Dispatch Policy Research Articles

Integrating release and dispatch policies in production models

Aggregate production planning for highly re--entrant production processes is typically generated by finding optimal release rates based on clearing function models. For production processes with very long cycle times, like in semiconductor production, dispatch policies are used to cover short term fluctuations. We extend the concept of a clearing function to allow control over both, the release rates and priority allocations in re-entrant production. This approach is used to improve the production planning problem using combined release and the allocation dispatch policy. The control parameter for priority allocation, called the push-pull point (PPP), separates the beginning of the factory which employs a push policy from the end of the factory, which uses a pull policy. The extended clearing function model describes the output of the factory as a function of the work in progress (wip) and the position of the PPP. The model's qualitative behavior is analyzed. Numerical optimization results are compared to production planning based only on releases. It is found that controlling the PPP significantly reduces the average wip in the system and hence leads to much shorter cycle times.

Ordering policy for non-instantaneous deteriorating items in two warehouse environment with shortages

This paper deals with non-instantaneous deteriorating items that have a shelf life and start deteriorating after a time lag. Under some special conditions, viz., attractive price discount, low storage cost, high demand, etc; the procurement of a large amount of such items at a time is decided. Because of the fixed capacity of the own warehouse (OW), a rented warehouse (RW) is used to store the excess quantity. Further, to enhance the usefulness of merchandise or goods, the first-in first-out (FIFO) dispatch policy is followed. In the present paper, a two-warehouse inventory model for non-instantaneous deteriorating items is developed under FIFO dispatch policy; shortages at OW are also allowed. The solution procedure has been illustrated with the help of a numerical example and a comprehensive sensitivity analysis is considered. Jaggi and Verma's (2010a) two-warehouse LIFO model with non-instantaneous deterioration is also compared with the proposed FIFO model.

A Method for “Sweet Point” Operation of Re-entrant Lines

Production systems with infinite buffers typically have the lead time (IT) vs. throughput (TP) characteristic curve (CC) with a knee-type shape. Operating the system below the knee is not efficient, since TP can be increased without an appreciable increase in IT. Operating the system above the knee is also counterproductive, since IT becomes extremely large without a significant increase of TP. Thus, the desirable operating point is at the knee, which is why it is often referred to in practice as the “sweet point”. In this paper, using an empirical/analytical approach, CCs of re-entrant lines described by the bottleneck workcenter- based model with exponential machines are analyzed, the positions of the sweet point are quantified, and an open- and closed-loop job release strategies that ensure the operation at the knee are developed. The development is carried out in terms of the First Buffer First Served and Last Buffer First Served dispatch policies, although CCs for other policies can be investigated in a similar manner.

A Tree-Structured Markovian Model of the Shipment Consolidation Process

This article studies the dispatch of consolidated shipments. Orders, following a batch Markovian arrival process, are received in discrete quantities by a depot at discrete time epochs. Instead of immediate dispatch, all outstanding orders are consolidated and shipped together at a later time. The decision of when to send out the consolidated shipment is made based on a “dispatch policy,” which is a function of the system state and/or the costs associated with that state. First, a tree structured Markov chain is constructed to record specific information about the consolidation process; the effectiveness of any dispatch policy can then be assessed by a set of long-run performance measures. Next, the effect on shipment consolidation of varying the order-arrival process is demonstrated through numerical examples and proved mathematically under some conditions. Finally, a heuristic algorithm is developed to determine a favorable parameter of a special set of dispatch policies, and the algorithm is proved to yield the overall optimal policy under certain conditions.

