Modelling and optimizing sequential imperfect preventive maintenance

This paper deals with the problem of scheduling imperfect preventive maintenance (PM) of some equipment. It uses a model due to Kijima in which each application of PM reduces the equipment's effective age (but without making it as good as new). The approach presented here involves minimizing a performance function which allows for the costs of minimal repair and eventual system replacement as well as for the costs of PM during the equipment's operating lifetime. The paper describes a numerical investigation into the sensitivity of optimum schedules to different aspects of an age-reduction model (including the situation when parts of a system are non-maintainable—i.e., unaffected by PM).     

A Stopping Criterion for Logarithmic Simulated Annealing

We perform a convergence analysis of simulated annealing-based search for the special case of logarithmic cooling schedules. Emphasis is put on the impact of structural parameters of the underlying configuration space on the number of transitions k≥L that is sufficient to achieve a certain probability (confidence 1−δ) of being in an optimum configuration. Since such a lower bound L of the transition number depends on some constants that are difficult to calculate, we evaluate a much simplified version L'≪L of the lower bound for the problem of finding short conjunctions representing a ``positive' Boolean vector and rejecting a set of ``negative' Boolean vectors. The evaluation is based on computational experiments where the frequency of occurrences of configurations is calculated for simulated annealing-based search that terminates after L' transitions. The experiments produce a good correspondence between frequencies of minimum configurations and the required confidence 1−δ, i.e., our study provides empirical evidence that the relation of basic parameters in the lower bound L, if calculated for small constants assigned to the parameters and thus resulting in L', can be used as a termination criterion in simulated annealing-based search.     

Promoting adolescent mental health in primary care: a review of the literature

High levels of psychological disturbance amongst adolescents have been linked to behaviours which can damage physical health, and with mental health problems in adulthood. The aim of this review was to see if published literature supports the hypothesis that primary care is a suitable setting in which mental health problems in adolescents can be prevented by early detection and treatment. Medline, BIDS, SIGLE and Psychlit databases (January 1990-February 1997) were systematically searched for English language studies on adolescent health promotion and mental health in primary care; reference sections were checked for earlier work. When offered, adolescent health checks and clinics have been well received with attendance rates of 73% and 83% reported, respectively. Primary care offers a setting for the prevention and detection of mental health problems in adolescents. Further research is needed to determine cost effective ways of using these opportunities.     

An experimental analysis of local minima to improve neighbourhood search

The paper reports the results from a number of experiments on local search algorithms applied to job shop scheduling problems. The main aim was to get insights into the structure of the underlying configuration space. We consider the disjunctive graph representation where the objective function of job shop scheduling is equal to the length of longest paths. In particular, we analyse the number of longest paths, and our computational experiments on benchmark problems provide evidence that in most cases optimal and near optimal solutions do have a small number of longest paths. For example, our best solutions have one to five longest paths only while the maximum number is about sixty longest paths. Based on this observation, we investigate a non-uniform neighbourhood for simulated annealing procedures that gives preference to transitions where a decrease of the number of longest paths is most likely. The results indicate that the non-uniform strategy performs better than uniform methods, i.e. the non-uniform approach has a potential to find better solutions within the same number of transition steps. For example, we obtain the new upper bound 886 on the 20×20 benchmark problem YN1.

