Design Method on Optimal Flexible Manufacturing Systems

SFC (Sequential Function Chart) is a kind of graphic representations, which is included in IEC standards for sequential control systems. It is introduced as a specific form of PC's (Programmable Controller's) programs. For the simplicity of graphic representations and the clarity of relations of sequences, SFC is much superior to Ladder Diagram that has been so far, widely used in sequential control systems. Each element of SFC is meaningful just by the connections and SFC is strong in the representation of a flow of control sequence. However, SFC has a weak point that it is difficult to describe logic by conditions in detail like interlock relations and to represent interrupt processing. But production rules are obviously suitable for representing logic by conditions and also for interrupting process.In this paper, we propose a method to translate SFC into production rules, and to implement SFC by production rules using RTPS (Real Time Production System) in which PSL (Production System Language) is used to describe production rules. SFC has three description elements: step representing states in systems, transition representing transition conditions of systems, and link representing the connections between step and transition. We classify the flows of SFC to three types: single sequence, selective sequence and parallel sequence, and, show that each sequence can be described by pairs of transition-step. Conceming a pair of transition-step, a rule for translating the pair of transition-step into production rules is presented. These production rules will be represented by PSL. Finally, we show an example that is implemented by an RTPS.

According to the requirements of IEC61131-3 standard, the system of Programmable Logic Controller (PLC) using Sequential Function Chart (SFC) as the main programming language is build. Aiming at the requirements of the SFC programming system, the framework of SFC is established, on this basis, a suit of data structure based on Object-Oriented thinking is designed. It describes the data structure of SFC mainly, and in this way, the functions of SFC, including edit, modify and compile, etc, are realized.The syntax check and logic check of SFC are achieved by traveling recursively and iterating the layers of the nested data structure of SFC. After making SFC into intermediate code, eventually the intermediate code is translated into the corresponding object code.

Proposed new troubleshooting methods for programmable controllers (PCs). Through studies of field applications in the tire industry the sequential function chart (SFC) is recognized as not only a powerful substitution for ladder logic diagram but also as useful in machine trouble diagnosis. Typical enhancements of SFC as self diagnostic function are; 1) to search the conditions which cause the machine to halt, 2) to collect logging table data for the historical program paths analysis, and 3) to specify the cause of the trouble by tracing back 1) and 2). This paper discusses the self diagnostic functions by SFC and its implementation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

This paper considers a whitelisting system for programmable logic controllers (PLCs). In control systems, controllers are final fortresses to continues the operation of field devices (actuators/sensors), but they are fragile with respect to malware and zero-day attacks. One of the countermeasures applicable for controllers is a whitelisting system which registers normal operations of controller behavior in a “whitelist” to detect abnormal operations via a whitelist. The previous research of the current author proposed a PLC whitelisting system with a control via a ladder diagram (LD). LD representations have a wide applicability because LDs can be implemented for all PLCs and security functions without hardware/firmware updates. However, the current status requires that all instances are manually entered in the whitelist. In this talk, we show how the setting up of the can be automatized whitelist from the PLC behavior. This paper introduces an auto-generation approach for the whitelist using sequential function chart (SFC) instead of the LD. SFC and LD are compatible representations for the PLC. Using Petri Net modeling, this paper proposes how to generate the whitelist from the SFC and how to detect abnormal operations via the whitelist. We call the SFC-based approach the model-based whitelist, the Petri Net based approach the model-based detection. Further, this paper carries out an experimental validation of the algorithms using an OpenPLC based testbed system.

Fault Detection and Isolation on Hybrid Systems by SFC

Validation of a SFC Software Specification by Using Hybrid Automata

Semantic Modelling of Hybrid Controllers for Robotic Cells

Adaptive SFC Based Supervision Algorithm on Flexible Production Systems

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.

