PLC vs PC Soft PLC Factory Control Systems

INDUSTRY TRENDS

PC Based Control

"The PLC vs the PC". This was the title of an article in a well-known US journal. However, it wasn't in a process control or factory automation journal. It appeared in the September 8, 1997 issue of the respected business publication Forbes Magazine. When the profile of a control system technology is raised to this level, it is timely to review the current status of PC based control and Soft PLCs.

What is PC Based Control?

Physically, a PC-based control system can take a number of forms, such as:

A PC with remote / distributed I/O networks running a "Soft PLC" control application
A PLC co-processor card installed in a PC backplane
A PC co-processor card installed in a PLC backplane
An open architecture PLC

Some think of the term Soft PLC as a metaphor that expresses a desire for the PLC industry to move down the same development path as the computer industry has since PCs appeared over 10 years ago. This would lead to more openness, lower cost hardware, reusable software components, interconnection of components from different vendors and workable standards.

Where are they being used?

There are numerous small-scale PC based control systems in operation today, in areas such as data acquisition, process monitoring and batch control. However, during the last few years, a number of major PLC users have decided to replace PLCs with industrial PCs in significant applications. General Motors ordered 800 industrial PCs for one of its largest transmission production lines. GM plants in 2 other locations have similar projects underway.

Ford Motor Co. has a smaller pilot replacement project underway at a six-station transfer line in its Sterling Heights, Michigan plant. In addition, BP Chemicals has initiated a major move toward PC-based process automation systems at its Hull Research & Technology Centre.

At Chrysler, a plant-wide plan states that all new control systems must be quoted with PC-based control as an option. The plant manufactures discrete parts and is an operation that Chrysler believes is presently well suited for PC control. The main reason for using PCs is for ease of trouble shooting. The factory staff are computer literate and the diagnostics provide a user friendly interface to track faults. Flow chart programming is also being used instead of ladder, which Chrysler hopes will be a benefit. Replacing several PLC boxes with software running on one PC for applications such as leak detection and press force monitoring is also saving premium floor space.

To date, use of PC-based control in Australia has been restricted to small scale, specialised applications. We are unaware of any significant applications of a similar scale as the GM or Ford systems mentioned above.

PCs at the "Early Adopter" Stage

In an article in the October 1996 edition of Industrial Computing Magazine, the founder and president of PLC Direct, Tim Hohmann, wrote that when a technology is adopted into the market, it follows a particular process called the Diffusion of Innovation. This implies that the following user types adopt a technology progressively, in sequence: innovators, early adopters, early majority, late majority and laggards.

Hohmann believes that the PC-based control market is at the early adopter stage, characterised by users who don't care as much about the technology as they do about the solution it offers. Developments such as MAP in the industrial-control market of the 1980s (initiated by GM, an early adopter as mentioned above) and the pen-based computer products of the 1990s are examples of products that have not made the jump to mass market. They are stuck in the Diffusion of Innovation process with the early adopters.

The innovator and early adopter users represent a small part of the total control system market. However, to move to the early majority phase, solutions must be practical, easy to apply, reliable and cost-effective. At this time, Hohmann contends that the PC comes up short with respect to these qualities as a PLC control replacement. The main problem is the cost to make a simple, rugged, real-time system at the same cost as a PLC.

The Place for PCs

Where do PCs make sense on the factory floor? The answer lies in the fact that not every control application has the same criteria for ruggedness, stringent real-time response and programming simplicity.

In places where the plant floor's electrical characteristics, mechanical characteristics, temperatures and dust levels approach office standards, rugged PLC hardware is not required. This reduces or eliminates the cost of PC survivability in a hostile environment.

Many industrial-control processes do not require the millisecond real-time performance available from PLCs. And many sites already have significant PC programming expertise on hand, so additional training requirements are not an issue.

Some control applications require additional data manipulation and storage and reporting characteristics that may be difficult to implement in a PLC. Batch control processing typically does not require stringent real-time performance but often has additional data handling/reporting requirements and does not require significant power off/restart handling.

At present, PC-based control systems are most likely to appear in the larger manufacturing companies. Smaller companies, who may not have dedicated PC staff, are more likely to stay with the traditional PLC solutions, which can be more easily maintained by electrical maintenance personnel.

The Longer Term

Technical differences notwithstanding, the PC and PLC industries are already starting to look more alike. Both PCs and PLCs are getting smaller and cheaper. Small, micro and nano PLCs costing a few hundred dollars, with 16 to 100 I/O are the largest growth area and represent 80% of new PLC sales worldwide. PLC-based systems have always had a price advantage over other control techniques because the hardware can be more closely scaled to the size of the application while providing robust and rugged industrial control.

While a PC based solution has no hope of competing with a single PLC costing $300, many control systems will also have a PC based Human Machine Interface (HMI) that can be used to run the Soft PLC application. Therefore, the hardware cost for even the smallest systems can be very similar for both options.

The PLC industry is now also following the PC lead and becoming more open. As a result of pressure from PC-based systems, vendors are offering PLC users open architectures that allow third-party PC hardware and software interfaces, a wider selection of models based on size and cost, easier Windows-based programming and increasing use of standard networking technology such as Ethernet TCP/IP. All this still comes with the speed, ruggedness, and reliability traditionally associated with PLCs.

Rather than one technology winning over the other, as the Forbes article title may imply, the consensus is that PLC and PC-based control technology will converge in the longer term. There are certainly some PLC applications today that could be better served with a PC-based solution. In the short term, however, there are a number of practical reasons why PC-based control will not replace PLCs in all applications. Instead, PC control should be thought of as another alternative in the implementation of a control system.