Tuesday, December 21, 2010

Sequential Function Chart PLC Programming

SFC (Sequential Function Chart) programming resembles the computer flowcharts that many will remember drawing up in their college. An initial step is always “action box”, the starting point of a flow chart is followed by a series of transitions and additional steps of action. The SFC concept is simple. An action box, with code written inside in any language of the programmer’s choice, is active until the transition step below it activates. The existing action box is turned off, and the next one in the sequence is active. The transition step is also has code to check that the necessary conditions are met to allow the program to advance for next step.

For appropriate applications which have a series of repeatable processes or repeatable multi steps process, this programming form is the easiest to implement. For instance would be a pick and place application, where product is constantly picked up from one area, moved through a specific path, and placed in another area.

While exception exist, since there is only typically one active piece of code and one transition to be concerned with, condition checking and the process control should be achievable without large rungs. The language is also easy to maintenance engineers because the program visual nature plus code segmentation makes it easy to troubleshoot. For instance, in the mechanism in a pick and place application has moved to the product but not picked it, the troubleshooter could bring up the program and look at the transition condition between the move to product box and the pick product box to see what is holding up the process.

This programming style is not suitable for every application, as the structure that is forced on a program could add unneeded complexity. A large time amount must be spent up front preparing and planning before any programming is attempted or else the function charts could become difficult to follow and unwieldy.

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