Profinet in the process industry Integrated network structures offer maximum benefit

Ethernet-based communication systems are gaining ground in automation engineering. Their ability to integrate network structures across whole companies provide important benefits. An Ethernet-based technology that takes account of the special requirements of the process industries is now available in the form of Profinet.

Related Vendors

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

TURCK GmbH

Glatt Ingenieurtechnik GmbH

Siemens AG

Ethernet is a success story in global corporate communications. The first Ethernet design arose in the 1970s, and in the 1980s the Institute of Electrical and Electronics Engineers (IEEE) declared Ethernet the official standard for local area networks (LANs) under the designation IEEE 802.3.

Ethernet is based on a data transmission algorithm known as CSMA/CD (Carrier Sense Multiple Access with Collision Detection). All the devices (“nodes”) on the network “listen in” to the data traffic, and if the line is free, any node can send data with equal priority. If two nodes attempt to transmit at the same time, the resulting collision causes each node to wait for a random period before trying to send the data again. This is repeated until the data have been transmitted without error. A crucial benefit of this procedure is that all nodes can transmit their data promptly.

Since Ethernet is based on point-to-point connections, active components are required to create a network containing more than two nodes. In modern Ethernet networks, this task is handled by “switches” — devices with several ports to which the nodes are connected. A switch stores the addresses of the nodes on the network, and if two nodes want to send data to each other, the switch functions as the exchange. Communication is “full duplex”: the nodes can send and receive simultaneously, and the use of switches avoids collisions.

Fundamental to the practical use of Ethernet are the various protocols, notably TCP/IP and UDP, that network devices use for different purposes. TCP/IP (Transmission Control Protocol/Internet Protocol) is used in most office applications, and forms the basis for other protocols such as those used by standard IT functions such as web browsers and e-mail. The OSI reference model (Reference Model for Open Systems Interconnection) represents the individual communication layers.

Another feature that makes Ethernet extremely flexible is known as data encapsulation. When data are transferred between user programs, they pass through the individual layers familiar from the OSI reference model. Each layer adds its own level of information, allowing the layers to operate independently.

In industrial use, the demands on Ethernet include ruggedness and suitability for extended temperature ranges. Siemens introduced Ethernet for industrial requirements as early as 1985.

Fieldbus advantages

Conventional automation technology, such as 4–20 mA analog signals and the Hart protocol, are still very common in the process industries, but fieldbuses like Profibus are becoming increasingly widespread. According to Profibus International, there are now more than 20 million installed Profibus nodes. Of these, 3.3 million are to be found in the process industries. A recent study by the ARC Advisory Group predicts that the market for fieldbus solutions in the process industries will grow at an annual average of 20 percent until 2011.

Potentially explosive gases and dusts create a serious challenge to automation in the process industries. Profibus is the right answer here, in the form of its PA version. Profibus PA is based on an interface corresponding to the IEC 61158-2 physical layer with Manchester coding (MBP), which means that the power supply for a field device, and communication with the device, share the same cable. This allows even intrinsically-safe applications to be implemented, and because no additional power supplies for the Ex area need to be installed in the field, costs are significantly reduced.

If high-speed I/O and drives need to be connected in non-Ex zones, Profibus DP is used. The only difference between Profibus DP and Profibus PA is in the interface and the associated data transfer rate. Profibus DP is based on RS 485, with a maximum data transfer rate of 12 Mbit/s. By contrast, Profibus PA is based on MBP, with a maximum transfer rate of 32.25 kbit/s. The DP protocol is used in both cases. The essential user benefit of Profibus is thus its ability to be used in both hazardous and non-hazardous areas of the plant. This provides advantages especially in hybrid applications where high-speed I/O and drives are used alongside process instrumentation. Profibus always works on the master/slave principle. The master polls its slaves cyclically to update process data. Acyclic communication is used to process parameter data.

Special application requirements such as those found in safety applications or drives are described in “profiles” (Profisafe, Profidrive, etc.). The profile used in process instrumentation is called PA Devices. Profiles are application-oriented specifications that are applied to a communication system, and they can be adapted to different communication systems.

