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Function Blocks Make Motion Easier

“This standardization integrates safety aspects and/or motion control with industrial control.”

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The PLCopen Motion Control Specification harmonizes the access of motion-control functionality across platforms, explains Eelco van der Wal, managing director of PLCopen (www.plcopen.org), Zaltbommel, The Netherlands. “In this way, the generated application program is much more hardware independent and reusable across different architectures—for instance, from centralized to distributed or integrated to networked control.” He adds that the integration of motion control and logic on one platform helps users to add motion-control functionality much more easily.

Noting that the specification provides a programming standard widely supported by industry, with around 30 platforms certified, van der Wal says his organization designed the specification for more than one application. “It will serve as a basic layer for ongoing definitions in different areas. As such, it is open to existing and future technologies.”

Through its Technical Committee 2—Task Force Motion Control, PLCopen defines common libraries of function blocks for specific applications such as safety and motion control. “This standardization integrates safety aspects and/or motion control with industrial control. As such, it provides a common look-and-feel to the users—the programmers as well as installation and maintenance people,” van der Wal states. With multiple implementations of this library, reusability of software and scaling of the control system are much easier, even across different architectures and/or controller brands, he explains.

One of PLCopen’s core activities focuses around the International Electrotechnical Commission’s IEC 61131-3 standard for programmable logic controllers. “It harmonizes the way people design and operate industrial controls by standardizing the programming interface,” van der Wal explains. That standard defines the Sequential Function Chart (SFC) language, used to structure the internal organization of a program, as well as four interoperable programming languages: Instruction List, Ladder Diagram, Function Block Diagram and Structured Text.

“The SFC is like a file cabinet specifically designed with enough pockets to hold the modular [software] code that describes a particular machine,” notes Tom Jensen, technical evangelist for Schaumburg, Ill.-headquartered Elau Inc. (www.elau.com). “If I’m troubleshooting, with ladder logic or Structured Text, I may have to read 1,000 lines of code before I understand how the machine operates,” he explains.

Provide program structure

But with SFC—which Jensen emphasizes is structure, not language—the end-user has a better alternative to get to a problem’s source. If, in the scenario above, which could be for a servo motor, the jump from the main ladder was to a subroutine with even more logic, the superior choice would be a Function Chart with those hundreds of lines of ladder logic in its subroutine, he asserts. “That subroutine would be a Function Block. Then, within the Sequential Function Chart, the Function Block becomes a unique file.”

Those PLCopen Function Blocks offer many advantages, emphasizes Rami Al-Ashqar, product manager, electric drives and controls, Bosch Rexroth Corp. (www.boschrexroth-us.com), Hoffman Estates, Ill. One major advantage is simplicity—or ease-of-use—for the motion-controller’s programmer and also the machine’s inheritors. Efficiency is another major advantage. “A ladder-logic program will be a lot more efficient if it is used in conjunction with PLCopen Motion Function Blocks,” Al-Ashqar asserts. Too, he says there’s more consistency because the PLCopen Motion Function Blocks conform to IEC 61131-3.

PLCopen continues to work on its motion-control Function Blocks. Van der Wal expects Version 0.99 of Part 4, Interpolation, to be released in late November, around the time of the SPS/IPC/Drives 2006 show in Nuremberg, Germany. “Interpolation brings the work in motion control to a new level,” he notes. “Part 4 is focused on the coordinated multi-axes motion in three-dimensional space.”

C. Kenna Amos,ckamosjr@earthlink.net, is an Automation World Contributing Editor.

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