According to ISO standards, a “machine tool is a mechanical device, which is fixed and powered, typically used to process workpieces by selective removal/addition of material or mechanical deformation (…). Machine tools operation can be mechanical, controlled by humans or by computers (…)”. There is a great variety of metalworking machine tools: milling machines, lathes, sheet metal forming machines, EDM machines and additive manufacturing machines are just some examples. Stainless steel, aluminium, titanium and copper are some of the main metals processed by these machine tools. In addition, to manufacture components for key industries like automotive, aerospace, energy and medical technology, machine tools enable the production of all the other machines, including themselves. This is why we call them the mother machines.
For the scope of the standard, mainly machine tools for machining metal and other hard materials were considered. Most of these machine tools are equipped with a CNC Control and a PLC. In order to represent these machine tools with a common OPC UA interface, several use cases were considered.
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 the MachineTool model, 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.
As an open standard, OPC UA is based on standard internet technologies, like TCP/IP, HTTP, 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 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.
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 as BrowseName. An Object representing a ‘Reservation’ is shown in Figure 2.
Object and Variable Nodes represent instances and they always reference a TypeDefinition (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 reconfiguration of systems if a Client uses types that multiple Servers implement.
OPC UA also supports the concept of sub-typing. This allows a modeller to take an existing type and extend it. There are rules regarding sub-typing defined in OPC 10000-3, but in general they allow the extension of a given type or the restriction of a DataType. For example, the modeller may decide that the existing ObjectType in some cases needs an additional Variable. The modeller can create a subtype of the ObjectType and add the Variable. A Client that is expecting the parent type can treat the new type as if it was of the parent type. Regarding DataTypes, subtypes can only restrict. If a Variable is defined to have a numeric value, a sub type could restrict it to a float.
References allow Nodes to be connected in ways that describe their relationships. All References have a ReferenceType that specifies the semantics of the relationship. References can be hierarchical or non-hierarchical. Hierarchical references are used to create the structure of Objects and Variables. Non-hierarchical are used to create arbitrary associations. Applications can define their own ReferenceType by creating subtypes of an existing ReferenceType. Subtypes inherit the semantics of the parent but may add additional restrictions. Figure 4 depicts several References, connecting different Objects.
The figures above use a notation that was developed for the OPC UA specification. The notation is summarised in Figure 5. UML representations can also be used; however, the OPC UA notation is less ambiguous because there is a direct mapping from the elements in the figures to Nodes in the AddressSpace of an OPC UA Server.
OPC UA specification defines a very wide range of functionality in its basic information model. It is not expected that all Clients or Servers support all functionality in the OPC UA specifications. OPC UA includes the concept of Profiles, which segment the functionality into testable certifiable units. This allows the definition of functional subsets (that are expected to be implemented) within a companion specification. The Profiles do not restrict functionality, but generate requirements for a minimum set of functionality (see OPC 10000-7)
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. Namespaces in OPC UA have a globally unique string called a NamespaceUri and a locally unique integer called a NamespaceIndex. The NamespaceIndex is only unique within the context of a Session between an OPC UA Client and an OPC UA Server. The Services defined for OPC UA use the NamespaceIndex to specify the Namespace for qualified values.
There are two types of values in OPC UA that are qualified with Namespaces: NodeIds and QualifiedNames. NodeIds are globally unique identifiers for Nodes. This means the same Node with the same NodeId can appear in many Servers. This, in turn, means Clients can have built in knowledge of some 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.
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.