Unleash the Power of OPC UA: Get Information on Custom Struct on the OPC UA Server via Eclipse Milo Client

In the world of industrial automation, OPC UA (Unified Architecture) has emerged as a widely adopted standard for secure, reliable, and platform-independent data exchange. One of the most popular OPC UA client libraries is Eclipse Milo, which provides a Java-based implementation for interacting with OPC UA servers. In this article, we’ll delve into the process of retrieving information on custom structs from an OPC UA server using the Eclipse Milo client.

What are Custom Structs in OPC UA?

In OPC UA, a custom struct is a user-defined data type that consists of multiple elements, such as integers, floats, strings, and other data types. Custom structs are often used to represent complex data models, like device configurations, sensor readings, or alarm information. By defining custom structs, OPC UA servers can provide a more detailed and structured view of the underlying data, making it easier for clients to interpret and utilize the information.

Why Use Eclipse Milo Client?

Eclipse Milo is a popular choice for OPC UA client development due to its robust feature set, ease of use, and Java-based implementation. Some of the key benefits of using Eclipse Milo client include:

• Platform independence: Eclipse Milo client can run on any platform that supports Java, making it an ideal choice for cross-platform development.
• Extensive feature set: Eclipse Milo client provides a comprehensive implementation of the OPC UA specification, including support for custom structs, methods, and events.
• Easy to use: Eclipse Milo client comes with a simple and intuitive API, making it easy to integrate with OPC UA servers and retrieve data.

Step-by-Step Guide to Retrieving Custom Struct Information using Eclipse Milo Client

Now that we’ve set the stage, let’s dive into the step-by-step process of retrieving information on custom structs from an OPC UA server using the Eclipse Milo client.

Step 1: Set up the Eclipse Milo Client

Before we begin, make sure you have the Eclipse Milo client library installed in your project. You can add the following dependency to your `pom.xml` file (if you’re using Maven) or download the JAR file manually:

<dependency>
    <groupId>org.eclipse.milo</groupId>
    <artifactId>opc-ua-client</artifactId>
    <version>0.6.1</version>
</dependency>

Step 2: Create an OPC UA Client Instance

Create an instance of the OPC UA client using the Eclipse Milo client library:

OpcUaClient client = OpcUaClient.create();
client.connect("opc.tcp://localhost:4840").get();

In this example, we’re connecting to a local OPC UA server at `opc.tcp://localhost:4840`. Replace this with the address of your OPC UA server.

Step 3: Browse the OPC UA Server for Custom Structs

Use the `browse` method to retrieve a list of nodes on the OPC UA server:

BrowseResult browseResult = client.browse(
    new BrowseDescription(
        BrowseDirection.Forward,
        identifyNode(NodeId.get(BrowsePath.forward(OpcUaStandardNamespace.Types Folder))))
).get();

List<BrowseResult> browseResults = browseResult.getResults();
for (BrowseResult br : browseResults) {
    for (ReferenceDescription rd : br.getReferences()) {
        if (rd.getTypeDefinition().equals(StructureType.get(OpcUaStandardNamespace.DataItemType))) {
            // Found a custom struct node
            NodeId customStructNodeId = rd.getNodeId();
            // ...
        }
    }
}

In this example, we’re browsing the OPC UA server for nodes of type `DataItemType`, which is a base type for custom structs. We iterate through the browse results and references to find the custom struct nodes.

Step 4: Read the Custom Struct Attributes

Once you’ve found the custom struct node, use the `read` method to retrieve the attributes:

ReadValueId readValueId = new ReadValueId(
    customStructNodeId,
    AttributeId.All,
    null,
    null
);

ReadResponse readResponse = client.read(null, 0, TimestampsToReturn.Both, readValueId).get();

DataValue dataValue = readResponse.getValues().get(0);
Variant variant = dataValue.getValue();

// Get the custom struct attributes
ExtensionObject xo = (ExtensionObject) variant.getValue();
StructuredType structuredType = (StructuredType) xo.getBody();

// Iterate through the custom struct attributes
for (Field field : structuredType.getFields()) {
    // Process the attribute value
    Object value = field.getValue().getValue();
    System.out.println(String.format("Attribute name: %s, value: %s", field.getName(), value));
}

In this example, we’re reading the custom struct attributes using the `read` method and then iterating through the attribute values using the `StructuredType` class.

Best Practices for Working with Custom Structs in OPC UA

When working with custom structs in OPC UA, keep the following best practices in mind:

1. Follow the OPC UA specification: Adhere to the OPC UA specification when defining custom structs to ensure compatibility and interoperability with other OPC UA clients and servers.
2. Use meaningful names and descriptions: Choose descriptive names and descriptions for your custom structs and attributes to make it easier for clients to understand and utilize the data.
3. Document your custom structs: Provide detailed documentation for your custom structs, including the structure, attributes, and data types, to facilitate client development and integration.
4. Test and validate your custom structs: Thoroughly test and validate your custom structs to ensure they are correctly defined, written, and read by the OPC UA server and clients.

Conclusion

In this article, we’ve explored the process of retrieving information on custom structs from an OPC UA server using the Eclipse Milo client. By following the steps and best practices outlined above, you’ll be well on your way to unlocking the power of OPC UA and custom structs in your industrial automation applications.

Keyword	Frequency
Get Information on custom struct on the opcua server via Eclipse Milo Client	5
OPC UA	7
Eclipse Milo Client	4
Custom Structs	6

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