Ramadge-Wonham Research Articles

Safe Performance of an Industrial Autonomous Ground Vehicle in the Supervisory Control Framework

A Cyberphysical system, being an autonomous guided vehicle (AGV) and having diverse applications such as thematic parks and product transfer in manufacturing units, is modeled and controlled. The models of all subsystems of the AGV are provided in discrete event systems (DES) form following the Ramadge–Wonham (R–W) framework. The safe performance of the AGV, being the desired behavior of the system, is presented in the form of desired rules and translated into a set of regular languages. Then, the regular languages are realized as supervisory automata in the framework of Supervisory Control Theory (SCT). To ease implementation and coordination of the control architecture, the supervisors are designed to be in two-state automata forms. The controllability of the regular languages, regarding the AGV, will be proved, using the physical realizability (PR) of the synchronous product of the automata of the system and the supervisors. Also, the nonblocking property of all the controlled automata will be proven to be satisfied. Simulation of the controlled AGV will validate the proposed method.

Modelling and modular supervisory control for the AODV routing protocol

The model of the Ad-hoc on Demand Distance Vector (AODV) routing protocol, being favorable in several wireless networks (Vehicular Ad-Hoc Networks (VANET), ZigBee etc.) is presented for a network with parametric number of nodes in the Ramadge Wonham (RW) framework of discrete event systems (DES). For a generic network, consisting of n nodes, the first goal is to introduce two specification rules, per node. The first rule guarantees the hierarchy of the requests of the nodes and the routing paths. The second rule offers a first level security regarding requests’ replies. The two rules are expressed as two regular languages. Each regular language is realized by a supervisor automaton. The two supervisor automata of each node compose the local controllers of a distributed supervisory control architecture controlling the total network. The physical realizability of the local controlled automaton is proved. Also, the nonblocking property and the satisfactory marked behavior of the total controlled automaton are proved. The complexity of the proposed distributed architecture is computed. To illustrate the effectiveness of the present control scheme, a four-node network simulation of the controlled automaton of the protocol is presented.

Modular supervisory control for multi-floor manufacturing processes

Due to space availability limitations and high land costs, there is an increasing development of multi-floor manufacturing (MFM) systems in urban and industrial areas. The problem of coordination in a multi-floor manufacturing process, in the Ramadge Wonham framework, is introduced. The manufacturing chain of each floor and the elevator system are modeled in the form of finite deterministic automata. The models of the multi-floor manufacturing process are parametric with respect to the number of floors and the number of manufacturing machines on each floor. The coordination desired performance is formulated in the form of desired regular languages in analytic forms. The languages are realized by appropriate supervisors in the form of finite deterministic automata. The models of the supervisors are also parametric with respect to the number of floors and the number of manufacturing machines on each floor. The total control of the coordination of the multi-floor manufacturing process is accomplished via a modular supervisory control architecture. The complexity of the supervisors as well as the complexity of the total modular supervisory architecture are determined in analytic forms with respect to the number of floors and the number of manufacturing machines on each floor. The special case of a two floor manufacturing process is presented as an illustrative example.

Privacy-preserving co-synthesis against sensor–actuator eavesdropping intruder

In this work, we investigate the problem of privacy-preserving supervisory control against an external passive intruder via co-synthesis of a dynamic mask, an edit function, and a supervisor. We attempt to achieve the following goals: (1) the system secret cannot be inferred by the intruder, i.e., opacity of secrets against the intruder, and the existence of the dynamic mask and the edit function should not be discovered by the intruder, i.e., covertness of dynamic mask and edit function against the intruder; (2) some safety and nonblockingness requirement should be satisfied. We assume the intruder can eavesdrop both the sensing information generated by the sensors and the control commands issued to the actuators. Our approach is to model the co-synthesis problem as a distributed supervisor synthesis problem in the Ramadge–Wonham supervisory control framework, and we propose an incremental synthesis heuristic to incrementally synthesize a dynamic mask, an edit function and a supervisor. The effectiveness of our approach is illustrated on an example about location privacy.

A new modeling framework for networked discrete-event systems

This paper proposes a new framework for modeling networked discrete-event systems (DES) with channel delays and losses. We construct a more elaborated model for the plant G and supervisor S by endowing them with an embedded module to preprocess the received messages and defining state transition mapping and output mapping to model the dynamics of G and S, which could provide users with flexibility to realize different mechanisms, and thus could potentially generate more permissive controllers. Two sets of new formulations are established to capture the temporal relationships between channel inputs and outputs without assuming the first-in-first-out (FIFO) property, which naturally contains the model without delays and losses in the Ramadge–Wonham framework. Based on the defined mappings of each component, the closed-loop behavior is defined to describe the concurrent system dynamics. We hope the new framework could serve as an analysis tool, and possibly the building block for a synthesis tool, for future theoretical explorations and practical implementations of networked DES control.

