Formal System Model Research Articles

On formal modeling, analysis and optimization of reconfigurable manufacturing systems

ABSTRACT This paper deals with the formal specification and multi-objective optimization of reconfigurable manufacturing systems (RMSs). RMSs are characterized by their dynamic structure, crucial for responding to functional variations, adjusting production capacity and overcoming operational dysfuntions promptly and cos-effectively. The effective utilization of this dynamic capability depends on thorough study and optimization of RMS configurations. To tackle this challenge, a novel formalism called genetic reconfigurable object nets (Gen-RONs) is introduced. Gen-RONs formalism integrates the modeling, verification and reconfiguration capabilities of reconfigurable object nets with advanced multi-objective techniques from genetic algorithms. Within Gen-RONs, Petri nets serve to: (i) encode RMS configurations, (ii) manage RMS scheduling and (iii) employ graph transformation theory rules as genetic algorithm operators for real-time system reconfiguration. Thus, Gen-RONs formalism provides a robust framework for optimizing RMSs using genetic algorithms. To ensure deadlock-free RMS configurations, Gen-RONs incorporate a binary tree-based technique. Furthermore, Taguchi’s method is employed to fine-tune genetic algorithm parameters. The contribution of this paper is demonstrated through a case study involving an RMS and the widely used genetic algorithm NSGA-II. Significant gains in terms of the number of solutions (i.e. configurations) and the criteria values are achieved.

Formal Modeling and Verification of Lycklama and Hadzilacos’s Mutual Exclusion Algorithm

This study describes our thorough experience of formal modeling and exhaustive verification of concurrent systems, particularly mutual exclusion algorithms. The experience focuses on Lycklama and Hadzilacos’s (LH) mutual exclusion algorithm. LH rests on the reduced size of the shared state, contains a mechanism that tries to enforce an FCFS order to processes entering their critical section, and embodies Burns and Lamport’s (BL) mutual exclusion algorithm. The modeling methodology is based on timed automata and the model checker of the popular Uppaal toolbox. The effectiveness of the modeling and analysis approach is first demonstrated by studying the BL’s solution and retrieving all its properties, including, in general, its unbounded overtaking, which is the non-limited number of by-passes a process can suffer before accessing its critical section. Then, the LH algorithm is investigated in depth by showing it fulfills all the mutual exclusion properties when it operates with atomic memory. However, as this study demonstrates, LH is not free of deadlocks when used with non-atomic memory. Finally, a state-of-the-art mutual exclusion solution is proposed, which relies on a stripped-down LH version for processes, which is used as the arbitration unit in a tournament tree (TT) organization. This study documents that LH’s TT-based algorithm satisfies all the mutual exclusion properties, with a linear overtaking, both using atomic and non-atomic memory.

Atomic cross-chain swap based on private key exchange

Atomic Cross-Chain Swap (ACCS) is one important topic in cryptocurrency, where users can securely and trustlessly exchange assets between two different blockchains. However, most known ACCS schemes assume specific scripting functionalities of the underlying blockchains, such as Hash Time Locked Contracts (HTLC). In addition, these schemes are typically only applicable to certain digital signature schemes, like Schnorr or Elliptic Curve Digital Signature Algorithm (ECDSA) signatures. In this paper, we propose a generic ACCS scheme, independent from the underlying blockchains. To the best of our knowledge, this is the first solution of this kind. Our results are as follows. First, we define a formal system model of ACCS. Next, we present a generic ACCS scheme meets our model. This scheme admits atomicity in cross-chain swaps without the need for a Trusted Third Party (TTP) and protects users’ privacy. Finally, by using the Non-Interactive Zero-Knowledge (NIZK) proof protocol as a tool, we instantiate our generic scheme for Elliptic Curve Discrete Logarithm Problem-based (ECDLP-based) signatures. In addition, we implement our scheme, and the experimental results show that our protocol outperforms the existing ACCS schemes, such as the HTLC-based schemes.

