Fieldbus systems have been around for 15 years or so and are hardly questioned any more: most discrete manufacturing and process control applications make use of this technology.
Abstract
Now a new generation of Ethernet-based industrial communication systems is emerging. The paper introduces a realtime Ethernet technology named EtherCAT, which overcomes the performance and length limitations of classical fieldbus systems while undercutting their cost levels. At the same time, Internet technologies such as web services are made available to field level devices.
EtherCAT employs a unique functional principle called ‘Ethernet on the fly’. This results in high communication performance since it overcomes the traditional requirement for one frame per node per cycle. Typical EtherCAT cycle times are in the 50…500 μs range and typical network-wide jitter values are smaller than ±30 ns.
The system allows users to combine line, tree and star topologies and works with and without switches, leading to significantly reduced infrastructure costs. A network segment can contain up to 65 535 devices and span several kilometres. Thus the technology is well suited for widely distributed applications as commonly found in the mining and process industries.
The technology introduced in the paper is an IEC and ISO standard as well as a SEMI standard. EtherCAT is supported by the EtherCAT Technology Group (ETG), which has more than 770 member companies from 42 countries.
Introduction
Fieldbuses have become an integrated component of automation technology. They have been tried and tested and are now widely established. It was fieldbus technology that enabled the wide-scale application of PC-based control systems. While the performance of controller CPUs – particularly for IPCs – is increasing rapidly, conventional fieldbus systems tend to represent 'bottlenecks' that limit the performance control systems can achieve. An additional factor is the layered control architecture consisting of several subordinate (usually cyclic) systems: the actual control task, the fieldbus system and perhaps local expansion buses within the I/O system or simply the local firmware cycle in the peripheral device. Reaction times are typically 3–5 times higher than the controller cycle time – an unsatisfactory solution.
Above the fieldbus system level, ie, for networking controllers, Ethernet has already been the state-of-the-art for some time. What is relatively new is its application at the drive or I/O level, ie, in areas that were dominated by fieldbus systems in the past. The main requirements for this type of application are high realtime capability, suitability for small data quantities, and naturally cost-effectiveness.
EtherCAT meets these requirements and at the same time makes Internet technologies available at the I/O level.
With EtherCAT the controller can update the input and/or output information at the time when the data is needed. The I/O response time contains all hardware relevant delays (IPC, EtherCAT and I/O system) from the physical input signal to the physical reaction on the output.
With a time of ≤100 μs this offers a performance to the PLC level that has been available up to now only within servo drives with a digital signal processor (DSP), (see Figure. 1).
Figure 1. EtherCAT: standard frames according to IEEE 802.3
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