Timed Statecharts Research Articles

Semantics and Execution Time of New Timed Statecharts

Semantics and Execution Time of New Timed Statecharts

Timing analysis of safety properties using fault trees with time dependencies and timed state-charts

Behavior in time domain is often crucial for safety critical systems. Standard fault trees cannot express time-dependent behavior. In the paper, timing analysis of safety properties using fault trees with time dependencies (FTTDs) and timed state-charts is presented. A new version of timed state-charts (TSCs) is also proposed. These state-charts can model the dynamics of technical systems, e.g. controllers, controlled objects, and people. In TSCs, activity and communication times are represented by time intervals. In the proposed approach the structure of FTTD is fixed by a human. Time properties of events and gates of FTTD are expressed by time intervals, and are calculated using TSCs. The minimal and maximal values of these time intervals of FTTD can be calculated by finding paths with minimal and maximal time lengths in TSCs, which is an NP-hard problem. In order to reduce the practical complexity of computing the FTTD time parameters, some reductions of TSCs are defined in the paper, such as sequential, alternative, loop (iteration), and parallel. Some of the reductions are intuitive, in case of others—theorems are required. Computational complexity of each reduction is not greater than linear in the size of reduced TSC. Therefore, the obtained results enable decreasing of the costs of FTTD time parameters calculation when system dynamics is expressed by TSCs. Case study of a railroad crossing with a controller that controls semaphores, gate, light-audio signal close to the gate will be analyzed.

Requirements Traceability within Model-Based Testing

Requirements traceability enables the linkage between all development artifacts during the development process. Within model-based testing, requirements traceability links the original requirements with test model elements and generated test cases. Current approaches are either not practical or lack the necessary formal foundation for generating requirements-based test cases using model-checking techniques involving the requirements trace. This paper describes a practical and formal approach to ensure requirements traceability. The descriptions of the requirements are defined on path fragments of timed automata or timed state charts. The graphical representation of these paths is called a computation sequence chart (CSC). CSCs are automatically transformed into temporal logic formulae. A model-checking algorithm considers these formulae when generating test cases.

Specification and verification of reactive system behaviour: The Railroad Crossing example

In this paper we present an approach to the specification and verification of reactive systems. The approach uses Timed Statecharts and Real Time Logic for the specification of temporal behaviour, and theorem proving techniques for the verification of safety and utility properties. Formal verification is achieved through the automation of semi-formal (rigorous) proofs using a theorem prover (Proofpower HOL). To illustrate the approach, we use the Railroad Crossing Problem, which has been proposed, along with a set of criteria for assessment, as a benchmark for the comparison of real-time formalisms. We conclude with our assessment of the approach against the proposed criteria.

Specification of Real-time Job Scheduling Systems Using Timed Statecharts

A real-time job scheduling system receives jobs from the environment, which must be queued and scheduled such that job deadlines and priorities are satisfied and processor usage is optimized. In order to compare different scheduling algorithms, formal methods that help to model the system are highly useful. In a previous paper, we developed a statecharts-based specification of real-time job scheduling systems. This paper uses a recent version of timed statecharts for the same problem and compares them to the earlier approach. Some surprising conclusions are reached.