Sequential Model-Based Diagnosis by Systematic Search (Abstract Reprint)

Model-based diagnosis aims at identifying the real cause of a system's malfunction based on a formal system model and observations of the system behavior. To discriminate between multiple fault hypotheses (diagnoses), sequential diagnosis approaches iteratively pose queries to an oracle to acquire additional knowledge about the diagnosed system. Depending on the system type, queries can capture, e.g., system tests, probes, measurements, or expert questions. As the determination of optimal queries is NP-hard, state-of-the-art sequential diagnosis methods rely on a myopic one-step-lookahead analysis which has proven to constitute a particularly favorable trade-off between computational efficiency and diagnostic effectivity. Yet, this solves only a part of the problem, as various sources of complexity, such as the reliance on costly reasoning services and large numbers of or not explicitly given query candidates, remain. To deal with such issues, existing approaches often make assumptions about the (i) type of diagnosed system, (ii) formalism to describe the system, (iii) inference engine, (iv) type of query to be of interest, (v) query quality criterion to be adopted, or (vi) diagnosis computation algorithm to be employed. Moreover, they (vii) often cannot deal with large or implicit query spaces or with expressive logics, or (viii) require inputs that cannot always be provided. As a remedy, we propose a novel one-step lookahead query computation technique for sequential diagnosis that overcomes the said issues of existing methods. Our approach (1) is based on a solid theory, (2) involves a systematic search for optimal queries, (3) can operate on implicit and huge query spaces, (4) allows for a two-stage optimization of queries (wrt. their number and cost), (5) is designed to reduce expensive logical inferences to a minimum, and (6) is generally applicable. The latter means that it can deal with any type of diagnosis problem as per Reiter's theory, is applicable with any monotonic knowledge representation language, can interact with a multitude of diagnosis engines and logical reasoners, and allows for a quality optimization of queries based on any of the common criteria in the literature. We extensively study the performance of the novel technique using a benchmark of real-world diagnosis problems. Our findings are that our approach enables the computation of optimal queries with hardly any delay, independently of the size and complexity of the considered benchmark problem. Moreover, it proves to be highly scalable, and it outperforms the state-of-the-art method in the domain of our benchmarks by orders of magnitude in terms of computation time while always returning a qualitatively as good or better query.

Reusable formal models for concurrency and communication in custom real-time operating systems

In embedded systems, the execution semantics of the real-time operating system (RTOS), which is responsible for scheduling and timely execution of concurrent processes, is crucial for the correctness of the overall system. However, existing approaches for the formal verification of embedded systems typically abstract from the RTOS completely, or provide a detailed and synthesizable formal model of the RTOS. While the former may lead to unsafe systems, the latter is not compatible with industrial design processes. In this paper, we present an approach for reusable abstract formal models that can be configured for custom RTOS. Our key idea is to formally capture common execution mechanisms of RTOS like preemptive scheduling, event synchronization, and communication abstractly in configurable timed automata models. These abstract formal models can be configured for a concrete custom RTOS, and they can be combined into a formal system model together with a concrete application. Our reusable models significantly reduce the manual effort of defining a formal model that captures concurrency and real-time behavior, together with the functionality of an application. The resulting formal model enables analysis, verification, and graphical simulation. We validate our approach by formalizing and analyzing a rescue robot application running the custom open source RTOS EV3RT.

Sequential model-based diagnosis by systematic search

Model-based diagnosis aims at identifying the real cause of a system's malfunction based on a formal system model and observations of the system behavior. To discriminate between multiple fault hypotheses (diagnoses), sequential diagnosis approaches iteratively pose queries to an oracle to acquire additional knowledge about the diagnosed system. Depending on the system type, queries can capture, e.g., system tests, probes, measurements, or expert questions.As the determination of optimal queries is NP-hard, state-of-the-art sequential diagnosis methods rely on a myopic one-step-lookahead analysis which has proven to constitute a particularly favorable trade-off between computational efficiency and diagnostic effectivity. Yet, this solves only a part of the problem, as various sources of complexity, such as the reliance on costly reasoning services and large numbers of or not explicitly given query candidates, remain.To deal with such issues, existing approaches often make assumptions about the (i) type of diagnosed system, (ii) formalism to describe the system, (iii) inference engine, (iv) type of query to be of interest, (v) query quality criterion to be adopted, or (vi) diagnosis computation algorithm to be employed. Moreover, they (vii) often cannot deal with large or implicit query spaces or with expressive logics, or (viii) require inputs that cannot always be provided.As a remedy, we propose a novel one-step lookahead query computation technique for sequential diagnosis that overcomes the said issues of existing methods. Our approach (1) is based on a solid theory, (2) involves a systematic search for optimal queries, (3) can operate on implicit and huge query spaces, (4) allows for a two-stage optimization of queries (wrt. their number and cost), (5) is designed to reduce expensive logical inferences to a minimum, and (6) is generally applicable. The latter means that it can deal with any type of diagnosis problem as per Reiter's theory, is applicable with any monotonic knowledge representation language, can interact with a multitude of diagnosis engines and logical reasoners, and allows for a quality optimization of queries based on any of the common criteria in the literature.We extensively study the performance of the novel technique using a benchmark of real-world diagnosis problems. Our findings are that our approach enables the computation of optimal queries with hardly any delay, independently of the size and complexity of the considered benchmark problem. Moreover, it proves to be highly scalable, and it outperforms the state-of-the-art method in the domain of our benchmarks by orders of magnitude in terms of computation time while always returning a qualitatively as good or better query.

