Thesis (MScEng)--Stellenbosch University, 2001. / 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. / AFRIKAANSE OPSOMMING: Wêreldwyd is stelsels besig om meer ingewikkeld te raak. Dit bring mee dat nuwe metodes
benodig word om hierdie ingewikkelde stelsels te beheer. Gekoppelde beplanning en beheer
("On-line planning and control") is 'n relatiewe nuwe metode om stelsels te beheer en het baie
moontlike voordele vir 'n verskeidenheid van gebruikers, veral in die vervaardigingsomgewing.
Davis [3] het 'n raamwerk ontwikkel vir gekoppelde beplanning en beheer, maar die raamwerk is
tans onvolledig. Hierdie projek het gepoog om een van die gapings in die raamwerk te vul deur
een van die funksies te outomatiseer en sodoende die behoefte vir 'n menslike waarnemer te
elimineer. Hierdie funksie, die intydse-kompromie-analise-funksie ("real-time compromise
analysis function"), is verantwoordelik vir die vergelyking van die statistiese prestasieskattings om
'n beheerbeleid te kies wat geïmplementeer moet word in die stelsel wat beheer word (die regtewêreld
-stelsel).
Die projek het twee tegnieke ontwikkel om die funksie te outomatiseer. Die eerste tegniek is
gebaseer op 'n algemene tegniek om twee stelsels statisties met mekaar te vergelyk, naamlik die
gepaarde-t vertrouensinterval-tegniek. Die gepaarde-t vertrouensinterval-tegniek word gebruik
om die beheerbeleide te vergelyk deur vertrouensintervalle te bou van die verwagte verskille vir
die verskillende vertoningskriteria en om die hipotese te toets dat die statistiese prestasieskattings
van die een beheerbeleid beter is as dié van 'n ander beheerbeleid. Die resultate van hierdie
vergelykings word dan gekonsolideer in 'n kompromiefunksie wat gebruik word om te bepaal
watter beheerbeleid tans geïmplementeer moet word in die regte-wêreld-stelsel.
Die tweede ontwikkelde tegniek is afgelei, maar verskil baie, van Davis [3] se oorheersende
waarskynlikheidsdigtheid-funksie ("dominance probability density function") -benadering en
gebruik ook idees van die gepaarde-t vertrouensinterval-tegniek. Dit vergelyk die beheerbeleide
deur die waarskynlikheid (vertrouensvlak) te bereken waarmee aanvaar kan word dat die
vertoningskriterion van een van die beheerbeleide 'n beter vertoningswaarde sal hê as die ander,
en omgekeerd. Hierdie vertrouensvlakke word dan gekonsolideer in 'n kompromiefunksie wat
gebruik word om te bepaal watter beheerbeleid tans géimplementeer moet word in die regte
wêreld stelsel.
Nadat die tegnieke ontwikkel is, was dit nie moontlik om hulle effektiwiteit wiskundig te evalueer
nie, want hulle statistiese basis is verdag. Dus moes die tegnieke geïmplementeer word voordat
hulle geëvalueer kon word. Daar is besluit om 'n emuleerder van die proses van gekoppelde
beplanning en beheer te ontwikkel volgens die raamwerk wat deur Davis [3] ontwikkel is sodat
die tegnieke geïmplementeer kan word. Hierdie Emuleerder is 'n Visual Basic* program wat
Arena" modelle gebruik. Om die Emuleerder moontlik te maak, was sekere afwykings van die
raamwerk nodig. Die eerste hiervan is dat die stelsels wat beheer word met gekoppelde
beplanning en beheer, komplekse stelsels is, maar dat die stelsel wat deur die Emuleerder beheer
word, slegs 'n eenvoudige MIMI l/EIEBI 00 I 00 sisteem is. Dit maak dit moontlik om aspekte
wat nog nie genoegsaam aangespreek is nie, byvoorbeeld die inisiëring van die stelselmodelle, te
omseil. Tweedens bevat die Emuleerder nie al die dele van die raamwerk nie en dele waarvoor die
tegnologie tans nog nie bestaan nie, is uitgelaat. Derdens, die regte wêreld stelsel is vervang met
'n model, want 'n regte wêreld stelsel was nie beskikbaar nie. Laastens is operasies wat eintlik
gelyktydig gedoen moes word, sekwensieel gedoen, maar op so 'n marrier dat dit lyk asof hulle
gelyktydig gedoen is, sodat die resultate nie beïnvloed word nie.
