A refinement development approach for enhancing the safety of PLC programs with Event-B

Abstract

Programmable Logic Controllers (PLC) are widely used in Industrial Control Systems (ICS) with strict safety assurance requirements. Unfortunately, traditional techniques for debugging prefer to use post-development approaches, such as simulation and black-box testing, rather than enhancing safety before programing. In this paper, we propose a refinement-based approach to model and verify PLC systems, aiming to assure safety properties by construction. It uses the Event-B formalism and focuses on the levels of requirement analysis, specification refinement, and system development. This approach takes a three-layer framework stepwise to specify the behaviors and properties of PLC programs, thereby reducing the modeling complexity. The basic firmware layer models the general mechanisms of PLC firmware, such as periodical instruction execution and centralized I/O scanning, which are application-independent models with fundamental safety properties at an abstract level. The middle layer establishes configuration models. These models correspond to the PLC settings and interactive environments of a specific system, such as I/O addresses and peripheral devices. The business layer models business logic with more specific system-level safety requirements. With our approach, the safety properties of PLC systems can be verified throughout the modeling and refinement process. In addition, rules are proposed to convert the most concrete Event-B model into PLC code satisfying the IEC 61131-3 standard. We demonstrate this approach with a real-world running example of a pump control system for gas transmission. • A refinement-based approach for modeling PLC systems using Event-B. • Defining PLC features are modeled with a 3-layer framework. • It decomposes the complexity of modeling and verification by stepwise refinement. • The safety requirements of PLC systems can be verified during the development. • It generates PLC program code compatible with IEC 61131-3 standard from the model.