Power of convergence16 January 2013

It's not that many years since industry was being pushed from pillar to post over the power of PLCs versus PCs, but all that has changed.

Talk to any of the factory automation system giants and it's clear that the old PLC (programmable logic controller) versus PC – and, for that matter, PLC versus DCS (distributed control system) – debates are just that: old news. Have that conversation with some of the original DCS developers and, although it's not quite the same story, they too will concede that the overlap, in terms of functionality, redundancy, scale, engineering environments and even experience and expertise, is now considerable and growing.

One of the reasons is technology. Massive increases in on-chip processing power and memory mean that systems capable of running multi-axis motion control, with ratioing and interpolation, are, broadly, just as capable of executing control functions, involving complex algorithms. Yes, sequential and continuous control remain very different, but systems are routinely now built with capabilities that span both, also supporting everything from ladder logic to function libraries.

Strategic acquistions
Another factor is business, with the few remaining consolidated firms in the PLC camp having worked hard on strategic acquisitions. Consider Rockwell's purchase of triple-redundant systems developer ICS Triplex and MPC (model predictive controls) firm Pavilion – both advanced process industry specialists and each with considerable systems engineering arms – not to mention Irish system integrator ProsCon.

That said, today, there's also no argument that PC-based control systems are ever likely to supersede PLCs. That 20 year-old prediction has long since been laid to rest: indeed, growth of the latter still far surpasses the former. Nowadays each system type is played to its strengths, with PCs reserved for running multiple applications, number crunching and visualisation, while PLCs and their I/O and subsystems handle the sharp end of automation. In fact, increasing numbers of what should probably be termed PLC systems – 'probably', only because so many include PC chipsets and functionality but in PLC form factor – now look after all aspects of monitoring and control.

Why PLCs? Partly price but primarily because they're based on dedicated and proven operating systems, specifically developed for control purposes. Unlike Microsoft Windows and similar commercial environments, you're not looking at operating systems subject to cycles of change and obsolescence. And that same argument applies to DCSs. As Roel Mulder, Emerson Process Management's director of system migration and modernisation, says: "People in industry don't want to upgrade every five years... They also want stability and redundancy, particularly with major plants like refineries, which cannot shut down for system maintenance. A decade ago, everyone was talking about open systems, but now people want a proper proprietary control environment, with long-term support."

And that's not all: Brian Holliday, divisional director with Siemens Industry Automation, also points to PLC developers' focus on the real worlds of plant and factory automation. "For example, we're constantly growing our portfolio of global and project-specific object libraries that can be shared, re-used and enforced in multi-site organisations. And while those provide the power of high level language environments such as C++ – with structured programming, etc, in our TIA [Totally Integrated Automation] portal – they do so with the discipline you need for real plant."

Returning to the overlap between PLCs and DCSs, it's not just about turf wars – although there is inevitably plenty of that. As Richard Sturt, Rockwell's business development manager for the process industries, puts it: "There's considerable value in a single plant- or factory-wide system handling everything from production line automation to plant control loops. Not least the fact of a single database and one engineering platform."

Hybrid solutions
That's particularly the case for the middle ground of manufacturers, typified by those in the pharmaceuticals or food and beverage industries. Essentially hybrid in nature, their factories generally involve 'wet' batch processing and mixing at the front end, under recipe control, followed by sorting, conveying, and bottling, canning, labelling and packing machines, under fast sequential management, at the other. "That's why we have built everything into one overarching automation system," comments Sturt. "The main issue managers should be challenging today is not which system to run, but the total cost of automation. So many plants and factories still have a mix of PLC and DCS equipment, but that means they're paying twice – working with and maintaining two separate systems."

So far, so good. But what about machine-to-machine integration? Whether operations wants real-time insight into OEE (overall equipment effectiveness), energy consumption and the like, or, at a more basic level, it's just about production line, cell or machine automation involving equipment from multiple vendors, everyone needs reliable communications. And, here, industry pundits are split. All agree that life is much easier than it used to be, but the devil is still somewhat in the detail.

Rockwell, for example, has recently moved to a converged network. It has pulled away from the three-layer approach of DeviceNet at the field level, ControlNet for the controller infrastructure and Ethernet for the enterprise – to plant-wide Ethernet/IP (with Cisco) throughout. Siemens, on the other hand, favours a two-level network comprising Profibus (RS485-based) and all its flavours for the factory, and ProfiNet (on Ethernet) notionally above that. Emerson, meanwhile, favours Foundation Fieldbus and Ethernet/IP (also with Cisco).

Mark Daniels, Rockwell's business manager for architecture and software, suggests that Ethernet/IP now provides for all the speed, security, IS (intrinsically safe), etc, capabilities you would expect – although he concedes it wouldn't be first choice yet for an oil and gas plant. "The big deal here is that these days Ethernet is so well understood. It runs everywhere, even over power, and aspects such as safety, determinism and IS have all been solved. So the reasons for separate networks are gradually disappearing. Over time it will only be about which flavour of Ethernet."

For now, though, you need to be aware of the differences. PLCs mostly come with information hooks, and there are plenty of interfaces and system integrators willing and able to make systems talk. However, experience suggests that, while most of the time you plug in and the connection just works, sometimes it doesn't.

That's why smart negotiators will insist that machine builders deliver equipment that fits their chosen infrastructure – and the same all the way up to prime contractors. As Siemens' business manager for automation Simon Keogh says: "While industry has made a lot of progress, and there is a lot of consolidation around protocols, operations people still need to look at their potential control system suppliers and make sure there aren't any show stoppers."

Don't worry too much: Dean Chittem, engineering director with Wirral-based conveyor manufacturer Sovex (formerly head of engineering at DHL UK and chief engineer at LPC, Europe's largest independent tissue manufacturer), believes standardisation and interface software have seen huge improvements. "This hasn't been an issue for the last couple of years. If you want to bolt on equipment, undertake production line modifications or do some incremental upgrades, it's easy to do and certainly not governed by what you already have on site."

And so, finally, to security – and Siemens' Holliday is good value here. "Automation system manufacturers have been working on this for years, but everyone had to respond to Stuxnet. So control systems that weren't previously considered in the same way as IT systems now are. Latest generation security concepts cover everything from the physical protection layer to user authentication. That's important, because control systems now are inevitably participants in an Ethernet/IP network.

"And remember, it's not just PLCs that are web enabled. Intelligent motor protection devices, variable speed drives and so on also have web servers for ease of remote monitoring. So security is critical."

Brian Tinham

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