System verification via Model‐Checking: A case study of an autonomous multi‐differential drive robot

AbstractModel‐Based Systems Engineering (MBSE) has been utilized in practice to design and behavioral modeling cyber‐physical systems. The Vee model helps frame MBSE's lifecycle approach, with system verification a vital aspect of the qualification process. However, popular modeling language tools in MBSE, such as Systems Modeling Language (SysML), are incapable of formally verifying these systems. Model checking allows for the development of formal system models similar in abstraction to SysML models for automatically checking if these formal models satisfy formal specifications. We propose an approach to translate behavioral diagrams in SysML, such as state‐machine diagrams, to the popular symbolic model checker NuSMV for formal verification. As a case study, we apply this process to autonomous multi‐differential drive robots (DDR). Subsequently, the NuSMV model is verified against some formal operational specifications obtained from the requirements diagram of the DDR. This system verification approach can help System Engineers identify design flaws or incorrect modeling or specifications that could be missed during the design phase through the results of the model checking process.

MBSE WITHIN THE ENGINEERING DESIGN COMMUNITY – AN EXPLORATORY STUDY

AbstractObjective of this contribution is to present the use of Model-based Systems Engineering within the engineering design community. Based on a differentiation between SE and MBSE, the definition of three core MBSE elements, namely modelling method, modelling language, and modelling tool as well as the three major aspects of a consistent system model (requirements, behaviour, and structure) a structured review is conducted, focussing on the understanding and motivation as well as the modelling of systems. The review includes 93 publications from Design Society library and proceedings of the CIRP Design conferences during the period from 2011 to 2022. The review points out, that there is an increasing application of MBSE within the engineering design community, mainly focussing on architecture definition or combined engineering activities. Only a small portion of works (16 publications) are providing a consistent approach as these publications link all aspects of the system model and consider all three MBSE elements. It can be concluded, that there is a diffuse understanding of MBSE and different motivations are given to apply more formal system models as well as modelling tools.

Finding Locally Smallest Cut Sets using Max-SMT

Model-based development (MBD) is increasingly being used for system-level development of safety-critical systems. This approach allows safety engineers to leverage the system model created during the MBD process to assess the system's resilience to component failure. In particular, one fundamental activity is the identification of minimal cut sets (MCSs), i.e, minimal sets of faults that lead to the violation of a safety requirement. Although the construction of a formal system model enables safety engineers to automate the generation of MCSs, this is usually a computationally expensive task for complex enough systems. We present a method that leverages Max-SMT solvers to efficiently obtain a small set of faults based on a local optimization of the cut set cardinality. Initial experimental results show the effectiveness of the method in generating cut sets that are close or equal to globally optimal solutions (smallest cut sets) while providing an answer 5.6 times faster on average than the standard method to find a smallest cut set.

Graded Refinement, Retrenchment, and Simulation

Refinement of formal system models towards implementation has been a mainstay of system development since the inception of formal and Correct by Construction approaches to system development. However, pure refinement approaches do not always deal fluently with all desirable system requirements. This prompted the development of alternatives and generalizations, such as retrenchment. The crucial concept of simulation is key to judging the quality of the conformance between abstract and more concrete system models. Reformulations of these theoretical approaches are reprised and are embedded in a graded framework. The added flexibility this offers is intended to deal more effectively with the needs of applications in which the relationship between different levels of abstraction is not straightforward, and in which behavior can oscillate between conforming quite closely to an idealized abstraction and deviating quite far from it. The framework developed is confronted with an intentionally demanding case study: a model active control system for the protection of buildings during earthquakes. This offers many challenges: it is hybrid/cyber-physical; it has to respond to rather unpredictable inputs; and it has to straddle the gap between continuous behavior and discretized/quantized/numerical implementation.

Linear Temporal Public Announcement Logic: A New Perspective for Reasoning About the Knowledge of Multi-classifiers

In this paper, a formal transition system model is presented called Linear Temporal Public Announcement Logic (LTPAL) to extract knowledge in a classification process. The model combines Public Announcement Logic (PAL) and Linear Temporal Logic (LTL). For this purpose, first, an epistemic logic model is created to capture information gathered by classifiers in single-framed data input. Next, using LTL, classifiers are considered for data stream inputs. Then, a verification method is proposed for such data streams. Finally, we formalize natural language properties in LTPAL with a video-stream object detection sample.