Best of both worlds

Combining many of the advantages of Profibus and Ethernet, Profinet may be described as industrial Ethernet with real-time capability and determinism, based on the widely-used standards described above. This openness guarantees that the technology is vendor-independent, giving customers and end-users freedom of choice when it comes to components. As described above, Ethernet is based on a procedure that is not deterministic. Determinism, however, is a core requirement of communication systems in automation engineering. Another important demand is for real-time capability. Profinet provides these system properties in addition to the Ethernet standards.

Profinet has a modular design that allows users to choose the functionality they need. There are two basic varieties: Profinet CBA (component based automation) and Profinet I/O.

With Profinet CBA, plant sections are represented by “technological modules”. These modules can contain program code as well as configuration data, so even larger projects involving different subcontractors can be managed easily. Each individual instance can process its plant section independently, while interface conventions between the technological modules ensure that the overall application runs smoothly. Another advantage of this technology is the re-usability of technological modules, which can mean significant savings in programming time for larger projects.

The other flavor of Profinet, Profinet I/O, has been designed to handle distributed I/O and drives. Profinet I/O distinguishes three types of communication: TCP/IP, RT (real time) and IRT (isochronous real time). An example of the use of TCP/IP is accessing the internet using a web browser. RT communication is used for cycle times of the order of 10 ms. With IRT, high-speed motion control applications can be implemented with cycle times down to 250 µs and jitter less than 1 µs.

The three versions can operate in parallel on the same medium. A crucial advantage of this scalability is that users only have to operate and maintain a single system. The same communication technology can be used from the field level to the MES/ERP level.

This flexibility opens up new fields of application. For instance, plants can now use web-based IT services alongside classic automation tasks. In conjunction with industrial wireless LANs (IWLANs), mobile workstations can be set up for service personnel.

The application profiles used by Profibus — Profisafe, Profidrive, PA Devices and others — are also specified by Profibus International for use with Profinet. It is possible to implement a safety connection over an IWLAN with Profinet, for example.

Fieldbus integration in Profinet is also important for customers in securing existing investments, and to handle hazardous areas. As mentioned above, hazardous areas need special measures up to and including intrinsic safety, and field devices must be able to get their power from the communication cable. Work has been underway for some time on ways to provide this functionality for Ethernet, based on a technology called Power over Ethernet (POE). However, there are currently no certified POE products on the market for use in Ex areas.

For this reason, current Profinet installations for hazardous areas rely on gateways that allow networks based on Profibus or other fieldbuses to be integrated into the Profinet network. Integration of fieldbuses including Profibus, Hart, Foundation Fieldbus H1 and DeviceNet is either available or planned.

Fast and flexible

The integrated network structures possible with Profinet create a high degree of flexibility. An analyzer that needs to transfer large volumes of data, for instance, could communicate direct with a control system via Profinet. A temperature transmitter or flowmeter would typically continue to operate on Profibus PA or Hart. Web technology can be used to create completely new services that increase flexibility and productivity. Although Profinet communication works differently to that of Profibus, the configuration appears very similar to the user. It is therefore easy to make the transition from Profibus to Profinet, significantly cutting costs for employee training.

Profinet is ideal for large systems. The number of nodes in one network segment can exceed 1,500, depending on the application. Since the data transfer rate is 100 Mbit/s with full duplex, higher-performance data transfer is possible, making it possible to achieve higher data throughput at all network levels where Profinet is used. The design of Profinet allows flexible use in all areas of a company’s infrastructure. Since Profinet is based on standard Ethernet, networking is guaranteed up to the ERP level of a company. The same medium can be used from the field level to the enterprise level, building on the ubiquity of Ethernet in corporate infrastructures.

The simpler network structures thus created significantly increase plant transparency, and also resulting make engineering and maintenance more user-friendly. At the same time, the breadth of possible applications makes Profinet the most flexible communication system on the market.

Prof. Dr. M. Bruns is Head of Process Automation, N. Heise is Marketing Manager, both with Siemens Automation & Drives, Nuremberg/Germany.

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