Scheduling of distributed autonomous manufacturing systems

Deposited with permission of the author. © 1995 Dr. Ambalavanar TharumarajahThis thesis addresses the scheduling and control of shop-floor production units that operate in a highly autonomous and distributed environment. The distinct feature of this environment is the heterarchical nature of the control where the scheduling function is quite independently carried out by the units. The units solve only part of the overall problem while resolving conflicts to maintain consistent global schedules. The need for communication and coordination, in such circumstances, introduces many complexities that affects the quality of the schedules produced. These include lapses of open-loop control due to uncertainty of up-to-date status information, asynchronous behaviour, and uncontrollable propagation of conflicts. A behaviour-based approach is introduced to solve these problems. Using this approach, the organisation of the shop-floor is viewed as similar to a colony of ants or an eco-system. The units operate quite independently but continue to adapt their schedules to changes in their environment. While they may not directly negotiate to resolve conflicts, their cooperation is innate or in-built through their local adaptive actions. This individual cooperative action of the units brings about a collective behaviour that produces the desired emergent global schedules. The major focus of this research is in examining the link between the individual and collective behaviours and developing a model that realises the desired scheduling functionality at the shop level. In order to achieve high scheduling performance (both locally and globally) a model of a unit incorporating dynamic problem decomposition, allocation algorithms and adaptation mechanisms is developed. For the latter, a reinforcement learning model is used to adapt the scheduling horizon. In fact, an important contribution if this research is the novel view we take of the problem and the manner of adaptation. In addition, a communication model for simulating the scheduling behaviours is designed using concepts of Holonic and other emerging concepts of manufacturing systems. The model is tested for a number of scheduling problems representing a variety of production situations. Preliminary results indicate an impressive scheduling performance comparable to well-known heuristics. Further examination indicates the types of dynamic behaviour that can be expected of such a model, including the levels of unresolved conflicts, the adaptability in the face of uncertainty, consequence of alternative communication policies and the sensitivities to adaptation. This thesis has also a strong qualitative theme in reviewing and consolidating the concepts underlying the design and operational attributes of autonomous distributed organisations of the shop-floor

Centralised on-line scheduling of a flexible manufacturing system

Abstract not available

Intermetallic Phase Formation And Growth In The Mg-Y System

Rare earths have been added to magnesium alloys in order to improve the creep resistance, corrosion resistance and strength. Solid-to-solid diffusion couples were assembled between Mg (99.9%) and Y (99.9%) to investigate the formation and growth of intermetallic phases and interdiffusion in the Mg-Y system. The diffusion anneals were performed at 450, 500 and 550°C for 360, 240 and 120 hours, respectively. The intermetallic layers that developed were the δ-Mg2Y and ε-Mg24Y5 phases, however the MgY phase did not form. A substantial penetration of Y in Mg was observed, however along with Kirkendall porosity that indicates faster diffusion of Mg than Y in Mg solid solution. The activation energies for parabolic growth in ε-Mg24Y5 and δ-Mg2Y were calculated to be 84 kJ/mol and 77 kJ/mol, respectively

A knowledge-based approach for on-line scheduling of an FMS

Abstract not available

Holonic scheduling of a flexible manufacturing system

Abstract not available

Environmental life-cycle comparisons of steel production and recycling: Sustainability issues, problems and prospects

This paper reports on historical analysis of the steel industry in which crude steel production trends are quantified for the period from 1950 to 2006. On the basis of this analysis, the future production of steel for the world is estimated using regression analysis. The historical analysis shows that the world steel production increased from 187. Mt to 1299. Mt in that period. In addition, the paper also reports on historical (1950-2006) steel scrap consumption and was compared with crude steel and electric arc furnace (EAF) steel production. Since 1950, scrap consumption by steel industry worldwide has been growing at 12% per annum whereas the EAF share of steel production has been increasing at 66% per annum. Furthermore, since 1987 iron ore prices have increased at 24% per annum whereas scrap prices have grown by 13% per annum.From the analysis on environmental benefits of steel recycling, it was established that there are numerous advantages of scrap utilisation. The major environmental benefits of increased scrap usage comes from the very fact that production of one tonne of steel through the EAF route consumes only 9-12.5GJ/tcs, whereas the BOF steel consumes 28-31GJ/tcs and consequently enormous reduction in CO2 emissions. In addition, a discussion on various alloying elements in steel and their presence in residual concentrations in the scrap on steel properties is also presented. Finally, this paper presents a discussion on policy issues that could enhance the use of scrap in steel-making is also presented