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By Don Talend
The International Electrotechnical Commission (IEC) 61131-3 standard covering programming languages for programmable logic controllers (PLCs)might not yield true practical uniformity in industrial machine control programming, but perhaps that shouldn't be a realistic expectation in the first place.
Some machine controls engineers and software developers indicate the standard is beneficial in terms of machine control design, transferability of knowledge and programming efficiency.
Paper Converting Machine (PCMC, www.pcmc.com), Green Bay, Wisconsin, builds tissue-converting machines for the variety-oriented paper towel and bath tissue markets. Product variations include the number of plies, product strength and absorbency, and smooth or quilted textures. To handle increasingly variable products, the company developed a surface rewinder machine designed for flexibility and speed.
PCMC wanted to design a new model that yielded high output rates, precise handling of fragile tissue web and quick product changeover.
Mark Kralovec, PCMC controls engineer, and the design team used Rockwell Automation's Logix control platform that combines motion and sequential control. A key component of the platform is its power programming, a method based on the ANSI/ISA S88 batch standard. These techniques use standardized programming methods and best practices that allow programmers to reuse codes for greater efficiency.
Kralovec notes that the rewinder's control platform uses elements of object-oriented programming, which made programming easier for the engineering team in Green Bay and at another operation in Italy. "Power programming techniques have user-defined data types for code objects that can be reused," he says. "We made one code object for our standard axis. We made one template for that, and then everyone used that same template. It allows us to basically work independently. People could create specific machine applications on their own and then easily bring that into the master system. By reusing the same template, we can do a lot of testing up front on the first one and know it's going to be the same on the others. Now that we've developed this architecture for overall control of the machine being managed—creating a command position interface down to these equipment modules that interface with our standard position axis control—it's very generic in the sense that about 80–90% of that can be reused on any machine."PCMC reports the new machine was developed with a 25% shorter lead time with more efficient assembly and testing, and a testing time two-thirds of previous.
Thermoforming Systems (www.tslusa.biz), Union Gap, Washington, a manufacturer of plastic-sheet thermoforming machines that serve the food service industry, uses a hybrid function block/flowchart programming language for machine control. Since 2004 the company has used software from Opto 22, which uses graphical interfaces to represent the control environment. The idea is to represent complex controls with symbols and simple command functions for less-experienced programmers, while devising the process to have a logical flow using a flowchart concept.
"Whenever we create a new function in the control or a new option, we try to make it as modular as possible and integrate it into the main control structure," notes Chuck Phillips, Thermoforming Systems' director of electrical engineering. "The software combines the use of function blocks, which can be used to express instructions via intuitive labeling, with flowcharts that provide a visual instruction sequence. This basic software platform is designed to allow programmers with little experience to use the program, while an additional scripting tool can be used for advanced if-then procedures or complex mathematical functions by programmers familiar with advanced scripting languages such as Visual Basic and C++." Phillips says that, although he had little familiarity with standard PLC/PAC languages or scripting languages before using the program, he uses the scripting tool to make programming changes.
"When that scripting language was introduced some years ago, I wasn't against it," adds Phillips. "I was just accustomed to flowchart. Now, for the past several years, on every new project, I almost exclusively use script blocks in the flowchart. If I have to touch any particular function or spread, I'll change it to a script."
A major catalyst in this evolution of industrial machine control toward greater programming efficiency and more complex functions was the development of the IEC 61131-3 standard, developed in conjunction with the International Organization for Standardization (ISO) for uniformity of PLC control. Five languages are noted in the standard.
Ladder—or relay ladder logic—is the most commonly used PLC programming technique in the U.S. "Ladder logic describes in a very precise way a parallel set of interlocking logical expressions that can be very complex in machine control," says Rick Morse, software business manager for automation and simulation at Rockwell Automation (www.rockwellautomation.com). "To be able to describe that in a cascading if-then format gets very complex and hard to read."
Ladder is very effective to describe digital logic in a form that's understandable to technicians and electricians, says Todd Walter, senior product manager, general industrial and embedded products for National Instruments (www.ni.com). "That's really its greatest strength," he states. "However, it's fairly cumbersome if you try to do complex algorithms, process control or complex sequencing."