Product Information

[20.04.09]

SERCOS III – Real-time Ethernet

SERCOS has become the standard as a digital drive interface for real-time communication in high-performance motion control applications. At the same time Ethernet TCP/IP has established itself as the standard for non-real-time communication on all levels on automation. Peter Harlow, Industry Manager for Machine Tools at Bosch Rexroth, takes a look at the combination of both these technologies – SERCOS III.

Factory automation has come a long way in terms of communication - twenty years ago all that was required was for simple field buses to transfer measurement values reliably and activate modules with precision. The more multi-layered the tasks, such as coordinating several drives (motion control), the greater the requirements became, and not every field bus was able to handle them effectively.

Today production systems demand networks which are also capable of enabling data to be exchanged between different processes and machines – hence the migration to the TCP/IP protocol, which enables computers, control systems and machines to exchange information both internally and externally. It eliminates any number of barriers in the corporate environment, making interaction between a wide range of applications feasible.

However, Ethernet in accordance with IEEE Standard 802.3 is not suitable for horizontal communication in production because it allows a single device to block the network traffic even when another device needs to send an urgent message. In the finely-tuned process typical of an extruding machine, any such delay in a signal to a valve would produce scrap parts. In contrast, Industrial Ethernet (IE) is a plant, production, process and control focused technology targeted specifically to the industrial environment, for automation and production machine control. It offers real-time communication so guaranteeing that every device in the network can receive and send data at any time.

At the same time, however, not all real-time communication IE technologies are alike. There are numerous solutions available that offer the promise of real-time compatible Ethernet – in a number of cases this is with enormous differences. With the introduction of SERCOS III in 2003, formally standardised by IEC, the best of both worlds is brought together – it integrates the proven real-time mechanisms of the SErial Realtime COmmunication System (SERCOS) interface with the Ethernet TCP/IP standard for non real-time communication.

SERCOS has been used in the industry for almost 20 years, providing real-time communications between controllers, drives and distributed peripherals. It has established itself as one of the world’s leading digital interfaces, with more than 80 different manufacturers offering SERCOS-enabled products. As an active member of the SERCOS International organisation, Rexroth has worked closely with its customers and partners to implement SERCOS III across its platform of drive and control solutions.

At the field level – which is where communication between individual drives, sensors and controllers takes place – SERCOS III real-time functions guarantee the required level of precision. Connectivity to higher level control systems is extremely simple with direct access to data at the lowest production level, and every Ethernet-enabled computer can be connected to a SERCOS III port without any additional hardware or software.

Thanks to the full-duplex mode, every node can call on the full range of 100 MBit per second in real-time mode. Full TCP/IP consistency is guaranteed at the same time because non-real-time data packets are forwarded via a special NRT channel (non-real-time), without compromising the guaranteed cycle times of the real-time data. This allows parameterization to be undertaken, for example, without the control system running using a standard notebook and Ethernet interface.

Third-generation SERCOS marks the introduction of Ethernet into the fieldbus world, and at data rates of 100 Mbit/s, it sets new speed benchmarks. Other new features include the use of CAT 5 copper cable in addition to the fibre optic technology which is already in use in previous generations. This alleviates the apprehension engineers have of handling fibre optic cable in the industrial environment, where termination and replacement are delicate processes. Highly efficient networks can be installed using ring or linear topologies and peer-to-peer communications.

At the same time, the cycle time of 31.25 μs in real-time mode does not mean that one individual module would take up the complete range for itself alone. Instead, it is possible for up to eight drives in Motion Control applications to be supplied with 8 bytes of cyclical data and for them to send the same number of diagnoses as well. This is sufficient for even the most demanding tasks – even high-precision CNC machines currently “only” have minimal cycle times of 1,000 μs. Thanks to the greater efficiency of SERCOS technology it will not be long before there will be no applications which would make it necessary to have a higher network speed. Moreover, Fast Ethernet also offers sufficient reserves for TCP/IP communication in production – especially because SERCOS is very efficient in the use of resources.

As a general rule individual components (slaves) are controlled by a controller (master). This results in the typical linear bus structure of a section of production. In turn, the individual controllers communicate with each other via a joint network, or gateway, segment. However, sometimes it is more efficient for one component to inform another one directly. SERCOS III permits cross-communication between masters and/or slaves. This makes it possible for data to be exchanged directly between a sensor and a drive which are controlled by separate controllers. This reduces the load on the central control system and also cuts data flows in the network. This so-called C2C (control-to-control) cross-communication between masters, such as two PLCs, for example, provides the basis for the distributed control of complex production lines. This flexibility in communication, which leads to reduced reaction times between master or slave devices and, therefore, in the overall process, permits the synchronous actuation of axes at any time, even where several SERCOS networks are involved.