Synthesis of covert actuator and sensor attackers

In this work, we shall investigate the problem of covert attacker synthesis in the framework of supervisory control of discrete-event systems. Intuitively, the covertness property says that the attacker cannot reach a situation where its existence has been detected by the supervisor while no damage can be caused. We consider covert attackers that can exercise both actuator attacks (including enablement attacks and disablement attacks) and sensor attacks (restricted to sensor replacement attacks), where the (partial-observation) attackers may or may not eavesdrop the control commands issued by the supervisor. We shall develop an exponential time reduction from the covert attacker synthesis problem to the well studied Ramadge–Wonham supervisor synthesis problem, which generalizes our previous work on a reduction based approach for covert actuator attacker synthesis, for both the damage-reachable goal and the damage-nonblocking goal. We also provide discussions on conditions under which the exponential blowup in state sizes, due to the reduction construction, can be avoided.

Modeling and Control of Heterogeneous Agricultural Field Robots Based on Ramadge–Wonham Theory

Cooperation among heterogeneous agricultural field robots in an agricultural environment guarantees the advantages of effectiveness and scalability. However, traditional control theories for coordination among multiple heterogeneous robots lack a systematic modeling method and a control strategy under unstructured and uncertain environments. To handle these limitations, a novel approach based on the Ramadge–Wonham theory and discrete event system is proposed in this letter. Specifications and a supervisory controller based on discrete event models were defined from an agricultural perspective considering cooperation among heterogeneous agricultural field robots. Discrete event systems were modeled through the automata theory and the behavior of heterogeneous field robots satisfied the designed specifications. The resulting supervisor ensures that the control objectives of formation control, obstacle avoidance, movements, and path following are satisfied. The approach and architecture proposed in this study were validated using a physics-based simulator and field experiments.

Time Optimal Synthesis Based Upon Sequential Abstraction and Its Application to Cluster Tools

The Ramadge–Wonham supervisory control paradigm has been shown effective in dealing with logic control. Nevertheless, time-related performance is always one of the major concerns in industry. Current methods for synthesizing time optimal supervisors are incapable of dealing with large discrete-event systems (DESs) with massive state spaces. This paper proposes an approach for finding a time optimal accepting trace for large DESs based upon sequential language projection, and pruning. The algorithms are tested on a linear cluster tool to show their effectiveness.

Supervisory control of concurrent discrete-event systems

Concurrency is a common feature in most industrial systems, where several components can execute different actions simultaneously. In this paper, we first introduce a new feasible nonblocking concurrent supervisory control map for a concurrent system, and subsequently, new concepts of concurrent controllability and concurrent observability. Then we present a sufficient and necessary condition for the existence of a feasible nonblocking concurrent supervisory control map to achieve a given concurrent language. After introducing a new concept of prefix-closed concurrent normality, we show that the supremal concurrently controllable and prefix-closed concurrently normal sublanguages exist and are computable. Our emphasis here is not to address the computational efficiency of solving concurrent supervisory control problems, which is still a technical challenge, but to show that the commonly used Ramadge–Wonham asynchronous control is simply a special case of our proposed concurrent control framework by providing an insightful treatment on compound events.

The synthesis and PLC implementation of hybrid modular supervisors for real time control of an experimental manufacturing system

In this paper, synthesis and PLC based implementation of hybrid modular supervisors for real time supervisory control of an experimental manufacturing system are proposed. The hybrid approach couples Ramadge–Wonham (RW) supervisors in the form of automata to uncontrolled PN models through inhibitor arcs. The RW supervisors can be obtained in monolithic or modular forms. In the monolithic case, there is only a single supervisor that has complex structure and huge number of states and events. The modularity of supervisors provides simple and small control structures compared to monolithic ones. Modular hybrid approach offers fewer states for the PLC implementation of the hybrid controller with less memory requirements. The applicability and effectiveness of the modular hybrid approach are demonstrated by the PLC based real-time control of an experimental manufacturing system for different cases. The obtained results show that modular supervisors require less memory space compared to monolithic counterparts.

