Soft-testing of industrial control systems programmed in IEC 1131-3 languages

This paper presents a technique and the means for testing PLC-based control software outside the actual plant environment with the purpose of increasing the confidence level on the compliance of the software to functional and temporal requirements. The need to obtain a high confidence level on the correct software operation arises from the fact that in most of the cases it is quite dangerous and expensive to test unproved PLC operation by linking it with the actual facilities that it is going to control. The proposed technique relies on a combined simulation of the controlled plant and the PLC system and an analysis of the plant responses. The PLC system simulation imitates the way software is executed on a PLC that is programmed in the languages of the IEC1131-3 standard. It is based on the programming model of the IEC standard and analytical formulae for estimating the program runtime. The simulation of the plant is based on a discrete convolution model that is solved at the same rate with the rate determined by the control algorithms. A tool realizing these concepts has been developed and its use in testing the control software of three critical outputs of a distillation column is demonstrated.