Conversion between ladder diagrams and PNs in discrete-event control design-a survey

Extended labeled Petri nets (ELPNs), i.e., labeled Petri nets with inhibitor arcs, are usually used to model the desired closed-loop behavior of a controlled discrete-event system, and, as such, their states are formed with both the controller and the plant states. However, the control logic is based on the controller states only and the interaction between controller and plant is carried out through sensor readings from the plant and control actions (forced events) from the controller. This makes ELPN not suitable for modeling the controller. Control interpreted Petri nets (CIPNs), on the other hand, include control actions in the places and sensor readings in the transitions as part of their formal structure, and so provide a better formalism for controller modeling. In this paper, we propose a two-step approach to discrete-event controller implementation, as follows: (i) we first propose a set of transformation rules to convert the initial ELPN to an equivalent CIPN, therefore extracting the control logic from the desired closed-loop behavior and (ii) we present a straightforward systematic way to translate the CIPN into a ladder diagram. We apply the results presented here to the implementation of the automation system of a plastic molding machine.

Automated guided vehicles (AGVs) are being extensively used due to their attributions of high efficiency and low costs. To assure their added value, taking a typical AGV transport system as an example, the reliability issues in AGVs are investigated in this paper. First of all, the AGV transport system was mod-elled as a phased mission that comprises a few key phases. Then, the Petri net (PN) method is applied to describe the logic of the whole phase mission and based on this, the reliability of the mission is assessed via Monte-Carlo simulation. In order to validate the reliability assessment result by the PN method, the theoretical reliability of the AGV system is also assessed through performing fault tree analysis (FTA). The comparison indicates that both methods give very similar results. Thus, it can be concluded that apart from FTA, the PNs method is also a reliable tool for AGV system reliability assessment.

As automated manufacturing systems become more complex, the need for an effective design tool to produce both high-level discrete event control systems (DECS) and low-level implementations becomes more important. Petri nets represent the most effective method for both the design and implementation of DECSs. In this paper, automation Petri nets (APN) are introduced to provide a new method for the design and implementation of DECSs. The APN is particularly well suited to multiproduct systems and provides a more effective solution than Grafcet in this context. Since ordinary Petri nets do not deal with sensors and actuators of DECSs, the Petri net concepts are extended, by including actions and sensor readings as formal structures within the APN. Moreover, enabling and inhibitor arcs, which can enable or disable transitions through the use of leading-edge, falling-edge and level of markings, are also introduced. In this paper, the methodology is explained by considering a fundamental APN structure. The conversion of APNs into the IEC1131-3 ladder diagrams (LD) for implementation on a PLC is also explained by using the token passing logic (TPL) concept. Finally, an illustrative example of how APNs can be applied to a discrete manufacturing problem is described in detail.

Petri nets have been extensively applied for specification, design, verification, performance evaluation, and simulation of discrete event control systems (DECS). The usual way to verify the dynamic behaviour of DECS design is to employ a process-based model, which proves to be impractical in application because the process-based model does not usually fully correspond to the control programming behaviour. This paper proposes a sensor-based stage Petri net (SBSPN) to represent a programmable logic controller (PLC) stage logic diagram (LD) for discrete-event control design. The new technique is able to simplify the modelling process of the complex PLC program without considering the interlocking problem within the ladder diagram. An illustrated example is used to explore the new methodology. The proposed method contains the power and flexibility of the Petri net analysis prior to the PLC implementation. It is believed that this technique is significant for industry practice.

Algebraic properties of automata associated to Petri nets and applications to computation in biological systems

Petri nets have been extensively applied for specification, design, verification, performance evaluation, and simulation of discrete-event control systems. A real-time Petri net is a further improvement over the timed Petri net. The paper proposes to use a real-time Petri net to represent the stage programming of a programmable logic controller for a discrete-event control system. An illustrated example is used to explore the Petri net-based PLC stage programming methodology.

On the Development of Matrixc, a Discrete Event Controller Supervisor: Implementation of a Fuzzy Logic Dispatching Approach

To simulate and debug or diagnose manufacturing systems is always challenging. This paper presents a hybrid PLC-controlled and PC Visual Basic (VB)-based Human-Machine Interface (HMI) platform based on Petri net (PN) theory for Manufacturing Systems. An augmented PN model was adopted to design and simulate the operational sequence of the Flexible Manufacturing System (FMS) at the National Kaohsiung First University of Science and Technology (NKFUST). With the simulated PN model, the control sequences were quickly translated into PLC programs. A PC-based HMI monitoring system based on the PN model was developed to provide an intuitive offline simulation and online diagnostic tools for the FMS. This hybrid system shows many of advantages by using the economical discrete control unit with PLC, and PC-based HMI using PN model as a base. The discrete event control sequence of an FMS can be easily modeled and evaluated before its build-up. More importantly, it can be diagnosed online when the system is malfunctioning. That is, the presented system provides an adequate means for offline system design, simulation, performance evaluation, and quick online system diagnostic.

