PC versus PLC controls 08 June 2012

PC-based automation, in various forms, has been around for decades, but PLCs and related systems remain as popular as ever. Brian Tinham examines the differences and establishes which approach makes most sense where

It's probably fair to say that the vast majority of plant managers neither know nor care much about the mud-slinging that was de rigueur around a decade ago, concerning computer-based automation. Back then, and for many years previously, debates raged over whether it was better to use PC- or PLC-based control – and, in extremis, whether one signalled the demise of the other. Fact is, it was fairly arcane stuff even then, since both had already become quite capable of dipping into each other's 'territory' and doing the job tolerably well. However, for some the legacy lives on, so it's worth just getting some context.

In a nutshell, PLCs were initially developed to automate factory equipment and lines, using electronic logic to replace earlier relays, cam timers, drum sequencers and hard-wired control panels. So, suffice to say they were masters of managing discrete and sequential (discontinuous) logic, with all-important flexibility provided by their programmability, processor ranges, function libraries and input/output capabilities. Before long, they could manage everything from simple, single-controller machines to complete multi-product production lines, using networks of PLCs. Development since has centred on miniaturisation and extending intelligence.

Meanwhile, PC-based control grew out of test and QA labs, as well as 'low-end' process plants and the utilities – the latter admittedly mostly SCADA (supervisory control and data acquisition), often with PLC front-ends. Most important, however, those test and process roots fostered a focus on continuous control of analogue (non discrete) parameters, such as temperature, pressure and flow.

Indeed, for a time ruggedised PCs were the low-cost pretenders to the throne hitherto dominated by the mighty DCS (distributed control system) giants that had made their names, and their fortunes, in the high-value petrochemical, and oil and gas industries. They didn't offer anything like the scope of their bigger brothers, but the provenance was clear – with more than a nod to the pneumatics, analogue electronics and subsequently digital loop controllers that had gone before.

So, went the argument, why would you use PC-based controls to automate factory production lines? They simply weren't fit for purpose. And vice versa: why PLCs on a process plant? That infallible logic started to crumble when plants and factories alike – particularly those with hybrid production lines involving, say, mixing and/or cooking, followed by high-speed filling and packing machines – saw the potential for cost cutting inherent in single source automation.

It was further eroded when automation system vendors on each side of the divide saw the opportunity and not only added appropriate functionality, but also started acquiring one another. And it collapsed almost entirely when virtually everything, from the chipsets to the programming languages and even the HMIs, became practically (but not quite) indistinguishable.

As a result, the old dogma only now stands up at the extremes of, say, managing a petrochemical plant at one end of the spectrum (where PC-based DCSs would routinely be applied) and OEM automation for a modest machine, at the other (PLC).

Well almost: three more points remain. First, it's worth noting that PLCs are also available in very small automation increments, as micro- and even nano-PLCs – almost intelligent relays.
Secondly, this is a mature market, so suppliers such as Siemens have long since also offered automation products that amount to a halfway house between PLCs and PCs. They are what we used to term 'soft controllers', effectively Windows-based PCs in a PLC form factor. And thirdly, some of the suppliers have also unified the traditionally diverse engineering and training environments for the PC, PLC and event DCS camps – effectively deferring the platform deployment decision right down to the application.

So, clearly, best advice today is that selection of one approach over the other is a 'horses for courses' issue, where course criteria include not only the application, but also the skill sets available, existing systems, spares inventory and comfort zone of the plant, factory or business concerned. Put that another way, and it's the scope and detail of the intelligence required, whether it is mission critical, the preferred language and where best to house it.

Horses for courses
As Paul Herron, business development manager for PC-based automation with Siemens, puts it: "It's not one versus the other. There's a set of automation solutions required, and PCs cover some of them better, and PLCs others. For many applications, though, either could be fine." For him, it's about understanding the implications of the underlying technology. PCs, he observes, have been the beneficiaries of solid state devices, for example. So, given the right components and protective packaging, their reliability can match that of PLCs, even in harsh industrial environments.

"At an application level, then, if you want to run open technologies and Microsoft software with highly performant parallel tasks running on the same machine [such as control and visualisation] then a PC-based system may well be the obvious choice." Herron gives the example of a cigarette packaging machinery manufacturer, where the sheer speed and sophistication of its latest equipment effectively ruled out a PLC and HMI approach. "We looked at a PC-based solution and helped them to make a tenfold performance improvement. That was achieved primarily because of the multi-core processors power available."

That said, PLCs, he confirms, still score in terms of their fitness for purpose for factories. "They start-up without any intervention, provide focused diagnostics, are designed for plug-in maintenance, and are configured using familiar languages, such as ladder logic," he explains. "That's why, for a lot of factories, the engineering team favours PLCs. Quite simply, the IT skills may not be there at the shopfloor level to support PCs."

And there are other persuasive arguments for PLCs. As Mark Daniels, Rockwell Automation business leader for systems architecture and software, points out, equipment longevity and security are still better served by PLCs. "Most PLC manufacturers, like ourselves, have stuck to traditional processor platforms, with our own chipsets, so when customers ask us for support terms that may extend up to 20 years, we're in a better position to confirm acceptance," he asserts.

It's a similar story with security: closed operating systems, typically employed to run PLCs, still attract far fewer viruses than their PC counterparts. "Stuxnet [the first worm to attack industrial systems] heightened our awareness and the PLC industry has been upping its game in security ever since. But, for the most part, with PLC operating systems it's a matter of closing the doors that have been left open. We won't need to install antivirus software any time soon," states Daniels.