An Integrated Load-Planning Algorithm for Outbound Logistics at Webb Wheel

We present an integrated model for simultaneous optimization of the loading and routing decisions associated with an automotive supplier’s outbound supply chain. The supplier, Webb Wheel (WW), is a manufacturer of brake drums, rotors, hubs, and spoke wheels. WW accepts new orders from customers each day. Given sufficient inventory, it combines these orders into loads, releases them based on various dispatch criteria (e.g., truck-utilization, route-utilization, or penalty-based dispatch policies) and due-date considerations, and ships them in truckloads, less-than-truckloads, and containers. Dynamically changing demand information, inventory rationing, inventory interactions among orders, and lead-time considerations are some of the challenging aspects of the problem. Our optimization model is based on the decomposition of the problem into assignment and routing subproblems. The assignment subproblem determines the transportation mode and carrier choices, while considering total transportation costs. These costs depend on a variety of factors, including destination, number of drop locations on the route, and needs of customers on the route. Given the customer clusters and transportation modes from the assignment subproblem, the routing subproblem determines the sequence of drops and the true cost of the shipment using a modified traveling salesman problem. A scalable database with a graphical user interface supports the optimization model. We test our algorithm using four months of WW data and compare these data to the company’s practice. Our results demonstrate the impact of transportation mode-specific capacities, customer locations, inventory availabilities, and due-date restrictions on outbound logistics costs. Since implementing our load-planning algorithm, WW has achieved cost savings of 4.4 percent over its previous load-planning process.

A markov decision process model for the optimal dispatch of military medical evacuation assets.

We develop a Markov decision process (MDP) model to examine aerial military medical evacuation (MEDEVAC) dispatch policies in a combat environment. The problem of deciding which aeromedical asset to dispatch to each service request is complicated by the threat conditions at the service locations and the priority class of each casualty event. We assume requests for MEDEVAC support arrive sequentially, with the location and the priority of each casualty known upon initiation of the request. The United States military uses a 9-line MEDEVAC request system to classify casualties as being one of three priority levels: urgent, priority, and routine. Multiple casualties can be present at a single casualty event, with the highest priority casualty determining the priority level for the casualty event. Moreover, an armed escort may be required depending on the threat level indicated by the 9-line MEDEVAC request. The proposed MDP model indicates how to optimally dispatch MEDEVAC helicopters to casualty events in order to maximize steady-state system utility. The utility gained from servicing a specific request depends on the number of casualties, the priority class for each of the casualties, and the locations of both the servicing ambulatory helicopter and casualty event. Instances of the dispatching problem are solved using a relative value iteration dynamic programming algorithm. Computational examples are used to investigate optimal dispatch policies under different threat situations and armed escort delays; the examples are based on combat scenarios in which United States Army MEDEVAC units support ground operations in Afghanistan.

Credit financing in a two-warehouse environment for deteriorating items with price-sensitive demand and fully backlogged shortages

In the current global market, organizations use many promotional tools in order to increase their sales. One such tool is permissible delay in payments, i.e., the buyer does not have to pay for the goods purchased immediately rather can defer the payment for a prescribed period given by the supplier. This phenomenon motivates the retailer/buyer to order a large inventory lot so as to take full benefit of credit period. But the well decorated showroom (OW) with modern facilities has a limited storage capacity. Thus the retailer has to hire a rented warehouse to store the excess units. In this scenario, retailer usually adopts two types of dispatch policy: FIFO & LIFO, depending upon the situation, e.g., nature of items/deteriorating items, location of warehouse. Further in order to survive in the market, the retailer dynamically adjusts the prices of the goods to boost the demand and enhance the revenues.In the light of these facts, this paper develops an inventory model for deteriorating items with price-sensitive demand under permissible delay in payment in a two warehouse environment. Shortages are allowed and fully backlogged. The objective of this study is to find the optimal inventory and pricing policies so as to maximize the total average profit. Further, the different trade credit scenario has been exhibited with the help of a numerical example. A comprehensive sensitivity analysis has also been carried out to advocate the implication of FIFO and LIFO dispatch policy.