Programmable logic controllers (PLCs) are widely used in industry. Thus PLCs usually use the IEC 1131-3 standard programming language. The ladder diagram is the most popular language which does not require a lot of the steps and sequences of the operation. Therefore it is difficult to understand the program structure and developing the system. The Petri net is a graphical model that can explain the system operation easily and clearly. This paper presents the transformation structures from Petri nets to ladder diagram and possible implementation to control the process through the PLC's input and output.

Modelling and Verification of PLC-Based Systems Programmed with Ladder Diagrams

There are several critical processes in the industry that are automated with the goal of optimizing available resources and increasing production of products such as gas and oil. Ladder diagrams is a language used in systems developed to manufacturing industry that uses Programmable Logic Controllers. Such systems have reliability and security issues. To simulate and verify the properties of systems, mathematical tools like Petri Nets (PN) are used. Some researchers propose methods to convert Ladder in some PN class. This work presents some conversion methods, compares them briefly, and presents an implementation of one of them. Such an implementation converts Ladder code to Colored Petri Nets model.

The petri net (PN) simulation method is a common-used methodology for mission reliability analysis of phased mission system (PMS). However, traditional PN sometime become unviable for PMS modeling. To solve such problems, an extension of PN named Extended Object-Oriented Petri Net (EOOPN) has been introduced, which proves to be efficient for PMS modeling. In this paper, a software named EOOPN Tool is exploited to trace the functions of PMS mission reliability modeling and simulation. The software bases on EOOPN model. It is developed in WINDOWS-7 Operating System using C# language. A user-friendly interface is provided for EOOPN modeling and result representing. After modeling, a validity checking is permitted then a simulation could be performed. To be specific, several logical transitions are added to fulfill the need of PMS mission logic modeling. Endue transitions with color to express a firing rate or a distribution of the time delay. Furthermore, an EOOPN sub-model base is provided for OOPN modeling and a XML document that complying with the PNML standard is used for input/output file.

The optimal controller synthesis problem for discrete event control of a flexible manufacturing system (FMS) is presented in this paper. To explicitly formulate concurrent activities, multiple resources sharing, precedence constraints and dynamic routing in FMS operation, we adopt timed (place) Petri nets for problem representation. The A* based heuristic search algorithm is proposed to search for an optimal event sequence to achieve minimum-time and deadlock-free discrete event control. Based on the obtained event-driven sequence, we use two levels of specification to design the optimal sequence controller. The coordination control level consists of synchronization and parallelism of different sub-systems and is specified by decision-free Petri nets (marked graphs). The local control level consists of running elementary sequences for subsystems and is specified by sequential function charts.

In this paper, a new method is proposed for digital hardware implementation of Petri net-based specifications. The purpose of this paper is to introduce a new discrete event control system paradigm, where the control system is modeled with extended Petri nets and implemented as an asynchronous controller using circuit elements. The applicability of the proposed method is demonstrated by an asynchronous implementation of a Petri net-based discrete event control system (DECS) for an experimental manufacturing system using a Xilinx field programmable gate array (FPGA). Unlike microprocessor, microcontroller or programmable logic controller (PLC)-based software implementations or hardware-based synchronous implementations, the implementation method used in this paper is asynchronous and based on hardware offering very high speed to control fast plants at low cost. This paper is expected to serve as a guideline to show how to obtain very high speed, concurrent and asynchronous Petri net-based controllers.

Modern complex systems require intensive application of complicated supervisors. Structural simplification techniques are one of the fundamental researches in the context of automated manufacturing systems (AMSs). They can reduce implementation cost, mitigate fabrication complexity, and improve reliability. Several typical methods are developed along this direction. In order to thoroughly explore their effectiveness and performance, we make a comparison and contrast investigation among them. Several analytical results and performance measures are proposed for their qualitative and quantitative comparison. Our approach can assist researchers and practitioners better comprehend the inherit mechanisms and relative merits of these simplification methodologies and their applicability in AMSs. This research is motivated by AMSs' control; however, they are also applicable to other systems with discrete event controllers.