The future PLC

The debate about PLC versus PC had been going on for a while when a new name appeared, the PAC (programmable automation controller). PAC promises to bring the best of both worlds, the functionality of the PC and the reliability of the PLC. As PAC is reaching the top of the hype curve, one may wonder what's going to happen to the old faithful and reliable automation warrior, the PLC. Let's do a health check on our PLC friend, starting with its family history.

A bit of history

Bedford Associates, founded by Richard Morley, introduced the first programmable logic controller in 1968.This PLC was known as the Modular Digital Controller from which the Modicon Company derived its name. It was developed to replace large relay-based control panels which were inflexible, requiring major rewiring or replacement whenever the control sequence had to be changed. The first challenge in those early days, according to Dick Morley (http://www.barn.org/FILES/historyofplc.html), was to convince people that a box of software, even though cased in cast iron, could do the same thing as 15 metres of cabinets, associated relays and wiring. To do so it had to be fast, reliable and (relatively) inexpensive.

Engineers achieved reliability and ruggedness by using first grade components and avoiding low MTBF (mean time between failures) parts such as cooling fans or overheating devices. Outside air was not allowed to enter the cabinet to avoid corrosion and contamination. Electronic circuits were designed for low power dissipation and to withstand almost any type of external interference. At that time, testing for industrial ruggedness involved rubber hammers and spark coils!

In the mid 1970s, the advent of the microprocessor gave another boost to the PLC, allowing it to take on more complex tasks and more functions. New PLC manufacturers appeared on the market as the microprocessor's speed and capabilities continued their exponential growth. PLC designers created as many models as they needed to cater for their clients' increasing demands. This plethora of brands and models provided end users with a formidable range of options to choose from but at the same time, created some challenging problems. The first one was the software.

The software

Based on relay and contact symbols used to draw wiring diagrams, the Ladder Diagram programming language appealed to electrical maintenance personnel and was soon adopted as the de facto PLC language. Control applications were also developed in BASIC, FORTH, C, Structured English, Instruction List and numerous other proprietary languages including various dialects of ladder programming. The documentation for early PLC programs was either non-existent or very poor, making large programs difficult to understand and debug. The development of PLC programming packages improved this situation; however, until the early 1990s there was no formal programming standard for PLCs.

The international industrial community recognised the need for a standard and established a working group within the IEC (International Electro-technical Commission) to look at the complete design of the PLC. This included hardware design, installation, testing, documentation, programming and communications. During the early 1990s, the IEC published various parts of the IEC61131 standard that covers the complete lifecycle of the PLC. Part 3 specifies the programming languages and the software structure.

The IEC 61131-3 standard encourages well structured program development, strong data typing and provides flexible language selections. Briefly, the standard requires a single software package to deliver five languages for programming:

• Structured Text (ST) is a high level textual language that encourages structured programming. Its syntax strongly resembles PASCAL and supports a wide range of standard functions and operators;
• Function Block Diagram (FBD) is a graphical language for depicting signal and data flows through function blocks and reusable software elements. FBD is very useful for expressing the interconnection of control system algorithms and logic;
• Ladder Diagram (LD), as we know it. However, the IEC Ladder Diagram language also allows the connection of user-defined function blocks and functions and so can be used in a hierarchical design;
• Instruction List (IL) is a low level assembler-like language that is based on similar instruction-list languages found in a wide range of today's PLCs;
• Sequential Function Chart (SFC) is a graphical language for depicting sequential behaviour of a control system. It is used for defining control sequences that are time and event driven.

Communications: the real battlefield

In the 1960s, the 4-20 mA analog signal standard was introduced for instrumentation, to communicate analog values between field devices and controllers. In most applications the signal varies within a range of 4-20 mA in proportion to the process variable being represented. In the 1980s, smart sensors began to be developed and implemented. The dream to remain compatible with the 4-20 system while reaping the advantages of digital information became reality in 1986 with the development of the HART (highway addressable remote transducer) protocol. The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals on top of the 4-20 mA. HART enables a smart field instrument to exchange additional information, beyond just the normal process variable, with its controller. It became a de facto standard in the process industries and is still very much in use today.

More recently, industries started to understand the benefits of multi-drop connections, so many devices could share the same pair of wires and safety barrier. This prompted the need to integrate the various types of digital devices into field networks to optimise system performance. The Fieldbus was born.

With a large number of different tasks to be controlled and many types of PLCs and controllers, engineers' imaginations ran wild. One researcher has identified over 400 types of industrial networks (including higher lever networks)! One can imagine the dilemma end users faced when they had to build a control system requiring devices from various manufacturers... Fortunately, over the past five years, the battle over networking protocols for plant automation systems has settled down. The days of highly proprietary protocols are gone. Major open-standard networking protocols have emerged. Networks such as Foundation Fieldbus, DeviceNet, ControlNet, Modbus, Profibus, CANopen and InterBus belong to this category. They can handle a mix of devices from various vendors, and all of these protocols have substantial user groups sharing information and contributing to the technology.

PLCs also needed to communicate at a higher level with host computers or peer-to-peer with other controllers. Ethernet and the de facto TCP/IP protocol running on top appealed to industries because of their wide deployment in existing company local area networks. Their advantages in industrial applications are many. User interfaces to back-office functions can be easily made with a web browser. Email capabilities may be used to alert off-site staff of problems on the factory floor. XML can be used to share data between the industrial network and office automation equipment. Management of industrial devices or complete systems can be done remotely over the internet. Many standard protocols that were previously run on two-wire serial interfaces are now being run on top of TCP/IP. Protocols such as Modbus TCP and Profinet fit into this category. Further development in wireless networking will simplify plant wiring, resulting in considerable savings and increased flexibility.

Will ethernet find its way down to the device level and compete with the various Fieldbuses? The battle is still on.

The future

The PLC is not dead but has started a new life integrating several of the features that have allowed PCs to crawl down to the plant floor. PLCs have indeed undergone a transformation in the last few years. Besides improvements in the software programming language, many PLC models now offer higher performance, ethernet communications capability and sophisticated self-diagnostic tools - all while being cheaper than ever. PLCs have stepped up to the challenge presented by soft control, offering similar features for the most demanding environments.

The PLC industry is now following the PC lead and becoming more open. 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. And this still comes with the speed, ruggedness and reliability traditionally associated with PLCs.

Interfaces to memory devices such as USB memory devices and Compact Flash give the PLC enhanced data sharing capabilities, including gigabyte volumes of data storage, portability and control over data syntax. Instruction sets in today's IEC-61131 programming development systems support data formatting, read and write file operations, and other data-organisation tools for conveniently sharing data with analysis tools such as spreadsheet and database applications.

The question is no longer which one, PLC, PC or PAC, will win the battle. Rather than one technology winning over the other the consensus is that PLC- and PC-based control technology will converge in the longer term. Perhaps the only remaining question one may ask is: what will be the name of the future PLC?