A Study of Decision-Making in Troop Dispatch Policy Based on the Two-Level Theory : Focusing on the win-set changes on the dispatch of Korean troops to Iraq

A Study of Decision-Making in Troop Dispatch Policy Based on the Two-Level Theory : Focusing on the win-set changes on the dispatch of Korean troops to Iraq

Proposal of Block Bidding for Large-Scale Wind Power Energy

It is necessary for power systems to provide flexible power to balance the fluctuation and intermittency of wind power. For the large-scale and central-integrated wind generation (such as that in China), the required flexible power may be very significant and expensive. In market environment, it is important to design a proper market mechanism and price signal which can promote conventional power and load demand to integrate wind power and gain corresponding profit. Firstly, the paper analyzes the current economy and dispatch policy for wind power in China. Then, a Block Bidding mode for wind power is presented. Unlike Timely Bidding mode that power is auctioned hourly by hourly, for Block Bidding the wind power energy is divided into different energy block. And these blocks are auctioned by conventional power resources and load demands. The Block Bidding mode has some advantages: 1) subdivides wind power by energy quality, and then the value of different parts can be revealed; 2) provides incentive for conventional power resource and load demand to integrate wind power; 3) is convenient for trading and dispatching wind power. Finally, a simple numerical case is given to illustrate the validity of Block Bidding mode.

Política de despacho para evitar bloqueos en celdas de manufactura flexible del tipo multi-reentrante

<p>Una Celda de Manufactura Flexible (CMF) se puede modelar usando la teoría pertinente a sistemas de eventos discretos. La CMF integra un conjunto de recursos, operaciones y materias primas que se combinan para la elaboración de productos. Si esos recursos no se controlan adecuadamente, la CMF se bloqueará. Con el fin de evitar dicho bloqueo, en este trabajo se presentan cuatro lineamientos para construir una política de asignación de recursos a partir del estudio básico de esperas circulares y prioridades de producción. Para elaborar una CMF teórica planteada por Peng y Zhou (2003), y desde los lineamientos propuestos, se construyó una política de despacho y se efectuaron simulaciones de la celda de manufactura con y sin controlador. Como resultado, la política de despacho logró realizar las asignaciones de recursos en forma adecuada para el ensamblaje de las partes.</p>

A hybrid method of incorporating extended priority list into equal incremental principle for energy-saving generation dispatch of thermal power systems

The energy-saving generation dispatch (ESGD) policy released by Chinese Government in 2007 is a new code for optimally dispatching electric power generation portfolio in the country with the dual objectives of improving energy efficiency and reducing environmental pollution. The ESGD is substantially different from the competitive market in the developed economies, the traditional economic dispatching or the rational dispatching principle implemented in China prior to the new policy. This paper develops a hybrid method that integrates the extended priority list (EPL), the equal incremental principle (EIP) and a heuristic method to optimize daily generation schedules under ESGD. The EPL is presented to search desirable units set that satisfies the complicated duration period requirements based on thermal unit generation priority list. The EIP is developed to allocate load among the committed units within the combined set. A heuristic method is proposed to deal with inequality constraints, which usually result in difficulty for power allocation, and used to improve these results. The algorithm has been embedded into a newly developed decision support system that is currently being used by operators of the Guizhou Province Power Grid to make day-ahead quarter-hourly generation schedules.

An integrated release and dispatch policy for semiconductor wafer fabrication

This paper presents an integrated release and dispatch policy for semiconductor wafer fabrication. In this policy, the concept of virtual production lines (VPLs) is first introduced to allocate production time for each process step, and the average queuing time information is derived from the target WIP distribution which is obtained by simulations. Then, a pull VPLs-based effective-workload control (EWL-n-Ctrl) policy is proposed to control the release process, and a queuing time ratio (QTR) rule is put forward to control the dispatch process. The behaviour of EWL-n-Ctrl is analysed using the Mini-Fab model. Results of the numerical experiments show that the integrated release and dispatch policy (EWL-n-Ctrl+QTR) outperforms common rule-based scheduling policies in terms of the average cycle time, the standard deviation of the cycle time and the work in process under a prescribed throughput level. A simulation study into the effect of capacity utilisation levels and product mix ratios is also carried out.

