Machine-to-machine communication has seen a rapid development in recent years. Brian Tinham examines some of the changing technologies and their implications.
Manufacturing managers struggling with digital machine communications, and hankering for the 'good old days' of 4-20mA or 0-10V wiring, are suffering a serious dose of rose-tinted spectacles. And the same goes for plant technicians. Analogue automation technology was neither more reliable nor more robust. Indeed, calibration errors were common on process plants – sometimes leading to measurement errors of 10-20%, despite far superior instrument capabilities – due to undetected drift. Engineers typically had to fudge the figures to keep processes on track.
Digital fieldbus technology of whatever flavour is quite different, not least because it enables the use of configurable 'intelligent' devices that provide both sensor and equipment data but also instrument health and diagnostics.
So calibration errors are immediately visible and readily rectified, even remotely. Not only that, but costly cable trays carrying hundreds of individual instrument wires are a thing of the past. Why? Because, with the exception of HART (Highway Addressable Remote Transducer, which carries digital diagnostics over existing analogue wiring), all fieldbus types and architectures address multiple sensors per cable.
So why the trips down memory lane? Andy Verwer, co-director of Verwer Training and Consultancy (formerly responsible for the Profibus Competency Centre at Manchester Metropolitan University), explains that fieldbus networks are accurate, reliable and infinitely more capable than their analogue predecessors. However, when there's a problem, it takes technicians with a good grounding in electronic engineering, decent tools and a capacity for logical thinking.
"You don't need degree-level people, but engineers must have a few days' training on high-speed networks. They also need decent waveform visualisation tools, because they have to understand what's going on in the physical infrastructure. Faults can be difficult to trace, because they don't necessarily knock out the nearest device.
In fact, that may be working perfectly, while another farther device has dropped off the bus." And his observation holds true whatever the fieldbus type: Foundation Fieldbus or Profibus PA for the process industries; Profibus DP and CC-Link for factory automation; DeviceNet for machine control; ASI for low-level sensors, and so on.
That said, another reason for the nostalgia is the proliferation of fieldbus types and their pace of development. Since being introduced 25 years ago to enable remote I/O on machines and plant, the big few have been equipped for increasingly sophisticated diagnostics and many now run wireless, too. Also, in the mid 1990s 'profiles' were added for intrinsic safety (in certified hazardous areas), functional safety came in the 2000s and, more recently, redundancy and energy management arrived (to power down the automation systems themselves).
And now there are the increasingly popular, much higher-performance Ethernet equivalents – Ethernet IP (Industrial Protocol), ProfiNet and CC-Link IE – which threaten to bring physical fieldbus kicking and screaming into the mainstream. Many pundits suggest these will quickly subsume the old guard of machine- and plant-specific fieldbuses. Why? Not only because they promise the benefits that economy of scale and global standards bring, but also due to their familiarity, power and openness.
Michael Loughran, solutions architect with Rockwell Automation, is certainly one who sees a convergence on Ethernet. "It's happening now," he asserts. "There's an explosion of Ethernet. And whereas five years ago, it was the automation system vendors pushing the technology, now it's the users." And he adds that, while Rockwell still supports its earlier three-tier automation networks model – DeviceNet, Modbus and Sercos at the fieldbus level for drives, controls and servo-drives; ControlNet for high-availability PLC communications; and Ethernet IP, formerly at the business/office level – the latter is by far the fastest growing.
More advanced technology
"We can do everything on Ethernet IP that five years ago we had to do with one or other of the specialised industrial networks," insists Loughran. "At the time, they were the best available, but technology has moved on. And, as far as we're concerned, we have the advantage that the same Ethernet IP protocol is used throughout – for everything from high-performance machinery operations to safety-related controls, including all the SILs [safety integrity levels]."
That's partly because Rockwell has been doing this stuff for a long time. But it's also because the automation giant gave its work to ODVA (the Open DeviceNet Vendors' Association), which meant it rapidly became 'open source'.
"We manufacture a wide range of automation systems and equipment, but there are plenty of specialists in barcode readers, instrumentation, etc, that also need to connect," comments Loughran. "Also, users are demanding openness in how they consume data from all this equipment – on Apple, Android and PC devices. So it was in ours and the industry's interests to make Ethernet IP universal."
But this is not just about Rockwell. Other very big players with millions of nodes connected include Siemens, with Profibus and latterly ProfiNet, as well as Mitsubishi, with CC-Link and now CC-Link IE – the latter being the first to offer gigabit speed connectivity – through CLPA (the CC-Link Partner Association). Both have similar stories and each sees at least partial convergence in the not too distant future.
As Mark Freeman, Siemens product manager for distributed I/O, puts it: "It's not just about performance: greenfield sites are turning to [high-speed] ProfiNet for its improved scalable redundancy, shared device capabilities and wireless technology, none of which can be done with Profibus. And with our gateway providing a hook into plants' existing Profibus infrastructures, and Profibus PA functionality coming soon, this is going to keep on growing."
John Browett, general manager of CLPA, says much the same, although conceding that CC-Link IE is still way behind its predecessor, in terms of uptake. "In the next five years, CC-Link IE will overtake CC-Link. Users get a Gigabit Ethernet infrastructure at the physical layer, with standard switchers and routers, but on top of that there's a real-time protocol running in the application layer to guarantees determinism."
And he continues: "In the last 12 months we've also added profiles for motion control and safety. This year we'll be launching energy management, so that everything can be monitored and controlled on one wire... In Asia, the automotive sector is moving up to it, but it's also seeing uptake in the electronics industry, for example in flat panel display manufacturing."
Nevertheless, all three Ethernet protagonists agree that, for some, a single network is going to be a longer time coming. In the process industries, particularly, Ethernet of any kind has its limits, primarily because of the requirement to power remote instruments over the loop. Although currently under development, power over Ethernet is not yet here.
Long-term roll out
Even when that functionality is rolled out, Ethernet won't be universal on plant for years. It's not difficult to envisage its adoption on sophisticated instrumentation – such as mass flow devices, where there are multiple parameters to handle. But it's much harder to imagine the same happening to the vast majority of relatively simple pressure, temperature and level sensors and transmitters. And it's not just about investment in devices.
Just as important for many is the perception of problems relating to stability and security. Given industry's conservative nature, the fieldbus classics of Foundation Fieldbus, Profibus, DeviceNet, etc, still hold considerable appeal.
That will change, insists Loughran. "To date, industry has not seen many cyber attacks on infrastructure and automation, primarily because we've enjoyed security by obscurity. However, nowadays everyone actively manages the issue, using approaches that closely match office concepts – with 'defence in depth' and, in our case, security built into the [network] switches."
But the last word goes to Siemens' Freeman: "Convergence will come, because the benefits outweigh any current perceived limitations. As intelligent field devices proliferate with built-in web server technology, the advantages of easy access down to the device and right across the network make it hard to ignore."