Automated Test Sequence Generation for Function Block Diagram Programs

As Function Block Diagram (FBD) programs are used to implement safety-critical systems such as nuclear reactor protection systems, it is crucial to be able to generate effective test cases. The FBD is one of programming languages that are used for programmable logic controllers (PLCs). PLC programs are repeatedly run within a particular scan time for every execution. Among the constituents of FBD programs, function blocks and feedback variables use the inputs and outputs of the previous scan cycle on which to operate. Researchers have recently developed an automated test generation technique that satisfied several structural test coverage criteria for FBD programs using symbolic execution. However, their work could not generate test sequences for consecutive scan cycle but test inputs for one scan cycle. Test sequences are essential for testing FBD programs that have function blocks and feedback variables. This paper extends previous work in the field by generating test sequences for FBD programs with function blocks in a fully automated manner. The key technique involves explicitly unwinding FBD programs and solving test requirements using an SMT solver. We conduct experiments on increasing the coverage of test requirements by unwinding cycles and evaluating the effectiveness of the test set using mutation analysis with several subject programs, including a real-world reactor protection system. The experimental results show that the proposed approach is able to generate effective test sequences for FBD programs.