Enough. What about real users' choices and experience? Take super yacht builder, Sunseeker International, which specified a monitoring and control system using just two AC500 PLCs, from ABB, for its new luxury vessel, launched at this year's London Boat Show. Its systems are used to monitor and control on-board systems and equipment, such as bilge pumps, windscreen wipers, lighting and air conditioning – with data displayed on a 15in colour touch screen integrated into a glass panel at the helm and a separate panel in the crew quarters.

The PLCs also monitor all alarm and status signals on the yacht, ranging from water level switches in the bilges to exhaust temperature alarms or pressure monitoring of fire systems. They either monitor signals directly or, where required, have been programmed with delays to avoid nuisance alarms. And they run logic functions around conditional alarms, while also providing automatic monitoring and control of the yacht's on-board power management system.

Why PLCs? "Features can be provided that would not have been practical without a PLC control system," explains Haydn Harper, electrical design engineer for Sunseeker. He gives examples ranging from hugely reduced cable runs, with flexibility over additional I/O, to the applicability of modern momentary action switches, instead of conventional hard-wired rocker switches, to manage equipment control from multiple control locations.

As for why ABB, Harper says simply that the range of CPUs, communications ports and I/O makes its PLCs extremely versatile and good value. "The system is very easy to expand. Additional I/O cards can be simply added to the system. We have had projects in the past where a specific boat needed more I/O and we were able to easily add remote IO to the project to provide the extra signals."

And it's a similar story with communications. The engine room PLC, for example, which is used to control all local equipment, has a Modbus data exchange link to the main PLC at the helm that, in turn, controls equipment in the forward half of the boat and interfaces to the helm console. Meanwhile, the same PLC also communicates with the power management system via RS485, gathering voltage, current and frequency data. There is also a data link to the NMEA2000 communication network used to provide data, such as tank levels and dc voltages and current. And the two touch screens are also linked to both PLCs via Modbus TCP so that control of equipment and data can be displayed simultaneously.

A good choice, then. Elsewhere, systems integrator Clint Johnson, of Control Freaks, went for Mitsubishi Electric PLCs when he took on the project to automate a machine for making biscuit crumbs, breakfast cereals and snack foods. "The basic process is to mix ingredients into uniform pellets and dry them," explains Johnson. "Sounds easy enough, but the devil was in the detail."

That detail includes two horizontal spiral blade mixers running backwards and forwards at different speeds to achieve a mix. "There is quite an art to the speed profiles we use," he says. "So we stored these as recipes on a Mitsubishi FX3U PLC, which controls a number of 15kW E700 and D700 variable speed drives on the mixer axes, over an RS485 network."

Other controls selected included a Mitsubishi WS safety controller, which defines a multi-level control network, and an E1163 HMI, linked via Ethernet to provide diagnostics. PLC I/O feedback loops read the current in the motors to gauge the mix consistency. The finished mix then flows through a series of hoppers and graters to form the desired pellet size. From there, a dosing mechanism drops the pellets onto a conveyor and a series of levelling systems before it moves into a dryer to form the final product.
"This was quite a complex job in itself, but there were two more related issues we had to address," recalls Johnson. "The first was machine safety: operators would regularly have to access the machine for product changeovers, cleaning and maintenance. The second was that the machine had to be built as several separate subsystems, then connected without compromising operator safety."

His solution was to define several discrete safety zones within the machine and connect them with a control system that could determine whether to stop a single zone, several zones or the whole machine. "We ended up with two-hand controls, multiple emergency stop zones, interlocks on the access doors and trapped key mechanisms for assembling the various sections together," explains Johnson. "In all, there are about 30 safety switches across the machine that all needed to be interconnected into one coherent system."

Another sound choice. Then again, it was also a Mitsubishi PLC that was used to develop an eight-head filling machine, aimed at improving the filling of catering sized bottles with flavouring syrups for coffee, ice cream etc at Kerry Flavours & Ingredients. Colin Jones, of SP Filling in Llandrindod Wells in Mid-Wales, makes the point that spills and drips can lead to product wastage, spoiled packaging and hygiene issues – and hence the need for a machine capable of getting syrup into the bottles and capping them securely in one smooth operation. He also says that, while performance was key, budgets were tight, and the system had to be flexible enough to cope with future changes.

"From the start we were committed to using stainless steel flexible impeller pumps and an eight-head filling geometry," explains Jones. "My initial thought was that we should use stepper motors because they were cheap. However, the Mitsubishi specialists suggested we look at the company's MRE servo motor, which met the cost criteria and whose closed-loop configuration would lead to guaranteed long-term accuracy and easy reprogramming."

Jones says he was surprised at Mitsubishi's suggestion for the control system. He had assumed a small PLC for each pump/motor, plus a larger supervisory unit. However, the firm recommended a single Q-series PLC. "The entry level Q-00J is a low cost but powerful machine controller that can be customised using specialised output cards," he confirms, adding that for the syrup filler, the system needed just one output card. That was an eight-channel pulse chain driver to control all the filling axes, which, he says, needed hardly any systems configuration and commissioning effort.

Jones says that, having been on site and returned with a technical specification that was "dauntingly long and detailed", he was delighted to find that his anticipated problems had already been addressed. "[Mitsubishi's] engineers had written a program that allowed the machine operators to simply select the syrup type and enter details, such as bottle size, production run and timings, and everything was set automatically for optimum performance," he states.

"The best thing was that we had saved so much money on the drives and controls that we could afford to fit a Mitsubishi GOT flat screen HMI, which makes setting and operating the machine as simple as possible."

Brian Tinham

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