Reducing Ambulance Response Times Using Discrete Event Simulation

Objectives. The objectives of this study are to develop a discrete-event simulation (DES) model for the Singapore Emergency Medical Services (EMS), and to demonstrate the utility of this DES model for the evaluation of different policy alternatives to improve ambulance response times. Methods. A DES model was developed based on retrospective emergency call data over a continuous 6-month period in Singapore. The main outcome measure is the distribution of response times. The secondary outcome measure is ambulance utilization levels based on unit hour utilization (UHU) ratios. The DES model was used to evaluate different policy options in order to improve the response times, while maintaining reasonable fleet utilization. Results. Three policy alternatives looking at the reallocation of ambulances, the addition of new ambulances, and alternative dispatch policies were evaluated. Modifications of dispatch policy combined with the reallocation of existing ambulances were able to achieve response time performance equivalent to that of adding 10 ambulances. The median (90th percentile) response time was 7.08 minutes (12.69 minutes). Overall, this combined strategy managed to narrow the gap between the ideal and existing response time distribution by 11–13%. Furthermore, the median UHU under this combined strategy was 0.324 with an interquartile range (IQR) of 0.047 versus a median utilization of 0.285 (IQR of 0.051) resulting from the introduction of additional ambulances. Conclusions. Response times were shown to be improved via a more effective reallocation of ambulances and dispatch policy. More importantly, the response time improvements were achieved without a reduction in the utilization levels and additional costs associated with the addition of ambulances. We demonstrated the effective use of DES as a versatile platform to model the dynamic system complexities of Singapore's national EMS systems for the evaluation of operational strategies to improve ambulance response times.

Traffic Dispatch Policy in a Power-Aware Router

In this paper, we present a new traffic dispatch policy for a power-aware router, which was named GreenRouter and was proposed in our previous publication [. Packet processing capacities, mainly including IP lookup and forwarding engines, are shared and modulated according to traffic loads, unoccupied capacities are powered off in order to save power. A flow based traffic dispatch policy was proposed to allocate traffic from network interfaces to processing engines dynamically. Significant power saving has been achieved while QoS constrains have been satisfied. We present a new packet-based dispatch policy for GreenRouter in this paper, which can achieve more power saving. Reordering is adopted to avoid out-of-order delivery.

Lookahead actions in dispatching to parallel queues

Applying the first policy iteration (FPI) to any static dispatching (task assignment) policy yields a new improved dynamic policy that takes into account the particular cost structure and the expected future arrivals. However, it is generally hard to go beyond that due to the complex state space and the resulting difficulty in computing the value function for a dynamic policy. For example, applying FPI to identical FCFS servers with Bernoulli split gives the Least-Work-Left (LWL) policy, for which no closed-form value function is known. In fact, LWL with identical servers is equivalent to an M/G/k queue, the performance measures of which have remained as open problems. The situation gets even more complicated with heterogeneous servers. In this paper, we take an intermediate approach and consider lookahead actions that concern not only the current job but also the job arriving next, after which a basic (static) policy is assumed to take over. This is important as the benefits from some decisions can only be reaped with appropriate subsequent actions. The lookahead enables sound estimates also for marginal admission costs, e.g., with respect to LWL. The superior performance of the new near-optimal dispatching policies is demonstrated numerically.

A Dispatching Model for Server-to-Customer Systems That Balances Efficiency and Equity

The decision about which servers to dispatch to which customers is an important aspect of service systems. This decision is complicated when servers must be equitably—as well as efficiently—dispatched to customers. In this paper, we formulate a model for determining how to optimally dispatch distinguishable servers to prioritized customers given a set of equity constraints. These issues are examined through the lens of emergency medical service (EMS) dispatch, for which a Markov decision process model is developed that captures how to dispatch ambulances (servers) to prioritized patients (customers). It is assumed that customers arrive sequentially, with the priority and location of each customer becoming known upon arrival. Four types of equity constraints are considered—two of which reflect customer equity and two of which reflect server equity—all of which draw upon the decision analytic and social science literature to compare the effects of different notions of equity on the resulting dispatching policies. The Markov decision processes are formulated as equity-constrained linear programming models. A computational example is applied to an EMS system to compare the different equity models.

