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SERCOS-III combines real-time and EthernetAt the Hanover Fair in 2003, the so-called Interests Group SERCOS (IGS) announced a new project involving linking the existing high-performance SERCOS interface and the Ethernet standard to form a new generation of SERCOS, named SERCOS-III. Work has proceeded as scheduled so, as promised, a progress report was issued at the SPS/IPC/Drives 2003 show in Nürnberg, Germany In the eighties, the German ZVEI and VDW organisations initiated a consortium to specify a digital open interface that would ease the transition from analogue to digital drive technology. Thus, the SERCOS interface was born. The first generation SERCOS interface supported transfer rates of 2 and 4 Mbit/s and was mainly used for advanced machine tool applications. In following years, the interface was widely accepted worldwide in many different industries, and in 1995 the SERCOS interface was approved as IEC standard 61491. The second generation followed in 1999. The transfer rate was increased to 8 and 16 Mbit/s and the service channel for the transfer of non-synchronous data was expanded. This technology has been available since 2001 in the SERCOS816 ASIC, with downward compatibility to the first generation. The collision-free data transmission based on a time slot mechanism, together with the highly efficient communications protocol of the SERCOS interface, ensures very high performance and the best possible determinism. Up to 40 axes can be synchronized with a cycle time of 1ms and a jitter less than 1µs.k. Superior performance With this performance, IGS sees the SERCOS interface clearly ahead of other standardised interfaces like Profibus or CAN and claims that not even the solutions are based on Fast-Ethernet with data rates of 100 Mbit/s show a better performance. For these reasons, it says, the SERCOS interface has become a de-facto standard in various industries, especially for multi-axis applications. The SERCOS interface was originally intended to be a drive interface, but now has become a universal motion control interface. The interface has revolutionised whole areas of machine building, the shaftless printing machine with its more than 100 axes synchronised via the SERCOS interface, packaging machines or multi-axis machine tools. Whilst most of the drive interfaces on the market are proprietary and can only be used with the manufacturer's own drive, more than 50 control manufacturers and 30 drive manufacturers offer SERCOS interface products and it has status as International Standard IEC/EN 61491 - any manufacturer can implement the SERCOS interface without having to pay license or membership fees. In SERCOS-III, the idea is explored of combining the proven mechanisms and properties of the SERCOS interface with the Ethernet, thus creating an advanced version of the most successful open motion control communication interface. SERCOS-III is based on the established real-time mechanisms of the original SERCOS interface and continues to work on the principle of cyclic data transfer with an exact time pattern. IGS maintains that a hardware-based synchronisation is the indispensable prerequisite for the reliable implementation of motion applications. SERCOS-III will have a ring structure like the current generation SERCOS interface. Conditional on the Ethernet physics, however, it is not a single, but a double ring structure (Fig. 1). This double ring structure offers the possibility of redundant data transfer. This is an enhancement to the second generation SERCOS. With SERCOS-III, in case of a break at any point in the ring, the communication continues. The plant works on while the integrated diagnostics tool signals the break, which can be repaired without interfering with the plants' performance. In addition to the ring structure, a linear structure is also possible. The linear structure doesn't offer the redundancy advantage, but it does save a wire connection. SERCOS-III does not use the star topology of the standard Ethernet. There are no hubs or switches needed. SERCOS-III has been defined such that any standard IP telegram (eg TCP/IP) can be transmitted in a non-real-time time slot, in parallel to the real-time transfer of the data required for motion control. The SERCOS-III controller is able to exchange these telegrams with superordinate devices in the network. Thus, SERCOS-III combines the established real-time mechanisms and diagnostic capabilities of the SERCOS interface with universal communication based on Ethernet (Fig 2) Direct communication between slaves is currently not possible with the SERCOS interface. However, this feature would be advantageous in some motion control applications. The Ethernet physics enable such a data transfer from slave to slave, and the new SERCOS-III will support this feature. The SERCOS interface has a service channel, which can be used for the transfer of communication data as well as parameter or diagnostic data. To keep SERCOS-III downward compatible, it too will have the service channel. An additional optional IP channel can be added, with which it is possible to transfer real time or non-real time data by standard Ethernet frames. The cyclic channel and the IP channel are configurable. SERCOS-III halves the minimum cyclic time of the current SERCOS interface from 62.5µs down to 31.25µs. Because of the greater bandwidth of the Ethernet physics, it is still possible to connect an adequate number of slaves, despite the short cycle time. Thus, it is possible to implement not only decentralised drive concepts but also concepts with centralised signal processing. With a decentralised drive concept, all control loops are closed in the drive. With centralised concepts, only the current loop is closed in the drive, whereas all other loops of several axes are implemented in the central control electronics. SERCOS-III is the only open motion control system that supports these structures. Modular concepts The trend towards modularisation requires motion control concepts that offer the possibility of synchronising several machine modules (Fig. 3). Some proprietary solutions, based on SERCOS interface physics, already exist. The aim of SERCOS-III is to standardise this type of communication. It is planned to define a profile for the synchronisation and communication between motion controls. Safety functions will be part of the future standard in motion controls and drives. Safe data transfer will be supported from the very beginning; safe and non-safe data will be transferred with the same mechanisms. SERCOS-III performance SERCOS-III will have a substantial increase in transmission speed. Some typical values are listed in Table 1. Figure 4 illustrates the types of communication needed by the automation industry, classified into four real time types. The figure shows the coverage provided by the ODVA, PNO and IGS solutions. The standard Ethernet communication is also included, if the IP channel is used without the cyclic real-time channel. When will SERCOS-III be available? The basic concept has been defined and approved by the SERCOS member organisations in Europe, North America and Japan. Detailed specifications are being developed by a technical working group, which includes various member companies. Since SERCOS-III is based on proven and tested profiles, work on the specifications will mainly concentrate on the hardware and additional features. First prototypes of the SERCOS-III hardware will be available in 2004, with first products envisioned for 2005.
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