Symbolic Interpretation and Execution of Extended Finite Automata

We introduce a symbolic interpretation and execution technique for Extended Finite Automata (EFAs) and provide an interpreter that symbolically interprets and executes EFAs w.r.t. their (internal) variables. More specifically, the interpreter iterates over the EFA transitions, and by passing each transition, it symbolically interprets and evaluates the condition on the transition w.r.t. the known values of variables, and leaves other variables intact, and when it terminates, it returns the residual model. It is shown that the behavior of the residual system with respect to the original system is left unchanged. Finally, we demonstrate the effectiveness and necessity of the symbolic interpretation and execution combined with abstractions for the nonblocking supervisory control of two manufacturing systems

Supervisory Control of Extended Finite Automata Using Transition Projection

A limitation of the Ramadge and Wonham (RW) framework for the supervisory control theory is the explicit state representation using finite automata, often resulting in complex and unintelligible models. Extended finite automata (EFAs), i.e., deterministic finite automata extended with variables, provide compact state representation and then make the control logic transparent through logic expressions of the variables. A challenge with this new control framework is to exploit the rich control structure established in RW's framework. This paper studies the decentralized control structure with EFAs. To reduce the computational complexity, the controller is synthesized based on model abstraction of subsystems, which means that the global model of the entire system is unnecessary. Sufficient conditions are presented to guarantee that the decentralized supervisors result in maximally permissive and nonblocking control to the entire system

Automatic Generation of Controllers for Collision-Free Flexible Manufacturing Systems

A method for automatic generation of non-blocking controllers that generate collision-free flexible manufacturing cells is presented in this paper. Today, industry demands on flexible production sometimes require significant changes in location, orientation and configuration of industrial robots and other moving devices, when new products are introduced. All these changes pose a threat to the devices to collide while sharing workspace. To avoid this, a formal model of the operations in a manufacturing system is generated, and for each operation state a corresponding 3D simulation shape is created. A collision-free system is then achieved by considering pairs of colliding shapes as forbidden states. The automatic generation also includes a synthesis procedure, where a non-blocking and controllable supervisor is generated based on guard generation. The guards are computed by binary decision diagrams, which means that complex systems can be handled, still generating comprehensible restrictions that are easily included in PLC-code

Abstractions for nonblocking supervisory control of Extended Finite Automata

An abstraction method for Extended Finite Automata (EFAs), i.e., finite automata extended with variables, using transition projection is presented in this work. A manufacturing system modeled by EFAs is abstracted into subsystems that embody internal interacting dependencies. Synthesis and verification of subsystems are achieved through their model abstractions rather than their global model. Sufficient conditions are presented to guarantee that supervisors result in maximally permissive and nonblocking control. An examples demonstrate the computational effectiveness and practical usage of the approach

Reduced-order synthesis of operation sequences

In flexible manufacturing systems a large number of operations need to be coordinated and supervised to avoid blocking and deadlock situations. The synthesis of such supervisors soon becomes unmanageable for industrial manufacturing systems, due to state space explosion. In this paper we therefore develop some reduction principles for a recently presented model based on self-contained operations and sequences of operations. First sequential operation behaviors are identified and related operation models are simplified into one model. Then local transitions without interaction with other operation models are removed. This reduction principle is applied to a synthesis of non-blocking operation sequences, where collisions among moving devices are guaranteed to be avoided by a flexible booking process. The number of states in the synthesis procedure and the computation time is reduced dramatically by the suggested reduction principle

Incremental and Hierarchical Deadlock-Free Control of Discrete Event Systems with Variables: A Symbolic and Inductive Approach

Today\u27s industry trend towards agile product development cycles and the ambition to shorten the time-to-market, represents an extremely competitive marketplace. This has driven the industry to use very complex and highly flexible manufacturing systems. In these systems, a large number of manufacturing operations need to be coordinated in order to fulfill the manufacturing requirements and assuring safety and deadlock-free behavior of the entire system.Supervisory control theory (SCT) is one of the formal methodologies that promises a systematic and automatic computation of controllers for coordination of manufacturing operations, more broadly, discrete event systems (DES). However, by increasing the number of operations, synthesizing controllers soon becomes unmanageable and can lead to state space explosion. This is one of the main reasons that the industrial acceptance of the SCT framework is scarce.In this thesis, we investigate this fundamental issue in the SCT framework and propose different approaches to cope with this problem. One of the challenges in synthesizing a controller for a DES is that we need to explore and examine all its possible behavior. However, for a DES with infinite behavior this process is not feasible due to time and memory limits. In this thesis, we propose a novel symbolic synthesis technique based on the IC3 algorithm, one of the most effective SMT-based model checking algorithm, for safe and maximally permissive supervisory control of infinite-state DES with variables. An evaluation of the proposed IC3-based technique on standard SCT benchmarks shows a radical improvement in computation of controllers for systems with large or infinite state space compared to BDD-based and SAT-based approaches.Furthermore, in practice, most of the manufacturing systems are distributed and often composed of several components. In this thesis, we also propose an incremental and hierarchical control architecture to obtain an effective and computationally efficient synthesis process for design of controllers for distributed DES. To this end, we exploit the efficiencies of symbolic techniques and synthesize controllers incrementally rather than at once for the entire system. Also, we use effective model abstraction techniques to abstract away unnecessary information to the synthesis process which, in turn, helps us to avoid building the entire state space of the systems. The computational effectiveness and practical usage of the introduced control architecture is illustrated by controller synthesis for a safe and deadlock-free coordination of operations in an industrial manufacturing cell

