Evaluation of the effect of a paver’s trolley on productivity, task demands, workload and local discomfort

The objective of this study was to determine the differences in the number of paving stones laid (productivity), task demands, energetic workload, body region discomfort and preference when laying paving stones with or without use of a paver’s trolley (n = 8) in a within-subject controlled study of pavers. The number of paving stones laid and the task demands were measured by means of systematic observations at the workplace. The energetic workload was determined using the percentage of heart rate reserve (%HRR). Body region discomfort was measured using visual analog scales, and the workers’ preference was ascertained via interview. The use of a paver’s trolley had no effect on productivity, %HRR or body region discomfort compared to working without a paver’s trolley. The duration of knee-straining activities did not differ between working with (141 min) and without (146 min) the paver’s trolley. However, six of the eight pavers indicated that, given suitable circumstances, they wanted to use the paver’s trolley.

Relevance to industry

To reduce the chance of work-related low back and knee complaints among pavers, the duration, frequency and intensity of lower back and knee-straining activities should be limited by means of technical measures such as mechanical paving. The paver’s trolley does not appear to reduce knee-straining activities and therefore does not appear useful in reducing the risk of knee complaints and disorders.     

Evolutionary pattern search algorithms for unconstrained andlinearly constrained optimization

We describe a convergence theory for evolutionary pattern search algorithms (EPSA) on a broad class of unconstrained and linearly constrained problems. EPSA adaptively modify the step size of the mutation operator in response to the success of previous optimization steps. The design of EPSA is inspired by recent analyzes of pattern search methods. Our analysis significantly extends the previous convergence theory for EPSA. Our analysis applies to a broader class of EPSA and it applies to problems that are nonsmooth, have unbounded objective functions, and are linearly constrained. Further, we describe a modest change to the algorithmic framework of EPSA for which a nonprobabilistic convergence theory applies. These analyses are also noteworthy because they are considerably simpler than previous analyses of EPSA     

Computing three-dimensional two-phase flows with a mass-conserving level set method

A method is described to compute three- dimensional two-phase flow, allowing large density ratios and coalescence and break-up of bubbles. The level set method is used to describe interfaces, and the volume-of-fluid method is used to ensure mass conservation. Efficiency in computing the interface dynamics is achieved by using a functional relation between the level set and volume-of-fluid functions. Difficulties and remedies in re-initialization of the level set function and inaccurate compution of surface tension are discussed. Test cases for validation are described, and demanding two-bubble computations to show the generality and the versatility of the method are presented.     

The use of hypertext in the writing of group papers

Abstract The Virtual Constructing Tool (VCT) was developed at the Nijmegen School of Management in 1998. The VCT is intended to facilitate the process of writing group papers through the use of construction and communication tools and is based completely on the Internet. The VCT allows a paper to be structured by means of hyperlinking and cooperation between students. Students are allowed to divide tasks, but they are held responsible for the quality of the arguments in the paper as a whole. They are invited to spend time in dealing with validity claims and in building a valid train of thought. Producing a group paper includes four interrelated working processes: exploring the subject, exchanging information, arguing and discussing and editing. Hyperlinking offers a contribution in each of these processes which varies from associative linking of pieces of information to strengthening linear structuring. Therefore, a different perspective has been taken to that of those authors who consider hyperlinking as a tool par excellence for the nonlinear production of information.     

State Observation and Diagnosis of Discrete-Event Systems Described by Stochastic Automata

The problems ofstate observation and diagnosis are solved for discrete–eventsystems, which are described by stochastic automata. As manysystems are not observable in the sense that it is possible toreconstruct the state unambiguously, the observation problemis set up as the problem of determining the smallest possibleset of states that are compatible with the measured input andoutput sequences. The diagnostic problem is shown to be, in principle,an observation problem. Conditions for the observability anddiagnosability of stochastic automata are presented. The resultsare illustrated by examples.     

Evolving combinatorial problem instances that are difficult to solve

This paper demonstrates how evolutionary computation can be used to acquire difficult to solve combinatorial problem instances. As a result of this technique, the corresponding algorithms used to solve these instances are stress-tested. The technique is applied in three important domains of combinatorial optimisation, binary constraint satisfaction, Boolean satisfiability, and the travelling salesman problem. The problem instances acquired through this technique are more difficult than the ones found in popular benchmarks. In this paper, these evolved instances are analysed with the aim to explain their difficulty in terms of structural properties, thereby exposing the weaknesses of corresponding algorithms.     

Dielectric properties of apples in the range 0.1–100 kHz

Capacitance and conductance spectra are measured between 100 Hz and 100 kHz for three varieties of apple. Analysis of the spectra for several types of electrode material indicates that electrode effects may be neglected for frequencies above 1 kHz. Power-law responses are observed withCf ^–0.3 for the capacitance andG _a.c.f ^+0.9 for the a.c. conductance. Activation energies were found to be of the order of 0.05–0.15 eV for the three apple varieties and found to be frequency dependent. Comparison is made with an apple in which the cellular structure has been damaged by freezing. Freezing produced a significant increase in the high-frequency activation energy for conductance and a decrease in the power-law exponent to +0.3 for a.c. conductance. It is suggested that the dielectric properties result from a combination of relatively free ionic diffusion in the extracellular medium and the hopping of counterions along trap sites on the cell wall.