SERCOS III delivers a level of performance which more than meets the requirements of today’s production machines to master complex automation tasks. For example, it can allow up to 256 axes on one ring. This particular feature has proved invaluable in the printing and converting industry – new printing press equipment, which typically require between 50 and 60 axes, now place the PLC I/O on the same ring saving the need for an extra Profibus/Interbus network.

The technology which makes cross-communication between the individual nodes possible not only contributes to the efficiency and flexibility of SERCOS solutions, it also increases safety because it enables a SERCOS III network to be organized on the basis of a ring structure. In the event of a cable failure, therefore, a redundant signal route is available. This is not the case in the classical Ethernet which uses switches and routers for coordinating data flows. The SERCOS network, in contrast, coordinates itself and offers flexible strategies: a classical linear structure in order to save materials or a redundant ring structure for greater safety. The engineers have the choice of selecting the most suitable cabling to meet requirements without having to think about additional elements for the network infrastructure.

The same applies to the safety of data transmission. To ensure that information is transferred safely, SERCOS offers a certified security protocol – called SERCOS safety – that complies with the requirements of the safety standard IEC 61508 up to Safety Integrity Level 3 (SIL 3). This covers risks arising out of or in connection with system failures and which may pose a risk to the health of employees, to the environment, etc. For example, a sensor used for detecting a dangerous axis movement or an overheated drive is also supposed to ensure that the machine is made safe in a controlled manner and also switched off. Previously this required separate cables. SERCOS III allows all safety-related information to be transferred via the existing data cables so that in the event of an emergency it can be guaranteed that the power supply will be shut off if the emergency stop button is actuated. The elimination of additional hardware reduces costs without any compromise on safety.

SERCOS safety is also protected against potential errors such as repeats, losses, insertions, incorrect sequences, corruption, delays and the confusion of secure and standard data. The security protocol is certified in accordance with IEC 61508 and was also tested by the German TÜV in respect of safety requirements. To ensure that data is transmitted securely SERCOS safety uses the CIP safety protocol of the ODVA, the organization that supports network technologies built on the Common Industrial Protocol. It is used by a range of communication standards such as DeviceNet, ControlNet and Ethernet/IP and allows engineers to use the same safety mechanisms on different platforms. This makes it possible to connect several CIP-based networks together on a consistent basis.

SERCOS has now definitely made the transition from a specific drive interface to a universal real time Ethernet solution. However, it must be recognized that in addition to SERCOS III, there are a number of other real-time Industrial Ethernet solutions available in the market – including PROFINET RT and EtherNet/IP – that have gained international support. Although these individual solutions all use TCP/IP as a standardised protocol, this does not mean that identical connections and control mechanisms are used. There are significant differences, in particular with regard to unrestricted real-time capabilities, transfer rate and network administration. This results in a dilemma for controls engineers.

Currently a unified standard is not in sight and many systems and components available on the market today cater for one single solution. Developers may therefore have to decide between the best Industrial Ethernet concept and the best components available for solving a task, or the engineers may be constrained to create costly gateways between the different protocols – neither of which is ideal.

Industry cannot afford to become dependent on the technology and strategy of any single manufacturer; Rexroth is committed to an open connectivity platform (OPC) that is based on universal, powerful, uncomplicated and above all non-proprietary systems. With this in mind, Rexroth can offer a way out of this dilemma – instead of just extending the power of SERCOS III, Rexroth has integrated multi-protocol capable connections in its components and systems.

Rexroth’s industrial automation solutions offers not just the classic field buses, such as PROFIBUS and DeviceNet, but also the master and slave interfaces for SERCOS IIII, PROFINET RT and EtherNet/IP. This means that Rexroth’s embedded control platform IndraControl can be integrated directly into most systems worldwide with multi-protocol-capable interface connections reducing development effort and simplifying the system design.

Furthermore, the IndraDrive family has slave interface connections for further Ethernet-based derivative and field buses. They allow addressing the high-precision servo drives via SERCOS III, PROFINET RT, EtherNet/IP and EtherCAT. IndraDrive supports nine communication interfaces in total, fitting seamlessly into the system world preferred by the user, so that developers are no longer forced to decide between network and components – they can simply select the best modules for their application.