Minimizing the expected flowtime in stochastic flow shops

In this paper we consider a flow shop with blocking, n jobs, m machines, and stochastic processing times. We show that if the jobs can be stochastically ordered, then a SEPT sequence minimizes the expected flowtime on two machines.     

Reduced outpatient waiting times with improved appointment scheduling: a simulation modelling approach

Abstract. An outpatient department represents a complex system through which many patients with varying needs pass each day. An effective appointment system is a critical component in controlling patient waiting times within clinic sessions. Current waiting times are often unacceptable and place great stress on clinic staff. This paper describes the development and use of a detailed simulation model of an Ear, Nose and Throat (ENT) outpatient department. The simulation allows various appointment schedules to be examined and their effects on the clinic evaluated. The model has been used to identify a number of critical factors that influence patient waiting times and the build up of queues in the clinic. Alternative appointment schedules have been shown to drastically reduce patient waiting times, without the need for extra resources, and enable the department to move towards meeting the UK Government's Patient's Charter. RID="*" ID="*" The authors would like to express their thanks for the valuable support provided by the anonymous participants (outpatient department staff). Thanks also to Helen Gilby, Dr Arjan Shahani and Professor Valter de Senna for their useful comments and help during the research. Correspondence to: P.R. Harper     

Minimizing the expected response time of an idled server on a line

We address the problem of where to locate an idle server in a one-dimensional system where the arrival of demands for service have both spatial and temporal uncertainty. In such a system it is reasonable to have an idle server make an anticipatory move in order to better position itself for the next demand. However, moving all the way to its home base, or even moving at all, might delay the service for a demand which arrives during the move. We develop an analytical model to optimize the destination of an anticipatory move based upon the location of the server. Insights into the impact of system parameters as well as empirical examples are given.     

Minimizing the expected processing time on a flexible machine with random tool lives

We present a stochastic version of economic tool life models for machines with finite capacity tool magazines and a variable processing speed capability, where the tool life is a random variable. Using renewal theory to express the expected number of tool setups as a function of cutting speed and magazine capacity, we extend previously published deterministic mathematical programming models to the case of minimizing the expected total processing time. A numerical illustration with typical cutting tool data shows the deterministic model underestimates the optimal expected processing time by more than 8% when the coefficient of variation equals 0.3 (typical for carbide tools), and the difference exceeds 15% for single-injury tools having an exponentially distributed economic life (worst case).     

Minimizing and balancing setups in a serial production system

This paper addresses a problem arising in the coordination between two consecutive manufacturing departments of a production system, in which parts are processed in batches, and each batch is characterized by two distinct attributes. Due to limited interstage buffering between the two stages, the two departments have to follow the same batch sequence. In the first department, a setup occurs every time the first attribute of the new batch is different from the previous one. In the downstream department, there is a setup when the second attribute of the new batch changes. The problem consists of finding a common batch sequence optimizing some global utility index. Here we propose a metaheuristic approach to a bi-criteria version of the problem considering two indices, namely the total number of setups paid for by the two departments and the maximum number of setups paid for by either department.     

A fast procedure for computing the total system cost of an appointment schedule for medical and kindred facilities

Scheduling outpatients and medical operation rooms has the following structure: Nusers are given appointment times to use a facility, the duration required by the facility to service each user is stochastic. The system incurs a “user idle cost” if a user arriving at the appointed time finds the facility still engaged by preceding users, while a “facility idle cost” is incurred if the facility becomes free before the next user arrives. We develop an accurate procedure to compute the expected total system costs for any given appointment schedule. Compared to earlier related procedures, ours is much faster and can handle larger problems as well as very general service-time distributions. We then show that this fast computation procedure enables one to determine easily the “lowest-cost appointment schedule” for any given “job” (i.e., “user”) sequence. This in turn will enable one to search for the optimal job sequence that has the best “lowest-cost appointment schedule”.     

Finite waiting space bulk service system

Summary This paper discusses the ergodic queue length distribution of a bulk service system with finite waiting space by the method of the imbedded Markov chain. The system under consideration is a queuing system with Poisson arrivals, general service times, single server and where service is performed on batches of random size.     

A heterogeneous system with finite waiting space

Summary A simple queue with two heterogeneous servers is analyzed. The emphasis is on comparing the two-server heterogeneous and homogeneous systems with the restriction of an upper limitN on the queue size. The optimal service rates for both the servers are found in terms of the arrival rate and the traffic intensity. The average characteristics of the heterogeneous system are minimized, and their improvement over the corresponding homogeneous system characteristics is established. For different values ofN and, tables are given which compare the average characteristics of the two systems.     

