Net Paradigm Research Articles

A return to biased nets: new specifications and approximate Bayesian Inference*

ABSTRACT The biased net paradigm was the first general and empirically tractable scheme for parameterizing complex patterns of dependence in networks, expressing deviations from uniform random graph structure in terms of latent “bias events,” whose realizations enhance reciprocity, transitivity, or other structural features. Subsequent developments have introduced local specifications of biased nets, which reduce the need for approximations required in early specifications based on tracing processes. Here, we show that while one such specification leads to inconsistencies, a closely related Markovian specification both evades these difficulties and can be extended to incorporate new types of effects. We introduce the notion of inhibitory bias events, with satiation as an example, which are useful for avoiding degeneracies that can arise from closure bias terms. Although our approach does not lead to a computable likelihood, we provide a strategy for approximate Bayesian inference using random forest prevision. We demonstrate our approach on a network of friendship ties among college students, recapitulating a relationship between the sibling bias and tie strength posited in earlier work by Fararo.

Maximally Permissive Robustness Analysis of Automated Manufacturing Systems With Multiple Unreliable Resources

For real-world automated manufacturing systems (AMSs), the failures of multiple unreliable resources are very common. Maximal permissiveness is a key aspect for evaluating the performance of robust control strategies of failure-prone AMSs. However, there exist fewer works in the existing literature that can achieve maximally permissive robustness analysis. This may impede the applicability of existing robust control strategies in realistic AMSs. This article tackles the issue in the paradigm of Petri nets (PNs). We present an innovative mechanism to formalize unreliable resource failures by preventing all ingoing transitions of unreliable resource places from firing. Both reachability graph-based and integer linear programming-based methods are proposed to analyze the robustness of AMSs with either single or multiple unreliable resources. Several examples are proposed to illustrate the approach.

Robustness Analysis of Automated Manufacturing Systems With Uncontrollable Events Using Petri Nets

In this paper, we address the robustness analysis problem of automated manufacturing systems with uncontrollable events in the paradigm of Petri nets (PNs). First, we formalize unreliable resource failures as the removal of all ingoing transitions of unreliable resource places (denoted by unreliable transitions hereafter). Second, we obtain a necessary and sufficient condition to check the robustness of markings, so called robustness controllability theorem (abbreviated as RCT hereafter) in the paradigm of reduced reachability graph (abbreviated as R2G hereafter). All markings involved in the R2G of a PN are equivalent to that of its reachability graph, except that all arcs associated to unreliable transitions are removed from R2G. Based on RCT, the robustness of all markings in an R2G can be determined. An example is proposed to illustrate the approach. Note to Practitioners—In reality, it is an urgent issue to analyze the behaviors of automated manufacturing systems (AMSs) so as to guarantee their stable operation against resource failures, e.g., the missing of a signal or the failure of a sensor. In this connection, different methods are proposed to deal with the robustness analysis and control problem of AMSs with unreliable resources. The objective is to avoid any deadlock in the AMSs or to ensure the liveness of the subsystems that require only reliable resources when there exist resource failures. Due to limited actuating and sensing abilities, AMSs may be partially controlled, i.e., there exist events whose firing may not be inhibited by an external action. However, fewer research works consider this practical situation when handling robustness analysis and control issue, which renders the existing approaches impracticable. In this paper, we solve the robustness analysis problem of AMSs with uncontrollable events by using Petri nets. A necessary and sufficient condition is proposed to check the robustness of markings, called robustness controllability theorem (abbreviated as RCT hereafter) in the paradigm of reduced reachability graph. With the aid of RCT, the robustness of all markings can be determined so as to guarantee the flexibility of AMSs.

Robustness Analysis of Automated Manufacturing Systems With Unreliable Resources Using Petri Nets

