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Martin Müller
With its open protocol structures, its strengths in universal communication and its free choice of topology, Ethernet has long since established itself as the standard in office communication. In the field of industrial communications, on the other hand, various Fieldbuses have emerged to date as the defacto standard. This is largely due to geographical reasons or specific application requirements. In countless machines and systems worldwide, they serve to connect sensors and actuators with control technology. Ethernet is already used in the industry for communicating between various machines and subsystems. It is therefore natural and obvious that the industry should also be interested in using the global communication standard Ethernet for connecting sensors and actuators. However, this is not possible without modifying or adding to the standard Ethernet technology. The reasons for this include the complexity of the protocol, the processing power required in each simple device and the increased project efforts. As a result, various companies and organizations are working on Ethernet extensions, especially those that will enable real-time processing.
Fig.1 Fieldbus systems such as Interbus are currently the standard in industrial communications
Requirements on industrial communications
In industrial automation solutions, communications systems must primarily transport I/O signals (bits) between the control unit, sensors and actuators. Other key tasks include the synchronization of drive systems and the transfer of safety-oriented signals. Security technology is currently being integrated into Fieldbus systems and is now being used in applications for the first time. For Ethernet, there is no open and standardized method to facilitate this. A further key requirement on industrial communications systems is vertical integration, in other words the universal and transparent linking of the factory domain with the office domain. Ethernet is currently being used chiefly in this context where it is clearly advantageous.
In addition to the general requirements of industrial communications systems listed above, there are a few key criteria that an Ethernet-based communications system must meet before users will readily switch from the Fieldbus technology being used successfully at present to the Ethernet technology of the future.
First of all, an Ethernet-based communications system must satisfy real-time requirements in the sub-millisecond region. To facilitate this, a suitable protocol and possibly also special ASICS may be required that can also be integrated into simple devices. Various concepts exhibit tremendous differences, especially in this respect, since the existing Ethernet system is bent so far in places that interaction with standard components is no longer possible. So while excellent performance values are generally achieved with this solution, the demand for open, transparent communication is not met.
In order to reduce the complexity of future automation solutions, modular control and functional units are required that operate autonomously in the overall system and communicate with each other via suitable networks. This makes it possible to optimally automate the mechanical modules that are generally already present and assemble machines from the various functional units. In terms of the communications system, an architecture is required where the distributed control intelligence can be connected and integrated. The various communications concepts may well make it possible to distribute control functions around the system, but only very few systems offer a true architecture for distributing intelligence.
Fig.2 Future communications systems must support distributed automation architectures
To date, users have invested several billion dollars in systems and machines that are based on Fieldbus systems. According to a study carried out by Frost & Sullivan, Interbus alone represents an investment volume of more than 1 billion dollars. Hence there is a clear demand from users and manufacturers that new, Ethernet-based solutions allow seamless integration of Fieldbus systems. It is not sufficient to use simple gateways, at least not in the way they are normally used, since they would represent a change in the operating and diagnostic philosophy. Modern concepts provide for proxy structures in the communication architecture so that it is even possible to access data on Fieldbus devices without causing an interruption.
Having access to the main control systems is just as important as the technical performance data of the communications system. In this regard, it is worth mentioning the market leaders Siemens, Rockwell and Schneider, but also PC-based control technology. In the context of Ethernet-based communication systems, it is frequently only PCs that are mentioned as a control system. This overlooks the fact that they currently hold a less than 10% share of the control market.
A further key point in the evaluation of Ethernet-based communications systems for industrial use is whether they have a guaranteed future or not. Generally, it can be said that the more companies support a system and the larger market significance they gain, the more likely it is that the system will also still be available in ten or twenty years’ time wherein its technology is continuously developed.
To summarize, it can be stated that the following represent key assessment criteria when evaluating Ethernet-based communications systems:
(1) Real-time communication
(2) Architecture for distributed automation
(3) Architecture for the integration of Fieldbuses
(4) Support for connectivity to all key control systems
(5) Guaranteed future
Decision in favor of the most comprehensive concept
Profinet’s SRT (Soft-Real-Time) and IRT (Isochronous-Real-Time) concepts are designed for real-time communication and are graduated according to the needs of the various device classes. Simple I/O devices are handled with Profinet I/O in the conventional view of the local peripherals. Devices and systems with low real-time requirements can be operated easily and flexibly with SRT. In highly dynamic drive applications, IRT is the solution of choice when used in conjunction with suitable hardware support for the sub-millisecond applications with high synchronicity demands.
For intelligence distribution, Profinet offers CBA (Component-Based-Architecture), a component-based architecture model that can be used to take control of even complex and distributed automation structures. Key aspects in this regard include the component model, which is based on the Windows technology DCOM, together with switching of the functional components via circuit editors, for example.
Fig.3 Distribution of update times with PROFInet
With Profinet, Fieldbus systems are integrated via defined proxy structures so that devices and objects connected to the Fieldbus are seamlessly integrated into the Profinet concept.
None of the market-leading control systems enjoy support from all of the major control suppliers. Profinet is currently supported by Siemens, Bosch, Phoenix Contact and other control suppliers and expressed in terms of market share, this represents the widest support among all the systems.
As already mentioned in the context of the connection to control systems, the future of the Profinet concept is guaranteed thanks to support from market-leading firms. Profinet is not only supported by the Profibus user organization, but also by the Interbus Club, who represent two of the world’s leading organizations and systems in the Fieldbus sector.
If one examines the evaluation criteria noted above and compares all of the Ethernet-based communications concepts that are currently under discussion, it is clear that Profinet represents the most comprehensive concept. Many other concepts try to score points with maximum performance in the real-time sector, although they sacrifice other key aspects such as the distribution of intelligence or the integration of Fieldbuses, or are only available for PC-based control system, The criteria mentioned formed the basis for Phoenix Contact’s decision to choose Profinet, and will also provide a future guarantee that this system will prosper on the worldwide market.
Fig.4 Switches, gateways and I/O products from the Factory Line family bring the Ethernet to the field
北京市公安局海淀分局備案號：11010802023656號