Gašper Mušič

Hybrid fuzzy model-based predictive control of temperature in a batch reactor

Processes in industry, such as batch reactors, often demonstrate a hybrid and non-linear nature. Model predictive control (MPC) is one of the approaches that can be successfully employed in such cases. However, due to the complexity of these processes, obtaining a suitable model is often a difficult task. In this paper a hybrid fuzzy modelling approach with a compact formulation is introduced. The hybrid system hierarchy is explained and the Takagi–Sugeno fuzzy formulation for the hybrid fuzzy modelling purposes is presented. An efficient method for identifying the hybrid fuzzy model is also proposed. A MPC algorithm suitable for systems with discrete inputs is treated. The benefits of the MPC algorithm employing the proposed hybrid fuzzy model are verified on a batch-reactor simulation example: a comparison between MPC employing a hybrid linear model and a hybrid fuzzy model was made. We established that the latter approach clearly outperforms the approach where a linear model is used.

Original Articles: Coloured Petri net scheduling models: Timed state space exploration shortages

Abstract: The paper deals with the problem of timed state space generation and exploration in the frame of simulation-optimization approach for discrete-event systems. Coloured Petri net representation of a system is considered and corresponding techniques of timed state space generation and timed simulation are addressed. It is shown that the established simulation techniques do not perform adequately in some application relevant examples since in general, only a subset of a timed state space of a simulated system is represented. Two examples are provided to illustrate the effect of timed state space reduction. While the optimal solution is preserved within the reduced state space in one example, in the second example this is not the case and the optimum is missed. This indicates that the timed simulation technique has to be carefully designed in order to be suitable for the simulation-optimization approach.

Experimental design of an optimal phase duration control strategy used in batch biological wastewater treatment

The paper presents the design of an algorithm used in control of a sequencing batch reactor (SBR) for wastewater treatment. The algorithm is used for the on-line optimization of the batch phases duration which should be applied due to the variable input wastewater. Compared to an operation with fixed times of batch phases, this kind of a control strategy improves the treatment quality and reduces energy consumption. The designed control algorithm is based on following the course of some simple indirect process variables (i.e. redox potential, dissolved oxygen concentration and pH), and automatic recognition of the characteristic patterns in their time profile. The algorithm acts on filtered on-line signals and is based on heuristic rules. The control strategy was developed and tested on a laboratory pilot plant. To facilitate the experimentation, the pilot plant was superimposed by a computer-supported experimental environment that enabled: (i) easy access to all data (on-line signals, laboratory measurements, batch parameters) needed for the design of the algorithm, (ii) the immediate application of the algorithm designed off-line in the Matlab package also in real-time control. When testing on the pilot plant, the control strategy demonstrated good agreement between the proposed completion times and actual terminations of the desired biodegradation processes.

Production-process modelling based on production-management data: a Petri-net approach

During the development of a production control system, an appropriate model of the production process is needed to evaluate the various control strategies. This paper describes how to apply timed Petri nets and existing production data to the modelling of production systems. Information concerning the structure of a production facility and the products that can be produced is usually given in production-data management systems. We describe a method for using these data to construct a Petri-net model algorithmically. The timed Petri-net simulator, which was constructed in Matlab, is also described. This simulator makes it possible to introduce heuristics, and, in this way, various production scenarios can be evaluated. To demonstrate the applicability of our approach, we applied it to a scheduling problem in the production of furniture fittings.