This paper studies the maximally permissive robustness analysis of automated manufacturing systems (AMSs) with unreliable resources in the paradigm of Petri nets (PNs). Two types of robust markings, i.e., strongly robust markings and weakly robust markings, are defined in this paper. We propose robustness equivalence and non-robustness equivalence to characterize the markings that exhibit the same robustness and non-robustness, respectively. Reachability graph (or RG hereafter) is directly used to determine the robustness of markings; however, it is difficult to use in large-scale systems due to formidable computational difficulty. As an alternative, we present a reduced reachability graph (or R2G hereafter) based necessary and sufficient condition to check the robustness of markings, in terms of the liveness analysis of markings in R2G. We show that all safe markings of an R2G correspond to strongly robust markings of the corresponding RG, and deadlock markings as well as their bad markings and livelock markings as well as their bad markings of an R2G correspond to non-robust markings and weakly robust markings of the corresponding RG, respectively. Hence, the robustness of markings in an RG can be determined effectively and efficiently through the liveness analysis of markings in the corresponding R2G. Note to Practitioners—In practical manufacturing scenarios, it is urgent to analyze and control the automated manufacturing systems (AMSs) so as to ensure their continual production against any resource failure. If the failures of an AMS are not handled gracefully, the whole system may fall into a blocking. As a consequence, system production does not meet rapid manufacturing goals and objectives. In this paper, we focus on the maximally permissive robustness analysis of AMSs with unreliable resources in the paradigm of Petri nets. We define two types of robustness in terms of markings, i.e., strong robustness and weak robustness. From the viewpoint of reachability graph, we propose robustness equivalence and non-robustness equivalence among markings and present the procedures to check the robustness of markings. Furthermore, a set of necessary and sufficient conditions are established to provably ensure that the robustness of markings can be determined through liveness analysis in a reduced reachability graph. Therefore, the robustness of markings can be determined in a computationally efficient way.

Maximally Permissive Deadlock and Livelock Avoidance for Automated Manufacturing Systems via Critical Distance

The problem under consideration in this paper is how to avoid deadlocks and livelocks in the paradigm of Petri nets (PNs). Although deadlock and livelock avoidance has been extensively studied in existing literature, fewer results can be applied to practical automated manufacturing systems (AMSs) due to expensive computations and reduced permissiveness. In this paper, we propose an efficient and maximally permissive control scheme to avoid deadlocks and livelocks by using critical distance. From the perspective of the reachability graph of a PN, critical distance is equal to the maximum number of transitions among all transition sequences from any critical state to its corresponding deadlock or livelock state. First, we show how to calculate the critical distance of a PN in an efficient way by using a portion of reachable states. Then, local reachability graphs are established based on critical distance to avoid deadlocks and livelocks. The provided policy is maximally permissive since only the necessary minimum number of illegal transitions are forbidden at each state. Several representative examples are presented to illustrate our approach. Note to Practitioners—Deadlock and livelock avoidance is a crucial problem in automated manufacturing systems (AMSs). Various methods have been proposed to deal with deadlocks and livelocks by researchers and practitioners. However, there exist fewer works that focus on deep exploration regarding how long a deadlock or livelock will occur from a safe state. As a consequence, the existing works suffer from the disadvantages of being too aggressive or conservative, i.e., checking the whole state space or forbidding many acceptable states when avoiding deadlocks and livelocks. Through the analysis of a partial of reachable states, this paper derives the critical distance of Petri nets modeling AMSs, which is equal to the maximum number of transitions among all transition sequences from any critical marking to its corresponding deadlock or livelock marking. All deadlocks and livelocks, if exist, can be detected at any given marking within the critical distance. Our approach not only is maximally permissive but also explores only a partial of states, thereby mitigating state explosion problem.

Multi-Level Testing Approach for Multi-Agent Systems

The software development cycles need product testing. There is a crucial lack in testing phases of multi-agent systems. To this end, a call for an investigation of appropriate testing techniques is necessary to provide adequate software development processes and supporting tools. Among all existing solutions of the test, the model-based testing technique –MBT) has gained attention with the popularization of models both in software design and development. This technique uses a so-called abstract test model to generate abstract test cases. After their concretization, concrete test cases are submitted to the system under test. The systems outputs are finally compared to the abstract test model expected results. In this context, a model-based testing approach for multi-agent systems based on the Reference net paradigm is proposed in this paper. A running example supported by a multi-agent testing prototype, which aims at simplifying and providing a uniform and automated way for multi-agent systems testing is presented and discussed.

From strong to exact Petri net computers

Petri net paradigm of computing is concerned discussing basic types of Petri net computers and their relations. We developed a technique of parametric specification of Petri nets, built of a few connected components which are repeated depending on a parameter or a set of parameters, and composition of software to generate automatically the corresponding models in a vivid graphical form. We construct explicitly a Petri net that is a strong computer of double exponent after R.J. Lipton. Its nondeterministic computation is represented by a Petri net reachability graph to which a successful branch belongs. Then we minimise and transform the net to obtain twice smaller construct than the original one. Moreover, the obtained net possesses such a marvellous property that addingto it four priority arcs makes it an exact (deterministic) computer having a single firable sequence only. Same as the Lipton’s net, the minimized net allows the proof that the Petri net reachability problem is exponential in space. Besides, it represents an example when simple transformations allow guessing the successful branch of a nondeterministic computation. The practical significance of the approach in applications consists in a state-of-art heuristic technique of solving some NP problems in polynomial time.