South Korea's Overseas Troop Dispatch Policy

This article examines South Korea's foreign policy behaviors, focusing on its policy related to the issue of troop dispatches abroad. The analysis begins with an overview of troop dispatch overseas in international politics. After careful consideration of troop dispatch policies of other countries and the policy of South Korea, the study concludes that although there are various forms and declared objectives for Seoul's troop dispatch policy, the policy exhibits a consistent pattern: South Korea's relationships with and concern about the U.S. alliance is important; gaining economic benefits, especially winning construction projects, is an essential motivation; securing energy sources is also imperative; and Seoul's concern about its image and reputation around the world—and hence the promotion of soft power—is becoming increasingly important. The findings are both theoretically and empirically important given the lack of satisfactory research on nonmajor powers’ troop dispatch policies.

Emergency medical helicopter operations for avalanche accidents

BackgroundIn the European Alps emergency medical helicopter services are regularly involved in avalanche rescue missions. How the helicopter emergency medical system best supports avalanche rescue missions is controversial. Aim of the studyTo study advantages and limitations of the early dispatch of emergency medical helicopters after avalanche accidents. MethodsData on rescue mission characteristics and patterns and severity of medical emergencies were obtained for 221 helicopter avalanche rescue missions performed in the Austrian province of Tyrol between October 2008 and June 2011. ResultsA buried avalanche victim had to be searched for in only 12 (5.5%) of the 221 rescue missions, whereas medical emergencies were encountered at the scene in 24 missions (11%). Survival rate for totally buried victims extricated after helicopter arrival was significantly lower than for victims extricated before helicopter arrival (19% versus 74%, p=0.0002). In 124 missions (56%) no victim was present at the scene when the helicopter arrived. Medical emergencies involved normothermic cardiac arrest (n=11), severe accidental hypothermia (n=6), critical trauma (n=7) and hypothermia combined with critical trauma (n=1). Survival rate at hospital discharge was 27% for arrested normothermic patients and 50% for trauma and hypothermia patients. ConclusionsMedical emergencies are encountered at avalanche scenes twice as often as there is need to search for totally buried victims, clearly supporting the immediate dispatch of medical crew members to the accident site. The high rate of emergency medical helicopter operations to avalanche incidents where no victim is injured or buried may be characteristic for densely populated mountainous regions and can be reduced by a restrictive dispatch policy after avalanche accidents without clear information about human involvement.

The Formation and Development of Civilian Police Dispatch Policy in the Miyazawa Administration: Ensuring the Security of Dispatched Personnel

The Formation and Development of Civilian Police Dispatch Policy in the Miyazawa Administration: Ensuring the Security of Dispatched Personnel

Shipment Consolidation by Private Carrier: The Discrete Time and Discrete Quantity Case

This article studies the dispatch of consolidated shipments. Orders arrive to a depot at discrete time epochs following a discrete time batch Markov arrival process (BMAP). The weight of an order is measured in discrete units and may be correlated with the arrival time. As soon as the total weight of the accumulated orders reaches a threshold, which is a function of the time elapsed since the last dispatch, all orders are consolidated and a shipment is dispatched. A discrete time Markov chain for the accumulated weight of orders in the system is introduced and analyzed. The distributions of the accumulated weight at an arbitrary time, total accumulated weight in a consolidation cycle, and excess of weight per shipment are obtained. By introducing an absorption Markov chain and a terminating Markovian arrival process, we find the distributions of the consolidation cycle length, the waiting time of an arbitrary order, and the number of orders that occur in a cycle. An efficient computational procedure is developed for evaluating dispatch policies. The model with a quantity policy and a phase-type weight distribution is studied in detail. An extensive numerical analysis is conducted to test the efficiency of the algorithm and to gain insight into these shipment consolidation models.