Incremental and Hierarchical Deadlock-Free Control of Discrete Event Systems with Variables

In this report, we present an incremental and hierarchical deadlock-free control architecture for discrete event systems with variables. Given a system including several components that share alphabet and variables, we first introduce partial controllers that only control parts of each component that have local control behavior w.r.t. other components, and leave other parts intact. This enables us to compute a maximally permissive (or optimal) controller for the given system in an incremental way, rather than on the entire system at once.Second, to leverage the hierarchical supervisory control approach for systems with no variables using natural projections with observer and local control consistency (LCC) properties, we lift these concepts to systems with variables by introducing a new variable-observer condition. We show that these conditions are sufficient enough for optimal hierarchical control. Furthermore, similar to the observer condition and the LCC, we formulate the variable-observer condition in terms of a quasi-congruence. An industrial manufacturing example demonstrates the computational effectiveness and practical usage of the proposed control architecture

On the Computation of Natural Observers for Extended Finite Automata

Compared to finite automata, Extended Finite Automata (EFAs) allows us to efficiently represent discrete-event systems that involve non-trivial data manipulation. However, the complexity of designing supervisors for such systems is still a challenge. In our previous works, we have studied model abstraction for EFAs using natural projections with observer property on events as well as data. In this paper, we provide sufficient conditions for verifying the observer properties and further enhance the EFAs when the property does not hold. To this end, we introduce symbolic simplification techniques for data and generalize existing algorithms in the literature for the events to compute natural observers for EFAs. The importance of this combined abstraction and symbolic simplification method is demonstrated by synthesis of a nonblocking controller for an industrial manufacturing system

A Conceptual Model and Evaluating Experiments for Studying the Effect of Soil Deformation on Its Permeability

Soil structure and void ratio are the major factors that control the permeability changes during soil deformation. In this research, we proposed and tested a conceptual model considering these two factors based on the concept of permeability anisotropy. This model, which is expressed as k(e) graph, determines the total k values that soil can achieve and shows that as deformation proceeds, soil permeability passes through a specific zone in the k(e) graph. Thus, by deforming a soil sample, measuring its permeability during deformation, and comparing the results using the k(e) graph, it might be possible to predict deformation effects on the permeability. To evaluate this conceptual model, we designed and built a special apparatus to carry out two sets of experiments. The first set was performed to achieve the k(e) graph during static compression based on the conceptual model; and the second set was conducted to investigate the permeability changes relative to k(e) graph during simple shear deformation in constant volume condition. Our results show that the theoretical k(e) graph agrees more with the measured k(e) graph in medium to dense samples that might have no macropore. In addition, particles’ preferential orientation and/or anisotropic permeability were not changed during shear deformation due to three possible causes: deformation done in constant volume deformation, relatively low shear strain, and shearing along particle orientation. Void ratio and particle orientation are associated with each other, and soil shearing with constant void ratio might cause the anisotropy of permeability to be relatively constant. Thus, it is needed to design and build a new complex apparatus or use a special method for testing how permeability changes within the k(e) graph zone during soil deformation

On the Computation of Natural Observers for Extended Finite Automata

Compared to finite automata, Extended Finite Automata (EFAs) allows us to efficiently represent discrete-event systems that involve non-trivial data manipulation. However, the complexity of designing supervisors for such systems is still a challenge. In our previous works, we have studied model abstraction for EFAs using natural projections with observer property on events as well as data. In this paper, we provide sufficient conditions for verifying the observer properties and further enhance the EFAs when the property does not hold. To this end, we introduce symbolic simplification techniques for data and generalize existing algorithms in the literature for the events to compute natural observers for EFAs. The importance of this combined abstraction and symbolic simplification method is demonstrated by synthesis of a nonblocking controller for an industrial manufacturing system