Fault-Recovery and Robust Deadlock Control of Reconfigurable Multi-Unit Resource Allocation Systems Using Siphons

A multi-unit resource allocation system usually contains several processes and a number of resources with multiple units. Due to the competition for shared resources in these systems, deadlocks may occur. Recently, researchers have shown an increased awareness in deadlock control strategies for such a kind of systems without considering the dynamic changes such as processing failures and rework by using the Petri net paradigm. This article reports a new strategy for deadlock analysis and control in reconfigurable multi-unit resource systems (MRSs). We discuss a generalized class of Petri nets in which each stage of a process may require a number of units of different types of resources to perform a task. In this way, we can model more complex real systems. Thanks to a generalized class of Petri nets, i.e., the system of sequential systems with shared resources (S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> R), this article proposes an effective integrated strategy for designing robust supervisors for reconfigurable MRSs, and improves an S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> R model to achieve a new model, namely a system of sequential systems with shared resources and part-re-entry (S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> RP), which represents the procedure that a flawed product re-enters a system and is re-processed. We use a siphon-based max-controllability deadlock prevention policy (DPP) to supervise the evolution of the S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> RP, and present a comprehensive analysis to demonstrate that the controlled S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> RP is free of deadlocks. A net analysis tool (INA) is used to test and validate the resulting S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> RP.

Optimal enforcement of liveness for decentralized systems of flexible manufacturing systems using Petri nets

The decentralized supervisory structure has drawn much attention in recent years. Many studies are reported in the paradigm of automata while few can be found in the Petri net model. This paper proposes a new method for decentralized supervisory control using the Petri net paradigm. Two efficient Algorithms are developed in the proposed method. Algorithm 1 is used to compute decentralized working zones from the given LS3PR Petri net model for flexible manufacturing systems. Algorithm 2 is used to compute the decentralized controllers that enforced liveness to the decentralized working zones. The sequential assembling is used to reconnect and controlled the working zones via decentralized controllers. The decentralized controller is added to the decentralized working zones that have common elements, that is, common transitions. The proposed method has the following advantages: (i) it can be applied to a complex Petri net model for flexible manufacturing systems, (ii) the proposed methods has less computational complexity when compared with the previous methods, (iii) the proposed method can obtain a minimal number of decentralized controllers that enforce liveness of the uncontrolled Petri net model. Experimental examples are presented to explore the applicability of the proposed methods.

Global Supervisory Structure for Decentralized Systems of Flexible Manufacturing Systems Using Petri Nets

Decentralized supervisory structure has drawn much attention in recent years to address the computational complexity in designing supervisory structures for large Petri net model. Many studies are reported in the paradigm of automata while few can be found in the Petri net paradigm. The decentralized supervisory structure can address the computational complexity, but it adds the structural complexity of supervisory structure. This paper proposed a new method of designing a global controller for decentralized systems of a large Petri net model for flexible manufacturing systems. The proposed method can both reduce the computational complexity by decomposition of large Petri net models into several subnets and structural complexity by designing a global supervisory structure that can greatly reduce the cost at the implementation stage. Two efficient algorithms are developed in the proposed method. Algorithm 1 is used to compute decentralized working zones from the given Petri net model for flexible manufacturing systems. Algorithm 2 is used to compute the global controller that enforces the liveness to the decentralized working zones. The ring assembling method is used to reconnect and controlled the working zones via a global controller. The proposed method can be applied to large Petri nets size and, in general, it has less computational and structural complexity. Experimental examples are presented to explore the applicability of the proposed method.