Tool requirements planning in a flexible manufacturing system: Minimizing tool costs subject to a makespan constraint

We consider a tool requirements planning problem (TRPP) of determining the number of tool copies for each tool type in a flexible manufacturing system with an automatic tool transporter. In this paper, four heuristic algorithms are developed for the TRPP with the objective of minimizing the total tool purchase cost subject to a makespan constraint. In the algorithms, starting from an initial (infeasible) solution, the numbers of tool copies for certain tool types are increased at each iteration until the makespan constraint becomes satisfied. In these algorithms, information obtained from discrete event simulation is used to select tool types of which the numbers of copies are to be increased. Computational experiments are performed on randomly generated test problems and results show that good tool requirements plans can be obtained with the heuristic algorithms in a reasonable amount of computation time.     

Minimizing response time in a two-stage supply chain system with variable lead time and stochastic demand

We consider customer response time minimization in a two-stage system facing stochastic demand. Traditionally, the objective of representative mathematical models is to minimize costs related to production, inventory holding, and shortage. However, the highly competitive market characterized by impatient customers warrants the inclusion of costs related to customer waiting. Therefore we investigate a supply chain system in an uncertain demand setting that encompasses customer waiting costs as well as traditional plant costs (i.e. production and inventory costs). A representative expected cost function is derived and the closed form optimal solution is determined for a general demand distribution. We also provide examples to illustrate results for some common probability distributions. Our results indicate significant cost savings under certain assumptions when comparing solutions from the proposed model to the traditional newsvendor order/production quantity.     

Minimizing power dissipation in a disk file actuator

The ability to improve a disk file's access-time performance is severely limited by an inverse fourth-power increase in actuator-power dissipation. Two areas are germane to the actuator-power problem: the design of the actuator coil and the design of the control trajectory. Design considerations for optimal solutions in both of these areas are presented. Utilization of power-optimal control in concert with a power-optimized actuator makes possible a head/disk assembly with improved seek-time performance.<>     

Use of modulated excitation signals in medical ultrasound. Part I: Basic concepts and expected benefits

This paper, the first from a series of three papers on the application of coded excitation signals in medical ultrasound, discusses the basic principles and ultrasound-related problems of pulse compression. The concepts of signal modulation and matched filtering are given, and a simple model of attenuation relates the matched filter response with the ambiguity function, known from radar. Based on this analysis and the properties of the ambiguity function, the selection of coded waveforms suitable for ultrasound imaging is discussed. It is shown that linear frequency modulation (FM) signals have the best and most robust features for ultrasound imaging. Other coded signals such as nonlinear FM and binary complementary Golay codes also have been considered and characterized in terms of signal-to-noise ratio (SNR) and sensitivity to frequency shifts. Using the simulation program Field II, it is found that in the case of linear FM signals, a SNR improvement of 12 to 18 dB can be expected for large imaging depths in attenuating media, without any depth-dependent filter compensation. In contrast, nonlinear FM modulation and binary codes are shown to give a SNR improvement of only 4 to 9 dB when processed with a matched filter. Other issues, such as depth-dependent matched filtering and use of filters other than the matched filter (inverse and Wiener filters) also are addressed.     

On building a medical diagnostic system of acute exanthemas

Abstract

In this paper, we present a multi‐purpose medical diagnostic system named AEA — the Acute Exanthem Advisor, and the methodologies of its implementation. AEA provides an accurate diagnosis of acute exanthemas and a complete environment including a user‐friendly interface, reviewing function, record keeping function and explanation function. Therefore, it may serve as an assistant, a record keeper or an educational tool. A prediction program is also provided which is capable of predicting the number of potential patients who are going to have acute exanthemas in the near future. To illustrate the processes of the consultation and the prediction of the AEA, an example is given. Finally, for the 25 different test cases given to the diagnosticians and the AEA, the answers are almost the same, so we can conclude that the performance of the AEA is satisfactory. Now, we are trying to extend the AEA system to be a medical diagnostic net for acute exanthemas, which will be able to be remote accessed through network communications.     

Minimizing the mass of a cylindrical-laminar thermoprotective shell

The problem of minimizing the mass of heat insulation of a laminar cylinder, while ensuring the specified damping of external temperature perturbations, is considered.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 4, pp. 636–642, October, 1987.     

Minimizing production flow time in a process and assembly job shop

Scheduling is one of the most important issues in the planning and operation of production systems, but in medium to large shops, the generation of consistently good schedules has proven to be extremely difficult. The problem is that optimal scheduling solutions involve costly and impractical enumeration procedures. In the literature, most scheduling problems only address jobs with serial or sequential operations. Rarely do they consider jobs in which machining and assembly operations are simultaneously involved. This lack of attention to scheduling problems that involve both machining and assembly goes against what one would normally find in most job shops. In this paper, the problem of scheduling a set of N final products on M machines in a job shop environment that involve both machining and assembly operations is addressed. The objective pursued is the minimization of production flow time (makespan). A mathematical model is developed in an effort to obtain optimal solutions. Because this type of model grows exponentially as the size of the problems increases, an heuristic solution approach is developed to solve the problems more efficiently. The models are tested and compared on several test problems.