Browsing by Author "Morris, J. W."

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    The development of techniques to select a control policy during proactive on-line planning and control
    (Stellenbosch : Stellenbosch University, 2001-12) Morris, J. W.; Bekker, James F.; Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering.
    ENGLISH ABSTRACT: The worldwide trend for systems is to become more complex. This leads to the need for new ways to control these complex systems. A relatively new approach for controlling systems, called on-line planning and control, poses many potential benefits to a variety of end-users, especially in the manufacturing environment. Davis [3J developed a framework for on-line planning and control that is currently incomplete. This project aims to fill one of the gaps in the framework by automating one of the functions, eliminating the need for a human observer. This function, the real-time compromise analysis function, does the comparison of the statistical performance estimates to select a control policy for implementation in the system being controlled (the realworld system) at the current moment in time. In this project, two techniques were developed to automate the function. The first technique is based on a common technique for statistically comparing two systems, the paired-t confidence interval technique. The paired-t confidence interval technique is used to compare the control policies by building confidence intervals of the expected differences for the respective performance criteria and testing the hypothesis that the statistical performance estimates of the one control policy are better than those of the other control policy. The results of these comparisons are then consolidated into a compromise function that is used to determine the control policy to be implemented currently in the real-world system. The second developed technique is derived, but differs greatly, from Davis's [3J dominance probability density function approach, and it includes principles of the paired-t confidence interval technique. It compares the control policies by determining the probability (confidence level) with which one can assume that the performance criterion of the one control policy will provide a performance value that is better than the other's and vie-ursa. These confidence levels are then aggregated into a single compromise function that is used to determine the control policy to be implemented currently in the real-world system. After the techniques were developed, it was not possible to determine their efficiency mathematically, because their statistical base is suspect. The techniques needed to be implemented before they could be evaluated and it was decided to develop an emulator of the on-line planning and control process in accordance with the framework given by Davis [3J to implement them. This Emulator is in essence a Visual Basic" program that uses Arena" models. However, this Emulator needed certain deviations from the framework to make it possible. Firstly, while the systems that will be controlled with the on-line planning and control process will be complex systems, the system controlled in the Emulator is only a straightforward MlM/l/FIFO/OO / 00 system. This allowed for the conditions that have not been addressed sufficiently, e.g. the initialising of the system models, to be bypassed. Secondly, the Emulator does not include all parts of the framework, and parts for which the technology does not currently exist have been excluded. Thirdly, the real-world system is replaced with a model, because a real-world system was not available for the study. Finally, concurrent operations are actually done sequentially, but in a way that makes it seem that they were done concurrently, as not to influence the results. This Emulator was used to analyse both techniques for two different traffic intensities. The first part of the analysis consisted of an off-line non-terminating analysis of the individual control policies of the system. This was used as a base line against which the on-line planning and control process of the Emulator was evaluated. The findings of the evaluations were that, at the traffic intensities evaluated, the techniques provided results that were very similar to the results of the best individual control. From these results, it was speculated that at different traffic intensities, different control policies would be better than the techniques themselves, while the techniques will only give slightly worse results. In addition, because the on-line planning and control process attempts to respond to changing conditions, it can be assumed that the techniques will excel in those conditions where the input distribution is changing continuously. It is also speculated that the techniques may be advantageous in cases where it is not possible to determine beforehand which of the individual control policies to use because it is impossible to predict the input distribution that will occur. It is expected that the techniques will give good (but unfortunately, not necessarily the best) results for any input distribution, while an individual control policy that may give the best results for one input distribution, may prove disastrous for another input distribution. Three important conclusions can be made from the project. Firstly, it is possible to automate the real-time compromise analysis function. Secondly, an emulator can be developed to evaluate the techniques for the real-time compromise analysis. The greatest advantage of this Emulator is that it can run significantly faster than real-time, enabling the generation of enough data to make the significant statistical comparisons needed to evaluate the techniques. The final conclusion is that while initial evaluations are inconclusive, it can be shown that the techniques warrant further study. Three important recommendations cart be made from the project. Firstly, the techniques need to be studied further, because they cannot be claimed to be perfect, or that they are the only possible techniques that will work. In fact, they are merely techniques that may work and other techniques may still prove to be better. Secondly, because it would be foolhardy to assume that the Emulator is complete, the Emulator needs to be improved with the most critical need to develop the Emulator in a programming language and simulation package that allows concurrent operations and effortless initialisation. This will enable the Emulator to be much faster and a lot more flexible. The final recommendation is that the techniques need to be evaluated with other parameters in other increasingly complex systems, culminating in the evaluation of the on-line planning and control process with the techniques included in a real-world flexible manufacturing system. Only then can there be decided conclusively on whether the techniques are efficient or not. It is hoped that this project will form a valuable building block that will facilitate making on-line planning and control a viable alternative to controlling complex systems, enabling them to respond better to changing conditions that are currently becoming the norm.