Scope and purpose

The paper reports a number of experiments with local search algorithms applied to job shop scheduling (JSS). The JSS problem is defined as follows: Given a number of l jobs, the jobs have to be processed on m machines. Each job consists of a sequence of m tasks, i.e., each task of a job is assigned to a particular machine. The tasks have to be processed during an uninterrupted time period of a fixed length on a given machine. A schedule is an allocation of the tasks to time intervals on the machines and the aim is to find a schedule that minimises the overall completion time which is called the makespan. The scheduling problem is one of the hardest combinatorial optimization problems (cf. M.R. Garey, D.S. Johnson, SIAM J. Comput. 4(4) (1975) 397. Many methods have been proposed to find good approximations of optimum solutions to job shop scheduling problems; for an overview (see E.H.L. Aarts, Local Search in Combinatorial Optimization, Wiley, New York, 1998). In our paper, the main aim is to get insights into the structure of the underlying configuration space. We consider the disjunctive graph representation where the objective function of job shop scheduling is equal to the length of longest paths. In particular, we analyse the number of longest paths, and our computational experiments on benchmark problems provide evidence that in most cases optimal and near optimal solutions do have a small number of longest paths. For example, our best solutions have one to five longest paths only while the maximum number is about sixty longest paths. Based on this observation, we investigate a non-uniform neighbourhood for simulated annealing procedures that gives preference to transitions where a decrease of the number of longest paths is most likely. The results indicate that the non-uniform strategy performs better than uniform methods, i.e., the non-uniform approach has a potential to find better solutions within the same number of transition steps. For example, we obtain the new upper bound 886 on the 20×20 benchmark problem YN1.     

Robotic assistants in therapy and education of children with autism: can a small humanoid robot help encourage social interaction skills?

This article presents a longitudinal study with four children with autism, who were exposed to a humanoid robot over a period of several months. The longitudinal approach allowed the children time to explore the space of robot–human, as well as human–human interaction. Based on the video material documenting the interactions, a quantitative and qualitative analysis was conducted. The quantitative analysis showed an increase in duration of pre-defined behaviours towards the later trials. A qualitative analysis of the video data, observing the childrens activities in their interactional context, revealed further aspects of social interaction skills (imitation, turn-taking and role-switch) and communicative competence that the children showed. The results clearly demonstrate the need for, and benefits of, long-term studies in order to reveal the full potential of robots in the therapy and education of children with autism.

Ice analog halos

Crystals of sodium fluorosilicate are used to produce easy to set up visual displays of atmospheric halos, including the 22 degrees halo, the Parry arc, and upper tangent arcs. Scattering phase functions for single ice-analog rosettes, including a rough one, and a column aggregate, measured in randomized orientation, are also given. The phase functions show prominent halo features, with the exception of the rough crystal.     

On the difference of fatigue strengths from rotating bending,four-point bending,and cantilever bending tests

Comparisons of piping fatigue data demonstrate that the fatigue strength from rotating bending tests is lower than that from cantilever and four-point bending tests, especially in the low-cycle fatigue life range. The lower strength from the rotating bending test is generally believed to result from the fact that in this test all the points on the piping surface are subjected to the maximum stress range. Consequently, the weakest point in the specimen always initiates and causes failure. On the contrary, in cantilever and four-point bending tests, the maximum stress range occurs only at the top and bottom extreme fibers, which may not contain the weakest point in the specimen. Hence, the pipes in rotating bending tests usually fail earlier in comparison with the other two tests. Finite element analyses for the three tests revealed another and more compelling reason for the lower fatigue strength from the rotating bending test. The results demonstrated that, for the same prescribed bending moment range, the inelastic strain range in rotating bending is higher than the ranges in four-point and cantilever bending tests. Experimental data also demonstrate a similar trend. The new observation suggests that fatigue data from these three tests should be analyzed or compared in terms of strain range, instead of nominal stress range.     

Fundamental Bit-Error-Rate Limits of In-Line Optical Regeneration by Synchronous Intensity Modulation: An Analytical Gaussian-Statistics Model

Fundamental limits of optical regeneration by in-line synchronous intensity modulation are evaluated through a simple Gaussian-statistics analysis. Regeneration is shown to involve three processes: asymptotic stabilization of signal-to-noise ratio, asymptotic stabilization of timing jitter, and instrinsic conversion of timing jitter into intensity noise. The model also includes filtering, which is shown to reduce or suppress amplitude noise. Key system parameters, which are valid regardless the origin of noise and jitter (amplification, interaction, collisions), are identified for the regime of “infinite” transmission, where the bit-error rate becomes distance-invariant. These parameters make it possible to predict the level and distance at which the bit-error rate converges, thus enabling rapid analytical evaluation and optimization of system performance. Novel effects such as transition from intensity-noise-limited to timing-jitter-limited regimes (or the reverse) and eye reopening with distance have been identified and analyzed for the first time.