TC4 - Communication
PLCopen OPC-UA Server and Client architectures - What's in in for you?
Overview: Initiatives like Industry 4.0, Smart Factories, industrial Internet of Things, and others, have communication as key element. This means that efficiency in combining intelligent systems, sensors, and actuators becomes more and more important. This paper shows that current state-of-the-art solutions can fulfill already most demands, and with the extensions even the most demanding requirements will be able to be fulfilled with standard specifications, tools, and implementations. This will add in the acceptance of new technologies and architectures.
Communication in industrial control is not new.
The famous CIM triangle is around 30 years old and
today there are many old historically grown data
exchange protocols being used in various industries.
They offer little possibility to transport complex
data and the extensibility is very limited.
Introduction OPC UA Technology
OPC Unified Architecture, or OPC UA, offers the
basis for universal, open, hardware- and software
independent, secure and reliable network
communication i.e. provides the monitoring of
configurable timeouts and connecting interruptions
and encrypted communication. For this it uses a
scalable Client/Server architecture, in which the
Client initiates the data request and the Server
responds and delivers, possibly over a secure
channel, and out of the box, as shown in Fig. 1.
This overall was the basis for PLCopen to use the
OPC UA technology to harmonize the communication in
industrial control, and that in cooperation with the
Mapping OPC UA Server Technology in IEC 61131-3
The first step for the joined working group was
to map the definition of the OPC UA information
model to the IEC 61131-3 software model. The OPC UA
Information Model provides a standard way for
Servers to expose objects to Clients. Objects in OPC
UA terms are composed of other objects, variables
and methods. In this case the Objects are used to
represent IEC 61131-3 software model components like
CtrlProgram, CtrlTask, CtrlResource and CtrlFunction
Blocks, and Variables are used to represent values.
This leads to a standard way how controllers with
integrated OPC UA server technology expose data
structures and function blocks to OPC UA clients
like HMIs and ERP systems.
The specification for the PLCopen OPC UA Information model can be downloaded from the PLCopen website.
Adding OPC UA Client Technology in PLCopen / IEC 61131-3
In the modern world the strict separation of
levels and the top-down approach of the information
flow like defined in the CIM triangle, is losing its
dominance. In a smart network, every device or
service must be able to initiate independent
communication with all other services.
What is in it for you? New ways to communicate
By mapping the OPC UA information model to the
PLCopen / IEC 61131-3 Software model one gets a
unified look and feel to the OPC UA Server
technology embedded in the different brands of
controllers. This is the basis for communication “Out-of-the-Box”,
which can include security. If one harmonizes naming
conventions throughout the company, one can even
easily link a template in the HMI to the applicable
data (structure) in the controller independent of
the brand, as long as it support the PLCopen OPC UA
mapping. One example of these naming conventions are
the PackTags and the state machine as defined in the
OMAC for Packaging specification called PackML, as
part of the ISA 88 Technical Report on Machine and
Unit States. By using these conventions consistently,
one can connect a controller “out –of-the-box” via
the OPC UA Client mapping on the PLCopen
For instance, if one controller sets a specific velocity to a transportation system (like a belt) on which a product is moving which has to be picked up by a robot arm. The controller can do a method call to the robot controller for picking up the product while transferring the speed of the belt. The robot in turn can do a method call to the intelligent camera in order to get the rotation and translation values of the product. With these values the robot can calculate the trajectory to pick up the product smoothly and correctly. Also, communication with the ERP/MES system is possible initiated by the controller for retrieving recipe information as well as product tracing.
This also provides new possibilities, like the initiation of the communication as low as possible in the organization as the basis for production in Industry 4.0, as shown in fig. 5. This means that a product can have an RFID containing all the relevant information for its creation. This information can be read by the controller and the different working stations can do the appropriate actions, like adding 98 grams of yoghurt in the first station. In the end the inspection station can write back the actions that have been checked, as well initiate a method call to the ERP system to deliver the results of the processing done to the track-and-trace system. Such a system can even be in the cloud. And this is even possible today!
Future developments at OPC Foundation
And this is not the end of the story. Enhancements to OPC UA, like real-time extensions via TSN (Time Sensitive Network, making the Ethernet deterministic) will make real-time data communication possible, creating the basis to transport safety related information and allowing synchronization between the different intelligent devices, like robots. The latter esp. combined with the Publisher / Subscribe functionalities, making different communication philosophies possible. This again shows that further enhancements and improvements in programing, installation, and maintenance of communication based architectures are possible.
No matter what new initiatives like Industry 4.0, Smart Factories, industrial Internet of Things, or others will demand, the current solutions can fulfill already most demands, and with the extensions even the most demanding requirements will be able to be fulfilled with standard specifications, tools, and implementations. Communication has never been so easy as long as one is on the right track: PLCopen and OPC UA.
Technical specifications available for downloading:
Read more about the cooperation between PLCopen and OPC Foundation: