4 General information about PA-DIM and OPC UA

4.1 Introduction to Process Automation Device

Process Automation Devices are used within Industrial Automation Systems of Chemicals, Oil & Gas, Food & Beverage, Power generation, Metals, Cement, Mining & Minerals, Pulp & Paper, Water & Waste Water and measure pressure, temperature, flow, level, etc. or position valves with control actuators. Process Automation Devices are often connected to Control Systems and Plant Asset Management.

For the lifecycle commissioning, operation or maintenance, a minimum common set of Process Automation Device parameters and functions is necessary. With the concepts of Industrie 4.0 and IoT, the lifecycle is extended and starts already with procurement, which uses Common Data Dictionaries (CDD) like IEC 61987 and ECLASS. Each property (variable, parameter, etc.) within a CDD has a unique identifier (e.g. an IRDI).

This Process Automation Device Companion Specification defines the fieldbus protocol independent Information Model. This model includes a minimum set of parameters to provide interoperability and interchangeability for the main use cases of:

Identification,

Diagnostics,

Process Values and

Configuration

for Pressure, Temperature, Flow, Level, Control Actuator/Positioner applications. This Model includes support for the entire lifecycle.

In relation to this companion specification, the IEC 62769, Field Device Integration (FDI) specification defines the device configuration (online/offline) with a PC tool or mobile device including the definition of the User Interface (UI).

The collaboration of FieldComm Group and OPC Foundation aim is a protocol independent information model.

4.2 Introduction to OPC Unified Architecture

4.2.1 What is OPC UA?

OPC UA is an open and royalty free set of standards designed as a universal communication protocol. While there are numerous communication solutions available, OPC UA has key advantages:

A state of art security model (see OPC 10000-2)

A fault tolerant communication protocol.

An information modelling framework that allows application developers to represent their data in a way that makes sense to them.

OPC UA has a broad scope which delivers for economies of scale for application developers. This means that a larger number of high-quality applications at a reasonable cost are available. When combined with semantic models such as OPC UA for Process Automation Devices – PA-DIM, OPC UA makes it easier for end users to access data via generic commercial applications.

The OPC UA model is scalable from small devices to ERP systems. OPC UA Servers process information locally and then provide that data in a consistent format to any application requesting data - ERP, MES, PMS, Maintenance Systems, HMI, Smartphone or a standard Browser, for examples. For a more complete overview see OPC 10000-1.

4.2.2 Basics of OPC UA

As an open standard, OPC UA is based on standard internet technologies, like TCP/IP, HTTP and Web Sockets.

As an extensible standard, OPC UA provides a set of Services (see OPC 10000-4) and a basic information model framework. This framework provides an easy manner for creating and exposing vendor-defined information in a standard way. More importantly all OPC UA Clients are expected to be able to discover and use vendor-defined information. This means OPC UA users can benefit from the economies of scale that come with generic visualization and historian applications. This specification is an example of an OPC UA Information Model designed to meet the needs of developers and users.

OPC UA Clients can be any consumer of data from another device on the network to browser based thin clients and ERP systems. The full scope of OPC UA applications is shown in Figure 1.

Figure 1 – The Scope of OPC UA within an Enterprise

OPC UA provides a robust and reliable communication infrastructure having mechanisms for handling lost messages, failover, heartbeat, etc. With its binary encoded data, it offers a high-performing data exchange solution. Security is built into OPC UA as security requirements become more and more important especially since environments are connected to the office network or the internet and attackers are starting to focus on automation systems.

4.2.3 Information modelling in OPC UA

4.2.3.1 Concepts

OPC UA provides a framework that can be used to represent complex information as Objects in an AddressSpace which can be accessed with standard services. These Objects consist of Nodes connected by References. Different classes of Nodes convey different semantics. For example, a Variable Node represents a value that can be read or written. The Variable Node has an associated DataType that can define the actual value, such as a string, float, structure etc. It can also describe the Variable value as a variant. A Method Node represents a function that can be called. Every Node has a number of Attributes including a unique identifier called a NodeId and non-localized name called a BrowseName. An Object representing a ‘Reservation’ is shown in Figure 2.

Figure 2 – A Basic Object in an OPC UA Address Space

Object and Variable Nodes represent instances and they always have a HasTypeDefinition reference to a Node (ObjectType or VariableType Node) which describes their semantics and structure. Figure 3 illustrates the relationship between an instance and its TypeDefinition.

The type Nodes are templates that define all of the children that can be present in an instance of the type. In the example in Figure 3 the PersonType ObjectType defines two children: First Name and Last Name. All instances of PersonType are expected to have the same children with the same BrowseNames. Within a type the BrowseNames uniquely identify the children. This means Client applications can be designed to search for children based on the BrowseNames from the type instead of NodeIds. This eliminates the need for manual configuration of systems if a Client uses types that multiple Servers implement.

4.2.3.2 Namespaces

OPC UA allows information from many different sources to be combined into a single coherent AddressSpace. Namespaces are used to make this possible by eliminating naming and id conflicts between information from different sources. Each namespace in OPC UA has a globally unique string called a NamespaceUri which identifies a naming authority and a locally unique integer called a NamespaceIndex, which is an index into the Server's table of NamespaceUris. The NamespaceIndex is unique only within the context of a Session between an OPC UA Client and an OPC UA Server- the NamespaceIndex can change between Sessions and still identify the same item even though the NamespaceUri's location in the table has changed. The Services defined for OPC UA use the NamespaceIndex to specify the Namespace for qualified values.

There are two types of structured values in OPC UA that are qualified with NamespaceIndexes: NodeIds and QualifiedNames. NodeIds are locally unique (and sometimes globally unique) identifiers for Nodes. The same globally unique NodeId can be used as the identifier in a node in many Servers – the node's instance data may vary but its semantic meaning is the same regardless of the Server it appears in. This means Clients can have built-in knowledge of of what the data means in these Nodes. OPC UA Information Models generally define globally unique NodeIds for the TypeDefinitions defined by the Information Model.

QualifiedNames are non-localized names qualified with a Namespace. They are used for the BrowseNames of Nodes and allow the same names to be used by different information models without conflict. TypeDefinitions are not allowed to have children with duplicate BrowseNames; however, instances do not have that restriction.

4.2.3.3 Companion Specifications

An OPC UA companion specification for an industry specific vertical market describes an Information Model by defining ObjectTypes, VariableTypes, DataTypes and ReferenceTypes that represent the concepts used in the vertical market, and potentially also well-defined Objects as entry points into the AddressSpace.