PC or PLC based control – take your pick

26 March 2013

Automation suppliers may never agree on whether PC or PLC-based control offers the best solution – in reality it really depends on the application and user preference. However, with most PLC-based systems now offering the same ‘openness’ as PCs, they look like they will be around for some time.

Speaking last year to CEE’s parent magazine, Control Engineering, Hans Beckhoff, managing director and owner of Beckhoff Automation explained his belief that customers today are looking for ways to implement their own processing know-how, right down to the motor algorithms as opposed to a closed architecture solution. “Customers need platforms to implement their own know how,” he said.

Beckhoff Automation's platform offers PLC capabilities, HMI, motion, safety, input/output (I/O) and other functionality such as measurement, condition monitoring and robotics within one PC-based programming environment. The latest of these software platforms is TwinCAT 3, which integrates Microsoft-based tools such as Visual Studio, as well as programming using C++ and MathWorks Matlab/Simulink, for example.

Beckhoff Automation systems are software based and promote open control architectures. Hans Beckhoff went on to suggest that adding more hardware controllers (more PLCs) creates more points of failure, more communication hassles, more cost, and often more varied software to learn. “Traditional thinking can create a tangle of networks, many software platforms, layers of complexity, complex licensing requirements, and programming differences. With an industrial PC (IPC) platform, less is more,” he said.

Many suppliers of PLC-based control solutions would argue that today’s PLCs are equally as ‘open’ as PC-based systems. Siemens, for example, offers both PC-based and PLC-based solutions which puts it in a good position to offer comment on both technologies.

Heinz Eisenbeiss, head of Simatic Marketing at Siemens Division Industry Automation in Nuremberg, Germany explained that Industrial Control Systems (ICS) today are being used in an every wider variety of application areas – from building automation and infrastructure, micro-automation for simple machines, series and special machinery to plant control as well as process automation with DCS systems.

Eisenbeiss believes that very different demands are being placed on systems today, ranging from the cutting of price and performance in the lower segments, to additional flexibility in the midrange region. In high-end applications, key considerations will always be system availability and ease of maintenance over the life cycle of the plant. Eisenbeiss said: “PC-based control systems are gaining ground, especially in machine building applications, although this, and other fields of application, do still require dedicated PLCs. Their performance, reliability and modularity, means that there is still a large installed PLC base across most industries, and this continues to grow.

Siemens can offer both industrial PCs and PLCs so users are able to select the most appropriate ICS technology for a particular application – both by functional as well as by cost considerations. However, Eisenbeiss highlights an increasingly crucial consideration for the user, that of the automation engineering for ICS – the engineering software. “Siemens can offer its ‘Totally Integrated Automation Portal’, which ensures that the same user interface can be used across engineering disciplines and compatibility across the company’s ICS portfolio. “In the long term, this provides the user with investment security,” said Eisenbeiss.

Concentrating on connectivity
Commenting further on PLC-based systems, André Hartkopf, product manager PLC/HMI/Software at Mitsubishi Electric Europe B.V., Factory Automation European Business Group, said: “PLC based systems have been established for many years, but continuous development has meant that they still providing industry leading functionality, connectivity, security and ease of use. One area that Mitsubishi Electric has focused on is achieving easy connectivity to high level systems such as direct access to SAP BAPI's, or deep integration into databases such as IBMs DB2 or open systems like MySQL – all with simple parameterisation – no programming is needed, and all with less cyber security risks.”

He continued: “Users are able to program their PLC in C, or even in an open source language such as LUA if required. Even Codesys can be run in a traditional PLC environment today. Taken as a whole, the PLC is every bit as open, flexible and relevant to today’s automation environment as a PC-based alternative, in fact, in some instances, it may even be more flexible and open.”

Hartkopf goes on to say that, over the past 10 years or so, there has been an active move towards decentralised control. “This started with AS-i and its intelligent sensors offering diagnostic data as well as binary or analog signals. This has been further expanded with the release of IO-Link and again more intelligent sensors.” He explains further: “For example, in the past a load cell would give only an analogue signal related to the force applied to it. It would in return expect a ‘tare’ signal to reset. Today, it is possible to find intelligent devices that are customised to control the complete filling cycle – managing local outputs and filling profiles. So the use of embedded control devices, such as intelligent sensors, means that the instances of PLCs, standalone motion systems and other embedded intelligent devices is actually likely increase. Even inverters are now available with embedded PLC functions. The process of distributed and decentralised control is here to stay and this is thanks to advances in networking technology. In this context the idea of one PC controlling everything is perhaps a little dated.”

In conclusion, Hartkopf said: “In some respects you could argue the fourth industrial revolution is starting, with the concept of cloud-based operations, applied as a private cloud to a single manufacturing enterprise, beginning to take root. The use of PCs and IPCs will always be important, but perhaps we will also see a revolution there as well as the screen element becomes ‘detached.’ Why walk around your factory with a clipboard and pen when you could be using a smartphone or tablet PC and remain in contact with every part of your facility, including its distributed control devices and PLC-based control hubs.”

A redundant argument?
Mark Daniels, business manager Automation – UK & Ireland at Rockwell Automation, believes that the argument for PC- or PLC-based control is a redundant one. “Many areas of automation are seeing a convergence of technologies, resulting in many of the control systems in use today embodying attributes from both types of system,” he said. “We are finding that customers are now more interested in how a solution can impact their ability to be productive. Rockwell’s overall control strategy – Integrated Architecture – can help to provide productivity gains, and it is a strategy that includes the controller, the devices connected to it and the networks.”

Daniels went on to explain some of the areas that should be addressed when specifying a programmable control solution. “Reliability, productivity, support and visibility are all key areas for consideration.”

He explained that a reliable system needs to be secure. “The more open the system, in terms of how it connects to the world, then the more work will need to be done to secure the system. I believe that Process Automation Controllers (PACs) are much less open to cyber attack than PC-based systems, which sit on a very open, Windows or Linux-based operation system.

“Productivity should be considered not just in terms of plant downtime and uptime, but also how many engineering hours it takes to deliver a project. Further, most end-users would prefer code to be written to a standard – in an ordered manner – to allow troubleshooting and debugging to be easily undertaken. The IEC1131 standard, which is predominantly used by the PAC world, is inherently well-structured. PC-based systems, however, can offer more programmer ‘flexibility’ when creating code. This can mean more scope for flamboyant or independent programming, which may not be so good for longer-term system reliability or overall productivity.”

In contradiction of Hans Beckhoff’s argument that more hardware controllers creates more points of failure, Daniels argues that having a single point of failure is a double edged sword. “Yes, having less components means that there are less things that could fail. However, if you have different devices, that work together, you could lose one or even two devices without having to halt production. There will be a fault-tolerant element in the system so if you loose visualisation, for example, it is feasible for the system to still run, because the controller is still working and it is only the advisory element that has been lost. However, if you have just one device, and this does fail, production will definitely be halted.”

In conclusion, Daniels said: “I do not believe that Rockwell’s Integrated Architecture could ever be considered to be a ‘closed system’ because any vendor is able to connect to it via Ethernet/IP and our partner companies, such as CISCO and Endress + Hauser, have 100% native connectivity. The important discussion today really should be revolving around how to make the process more productive.”

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