Foundation officially defined what a node should contain by creating the base node class. This class acts as a template for creating all other nodes in the OPC UA system.

The base node class specifies mandatory characteristics that every node must have:

Node Identity: A unique identifier that distinguishes each node from all others in the system

Display Name: A human-readable name that client applications can show to end users

Node Class Type: The category of node this instance belongs to

Browse Name: The name clients use when navigating through the OPC UA server address space

The base node class also defines optional characteristics, such as a description field that provides additional context about the node's purpose or content.

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Eight Special Node Classes

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From the base node class template, the OPC Foundation derived eight specialized node classes. Application developers inherit from these classes to create objects that can be joined together in various ways to represent any physical entity.

Key specialized node classes include:

Variable Node Class: Represents a value that can be read from or written to, such as a temperature reading or speed setting

Method Node Class: Represents a function that clients can call to perform actions, like starting a motor or resetting a counter

Object Node Class: Represents real-world objects found in factories, such as machines, conveyor systems, or production lines

This collection of base types constitutes the OPC UA base information model. It provides the foundation for OPC UA application developers to build custom information models for their equipment with the guarantee that any OPC UA client will understand it.

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Creating Custom OPC UA Information Models

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To create a custom OPC UA information model, developers use one of two approaches:

1. Write a standardized XML file using a text editor
2. Use graphical tools that generate the XML file automatically

The XML file format provides a standardized way to define and package information models. This standardization allows organizations to document their information models and share them with partners, customers, or the broader OPC UA community.

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Built-In OPC UA Information Models

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For basic OPC UA server functionality, the OPC Foundation built several information models by adapting classic OPC specifications into OPC UA. These capabilities come standard with any OPC UA server and are built on top of the base information model.

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OPC UA Data Access

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This adaptation of the classic OPC Data Access specification handles continuous data acquisition from PLCs and industrial equipment. It provides core capabilities including read, write, browse, and subscribe operations.

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OPC UA Historical Access

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Adapted from classic OPC Historical Data Access, this model provides capabilities for processing and analyzing data collected from machines over time. It enables trend analysis and historical reporting.

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OPC UA Alarms and Conditions

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This adaptation of classic OPC Alarms and Events provides capabilities to receive alarms from equipment, respond to alarm conditions, and filter through alarm lists based on priority or other criteria.

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OPC UA Programs

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This information model provides capabilities to run stored sets of instructions, such as controlling batch processes or executing sequential operations on equipment.

Developers can build custom information models on top of these built-in models, extending their functionality for specific applications.

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The Challenge of Custom Information Models

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Custom information models present an interoperability challenge. While OPC UA clients can browse a server's address space and dynamically discover custom information models after connecting, this only happens after the connection is established.

True interoperability requires systems to anticipate the information structure they will encounter before making a connection. This advance knowledge empowers developers to build machines and software applications that understand exactly how they should interact with each other.

Without this predictability, each integration project requires custom development work to understand and map the unique information model of each vendor's equipment.

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OPC UA Companion Specifications

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To address the custom information model challenge, the OPC Foundation collaborates with various industry sectors to specify standardized information models for each vertical. These are called companion specifications.

When device and software vendors in an industry agree on the same information model specification, they enable plug-and-play systems within that sector. Equipment from different manufacturers can integrate seamlessly because they all speak the same information language.

Companion specifications build on top of the OPC UA base and built-in information models, adding industry-specific structure and semantics.

Current companion specifications cover several areas:

Industrial Protocol Mapping: Protocols like PROFIBUS and BACnet are mapped to OPC UA structures

Vertical Industry Standards: Specifications exist for oil and gas, energy, pharmaceuticals, and factory automation sectors

Equipment-Specific Models: Standards for specific equipment types like robotics, machine tools, and packaging machines

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Example: OPC UA Robotics Companion Specification

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The OPC UA companion specification for robotics was created through collaboration between the OPC Foundation and VDMA, the largest network of European robotics companies.

VDMA defines this specification as "an OPC UA information model for the representation of a complete motion device system as an interface for higher level control and evaluation systems."

The main use cases for OPC UA robotics include:

• Condition monitoring of robotic equipment
• Asset management and inventory tracking
• Preventive maintenance scheduling
• Vertical integration into production IT systems

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Structure of the Robotics Information Model

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The robotics information model is delivered as a NodeSet2 XML document. The document contains several key elements:

Namespace Declarations: Define the unique namespaces used in the model

Node Definitions: Specify each node with its properties and relationships

For example, the model includes an axis node that inherits from the object node class. This node has:

• A unique node ID
• A browse name for navigation
• A display name for user interfaces
• A description explaining its purpose
• Reference nodes specifying links to other nodes

The axis node has a component with a specific node ID that points to an axis identifier object node. This identifier has its own component pointing to a parameter set node. The parameter set contains a variable node holding the actual position value as a double data type.

The actual position variable node also has a component for engineering units, specifying whether the position is measured in millimeters, degrees, or other units.

This hierarchical structure continues throughout the model, creating a complete digital representation of a robotic system using interconnected nodes and references.

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Benefits of Standardized Information Models

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Standardized companion specifications provide significant advantages:

Reduced Integration Time: Systems integrate quickly because the information structure is known in advance

Vendor Interoperability: Equipment from different manufacturers works together without custom integration

Future-Proof Investments: Upgrading or replacing equipment maintains compatibility when vendors follow the same standard

Simplified Application Development: Software developers build applications once that work with any compliant equipment

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Summary

The OPC UA base information model provides the foundation for industrial system interoperability through standardized node classes and structure. The eight specialized node types derived from the base node class allow developers to create rich information models representing any industrial equipment.

Built-in information models provide standard functionality for data access, historical data, alarms, and program control. Companion specifications extend these foundations with industry-specific standards that enable true plug-and-play integration.

By using standardized XML formats to define and share information models, OPC UA creates an ecosystem where equipment from different vendors can communicate seamlessly, reducing integration costs and accelerating digital transformation in industrial environments.

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