Which Ethernet system is the right one?
24 February 2009
Ever more complex drive tasks are forcing a switchover to industrial Ethernet buses. But, asks motor and drive manufacturer, Lenze, which system will come out on top?
The theoretical answer is still being hotly debated, but in the end it will be practical situations and actual tasks which determine which industrial Ethernet bus is used.
Fieldbuses quickly reach the limits of their performance, especially when confronted with complex applications. With a maximum bandwidth of 1Mbit/s and a maximum line length of 25 metres between different devices, the technology already seems old-fashioned. Extensive plants with lots of network nodes and large amounts of data can no longer be set up economically using conventional fieldbuses. Although there are currently 26 industrial Ethernet solutions, three of them have asserted themselves and are to be found in the majority of applications. These three are Profinet, Powerlink and EtherCAT, and in Europe they have good prospects of complementing fieldbuses, and in the long term replacing them.
Profinet for simple drive tasks
All three Ethernet systems can be used in machines and systems in which a central processor controls several drives that perform different movements from one another. Neither a particularly large bandwidth nor real-time are required. The communication system is more likely to be based on the solution which the manufacturer of the controller prefers. That is why it will probably be the Siemens Profinet IO which comes out on top in the market for simple drive tasks in which a number of drives receive commands from a PLC. This system performs around as fast as Profibus. It is a configuration that is used especially in large plants (eg car production) in which a plant PLC controls numerous conveyor drives. ProfiSafe can be used to transmit additional safety information to the drives as well.
Powerlink for synchronised drives
The synchronisation of numerous axes is much more demanding. This form of application is best tackled with a decentralised architecture in which an intelligent drive acts as a master and generates control movements. The position of the master must be reliably transmitted, in genuine real-time, to the drives working in slave mode, and the publisher-subscriber concept is especially suitable for the kind of cross-communication this requires. This principle, familiar from CAN, also uses the Powerlink industrial Ethernet bus. It offers a bandwidth of 100Mbit/s and allows lines of up to 100m between two network nodes. Vertical communication is facilitated using asynchronous communication channels. What is especially nice for CANopen users is that Powerlink supports all of the CANopen device profiles, so that migrating towards Ethernet is a simple affair. Nor does the bus require any special hardware, it uses standard Ethernet controllers. Powerlink’s performance is highly dependent on the processors used and the memory with which the automation components are equipped.
EtherCAT for co-ordinated spatial movements
Three-dimensional movements are best dealt with by means of a central control architecture in which a powerful controller calculates and executes, in real time, the paths for all of the axes, which must be co-ordinated exactly. This is the domain of EtherCAT combined with powerful industrial PCs. Special ASICs process data quickly and keep node costs down. Users may have to be patient a while, for instance, their Gigabit Ethernet is adapted and a new ASIC version is ready, but EtherCAT interfaces have the advantage that the resources – those of the slave system, for example – are burdened much less than with a pure software solution. Because of the summation frame method it uses, EtherCAT transmits data particularly efficiently. With its minimal overhead, the entire bandwidth is used even for small amounts of data. It is also possible to modify data while running, so that the hold-ups are only short. EtherCAT is also suitable for numerous different topologies and combinations thereof, which makes many things easier when wiring complex plants. And, what is especially important for motion control applications is that EtherCAT also handles the CANopen protocol, which means that making a switch is not difficult.
Ethernet technology continues its rise to power, bringing things like greater TCP/IP transparency, boosted vertical integration, easier access to new technologies such as web and wireless systems – and being appreciably more efficient to operate than fieldbuses. That is why the industrial Ethernet buses Profinet, Powerlink and EtherCAT form the basis for many new automation concepts.
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