Marta Castilho Gomes

Optimal scheduling for flexible job shop operation

This paper presents a new integer linear programming (ILP) model to schedule flexible job shop, discrete parts manufacturing industries that operate on a make-to-order basis. The model considers groups of parallel homogeneous machines, limited intermediate buffers and negligible set-up effects. Orders consist of a number of discrete units to be produced and follow one of a given number of processing routes. The model allows re-circulation to take place, an important issue in practice that has received scant treatment in the scheduling literature. Good solution times were obtained using commercial mixed-integer linear programming (MILP) software to solve realistic examples of flexible job shops to optimality. This supports the claim that recent advances in computational power and MILP solution algorithms are making this approach competitive with others traditionally applied in job shop scheduling.

Reactive scheduling in a make-to-order flexible job shop with re-entrant process and assembly: a mathematical programming approach

A mixed-integer linear programming model is presented for the scheduling of flexible job shops, a production mode characteristic of make-to-order industries. Re-entrant process (multiple visits to the same machine group) and a final assembly stage are simultaneously considered in the model. The formulation uses a continuous time representation and optimises an objective function that is a weighted sum of order earliness, order tardiness and in-process inventory. An algorithm for predictive-reactive scheduling is derived from the proposed model to deal with the arrival of new orders. This is illustrated with a realistic example based on data from the mould making industry. Different reactive scheduling scenarios, ranging from unchanged schedule to full re-scheduling, are optimally generated for order insertion in a predictive schedule. Since choosing the most suitable scenario requires balancing criteria of scheduling efficiency and stability, measures of schedule changes were computed for each re-scheduling solution. The short computational times obtained are promising regarding future application of this approach in the manufacturing environment studied.

Mechanical Performance of Structural Concrete with the Incorporation of Coarse Recycled Concrete and Ceramic Aggregates

The paper presents the results of an experimental program to evaluate the viability of concrete made when various ratios of coarse natural aggregates (CNAs) are replaced with coarse recycled concrete aggregates (CRCAs), coarse recycled ceramic masonry and mortar aggregates (CRMMAs), or both. Results show that incorporation of CRCA and of CRCA and CRMMA simultaneously has no effect on compressive strength. However, this property decreases when only CRMMA is incorporated. A reduction of 23.6% in compressive strength was obtained when 50% of CRMMA was used. Splitting tensile strength is unaffected by incorporation of CRCA but it is affected by incorporation of CRMMA. The mixes with 50% of CRMMA showed a reduction of 20.1% in tensile strength. Every recycled aggregate concrete type suffered a linear decrease of modulus of elasticity as the replacement ratio of CNA by coarse recycled aggregates increased. The use of 100% CRCA caused a decrease in modulus of elasticity of 30%. In the mixes with 50% of CRMMA that decrease was 22.2%. Shrinkage was significantly affected by incorporation of course recycled aggregates, although the degree of shrinkage varied in accordance with their nature and incorporation ratio. For replacement ratios between 50 and 100%, the difference between a reference concrete (RC) and the mixes with CRCA remained constant, inside a 30% increase range.

Demolition waste generation for development of a regional management chain model

Even though construction and demolition waste (CDW) is the bulkiest waste stream, its estimation and composition in specific regions still faces major difficulties. Therefore new methods are required especially when it comes to make predictions limited to small areas, such as counties. This paper proposes one such method, which makes use of data collected from real demolition works and statistical information on the geographical area under study. Based on a correlation analysis between the demolition waste estimates and indicators such as population density, buildings ageing index, buildings density and land occupation type, relationships are established that can be used to determine demolition waste outputs in a given area. The derived models are presented and explained. This methodology is independent from the specific region with which it is exemplified (the Lisbon Metropolitan Area) and can therefore be applied to any region of the world, from the country to the county level. Generation of demolition waste data at the county level is the basis of the design of a systemic model for CDW management in a region. Future developments proposed include a mixed-integer linear programming formulation of such recycling network.

A discrete time reactive scheduling model for new order insertion in job shop, make-to-order industries

This research presents a new reactive scheduling methodology for job shop, make-to-order industries. An integer linear programming formulation previously developed by the authors to schedule these types of industries is extended to address the problem of inserting new orders in a predetermined schedule, which is important in order-driven industries. A reactive scheduling algorithm is introduced to iteratively update the schedules. Numerical results on realistic examples of job shops of different sizes illustrate the effectiveness of the approach. In each case, different alternatives for inserting a set of new orders in an initial schedule are optimally generated, enabling the user to choose the most convenient one. Solutions are characterised by measures of scheduling efficiency as well as stability measures that assess the impact of rescheduling operations in a previously defined scheduling solution.

