PLC Programming Goes Modular

Jan. 1, 2006
For PLC programming, U.S control engineers and technicians turn typically to ladder logic, a programming technique that’s been around since the birth of programmable logic controllers, or PLCs, in the 1960s.

Old habits die hard. But change is inevitable. And in this case, if proponents are right, change is on the way in the form of an open PLC programming standard known as IEC 61131-3.

Promulgated by the International Electrotechnical Commission, the IEC 61131 standard has already gained wide acceptance in Europe and Japan, points out Bill Lydon, a Franklin, Wisc.-based consultant who was recently named Managing Director, North America, for PLCopen (, a European-based worldwide organization that is the standard’s primary supporter. Lydon believes that once the benefits of 61131 are more widely understood here, the standard is certain to catch on broadly in North America as well.

The 61131 standard defines four programming languages, one of which is Ladder Diagram, drawn from the ladder logic that U.S. engineers are already familiar with. The others are Instruction List, Function Block Diagram and Structured Text.

“One beauty of the 61131 standard is that you can use best practice in terms of language selection,” observes Dr. Kenneth Ryan, director, Manufacturing Automation Research, at the Alexandria Technical College Center for Automation and Motion Control (, in Alexandria, Minn.

Ladder Diagram, or ladder logic, can still be used and works well for discrete logic programming, such as describing the action of a simple pneumatic circuit, Ryan says. But today’s advanced machines that increasingly integrate logic with complex motion control require algorithms that are difficult to program in ladder logic, and are best programmed using Structured Text, he notes. Likewise, for example, when the programming objective is the high-level description of a state-based machine, another 61131-defined method known as Sequential Function Charts is by far the superior approach, Ryan adds.

A major advantage of the 61131 standard is that any of the languages can be used to create Function Blocks, which encapsulate specialized control functions in modules, or software objects, that can be locked and reused in the same or different programs. Like Lego blocks, these Function Blocks can be plugged together in various configurations. “Function Blocks can save huge amounts of money in engineering time, because you can write them once and reuse them when appropriate,” says Ryan.

One company that is making extensive use of 61131-compliant Function Blocks is Elau AG (, a German-based packaging automation controls vendor that is part of Schneider Electric (, of Rueil-Malmaison, France. Elau offers a library of around 400 Function Blocks for use by machine builders and others using its PacDrive control system, says John Kowal, Elau global marketing manager, based in Schaumburg, Ill. These range from a complex servo motion control Function Block to others that handle temperature control and programmable limit switch functionality, as examples.

In line with a trend toward more use of embedded robot functionality in packaging machines, Elau recently added a robotic Function Block that allows control of multiple robot arms. With this Function Block, “you’re taking away between 50 percent and 80 percent of the programming work for the machine builder,” says Kowal. For end-users, he adds, “you’re eliminating the need for a proprietary robot controller,” and system performance is improved, due to tighter integration between machine and robot control.

The U.S. automation community is likely to hear more about IEC 61131-3 during 2006. PLCopen’s Lydon says the organization is currently developing plans for stepped-up North American promotion of the standard this year. And at Alexandria Technical College, Ryan says he is seeing a growing interest in 61131 by controls vendors, who are increasingly asking for customized sales force and engineering training programs covering the standard.

Wes Iversen, [email protected]