Business processes monitoring based on fuzzy cognitive maps

The subject matter of the article is a toolkit and monitoring processes of weakly structured business processes using fuzzy cognitive maps (FCMs). The goal is to create a model for monitoring and forecasting the course of business processes based on the FCMs, which provide flexibility and the ability to adapt to the conditions of changing circumstances in the process to be monitored, as well as insurance of the possibility of applying unstable FCM.s The task is to develop a formal monitoring system model; develop a computational FCM model; develop a method of using unstable FCMs: to develop a method for creating and using an FCM model. The methods used are graph modeling methods and computational experiment methods. The following results were obtained. A formal model of the system of monitoring and coordination of decisions regarding the course of business processes has been developed. A model of a weighted semantic graph was developed, which includes three types of vertices, namely, input vertices that will receive subjective evaluations of users regarding the course of the process, intermediate vertices that correspond to factors important from the perspective of the decision-maker, and final vertices that reflect integral evaluations of the quality of business process execution. A computational FCM model has been developed, which can adapt to the specifics of business logic due to the flexible adjustment of memory parameters and connections between FCM nodes. A method of using unstable FCMs has been developed. A methodology for creating and using FCMs designed for monitoring weakly structured business processes has been developed. Conclusions. The scientific novelty of the work is as follows: the model for monitoring and forecasting the course of business processes due to the use of a fuzzy cognitive map, in which the parameters of the importance and node memory and connections provide additional flexibility and the possibility of coordination and adaptation in the conditions of changing circumstances in the process to be monitored has been improved; a method of using unstable FCMs by setting limits on the values of the nodes excitation and using as a measure of excitation not only stable excitation values of FCM nodes but also the rate of growth of excitation values as an indicator of trends, which allows monitoring and forecasting the course of business processes has been proposed.

Formal Verification of Fractional-Order PID Control Systems Using Higher-Order Logic

Fractional-order PID control is a landmark in the development of fractional-order control theory. It can improve the control precision and accuracy of systems and achieve more robust control results. As a theorem-proving formal verification method, it can be applied to an arbitrary system represented by a mathematical model. It is the ideal verification method because it is not subject to limits on state numbers. This paper presents the higher-order logic (HOL) formal verification and modeling of fractional-order PID controller systems. Firstly, a fractional-order PID controller was designed. The accuracy of fractional-order PID control can be supported by simulation, comparing integral-order PID controls. Secondly, the superior property of fractional-order PID control is validated via higher-order logic theorem proofs. An important basic property, the relationship between fractional-order differential calculus and integral-order differential calculus, was analyzed via a higher-order logic theorem proof. Then, the relations between the fractional-order PID controller and integral-order PID controller were verified based on the fractional-order Grünwald–Letnikov definition for higher-order logic theorem proofs. Formalization models of the fractional-order PID controller and the fractional-order closed-loop control system were established. Finally, the stability of the fractional-order control systems was verified based on established formal models and theorems. The results show that the fractional-order PID controllers can be conducive to the control performance of control systems, and the higher-order logic formal verification method can ensure the reliability and security of fractional-order control systems.

BBARHS: Blockchain-Based Anonymous Ride-Hailing Scheme for Autonomous Taxi Network

In the past few years, ride-hailing platforms such as Uber, Waymo, and Baidu have built their own autonomous taxi system. Unlike public transit services, ride-hailing platforms raise severe privacy issues. To provide excellent autonomous taxi service, some significant security and privacy problems must be addressed. In this study, we present the security and privacy threats and first proposed blockchain-based anonymous ride-hailing scheme (BBARHS) for autonomous taxi network. We give the formal system model and security model of BBARHS. Then, we outline the concrete BBARHS scheme by making use of Monero and some efficient crypto tools. Through security analysis and performance analysis, the designed scheme is provably secure and efficient. The analysis results also show the designed BBARHS scheme is practical for autonomous taxi network.

Metaphorical Description and Model Description of Complex Systems.

From metaphorical description to model description is the symbol of the maturity of complexity science. This paper focuses on two model description methods of complex systems: formal system model and relevance theory model, and compares their advantages and disadvantages. These two models are descriptions of complex systems from different perspectives and have important and irreplaceable significance in the theoretical research and practical application of complex systems. The future research direction of complex systems is not the “integration” of the two models but the alternation of the two models, which scientifically reveals the essence of complex systems from multiple perspectives. In view of the complexity of education and the scientificity of educational evaluation, a fuzzy evaluation method is proposed. Fuzzy evaluation is a mathematical thinking mode based on fuzzy theory and mathematical methods, which studies fuzzy and complex phenomena, and provides quantitative relationship and description for educational evaluation.

