Majid M. Aldaihani

Hybrid scheduling methods for paratransit operations

We study an extension to the general routing problem, which deals with integrating fixed route service with the general pickup and delivery problem to create a hybrid routing problem. The primary application for such a service is a dial-a-ride system used by transit agencies to transport disabled or elderly individuals. The main aim of the integration is to reduce the vehicle miles of the on-demand vehicles while not significantly reducing the customer service level. Due to the combinatorial nature of the problem, we propose a heuristic algorithm that provides an approximate solution, which is computationally efficient for solving large sized problems. The proposed heuristic is tested using real data from a transit agency.

Modeling of machine failures in a flexible manufacturing cell with two machines served by a robot

In this study, a stochastic model is developed to analyze performance measures of a flexible manufacturing cell (FMC) under different operational conditions, including machine failures and repairs. The FMC consists of two machines served by a robot for loading and loading purposes, and a pallet handling system. The model is based on Markov processes and determines closed-form solutions for the probabilities of system states that are used to calculate system performance measures, such as production output rate and utilizations of system components under different parametric conditions and equipment failures and repairs.

Stochastic models for reliable and unreliable flexible manufacturing cells with two machines and two robots

Manufacturing systems are usually designed in automated units, called cells, and flexibility is introduced so that a variety of parts can be produced on the same set of equipment. In this paper, we consider a Flexible Manufacturing Cell (FMC) consisting of two machines, each served by a robot arm for loading and unloading purposes, and a common pallet handling system, which moves a batch of mixed parts into and out of the system. Two mathematical models are developed to study and compare operations of two cases of the described FMC. In the first case, both machines are assumed to be reliable all the time while in the second case both machines are assumed to be unreliable and subject to unexpected failures. Machining times, robot loading and unloading times, machine failure and repair times and pallet handling times are assumed to be random variables. Stochastic model of each system turns out to be a Markov chain with a transition matrix, whose size depends on the pallet capacity. Case examples are considered for each FMC, exact numerical solutions are obtained, and the production performance results, such as the production rate and equipment utilisations, are compared.

A stochastic model for the analysis of a two-machine flexible manufacturing cell

This paper presents a stochastic model to determine the performance of a flexible manufacturing cell (FMC) under variable operational conditions, including random machining times, random loading and unloading times, and random pallet transfer times. The FMC under study consists of two machines, pallet handling system, and a loading/unloading robot. After delivering the blanks by the pallet to the cell, the robot loads the first machine followed by the second. Unloading of a part starts with the machine that finishes its part first, followed by the next machine. When the machining of all parts on the pallet is completed, the handling system moves the pallet with finished parts out and brings in a new pallet with blanks. A model with these characteristics turns out to be a Markov chain with a transition matrix of size 5n+3, where n is the number of parts on the pallet. In this paper, we present exact numerical solutions and economic analysis to evaluate FMC systems, to determine optimal pallet capacity and robot speed that minimize total FMC cost per unit of production.

Mathematical models and a tabu search for the portfolio management problem in the Kuwait stock exchange

This article proposes two mathematical models to study and compare results of two cases of the portfolio selection problem in the Kuwait stock exchange (KSE) as an emerging market. The mathematical models attempt to balance the trade off between risk and return. Model-I maximises the expected return and maintains the risk to certain limits while Model-II minimises the portfolio correlation and constrains the expected return to a minimum acceptable level. Since, both models are turned out to be non-linear, a tailored tabu search algorithm is used to provide efficient solutions with reasonable amount of computational times. After testing the models using real data from KSE, the results indicated that Model-I is able to significantly beat the market using both quarterly and annual basis strategies.

