Connectivity Framework
7: Connectivity Standards
in the operations domain and is also being applied to information and application domains( see Figure 1-1).
OPC-UA is an evolution of the classic OPC( Object Linking and Embedding for Process Control) standards. It unifies the various original OPC specifications and is an evolution from an API to a network protocol. Adapters are available to bridge between OPC-UA and classic OPC. OPC is operational in thousands of factories globally. Traditionally, OPC was used to configure and query plant-floor servers( usually Programmable Logic Controllers( PLCs)). Actual device-device communication was then effected via a hardware-based fieldbus 1 such as Modbus or Profinet.
OPC-UA retains some of that flavor; it connects and configures plant-floor servers. The UA version adds better syntactical data typing( see section 4.1.3) and semantic information modeling capabilities. There are many companion specifications that define information models for various device types. For example, Field Device Integration( FDI) defines a model that represents all fieldbus device types. A remote client such as a graphical interface can browse the device data controlled by a server on the floor. By allowing this introspection across many servers, clients can build a directory with cross-references of all the devices on the floor. Additionally, OPC-UA also addresses the specific needs of device-device communication and therefore does not anymore rely on additional fieldbus solutions. Its scalability allows for implementation on devices with very restricted hardware resources, such as sensor and actuator devices.
OPC-UA divides system software into clients and servers. The servers usually reside on a device; they provide a way to access the device through a standard“ device model”. There are device models for dozens of types of devices from sensor to feedback controllers. Each manufacturer is responsible for providing the server that maps the generic device model to its particular device. The servers expose an object-oriented, remotely-callable API that implements the device model.
Generic device models are central to the OPC-UA architecture. For example, the object model for a motor starter includes methods for setting parameters, reading data and operating the starter. Thus, applications can control a starter directly without being dependent on the manufacturer’ s particular implementation.
OPC-UA is developing a“ pub-sub” capability. This will provide direct device-to-device connection. There will be several“ profiles” using different underlying protocols. The UDP profile supports multicast for efficiency. It targets simple implementation and does not attempt advanced functions like fragmentation control( sending large data types in pieces), reliability or quality-of-service control. Another profile is designed for connection to cloud-based data analytics. There is work on a DDS profile that will provide more sophisticated pub-sub functionality.
OPC-UA targets all kinds of manufacturing, including automotive, oil and gas, pharmaceuticals, food & beverage, medical machines, machine tools. It connects applications at the factory-floor
1
A“ fieldbus” is the name of a family of industrial computer network protocols used for real-time distributed control.
IIC: PUB: G5: V1.0: PB: 20170228- 50-