Verifying the Correctness of UML Statechart Outpatient Clinic Based on Common Modeling Language and SMV

Unified-modelling language (UML) is a standard general purpose modelling language, which is widely, used in system design of banking, biological, plantation and healthcare. Recently, there are many systems of healthcare are modeled using behavioral diagram such as UML statechart for design purposes. However, the behavior of healthcare statechart is rarely verified to ensure it is behaving as we needed. In software engineering, a software should be verified before it is transform to the further phases. In this paper, a statechart of outpatient clinic is verified to ensuring the correctness of its design. Therefore, to achieve our objective, we have applied Common Modeling Language (CML) and SMV model checker for verification formal system modeling and specification of property of statechart outpatient clinic. The result shows that the statechart of outpatient clinic is behave as required and the statechart is allowable to transform to the next phase.

Moving towards a Non-Dyadic View on Service Systems and its Operationalization – Applying the Hypergraph-based Service System Model

In today’s VUCA world, that is characterized by high volatility, uncertainty, complexity and ambiguity, service provisioning is required to realize flexible and adaptable reconfiguration of service delivery systems and its stakeholders’ resources. However, services are often embedded in greater service systems and the context information of both customer and service provider form both its boundary conditions the suitable solution service. To capture the complexity and leverage the dynamic of service systems, we propose the formal service system model (SSM) method. Following general systems theory, we define boundaries for service delivery and show SSM’s applicability for ad-hoc service operations. We show its usefulness for structuring a service system for service operations, specifically scheduling, planning, and pricing of service provisioning. We contribute to service systems engineering by applying one generalizable mathematical model for both structuring and operationalizing service systems and provide insights in-to capturing the complex relationships of its components

Tool Support for Validation of Formal System Models: Interactive Visualization and Requirements Traceability

Development processes in various engineering disciplines are incorporating formal models to ensure safety properties of critical systems. The use of these formal models requires to reason about their adequacy, i.e., to validate that a model mirrors the structure of the system sufficiently that properties established for the model indeed carry over to the real system. Model validation itself is non-formal, as adequacy is not a formal (i.e., mathematical) property. Instead it must be carried out by the modeler to justify the modeling to the certification agency or other stakeholders. In this paper we argue that model validation can be seen as a special form of requirements engineering, and that interactive visualization and concepts from requirements traceability can help to advance tool support for formal modeling by lowering the cognitive burden needed for validation. We present the VisualisierbaR tool, which supports the formal modeling of railway operations and describe how it uses interactive visualization and requirements traceability concepts to validate a formal model.

Rapid Formal Verification as Requirements Stage Verification and Validation Technique

Early formal verification of a system during initial stages of development has the potential to improve the confidence in the system specifications, in addition to improving the formal verifiability throughout the software development cycle. A formal verification methodology is derived to achieve system modelling and verification during the requirements specification and system design phase. This is achieved by using Focus Stream theory for developing a formal system model derived from requirements analysis leading to a preliminary software design. This results into component architecture and it incorporates the development of component behaviour using Finite Sate Machines. The models developed for real time system encompass modelling of infinite state domains. After model development, formal property enumeration under various categories is done. An enhanced safety property verification is proposed and implemented using Linear Temporal Logic propositions, pattern based properties and system safety based property patterns. Properties verification on the model is done in order to validate and refine the specifications. Derived properties are verified using NuXMV Satisfiability Modulo Theories based algorithms. Tool verification as mandated in RTCA DO-330 is incorporated in the proposed methodology. The proposed approach is practical, implementable by system designers and scalable to handle industrial systems.

PALOT: Profiling and Authenticating Users Leveraging Internet of Things.

Continuous authentication was introduced to propose novel mechanisms to validate users’ identity and address the problems and limitations exposed by traditional techniques. However, this methodology poses several challenges that remain unsolved. In this paper, we present a novel framework, PALOT, that leverages IoT to provide context-aware, continuous and non-intrusive authentication and authorization services. To this end, we propose a formal information system model based on ontologies, representing the main source of knowledge of our framework. Furthermore, to recognize users’ behavioral patterns within the IoT ecosystem, we introduced a new module called “confidence manager”. The module is then integrated into an extended version of our early framework architecture, IoTCAF, which is consequently adapted to include the above-mentioned component. Exhaustive experiments demonstrated the efficacy, feasibility and scalability of the proposed solution.