This chapter describes a case study for an embedded Light Rail Control System. This design is for a safety-critical system, in which the automated control of driverless trains must be done safely and in a timely manner. As is typical of embedded systems, the system interfaces with the external environment by means of several sensors and actuators. Control of each train is state dependent, which necessitates the design of a state machine to provide control of the train. As this system is an embedded system, the design approach benefits from starting with a systems engineering perspective of the total hardware/software system before the real-time software modeling and design. The Light Rail Embedded System refers to the total hardware/software system, while Light Rail Control System refers to the software system.

The problem is described in Section 21.1. Section 21.2 describes the structural modeling of the system, consisting of the structural model of the problem domain, followed by the system and software system context models. Section 21.3 describes the use case model from a software engineering perspective, describing both the functional and nonfunctional requirements of the safety-critical system. Section 21.4 describes the dynamic state machine modeling, which is particularly important to model the state dependent intricacies of this embedded system. Section 21.5 describes how the system structuring criteria are applied to this system, followed by Section 21.6, which describes how the object and class structuring criteria are applied to each subsystem. Section 21.7 describes how dynamic interaction modeling is used to develop sequence diagrams from the use cases. Section 21.8 provides an overview of the design model for the software system. Section 21.9 describes developing integrated communication diagrams, which leads to the design of the distributed software architecture in Section 21.10, and the component-based software architecture in Section 21.11. Section 21.12 describes system configuration and deployment.

PROBLEM DESCRIPTION

The Light Rail Control System consists of several trains that travel between stations along a track in both directions with a semi-circular loop at each end. Trains have to stop at each station. If a proximity sensor detects a hazard ahead, the train decelerates before stopping.