#LinkedInsights: Use of ladder and SFC

Control System


Today, we are back with the next #LinkedInsights to bring you again the latest discussions in the area of Instrumentation industry. Today, we analyze the use of ladder and SFC, while working on machines with multiple pneumatic cylinders that cycle in a certain sequence.

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#LinkedInsights: Automation Programming

A summer intern from Siemens India has been working on machines with many pneumatic cylinders, which cycle in a certain sequence. He believes that the PLC used there is completely IEC 61131 compliant. “I can program the sequence logic using ladder or SFC. I am very familiar with ladder logic, but new to SFC,” he asserts.

Based on his feedback, we wonder are there really any compelling reasons to use SFC over ladder logic? According to an operation superintendent, from Indonesia, there is no standard requirement for the usage of any specific programming method, as mentioned in IEC 61131. “It comes down to users’ preference. Complex sequence programming will be easier to comprehend and not troubleshoot by using SFC than any other methods (but others may not agree). Any programming method can be used (IL, ladder, FB or SL) with its pros and cons,” he adds.

Basic Ladder Diagram,Courtesy of plcacademy

Many agree that SFC is very common. As per a control specialist from California, technically SFC is quite easy to visualize. As such, there are less chances of making an error in the code. “I am not talking about getting the sequence wrong, but an unforeseen error that may occur under particular circumstances: Tracking step numbers and transitions in a ladder is messy. Once you use SFC. you will never go back, “ he says.

According to a senior control & system engineer based out of Netherlands, pneumatic valves are used to power up pressure at discharge. “You just need to open or close the valves in a correct sequence. The control can be done in the DCS. You need a functional description for the DCS programmer and confirmation of the machine vendor for a guarantee,” he stresses.

As a matter of fact, such programming depends a lot on geography too. A senior consultant from Germany says that the decision to use a ladder or SFC depends on which region of the world you are located in. “Accordingly, you can use structured text as well. In America, mainly ladder logic is used; in Europe, mainly SFCs are used. In the past, particularly for Siemens PLC, structured text were used. What you should use also depends on the knowledge and skills of the people who need to support the machine,” he shares.

SFC, Courtesy of automationforum

Highlighting the importance of maintenance in terms of safety and security issues, a CAP professional who is also a teacher in Instrumentation & SIS suggests using any language one cares for (except if PLC is a safety PLC). “Just remember, you may choose the language(s) you are more familiar with, but maintenance may be needed when you are not available. As such, you need to make sure the maintenance guys are also familiar with the languages you use,” he recommends.

Speaking more on the familiarity of languages, a senior systems integrator from Canada shares,“Programming is very flexible, but IEC 61131 standard outlines the guidelines to program on the most-used languages for a purpose. This means giving a language that applies to pretty much any control application. Programming sequences can be done in any language, normally it depends more on the programmer’s skills and preference. You need to keep in mind, though, that when you don’t use the proper language, then you need to produce code that controls the logic execution, and not to control the processes as it should be.”

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