Determinants of livelihood choice and implications for targeted poverty reduction policies: A case study in the YNL river region, Tibetan Plateau

Governmental policies that can develop advantageous endogenous livelihood transition are of great importance for anti-poverty. However, the existing research finds that in the Tibetan Plateau, there is a certain contradiction between the poverty alleviation policies and the local farmers’ needs, and thus the effect of these policies will be weakened and the goal of poverty alleviation be missed. It is therefore imperative to identify and analyze the factors influencing the livelihood choice so that policy makers and development practitioners can design policies that will tackle the poverty afflicting the majority of the local people. This paper provides a new method for anti-poverty policymaking and conducts a case study to enrich the safety net/cargo net paradigms. In this paper, household data in the YNL river region are used and a two-step clustering method is developed to classify these households into three types: hanging in (type I), stepping up (type II) and stepping out (type III). Then a multinomial logit model is adopted to identify determinants of households’ livelihood choice. The results indicate that drought poses a significant negative impact on farmers’ choice of type II, while household size, educational level of labor and medical expenditure have a significant positive impact on the choice of type II; spring frost, proportion of skilled training, borrowed money and the ratio of agricultural equipment have a significant positive impact on their choice of type III. Some anti-poverty policy implications are put forward in the last on the basis of these determinants.

On simulating Turing machines with inhibitor Petri nets

An inhibitor Petri net is constructed that simulates an arbitrary given Turing machine. The tape of the Turing machine, its program, and internal states are encoded by the marking of nine dedicated places of the Petri net. The rules of Turing machine work are encoded by a single control flow represented by a token passage within the inhibitor Petri net composed of the sequence, branch, and loop operators. Subnets that implement arithmetic, comparison, and copying operations are employed. In the Petri net paradigm of computation, a Petri net that simulates Turing machines provides the compatibility of concepts. It is a prototype of a co‐processor supplementary to the basic processor of Petri nets. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Inclusive Growth: Beyond Safety Nets?

Debates on inclusive growth have gained popularity within academic, public policy and private sector spheres. This article asks the question whether this has the potential to help move debates and policy beyond a still dominant safety nets paradigm. The emphasis on processes of growth – and hence livelihoods, employment and entrepreneurship – rather than re-distributing the benefits of growth is crucial in this. This reinforces the need to make institutions, formal and informal, an integral part of our understanding of growth, and inform policies that promote growth and inclusion simultaneously.

Supervisor Synthesis and Performance Improvement for Automated Manufacturing Systems by Using Petri Nets

For event-driven systems like automated manufacturing systems (AMSs), intelligent part dispatching, i.e., to assign various parts to proper processing routes, is crucial to gain high resource utilization and production efficiency. This work proposes an innovative dispatching mechanism, which can better these metrics by minimizing the cycle time, i.e., the mean time to produce a product, while avoiding any deadlock, i.e., no part can proceed due to circuit wait. Its prerequisite is liveness supervision; however, temporal information is associated to certain operation stages for the sake of performance evaluation and throughput maximization, i.e., to maximize the finished-part quantity per time unit. In the paradigm of Petri nets, we show the criticality to prioritize certain activities locally. Our method can well design ratio parameters among distinct processes to improve system productivity.

REINFORCED CONCRETE ELEMENTS SUBJECTED TO FIRE ATTACK

In these days of rapid urbanization the frequency of accidents due to fire are high. Structural sustenance thereafter is always doubtful. Non availability of technical literature on concrete behavior at elevated temperatures is the reason for this shortcoming. Even if such information is available, it is too inadequate to predict the longevity of the structure and its utility for occupation. In this backdrop the present study using FEM approach is taken up to evaluate the extent of damage due to fire attack making use of MSC Nastran. The accidents due to fire are more frequently caused by electric short circuiting in the control panel rooms in high raised buildings. The damage caused to column elements is often considered more critical as the columns are the principal load carrying members transferring loads from the superstructure to footings. In the present study heat propagation in a portion of a column member subjected to heat of 1000 0 C acting for duration of two hours is evaluated using MSC Nastran. The temperature variation across the thickness of concrete is studied considering the variation of thermal conductivity with temperature and porosity. The propagation of heat and the corresponding distress to concrete is determined from surface to the central core of the column along the shorter dimension. The output results obtained from the transient thermal analysis module of Nastran are fed as inputs to the structural analysis module to obtain the stresses and strains caused by fire. A large data of such information with varying parameters of temperature, duration of fire attack and concrete properties is proposed to be generated to be used later as set of training patterns to train a BOPN based neural net paradigm. The information from the neural net output would be used for working out the sizes of equivalent structural elements in the distressed structure. The revised structural configuration with the distressed RC element replaced by equivalent elements should facilitate the structural assessment.