The crew timetabling problem: An extension of the crew scheduling problem

In some urban transportation companies driving periods are short when compared with the total duty time, leading to long non-driving periods that can be used as cover time. This paper presents the Crew Timetabling Problem, an extension of the Crew Scheduling Problem in which crew timetables are obtained by levelling the cover crew resources. An objective function for this problem is proposed in order to balance the number of driving and cover crews. A Lisbon Underground case study is used to illustrate the Crew Timetabling Problem. The problem is represented in a multigraph and solved by a tabu search-based heuristic.

Optimization of Construction and Demolition Waste Management: Application to the Lisbon Metropolitan Area

This work presents a mixed integer linear programming (MILP) model of a construction and demolition waste (CDW) recycling network that includes a methodology for environmental impact assessment (Eco-indicator 99). Applied to a geographical area, this generic formulation can be used for feasibility studies and sensitivity analyses of the input parameters, while simultaneously defining the optimal network design, i.e., the location and capacity of the installed facilities and material flows. Furthermore, by using the e-constraint methodology, the Pareto Front is drawn, allowing the decision maker to choose the solutions presenting the best tradeoff between economic and environmental goals.

Scheduling of job shop, make-to-order industries with recirculation and assembly: a MILP approach

This paper studies the performance of two Mixed Integer Linear Programming (MILP) models to solve scheduling problems in a flexible job shop environment with recirculation and assembly using a due-date-based objective function. The models convey different approaches both in the modelling of time (discrete and continuous approaches) as well as in the assignment of jobs to machines. In the discrete time formulation, time is modelled explicitly through a time index with no binary variables being required for machine assignment. In the continuous time formulation, modelling of time is implicit and jobs are assigned to machines through binary variables. The formulations are compared by scheduling a job shop system considered close to the reality of the mould making industry. Both models were tested for sets that range from 10 to 40 jobs to be scheduled and for operational scenarios of (1) a single machine per operation and (2) two or more machines in parallel per operation. As a main conclusion it can be stated that in the first scenario the continuous time formulation performs better than its discrete counterpart while in the second scenario the discrete time formulation outperforms the continuous one.

Optimal reactive scheduling of multipurpose, make-to-order industries

Abstract This work presents a new, generalized mixed-integer linear programming (MILP) model for scheduling of multipurpose, make-to-order industries that accounts for the existence of recirculation (multiple visits to the same processing unit) and assembly to obtain final products. A reactive scheduling approach is built on the model and illustrated with an example, where different scenarios for the insertion of new orders in a predetermined schedule are optimally generated. Choosing the most suitable rescheduling policy requires balancing criteria of scheduling efficiency and stability.

Incapacidade Funcional e Ambiente Suburbano: Proposta de Tipologia para uma Relação Difícil

Introdução: Pretendeu-se conhecer a desigual distribuição dos indivíduos que apresentam incapacidades funcionais ligadas à audição, à visão e ao andar no espaço suburbano de Lisboa. Métodos: A articulação entre variáveis e unidades territoriais resultou numa tipologia que ajuda a diferenciar estes territórios e, finalmente, permitiu a criação de uma cartografia suburbana que relaciona, nas áreas mais problemáticas, episódios de incapacidade funcional e urbanismo. Resultados: Das 40 variáveis recolhidas verificou-se que com apenas oito foi possível descrever a realidade das 26 freguesias que apresentam uma taxa de risco bastante acima da média na concentração de pessoas com dificuldades funcionais. Estas freguesias estão assimetricamente distribuídas: 10 no norte e 16 no sul da Área Metropolitana. A aplicação da análise de clusters permitiu diferenciar estas freguesias em quatro tipos distintos: polarizador e envelhecido; desqualificado e residencial; antigo e rural; pouco acessível e diverso. Conclusão: Considera-se que os resultados obtidos a partir da investigação desenvolvida contribuem para o reforço do conhecimento do efeito acelerador ou inibidor do urbanismo na integração de populações com incapacidades funcionais e para a formulação de políticas urbanas mais adequadas, eficazes mas também diferenciadas, visando a inclusão sociourbana destes indivíduos.