Does openness spell the end of the PLC as we know it in automation? Can PLCs live on with PCs? BY david robinson

In recent months there has been a great deal of talk about PLCs having had their day, and increased use of open control architecture by traditional PLC users meaning its days are numbered.

Some have stated that by the turn of the century the PLC will have given way to open control systems, particularly PCs, in the vast majority of control applications. Before anyone makes a move, they should compare the capabilities of open systems against the current breed of PLCs in meeting their total control and operational needs.

Indeed, the road ahead with open solutions is not as easy or straightforward as it might appear. There are no clear definitions across the industry as to what an open control system actually is – what it consists of. Users must decide for themselves what `open’ means, which networks are applicable, which hardware, which operating system, which language, etc – obviously caus-ing much confusion.

Lots of options

In fact, open control systems are just one of a variety of options which can be used to meet today’s demanding automation requirements.

Indeed, it is clear that no one solution will solve all requirements, and there is a need for a range of solutions to fit a variety of applications. If we analyse some of the reasons why people are moving towards open, we can see that they are looking to reduce total cost and time to market – and gain vendor independence.

These are the issues perceived to be better catered for by an open system. In addition there is increased demand to leverage and extend control systems’ value by using third party products and technologies – and a need to move from processors with distributed I/O to highly distributed control for more flexibility, better performance and cost savings.

All of this, however, does not mean the end of PLCs and a cwholesale move to open systems. Both the PLC and the open architecture solution bring with them a host of benefits and issues. For instance, a PLC system typically benefits from the fact that it is optimised for I/O processing and logic execution, and that it has vendor-controlled technology. These allow for comprehensive vendor support and responsibility.

Further, the users are buffered from technical change, thus providing a longer PLC lifecycle. Typically, a PLC will provide predictable and repeatable performance.

However, some of the issues with the PLC are that it is not necessarily easily adaptable to functions not featured in the original design specification. Another is that there is not always full support for alternative vendors and I/O interfaces.

The pros and cons

The adoption of new technology and functionality is gated by the PLC manufacturer, and there is the reliance on a single supplier. Compare this with the open control system benefits where there is the scope to have rapid deployment of new technologies, the ability to leverage custom software, use of standard applications and functionality, plus being able to leverage the lower acquisition costs of commercial products (monitors etc).

Open systems appear to lead hands down. But there are issues associated with open architecture. Multi-vendor support can mean risk. Solutions are not necessarily optimised for an application. The interaction of computers is not necessarily predictable or repeatable. And, the product life cycles tend to be short, with associated upgrade costs.

So to conclude, open technology will play a major part in future automation solutions and in the PLC market.

As to whether the PLC will disappear, its unlikely, at least for some years. Instead it will evolve into a more open system, providing users with some of the benefits in a controllable environment.

Additionally, hybrid systems will provide even higher performance. In fact, you will have a range of systems providing different degrees of openness and performance, providing multiple solutions to give the users `choices in control’.

* The Author is with Rockwell Automation.