# STUDENTS' THESIS

## PhD Thesis

### Seyedfarhad Shafigh (2015). mathematical Models and Solution Procedures in the Design and Scheduling of Manufacturing Systems with Distributed Layouts

Numerous studies have been conducted to design facility layouts since the early 1950s. The majority of these studies have primarily focused on product layout, functional layout, cellular layout or their variants. Recent trend in manufacturing systems literature establishes the consensus that these conventional configurations do not meet the needs of today's multi-product enterprises working in dynamic environment. A promising approach to address changes in the production environment is to build facility layouts that can easily adapt to volatilities. Distributed layouts are among such facilities enabling industries to address volatilities and uncertainties.

This thesis addresses two distinct problems in facility design and scheduling for manufacturing firms operating in volatile environment and producing the multiple batches of products. In regards to the facility layout problem, a new comprehensive mathematical model that integrates layout configuration and production planning in the design of dynamic distributed layouts is formulated. The model incorporates a number of important manufacturing attributes such as demand fluctuation, system reconfiguration, lot splitting, work load balancing, alternative routings, machine capability and tooling requirements. In addition, the model allows the optimization of several cost elements in an integrated manner. These include material handling, machine relocation, setup, inventory carrying, in-house production and subcontracting costs. With respect to the scheduling problem, a mathematical formulation for scheduling of manufacturing systems with distributed layouts is developed. The objective of scheduling model is the minimization of the weighted sum of makespan and total traveling distance by the products. Thus on one hand, the problem is to find a schedule of operations on machines (the sequence and starting times of the various operations) which minimizes the overall finishing time or makespan. On the other hand, the problem is to find assignment of jobs to the machines such that total distance traveled by parts is minimized.

Optimal solutions for the proposed mathematical models can only be found for small size problems due to NP-complexity. To solve both models for larger-size problems, two hybrids metaheuristics (linear programming embedded a metaheuristic) for solving the facility design model and a genetic algorithm for the scheduling model have been developed. All proposed algorithms are thoroughly examined with an emphasis on solution convergence, solution quality and algorithm robustness. For both cases, we provide numerical results to support various managerial insights. In particular in facility design problem, we draw a managerial insight as to how high product variety and high volatility in the production environment can be accommodated without harm to operational efficiency or cost. Similarly in the scheduling study, we show that linking scheduling and material handling performance can contribute to the development of accurate models to obtain a schedule that can also greatly enhance system performance.

## Master Thesis

### Mohammad Moein Jalalian (2018). Application of Metaheuristics in Scheduling Continuous/Semi-continuous Process Industries and a Case Study

In today's competitive industry, scheduling plays a significant role in improving the efficiency of manufacturing systems. Hence, many scholars and practitioners have been researching to enhance the quality of scheduling methods. In this research, the focus is on solving a real-world scheduling problem in the food industry which was previously dealt with a very time-consuming manual method without high-quality solutions. The problem is to find the best schedule for producing multiple products on multiple machines in a semi-continuous manufacturing system. Having a continuous section in the system makes scheduling too complicated than the manual method could deal with properly. So, similar to many scheduling problems, in this thesis, metaheuristics (GA and MPSA) are applied to the problem in order to address the defects of the manual method. Selected methods show promising results and performance against the manual method used before. Statistical analysis shows better performance of the genetic algorithm while the other method is more robust to the selected parameters.

### Hayson C. H. Ko (2017). Permutation based Genetic Algorithm with Event-Scheduling/Time-Advance Algorithm as Decoder for a Flexible Job-shop Scheduling Problem

Today, numerous research support the growing scheduling problems that exist globally in competitive businesses. Scheduling needs to become efficient in order to remain relevant against competitors. Simulations need to provide results in short periods of time so that adjustments can be made and unnecessary costs can be avoided. Scheduling problems have become larger in size and greater in complexity given the rising product variations and increase in variety for manufacturing equipment. Hence, there is a practical need for genetic algorithms solving scheduling problems to be fast and versatile. This thesis introduces an event-scheduling/time-advance algorithm for the decoder to reduce the load on the genetic algorithm with a smaller global search space. Consequently, convergence can be reached sooner and larger problems can be tackled easily. The structure of this heuristic algorithm allows metrics to be easily implemented in order to give the user performance measures on the scheduling problem.

### Fatemeh Mohebalizadehgashti (2016). Balancing, Sequencing and Determining the Number and Length of Workstations in a Mixed Model Assembly Line

In this thesis, a mathematical model and a linear programming embedded genetic algorithm is developed to solve an assembly line balancing problem. The mathematical model addresses a multi-model-product manual assembly line situation where the stations are interconnected by a continuously moving convey. The objective function is to simultaneously minimize the length and number of workstation, and the cost of assigning common tasks of several models of a product to more than one station. The mathematical model can also be adjusted to handle assembly lines served by intermittent synchronous conveyor which are common in automated assembly lines. An off-the-shelf optimization packages can be used to solve the model to optimality for medium size problems (up to 3 models and 40 tasks). For larger problems, we developed a linear programming embedded multi-phased genetic algorithm. The computational performance of the hybrid genetic algorithm is very encouraging based on the results obtained by solving arbitrarily generated large size problems.

### Abenet Hodiya (2015). A Mathematical Model and a Simulated Annealing Algorithm for an Integrated Facility Layout and Cell Formation

In this thesis, we develop a mathematical model that integrates distributed layout and cell formation configurations in manufacturing systems. The proposed model incorporates a number of operational attributes such as sequence of operations, lot splitting, alternative process plans, and detailed relationships between pairs of locations along with material handling and product flow costs to determine Intra and Inter-cell layout configurations. Good solutions quality for the proposed mathematical model can only be found for small size problems because of NP-complexity. To solve the model for large size problems, an efficient simulated annealing algorithm is developed. A number of numerical examples of different sizes are presented to demonstrate the nature of the proposed model. In addition, some empirical studies to demonstrate the computational behaviors of the proposed solution procedure are presented.

### Saber Bayat Mohaved (2014). Linear Programming Assisted Genetic Algorithm for Solving a Comprehensive Job shop Lot Streaming Problem

The hybridization of metaheuristics with other techniques for optimization has been one of the most interesting trends in recent years. The focus of research on metaheuristics has also notably shifted from an algorithm-oriented point of view to a problem-oriented one. Many researchers focus on solving a problem at hand as best as possible rather than promoting a certain metaheuristic. This has led researchers to try combining different algorithmic components in order to design algorithms that are more powerful than the ones resulting from the implementation of a pure metaheuristic. In this thesis, a linear programming assisted genetic algorithm is developed for solving a flexible job-shop scheduling problem with lot streaming. The genetic algorithm searches over both discrete and continuous variables in the problem/ solution space. Linear programming model is used to further refine promising solutions in the initial population and during the genetic search process by determining the optimal values of the continuous variables corresponding to the values of the integer variables of these promising solutions. Numerical examples showed that the hybridization of the genetic algorithm with the linear programming greatly improves its convergence behavior.