To facilitate efficient procedure synthesis, the extended finite automata are adopted in this work to model all components in a chemical process. The intended operation is divided into several stages and each characterized with a set of unique features, e.g., stable operation, condition adjustment, material charging and/or unloading, etc. The control specifications of every stage should then be stipulated accordingly and also described with automata. The observable event traces (OETs) embedded in this system can be produced by synchronizing all aforementioned automata. The candidate operating procedures are summarized with sequential function charts (SFCs) on the basis of these OETs. The commercial package ASPEN Plus Dynamic has been used to validate these SFCs in simulation studies.

Test sequence construction from SFC specification

PLC 제어 시스템은 SFC 언어를 사용하여 설계할 경우, SFC 언어를 사용하면 제어의 흐름을 이해하기 쉽고, 유지보수가 용이하며 프로그램의 기술성이 뛰어나다. SFC 언어는 단일 시퀀스, 선택 시퀀스, 병렬 시퀀스로 나누어지며, 선택 시퀀스로 프로그래밍 하면 단일 시퀀스로 프로그램할 때보다 메모리의 크기가 커져야 한다. 본 논문에서는 선택 시퀀스의 기능을 단일 시퀀스로 구현하여 메모리의 크기를 줄여서 메모리의 효율을 향상시키는 방법을 제시하고, 실례를 통해 타당성을 확인하였다.

In this paper, the industrial boiler is the object to be controlled. Based on the analysis of its technical process and dynamic characteristics, taking into account the control requirements of security and stability, the boiler system control strategy has been designed. It is realized by Siemens PCS7 (process control system) and SMPT-1000 (super multifunction process control training system). The boiler system control strategy is implemented by Continuous Function Chart (CFC) and Sequential Function Chart (SFC). The experiment result shows that this boiler system control strategy not only ensures a smoothly running start-up process, it also meets control requirements perfectly and has good robustness.

Operation sequencing has been a key area of research and development for computer-aided process planning (CAPP). An optimal process sequence could largely increase the efficiency and decrease the cost of production. Genetic algorithms (GAs) are a technique for seeking to ‘breed’ good solutions to complex problems by survival of the fittest. Some attempts using GAs have been made on operation sequencing optimization, but few systems have intended to provide a globally optimized fitness function definition. In addition, most of the systems have a lack of adaptability or have an inability to learn. This paper presents an optimization strategy for process sequencing based on multi-objective fitness: minimum manufacturing cost, shortest manufacturing time and best satisfaction of manufacturing sequence rules. A hybrid approach is proposed to incorporate a genetic algorithm, neural network and analytical hierarchical process (AHP) for process sequencing. After a brief study of the current research, relevant issues of process planning are described. A globally optimized fitness function is then defined including the evaluation of manufacturing rules using AHP, calculation of cost and time and determination of relative weights using neural network techniques. The proposed GA-based process sequencing, the implementation and test results are discussed. Finally, conclusions and future work are summarized.

Grafcet or function charts for control systems is an international standard used for the specification and the implementation of logic controllers in manufacturing systems [1], [2]. This model is the basis of the Sequential Function Charts (SFC) international standard used for the implementation of logic controllers. Throughout the twenty years that have passed since it was defined [3], Grafcet is becoming widely used in the industry [4] and in education [5]. The main contribution of Grafcet, which uses a Petri-net like formalism, is that it allows a clear modelling of inputs and outputs and of their relations. It also allows modelling of concurrency and synchronisation. This makes Programmable Logic Controllers (PLCs) more tractable and simplifies the simulation of the control logic of the system. Many PLC builders today use the Grafcet as a specification and/or as a programming language. Among the large companies using it widely or recognising it as an internal standard are: Siemens, Renault, Peugeot, Michelin, and others. However, and in spite of its advantages, Grafcet has long been criticised because it is not supported by a formal foundation that allows to insure correctness and safety requirements on the one hand, and because it lacks adequate methodology that allows an efficient development of high quality models in the case of complex systems on the other.KeywordsDiscrete EventProof SystemDiscrete Event SystemProgrammable Logic ControllerLiveness PropertyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.