Feedforward control of a class of hybrid systems using an inverse model

Abstract: In this paper we describe the design of a control algorithm for MISO systems, which can be modelled as hybrid fuzzy models. Hybrid fuzzy models present a convenient approach to modelling nonlinear hybrid systems. We discuss the formulation of a hybrid fuzzy model, its structure and the identification procedure. This is followed by a derivation of the inverse model and its implementation in the control algorithm. The control scheme we are discussing splits the control algorithm in two parts: the feedforward part and the feedback part. In the paper, we deal with the feedforward part of the control algorithm, which is based on an inverse of a hybrid fuzzy model. Next, a batch-reactor process is introduced. The modelling of the batch reactor is tackled and the results of the simulation experiments using the proposed control algorithm are presented. The experiments involved controlling the temperature of a batch reactor using two on/off input valves and a continuous mixing valve. The main advantage of the proposed approach is that the feedforward part of the control algorithm can bring the system close to the desired adjusted feasible trajectory, which avoids the need for a very complex feedback part of the algorithm. Therefore, the control algorithm presents a low computational burden, particularly comparing to the standard model predictive control algorithms. These usually require a considerable computational effort, which often thwarts their implementation on real industrial systems. Nevertheless, we show that using the proposed control approach the hybrid fuzzy model framework presents a convenient option for modelling complex systems for control purposes in practice.

Analysis of multi-agent activity using petri nets

This paper presents the use of place/transition petri nets (PNs) for the recognition and evaluation of complex multi-agent activities. The PNs were built automatically from the activity templates that are routinely used by experts to encode domain-specific knowledge. The PNs were built in such a way that they encoded the complex temporal relations between the individual activity actions. We extended the original PN formalism to handle the propagation of evidence using net tokens. The evaluation of the spatial and temporal properties of the actions was carried out using trajectory-based action detectors and probabilistic models of the action durations. The presented approach was evaluated using several examples of real basketball activities. The obtained experimental results suggest that this approach can be used to determine the type of activity that a team has performed as well as the stage at which the activity ended.

Model predictive control systems with discrete inputs

The paper proposes a model predictive control algorithm that can be applied to discrete-time hybrid systems with discrete inputs. The algorithm is based on performance driven reachability analysis. The algorithm abstracts the behavior of the hybrid system by building a tree of evolution. Nodes of the tree represent reachable state of process, and the branches connect two nodes if a transition exists between the corresponding states. To each node a cost function value is associated and based on this value, the tree exploration is driven. As soon as the tree exploration is finished, the corresponding input is applied to the system and the procedure is repeated.

IEC 61131-3 Compliant Control Code Generation from Discrete Event Models

This paper describes a control logic implementation approach, which is based on discrete event models in form of finite state machines and Petri nets. Such models may be derived during supervisory control synthesis. The approach defines a transformation of the models into an IEC 61131-3 compliant code that can be translated and downloaded into a standard industrial programmable logic controller. This way, the development and implementation phases of industrial automation projects are shortened significantly. A well proven solution libraries may be built by developed and tested models and reused when necessary

Combined simulation for process control : extension of a general purpose simulation tool

Abstract Combined discrete event and continuous views of production processes are important in designing computer control systems for both process industries and manufacturing. The paper presents an extension of the popular Matlab–Simulink simulation tool to facilitate the simulation of the discrete sequential control logic applied to continuous processes. The control system is modelled as a combined system where the discrete and the continuous parts of the system are separated and an interface is introduced between them. The sequential control logic is represented by a Sequential Function Chart (SFC). A SFC blockset is defined to enable graphical composition of the SFC and its integration into the Simulink environment. A simulation mechanism is implemented which is called periodically from the standard Simulink simulation engine and carries out the correct state transition sequence of the discrete model and executes corresponding SFC actions. Two simulation case studies are given to illustrate the possible application of the developed simulation environment: the simulation of a batch process cell, as an example from the area of process control and an example of a manufacturing system, i.e. the control of a laboratory scale modular production system.

PETRI NET BASED SUPERVISORY CONTROL OF FLEXIBLE BATCH PLANTS

Abstract The application of supervisory control concepts in the control of batch process plants is Investigated in the paper. Supervisory control issues in flexible batch plants include co-Ordination: resource allocation and process management. The paper utilises the Petri net based Supervisory control framework to develop appropriate solutions. A straightforward approach of translating developed Petri net models to industrial implementation by sequential function charts is presented. The described concepts are illustrated by a simple resource allocation problem in a batch process cell.