I am new to using OPC UA, I would like me to clarify some doubts I have about OPC UA which are as follows:
– In what situations is the use of OPC UA.
– OPC UA Architectures over MQTT.
If there is any document that explains these two doubts, I thank you
OPC UA it is probably the de-facto standard for industrial M2M communication and it is very important in the context of Industrie 4.0.
Let's say you have an industrial machinery (like a PLC) that manages some others, like sensors. With OPCUA you can model into the PLC (which becomes an OPCUA server) some data, using an information model (object-structured and hierarchical, similar concepts to UML) built using rules defined by OPCUA standard (https://opcfoundation.org/developer-tools/specifications-unified-architecture/part-3-address-space-model/). So the PLC first gather data from these sensors using a specific industry protocol, then model in its address space some data that is considered relevant.
You can also build a (opcua) server on the sensors, imagine a temperature or humidity sensor in which you model data such as not only the value of the temperature, but also the manufacturer, engineering unit (Fahrenheit or Celsius for instance). But you can also insert methods within a server and associate to them some specific actions, for example turn on/off a specific functionality if some conditions occur. For all specifications you can look at https://opcfoundation.org/developer-tools/specifications-unified-architecture, where, after signing up, you can download specifications in detail. Another good documentation that I found is http://documentation.unified-automation.com/uasdkcpp/1.6.1/html/index.html where it is explained the main concepts.
Once you defined your opcua servers with an information model within its address space, you can start interacting with some others industrial machinery, in a standardized way. These machinery could be MES or HMI applications and they have to be opcua clients. They can query the opcua server above mentioned, browsing their address space, reading values, calling methods, monitoring some interesting variables or events (subscribing to them the server will send a notification when a change occurs). The main advantage is that all these operations are performed via the use of standardized messages: if you want to write a data you have to send a WriteRequest message, if you want to read the client will send a ReadRequest and so on. Since everything is standardized (from data types to serialization of messages), all clients can understand structure of opcua servers (even if they are from different manufacturers). Without that every manufacturer could use its own way to define services or variables and you have to create your application (let's say HMI) to fit to that particular vendor's APIs or conventions.
Regarding OPCUA over MQTT, in this you can find some useful information OPC UA protocol vs MQTT protocol. As I said before OPCUA has the advantage of defining a structured and a standard information model, accessible via standard services, so using MQTT is only one part of the whole.
Another good reference to understand information models in opcua server could be OPC Unified Architecture
