Saturday, February 26, 2011

Visual Validation of PLC Programs

It is important to understand the basic procedure used to construct a PLC Program (ladder diagram) to design the architecture of the PLC programming environment. Chuang et al. 1999 proposed a procedure for the development of an industrial automated production system that contains of nine steps. They are:
1) Define the process to be controlled;
2) Make a sketch of the process operation;
3) Create a written sequence listing of the process step by step;
4) On the sketch, add the sensors needed to carry out the control sequence;
5) Add the manual controls needed for operational checks or for the process-setup;
6) Consider the safety of the operating personnel and make additions and adjustments as needed;
7) Add the master stop switches required for a safe shutdown;
8) Create a ladder logic diagram that will be used as a basis for the PLC program; and
9) Consider the possible points where the process-sequence may go astray.

The most time-consuming task for the control logic designers is the 8th step, which is generally done by the repetitive method of ‘Code writing, testing and debugging’ until the control objectives are achieved. The bottleneck of the 8th step is that the conventional PLC programming environments are not especially intuitive, particularly for the testing and debugging of a PLC program, as they show only the status of a PLC without providing any links to the target system.

Engineers need to imagine the state changes of a production line from the input and output ports of a PLC for the validation of a PLC program. That is the reason conventional PLC programming environments are often prone and inefficient to human error. There is a strong need for a more intuitive PLC programming environment as the configurations of production lines and their control programs become more complicated. It is hoped that this article will take positive steps in this direction.

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