Focusing in on bus technology

22 November 2019

The communication network is the single most important technological component of a machine or plant control system. Martin Rostan offers some food for thought about how the fieldbus choice is made.

The network will ultimately determine the entire control architecture – a relatively slow network requires closing the faster control loops in decentralised devices, while a high-performance network enables engineers to freely decide whether to use centralised versus decentralised architectures. In addition, a well-accepted network provides the freedom to select components from a large variety of vendors while a network dominated by a single automation giant will often reduce the choice. 

So how do control system integrators or machine builders select their communication network? I do not believe that the majority of users actively select their fieldbus at all – they select the control system vendor and assume that whatever bus system this vendor provides will ‘do the job’.

But how about those users that start their control system evaluation by taking a closer look at the bus technology itself as the key component of their system? Many of those users that compare technologies in detail find the functional principle of EtherCAT compelling. With EtherCAT, the Ethernet packet is no longer received then interpreted with process data copied at every device. Instead, field devices read the data addressed to them while the frame passes through the node. Similarly, input data is inserted while the datagram passes through. With this ‘processing on-the-fly’ technique the frames are hardly delayed at all. As a result, an EtherCAT frame comprises the data of many devices both in ‘send’ and ‘receive’ direction within one Ethernet frame and the usable data rate increases to over 90%. 

Husky, an injection moulding machine manufacturer, reported that by using EtherCAT it was possible to reduce part weight variation which helped reduce the wall thickness of plastic cups produced on the machines. This has resulted in material cost savings of over $180,000 USD per machine, per year for Husky machine users. 

It is not just closed-loop controls that benefit from faster communications: in many applications there are so called ‘transitions’, where one has to wait for the part to arrive, the cylinder to reach its end position, or the pressure to reach a certain threshold. With a faster network, the waiting times are reduced and the throughput of the machine or plant is increased – increasing efficiency a few percentage points can be easily achieved this way. 

EtherCAT is not only fast, it is also precise: thanks to the distributed clock mechanism, measurement values can be sampled and outputs set in a synchronised manner network wide – and with a jitter smaller than one microsecond. This accuracy makes it well suited for use in synchronised motion control applications and for the integration of measurement tasks within the same network.

Increasingly, system integrators and machine builders want to integrate functional safety in their control architecture – and not just to reduce wiring. Conventionally, safety functions have been realised separately from the automation network – either via hardware or using dedicated safety bus systems. Safety over EtherCAT, however, enables safety-related communication and control communication on the same network.

Flexible and open
EtherCAT also offers flexible topologies that can simplify planning and installation. EtherCAT networks have no practical limitations regarding the topology – line, star, tree, redundant ring and all those combined with a practically unlimited number of nodes per segment. Even wireless technologies can be integrated and the ‘hot connect’ feature which allows users to connect and disconnect nodes or entire network segments during runtime. 

To address issues of migration from a legacy bus system a large selection of EtherCAT fieldbus gateways are available to help integrate existing devices with a legacy fieldbus connection into an EtherCAT network and establish interfaces to ‘neighbouring’ or higher-level systems. 

Martin Rostan is executive director at EtherCAT Technology Group.

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