ANALYSIS OF INFILL FRAMES USING NEURAL NETWORKS

High raised buildings are vulnerable to the lateral loads caused during natural calamities like earthquakes and cyclones. Lateral loads always affect the stability of structures. The shear walls, which are provided to resist these lateral forces, are often considered undesirable from utility point of view as they come in the way of vehicular movement in the parking area. Methods to evaluate the lateral strength from infill wall panels are studied here. In this paper, the contribution offered from infill walls to the lateral strength of the building is evaluated using a combination of FEM approach and Neural Networks. Since the integral action of the wall panels with the bounding frame is considered in this approach, the frame is called as infill frame. In the study the infill wall contribution is replaced using two diagonal struts joining the opposite corners of the frame. The Finite Element package MSC Nastran is used to evaluate the infill contribution. Frames with different aspect ratios of span to height, with and without wall openings are analysed. The strain energy of the infill frame is equated to the strain energy of frame with diagonal struts, from which the width of the equivalent struts is obtained. The equivalent diagonal struts are provided joining the opposite corners of the frame. In the present study the non linear behaviour of concrete has also been taken into consideration. To make the method more widely applicable, a large data bank with varying parameters of span/depth, beam/column sections, with and without infill openings and different grades of concrete is generated which shall be used for training a BOPN based neural net paradigm. The trained neural net is expected to furnish the width of equivalent strut, which can readily be used in the structural analysis of framed buildings. The utility of the study enhances as more training patterns are added to the neural net.

Efficient Reliability Analysis of Concurrent Software Applications Considering Software Architecture

Architecture-based reliability analysis of software applications is gaining prominence as it can provide valuable guidance to software architects during early design phases. Concurrent component execution is common among modern software applications, and hence, reliability analysis considering concurrency within the context of software architecture is essential. Our preliminary analysis approach considering concurrency suffers from state-space explosion; due to which it cannot be applied to practical software applications. This paper proposes solutions to the model specification and solution challenges arising from the state-space explosion problem. The specification challenge is alleviated using the Stochastic Reward Net (SRN) modelling paradigm which can intuitively and concisely represent concurrent software architecture at a higher level of abstraction. The computational challenge is alleviated by aggregating the original state space. Our illustrations show that aggregation introduces only a small inaccuracy into the reliability estimate, and hence, preserves the utility of the original model to produce design guidance via sensitivity analysis.

Simulation-based optimization of discrete event systems with alternative structural configurations using distributed computation and the Petri net paradigm

Decision-making on discrete event systems with alternative structural configurations is a field with application to the efficient design and operation of many systems, ranging from manufacturing facilities to communication networks. The solution of this problem may be afforded by its transformation into an optimization problem. A variety of statements for this optimization problem can be presented by using different formalisms able to describe the model of the system. These different statements allow developing diverse optimization algorithms for solving the problem, which may be very demanding for a computer. In this paper, several approaches are presented in order to reduce the computing requirements needed by the mentioned algorithms, some of them are implemented in one processor and others are based on distributed computing. In particular, this paper presents a new distributed methodology, which associates sets of alternative structural configurations of the system to different alternative aggregation Petri net (AAPNs), regarding the number of available processors. Under certain conditions, this methodology alleviates the computational requirements for every processor and speeds up the optimization process. A case-study is presented and different techniques are applied to solve it, for illustrating diverse distributed and non-distributed methodologies, regarding the available processors, as well as for comparing their relative performance.

Have the metabolic support pendulums in the intensive care unit stopped swinging?

Have the metabolic support pendulums in the intensive care unit stopped swinging?

SimQPN—A tool and methodology for analyzing queueing Petri net models by means of simulation

The queueing Petri net (QPN) paradigm provides a number of benefits over conventional modeling paradigms such as queueing networks and generalized stochastic Petri nets. Using queueing Petri nets (QPNs), one can integrate both hardware and software aspects of system behavior into the same model. This lends itself very well to modeling distributed component-based systems, such as modern e-business applications. However, currently available tools and techniques for QPN analysis suffer the state space explosion problem, imposing a limit on the size of the models that are tractable. In this paper, we present SimQPN—a simulation tool for QPNs that provides an alternative approach to analyze QPN models, circumventing the state space explosion problem. In doing this, we propose a methodology for analyzing QPN models by means of discrete event simulation. The methodology shows how to simulate QPN models and analyze the output data from simulation runs. We validate our approach by applying it to study several different QPN models, ranging from simple models to models of realistic systems. The performance of point and interval estimators implemented in SimQPN is subjected to a rigorous experimental analysis.