J. Rene Villalobos

Application of planning models in the agri-food supply chain: A review

The supply chain of agricultural products has received a great deal of attention lately due to issues related to public health. Something that has become apparent is that in the near future the design and operation of agricultural supply chains will be subject to more stringent regulations and closer monitoring, in particular those for products destined for human consumption (agri-foods). This implies that the traditional supply chain practices may be subject to revision and change. One of the aspects that may be the subject of considerable scrutiny is the planning activities performed along the supply chains of agricultural products. In this paper, we review the main contributions in the field of production and distribution planning for agri-foods based on agricultural crops. We focus particularly on those models that have been successfully implemented. The models are classified according to relevant features, such as the optimization approaches used, the type of crops modeled and the scope of the plans, among many others. Through our analysis of the current state of the research, we diagnose some of the future requirements for modeling the supply chain of agri-foods.

A tactical model for planning the production and distribution of fresh produce

We present an integrated tactical planning model for the production and distribution of fresh produce. The main objective of the model is to maximize the revenues of a producer that has some control over the logistics decisions associated with the distribution of the crop. The model is used for making planning decisions for a large fresh produce grower in Northwestern Mexico. The decisions obtained are based on traditional factors such as price estimation and resource availability, but also on factors that are usually neglected in traditional planning models such as price dynamics, product decay, transportation and inventory costs. The model considers the perishability of the crops in two different ways, as a loss function in its objective function, and as a constraint for the storage of products. The paper presents a mixed integer programming model used to implement the problem as wells as the computational results obtained from it.

America West Airlines Develops Efficient Boarding Strategies

In September 2003, America West Airlines implemented a new aircraft boarding strategy that reduces the airlines average passenger boarding time by over two minutes, or approximately 20 percent, for full and nearly full flights. The strategy, developed by a team of Arizona State University and America West Airlines personnel, is a hybrid between traditional back-to-front boarding and outside-inside boarding used by other airlines. Field observations, numerical results of analytical models, and simulation studies provided information that resulted in an improved aircraft-boarding strategy termed reverse pyramid. With the new boarding strategy, passengers still have personal seat assignments, but rather than boarding by rows from the back to the front of the airplane, they board in groups minimizing expected passenger interference in the airplane. The analytical, simulation, and implementation results obtained show that the method represents a significant improvement in terms of boarding time over traditional pure back-to-front, outside-inside boarding strategies.

Work allocation strategies for serial assembly lines under high labour turnover

Labour turnover can be very detrimental to the performance of traditional serial assembly lines. It has been shown that high turnover rates can reduce annual production throughput in serial assembly lines by more than 16% when these lines operate under a balanced work allocation strategy. This paper investigates whether alternative production methods, such as Bucket Brigades, can out-perform traditional lines by utilizing dynamic work allocation and active operator replacement policies in order to absorb better the variability introduced by labour turnover. The study performed was divided into two phases. Phase I consisted of using a simple three-station linear assembly line to compare the performance of the Bucket Brigades method against a traditional line (fixed work allocation, static operator replacement), and a method using fixed work allocation along with a dynamic replacement policy. In Phase II, a six-station serial assembly line was used to compare the performance of Bucket Brigades against the traditional balanced line method. The analyses were based on simulation models that used monthly turnover rates of 0, 6 and 12%. Phase I showed that the Bucket Brigade method outperformed the other two methods consistently when operating under moderate (6%) and high (12%) turnover rates. Phase II demonstrated that the Bucket Brigade method was better than the traditional production method in every instance, including the case with 0% turnover.

FLEXIBLE INSPECTION SYSTEMS FOR SERIAL MULTI-STAGE PRODUCTION SYSTEMS

An FIS is defined as an Inspection System where the inspection routines are determined just prior to performing the inspection. The goal is to determine the inspection routine so as to optimize global objectives (such as cost minimization) subject to global and local constraints (such as total inspection time available). The concept of an FIS provides the basis for the development of intelligent automated inspection systems. In this paper, the concept of a Flexible Inspection System (FIS) for Serial Multistage production lines is introduced, the modelling implications discussed, and numerical examples are presented.

Information-based inspection allocation for real-time inspection systems

The use of real-time Automated Visual Inspection (AVI) in the electronics assembly industry is subject to cycle time constraints determined by the manufacturing system. These constraints must be met to maintain the nominal production rate. A problem thus created is determining which components to inspect at each one of the AVI stations such that the inspection time available is used in an optimal way. This paper presents a real-time inspection allocation that is based on the information gained by inspecting one additional component. The selection of which components to inspect is modeled as an information maximization problem. A modified Knapsack greedy heuristic is used to find near-optimal solutions to this optimization problem within the required time constraints.

Vector classification of SMD images

Abstract The automated visual inspection of surface-mounted devices (SMD) requires the correct classification of an image as either “component present” or “component absent.” The inspection system must allow the classification to be fast and reliable while also assuring that the training of the classifier is simple and not time consuming. The traditional sequential approach to classifying images, while simple to implement, presents some disadvantages, including an increase in false alarm (type I) errors and the need for a time-consuming training phase. The method presented in this paper seeks to reduce classification errors by using a vector approach for the inspection of components. An experiment using real SMD inspection data is conducted to validate the performance of the proposed vector classifier. The results of the experiment support the hypothesis that the vector-based inspection approach renders fewer errors than the sequential inspection approach.

The use of dynamic work sharing production methods to reduce the impact of labour turnover in serial assembly lines

Labour turnover is one of the major complications that industry faces on a daily basis. Turnover is particularly harmful to serial assembly lines. The learning process that a new operator must undergo when first introduced to an assembly line station generates blockage and starvation in the previous and successive stations, causing a reduction in the lines throughput. In this paper, we analyse different work allocation strategies for serial assembly line designs in order to determine which designs perform best under the presence of labour turnover and a task-learning process. In particular, we compare the traditional balanced line with two other assembly line designs, a variant of the bucket brigades (BB) and a hybrid method – the modified work sharing (MWS) method. By making a judicious use of control buffers, the MWS method seeks to combine the positive characteristics of both the BB and traditional balanced lines to naturally adapt to different levels of labour turnover. The results presented in this paper show that the MWS outperforms the BB and the balanced designs.

The Stock Portfolio Game

One of the objectives of the stock portfolio game is to introduce undergraduate engineering students to the application of optimization techniques for stock portfolio selection. Ideally students playing the game would have basic background in linear algebra, calculus, and statistics. The general idea of the game is to present teams of students with real historical performance data for stocks with different levels of returns and variability so that they make decisions concerning the allocation of limited funds among these stocks. The game is played in four rounds within an hour-long session. The teams, comprised of no more than four students, are provided with an initial virtual investment fund, which they have to allocate or reallocate based on the information presented. The team with the most money in its investment account at the end of the game is declared the winner. Once the results of the game are known, one or more teams are asked to share their investment strategy with the rest of the group. This discussion forms the starting point for the last part of the session when the lecturer puts the investment game in the perspective of an optimization problem. At the end of the session basic principles of Markowitzs portfolio theory are presented and applied to the data given to the students.

Three dimensional automated visual inspection of surface mounted devices

Surface Mounted Technology (SMT) has created a demand for machine vision systems that can reliably verify component placement for screening inspection and process improvement. This paper describes a new 3-D computer vision system being developed at the Center for Electronics Manufacturing (CEM) of The University of Texas at El Paso (UTEP) for the inspection of Printed Circuit Boards (PCBs). A small range finder, utilizing a laser-based triangulation measurement method, is being employed for the development of an effective high resolution 3-D SMT inspection system.