Design of experiments for the analysis of the effects of pallet arrival patterns and maintenance policies on FMC productivity

This paper analyses the effects of various maintenance policies and part arrival patterns on the performance of a flexible manufacturing cell (FMC). A simulation analysis coupled with design of experiments is utilised in the study. In particular, the design of experiments includes maintenance policy as a factor with five levels and part release patterns as a factor with six levels. Effects of all the combinations of factors on the productivity of a typical FMC system are analysed using a simulation modelling approach. The results show that both factors have significant effects on manufacturing cell productivity and equipment utilisations. Releasing the pallets with longest processing time first into the cell and introducing preventive maintenance (PM) combined with corrective maintenance (CM) resulted in highest cell productivity. When a CM was started, it was found that the next scheduled PM should be combined with the CM if the time left to the next scheduled PM was 20% or less than the actual scheduled time.

Service level enhancement in newsvendor model

This paper considers a decision rule for newsvendor model with emergency order.The model involves service level revision during the consumption period of the item.Emergency order is placed if service level is dropped to a certain value. Nowadays, competition in global market forced companies to offer high service levels for customers with high expectations. An important type of service levels that businesses usually use in controlling inventory systems is the fill rate. Typically, in such businesses, a vendor orders a batch at the beginning of an inventory cycle such that a certain expected fill rate is met. An obvious shortcoming of this method of ordering is that the expected fill rate may drop to an unacceptable value during consumption period. This disruption in expected fill rate may lead to significant losses to a business. We, thus, propose a newsvendor model which allows the vendor to place an initial order that satisfies a predetermined fill rate. When the expected fill rate drops to a specific value during consumption, another order is placed to increase the expected fill rate. Propositions are developed to reduce the difficulty of the model and we devise a simple solution method which determines the initial optimal lot size, optimal lot size of the second order, and the reorder point.

Optimal wholesale price for producer in supply chain under newsboy environment

The lifecycles of technology-based products are rapidly becoming shorter through endless technological innovation. As a result, researchers and practitioners paid much attention on operations management issues for newsvendor-type products. However, due to the globalisation of market and keen competition, optimising supply chain decisions becomes very important and many researchers have turned their attention to the supply chain management for newsvendor-type products. In this paper, a model for supply chain with one producer and multiple retailers is considered. The retailers order quantities of the product based on newsvendor rule and the producer adjusts the production run accordingly. The purpose of the developed model is to find the optimal wholesale price which maximises the expected profit of the producer.

Economic selection of process mean and lot-sizing decisions for multi-newsvendor supply chain

Two related problems are integrated in this paper, the first is the targeting problem and another is production/inventory decisions in a supply chain. The supply chain under consideration consists of a supplier of raw material, a single producer, and multiple newsvendors. The producer can adjust the process mean before starting the production run. Once set to a certain target value, the process mean is not changed until the production lot is completed. At the end of a production run, the producer screens the lot and uses specification limits to evaluate the quality of the item. Nonconforming items are scrapped with no salvage value; however, conforming items are delivered to newsvendors who are subjected to random demand. If demand on a newsvendor in a season is lower than anticipated, surplus items will be returned to the producer at a certain transportation cost. We first develop a mathematical model that maximizes the expected total profit of the supply chain. Then, a table for two special functions is devised to simplify the solution method and is used to find the optimal solution of the proposed model. We also study the significance of this integration by comparing the performance of the proposed model with an independent model where the process mean selection and lot-sizing decisions are found separately.

Effect of preventive maintenance on supply chains with one producer and multiple newsvendors

It is well-known that the production decisions are influenced by the state of the production process. Therefore, the process must be maintained in top operating conditions through adequate maintenance programmes. A supply chain with one producer and multiple newsvendors is considered in this article. The production process in this supply chain is assumed to be imperfect in a sense that it may fail at any point in time during the production run. In the supply chain under consideration, if the production process fails, failure maintenance (FM) is conducted which brings the process to as-bad-as-old conditions. Regardless of the number of failures that have already occurred and after a fixed interval of time, the process undergoes preventive maintenance (PM) which brings the process to as-good-as-new conditions. The objective of the model is to determine the optimal production and maintenance schedules that maximise expected total profit of the supply chain. The proposed model is general in a sense that it is not specific to certain sector or supply chain. A closed form solution is obtained and numerical analysis is conducted on key parameters of the model. Moreover, the model is solved using diverse input data; the results show similar trends in all cases.