Evaluating the usability of a mobile guide: the influence of location, participants and resources

When designing a usability evaluation, choices must be made regarding methods and techniques for data collection and analysis. Mobile guides raise new concerns and challenges to established usability evaluation approaches. Not only are they typically closely related to objects and activities in the user's immediate surroundings, they are often used while the user is ambulating. This paper presents results from an extensive, multi-method evaluation of a mobile guide designed to support the use of public transport in Melbourne, Australia. In evaluating the guide, we applied four different techniques; field-evaluation, laboratory evaluation, heuristic walkthrough and rapid reflection. This paper describes these four approaches and their respective outcomes, and discusses their relative strengths and weaknesses for evaluating the usability of mobile guides.     

Characterisation Of Receding Horizon Control For Constrained Linear Systems

This paper characterises the geometric structure of receding horizon control (RHC) of linear, discrete‐time systems, subject to a quadratic performance index and linear constraints. The geometric insights so obtained are exploited to derive a closed‐form solution for the case where the total number of constraints is less than or equal to the number of degrees of freedom, represented by the number of control moves. The solution is shown to be a partition of the state space into regions for which an analytic expression is given for the corresponding control law. Both the regions and the control law are characterised in terms of the parameters of the open‐loop optimal control problem that underlies RHC and can be computed off line. The solution for the case where the total number of constraints is greater than the number of degrees of freedom is addressed via an algorithm that iteratively uses the off‐line solution and avoids on‐line optimisation.     

A Low-Power Programmable Bandpass Filter Section for Higher Order Filter Applications

We present a programmable, continuous-time bandpass filter that is extremely compact, power efficient, and can cover a wide range of frequencies (10 Hz-10 MHz). This capacitively coupled current conveyor (C ⁴) has a second-order bandpass transfer function and is capable of being used as a basic bandpass-filter element to create high-order filters. The use of floating-gate transistors helps to ease the difficulties of effectively utilizing G _m-C filters by providing precise, programmable current sources that set the filter's time constants. Additionally, we provide an algorithmic design approach for constructing these bandpass filters to meet any given specifications. This bandpass filter is ideally suited to large filter-bank applications because of its small size and low-power demands.     

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal. II. Effects of temperature and dopant concentration

The oxidation behavior of pure Cr and Cr implanted with Y was studied as a function of temperature (900 and 1025°C) and ion-implantation dose (1×10¹⁵ and 2×10¹⁶ Y ions/cm²). The microstructures of the Cr₂O₃ scales were affected by both of the variables studied. Yttrium ions segregated at the grain boundaries in the Cr₂O₃ scales formed on the implanted alloys and the concentration of Y at the grain boundaries decreased with a decrease in the dose of implanted Y. The mechanism of growth of the Cr₂O₃ scales was altered by the presence of the Y ions at the Cr₂O₃ grain boundaries only when a critical concentration of Y at the grain boundaries was exceeded.     

The growth and structure of oxide films on Fe. I. Oxidation of (001) and (112) Fe at 200–300°C

A study has been made of the oxidation of (001) and (112) Fe at 200–300°C in 5×10^–3 Ton O₂ to determine the influence of substrate orientation and surface pretreatment. Using oxidation kinetics, reflection electron diffraction, and electron optical techniques, it has been shown that the nature of the prior oxide film has a marked effect on oxidation behavior for a given orientation. The initial faster rate for a surface covered with a 16-Å prior oxide film formed by dry oxidation at room temperature is attributed to a smaller sub grain size in this film, compared to that for a prior film formed by electropolishing. This initial rapid rate is not sustained because of oxide separation from the metal. -Fe₂O₃ formation, which occurs at higher temperatures for either surface pretreatment, is enhanced by the oxide separation.     

Generic relaxation spectra of solid polymers. I. Development of spectral distribution model and its application to stress relaxation of polypropylene

An important aspect of designing polymeric articles for engineering applications and predicting their properties over their lifetime is the computation of their time-dependent viscoelastic behavior. A simplified numerical computational technique based on a Gaussian spectral distribution model was developed to describe this behavior over a wide range of time and temperature. The model was used to describe the stress-relaxation behavior of isotactic polypropylene (iPP) over a wide range of strain, time, and temperature. It appears that a spectrum with two components (one distribution for the amorphous zone and the other for the crystalline zone) is sufficient to describe the viscoelastic behavior of iPP. The parameters specifying the distributions (mean relaxation time, standard deviation, and relaxation strength) may be obtained by nonlinear regression analysis and the temperature dependence of the distributions may be evaluated experimentally. An excellent fit between experimental data and the mathematical model is observed. The method may be applied generally for any linear viscoelastic property (e.g., static and dynamic relaxation and creep in tensile or shear) and for any polymer. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1101–1115, 1997     

Preconditioning methods for very ill‐conditioned three‐dimensional linear elasticity problems

Finite element models of linear elasticity arise in many application areas of structural analysis. Solving the resulting system of equations accounts for a large portion of the total cost for large, three‐dimensional models, for which direct methods can be prohibitively expensive. Preconditioned Conjugate Gradient (PCG) methods are used to solve difficult problems with small (≪1) average element aspect ratios. Incomplete Cholesky (ILL^T) factorizations based on a drop tolerance parameter are used to form the preconditioning matrices. Various new techniques known as reduction techniques are examined. Combinations of these reduction techniques result in highly effective preconditioners for problems with very poor aspect ratios. Standard and hierarchical triquadratic basis functions are used on hexahedral elements, and test problems comprising a variety of geo‐metries with up to 50 000 degrees of freedom are considered. Manteuffel’s method of perturbing the stiffness matrix to ensure positive pivots occur during factorization is used, and its effects on the convergence of the preconditioned system are discussed. Copyright © 1999 John Wiley & Sons, Ltd.     

Optimal control of vortex shedding using low‐order models. Part I—open‐loop model development

An approach to developing active control strategies for separated flows is presented. The methodology proposed is applied to the incompressible unsteady wake flow behind a circular cylinder at a Reynold's number of 100. Control action is achieved via cylinder rotation. Low‐order models which are amenable to control and which incorporate the full non‐linear dynamics are developed by applying the proper orthogonal decomposition technique to data provided by numerical simulation. This process involves extensions to the usual POD approach and the results are therefore assessed for two ‘open‐loop’ test cases. The predictions are found to be satisfactory for control purposes, assuming the model can be reset periodically. The use of these models for optimal control is discussed in a companion paper, Part II. Copyright © 1999 John Wiley & Sons, Ltd.     

Linguistic reflection in Java

Reflective systems allow their own structures to be altered from within. Here we are concerned with a style of reflection, called linguistic reflection, which is the ability of a running program to generate new program fragments and to integrate these into its own execution. In particular, we describe how this kind of reflection may be provided in the compiler-based, strongly typed object-oriented programming language Java. The advantages of the programming technique include attaining high levels of genericity and accommodating system evolution. These advantages are illustrated by an example taken from persistent programming, which shows how linguistic reflection allows functionality (program code) to be generated on demand (Just-In-Time) from a generic specification and integrated into the evolving running program. The technique is evaluated against alternative implementation approaches with respect to efficiency, safety and ease of use. © 1998 John Wiley & Sons, Ltd.