An improved hybrid approach for the PLC-based implementation of reduced RW supervisors

A successful application of a hybrid (mixed Petri net/automaton) approach for the real-time supervisory control of an experimental manufacturing system was reported recently. The hybrid approach includes a Ramadge--Wonham (RW) supervisor in the form of an automaton. The reduced RW supervisor offers fewer states for the programmable logic controller (PLC) implementation of the hybrid controller with less memory requirements, but due to a problem called the avalanche effect, the PLC implementation of the hybrid controller with the reduced RW supervisor was not possible. This paper proposes a method to both detect and eliminate the avalanche effect problem for a RW supervisor in the form of an automaton, which enables the PLC implementation of the hybrid controller with the reduced RW supervisor. In addition, this paper improves the recently proposed hybrid approach by including the reduced RW supervisor and the avalanche effect detection and elimination method. The applicability of the improved hybrid approach is demonstrated by the PLC-based real-time control of an experimental manufacturing system.

A general technique for the PLC-Based implementation of RW supervisors with time delay functions

The Supervisory Control Theory (SCT) introduced by Ramadge–Wonham (RW) is a general theory to design supervisors (controllers) for discrete event systems. Although over the two decades SCT has received wide attention in academia, industrial applications are very few, due to the fact that there is a discrepancy between the abstract RW supervisor and its physical implementation. In this paper, an easy to use, general and practical technique is proposed for the PLC-based implementation of RW supervisors with time delay functions. The applicability of the proposed method is demonstrated by means of a PLC-based real-time control of an experimental manufacturing system.

Nonconflict check by using sequential automaton abstractions based on weak observation equivalence

In Ramadge–Wonham supervisory control theory we often need to check nonconflict of plants and corresponding synthesized supervisors. For a large system such a check imposes a great computational challenge because of the complexity incurred by the composition of plants and supervisors. In this paper we present a novel procedure based on automaton abstractions, which removes internal transitions of relevant automata at each step, allowing the nonconflict check to be performed on relatively small automata, even though the original product system can be fairly large.

Hierarchical Supervisory Control Based on Discrete Event Systems With Flexible Marking

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper proposes a two-level hierarchical supervisory control scheme for a discrete event system (DES), where the low-level is represented in the standard Ramadge–Wonham framework and the high level is represented by a DES with flexible marking (DESFM). As shown, the flexibility in the definition of marking provided by a DESFM simplifies the modeling of the high-level marking behavior. This approach, when compared with other approaches, requires less events and states to model the high-level, while guaranteeing hierarchical consistency. </para>

On-line scheduling and control of flexible manufacturing cells using automata theory

An effective flexible-manufacturing-cell (FMC) controller can be synthesized using classical automata and Ramadge–Wonham (R–W) supervisory-control theories. Such a discrete-event-system controller/supervisor would enable or disable (controllable) events for maximum deadlock-free, correct behaviour of the FMC. However, in cases of multiple part-routes, machine redundancy, etc. the R–W supervisor could include states with subsequent multiple controllable events. The question of choice arises at such states necessitating the use of an on-line decision-making agent, which, consequently, would determine the overall performance of the FMC. In the above context, this paper presents a novel methodology for the on-line, deadlock-free scheduling and control of FMCs. A two-phased method is proposed. During the off-line phase, the FMC is first modelled using time-augmented automata and a deadlock-free supervisor is synthesized using R–W control theory. Subsequently, an off-line decision-making plan is constructed. During the on-line phase, based on the latest state of the workcell and the off-line plan, the best-possible scheduling decisions are made using a real-time optimization search technique. The proposed novel approach is illustrated through a typical manufacturing-cell simulation example and compared with a random-based decision-making policy. It is clearly shown that significant improvement is achieved when using the proposed approach.

A hybrid approach to supervisory control of discrete event systems coupling RW supervisors to Petri nets

In this paper a hybrid approach is proposed for supervisory control of discrete event systems (DES) subject to forbidden states. Assuming that an uncontrolled bounded Petri net (PN) model of a (plant) DES and a set of forbidden state specifications are given, the proposed approach computes a maximally permissive and nonblocking closed-loop hybrid model. The first step is to simplify the given PN model by means of PN reduction rules. The simplified model and the specifications are then represented as buffers, and supervisory control theory (SCT) is applied to obtain a Ramadge–Wonham (RW) supervisor in the form of an automaton. After reduction of the latter’s state size by a ‘control congruence’, the simplified RW supervisor is represented by a so-called auto-net and coupled to the given uncontrolled PN plant model by means of inhibitor arcs to represent the disabling actions. The plant model and supervisor auto-net run concurrently, synchronizing on shared events. This procedure provides a maximally permissive and nonblocking ‘hybrid’ (mixed PN/automaton) closed-loop controlled system. The method is straightforward logically, graphically, and technologically. Its applicability is shown by two examples, one of them a workcell from the PN control literature.