Die Emuleerder is gebruik om beide tegnieke te analiseer vir twee verskillende verkeersdigthede.
Die eerste deel van die analise het bestaan uit 'n nie-terminerende analise van die individuele
beheerbeleide van die stelsel. Dit is gebruik as 'n basislyn waarteen die Emuleerder se proses van
gekoppelde beplanning en beheer geëvalueer is.
Die bevindinge van die evaluasie was dat vir die verkeersdigthede wat geëvalueer is, die tegnieke
resultate lewer wat vergelykbaar is met die van die beste individuele beheerbeleide. Oor hierdie
resultate is daar gespekuleer dat by verskillende verkeersdigthede, verskillende beheerbeleide
beter sal vaar as die tegnieke, terwyl die tegnieke slegs marginale swakker resultate sal lewer. En
omdat gekoppelde beplanning en beheer poog om te reageer op veranderende omstandighede,
kan dit aanvaar word dat die tegnieke sal presteer in omstandighede waar die toevoerverdeling die
heeltyd verander. Dit word ook beweer dat die tegnieke tot voordeel sal wees in gevalle waar dit
nie moontlik is om vooraf te bepaal watter van die individuele beheerbeleide om te gebruik nie,
omdat dit onmoontlik is om te voorspel watter toevoerverdeling gerealiseer gaan word. Dit word
verwag dat die tegnieke goeie (maar ongelukkig nie noodwendig die beste nie) resultate saliewer
vir enige toevoerverdeling, terwyl 'n individuele beheerbeleid wat moontlik die beste resultate vir
die een toevoerverdeling sal gee, katastrofies kan wees vir 'n ander toevoerverdeling.
Drie belangrike gevolgtrekkings kan gemaak word van die projek. Eerstens, dit is moontlik om
die intydse-komprornie-analise-funksie te outomatiseer. Tweedens, 'n emuleerder kan ontwikkel
word om die tegnieke vir die intydse-komprornie-analise te evalueer. Die grootste voordeel van
die Emuleerder is dat dit heelwat vinniger as reële tyd kan opereer, wat dit moontlik maak om
genoeg data te genereer om die betekenisvolle statistiese vergelykings te maak wat benodig word
om die tegnieke te evalueer. Die laaste gevolgtrekking is dat, alhoewel die aanvanklike evaluasie
nie beslissend is nie, dit gewys kan word dat die tegnieke verdere studie verdien.
Drie belangrike aanbevelings kan gemaak word vanuit die projek. Eerstens, die tegnieke moet nog
verder bestudeer word, omdat daar nie beweer kan word dat hulle perfek is of dat hulle die
enigste tegnieke is wat kan werk nie. Om die waarheid te sê, hulle is slegs tegnieke wat moontlik
kan werk en ander tegnieke kan steeds bewys word om beter te wees. Tweedens sou dit onsinnig
wees om te beweer dat die Emuleerder volledig is, en moet die Emuleerder nog verbeter word.
Die mees kritiese vereiste is om die Emuleerder te ontwikkel in 'n programmeringstaal en
simulasiepakket wat gelyktydige operasies en moeitelose inisiëring toelaat. Dit sal die Emuleerder
toelaat om baie vinniger en meer buigsaam te wees. Die laaste aanbeveling is dat die tegnieke
geëvalueer moet word met ander parameters in ander stelsels van stygende kompleksiteit, wat die
hoogtepunt bereik in die evaluasie van die proses van gekoppelde beplanning en beheer met die
tegnieke ingesluit in 'n regte-wêreld buigbare vervaardigingstelsel ("flexible manufacturing
system"). Slegs dan sal dit moontlik wees om onomwonde te sê of die tegnieke effektief is of nie.
Daar word gehoop dat hierdie projek 'n waardevolle boublok sal vorm wat sal bydra om
gekoppelde beplanning en beheer 'n uitvoerbare alternatief te maak vir die beheer van komplekse
stelsels, omdat dit hulle sal toelaat om beter te reageer op die veranderende omstandighede wat
deesdae die norm is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52513 |
| Date | 12 1900 |
| Creators | Morris, J. W. |
| Contributors | Bekker, J., Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 138 p. : ill. |
| Rights | Stellenbosch University |
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