Spatial coherence of backscatter for the nonlinearly produced second harmonic for specific transmit apodizations

To be successful, correlation-based, phase-aberration correction requires a high correlation among backscattered signals. For harmonic imaging, the spatial coherence of backscatter for the second harmonic component is different than the spatial coherence of backscatter for the fundamental component. The purpose of this work was to determine the effect of changing the transmit apodization on the spatial coherence of backscatter for the nonlinearly generated second harmonic. Our approach was to determine the effective apodizations for the fundamental and second harmonic using both experimental measurements and simulations. Two-dimensional measurements of the transverse cross sections of the finite-amplitude ultrasonic fields generated by rectangular and circular apertures were acquired with a hydrophone. Three different one-dimensional transmit apodization functions were investigated: uniform, Riesz, and trapezoidal. An effective apodization was obtained for each transmit apodization by backpropagating the values measured from within the transmit focal zone using a linear angular spectrum approach. Predictions of the spatial coherence of backscatter were obtained using the pulse-echo Van Cittert-Zernike theorem. In all cases the effective apodization at 2f was narrower than the transmit apodization. We demonstrate that certain transmit apodizations result in a greater spatial coherence of backscatter at the second harmonic than at the fundamental.     

The equivalence of three data collection methods with field test data: a FACETS application

The present study demonstrated the utility of the FACETS software for evaluating items in the field test stage of item development for a clinical early childhood instrument. The research focus was the equivalence of the parent/caretaker interview, structured assessment, and observational methods for data collection for a developmental inventory for children from birth to age seven. Data for this study were from a field test with some missing responses. The Rasch-based software FACETS was used to test the equivalence of the methods of data collection as well as to identify items for which the methods did not provide equivalent information. Thirty-five items were studied from the adaptive domain, 26 items from the communication subdomain, 34 items from the motor domain, and 31 from the personal-social domain. When the methods were unconstrained, the overall test indicated that at least two of the methods were not equivalent. However, FACETS bias analyses with the method measures constrained to equality allowed the identification of a limited number of items and associated methods that were possibly problematic. The use of FACETS allowed test developers to focus on items from a field test event that were inconsistent with the targeted test development model.     

Taking the load off: investigations of how adaptive cruise control affects mental workload

It has been posited that Adaptive Cruise Control (ACC) represents a new generation of vehicle automation, in that it has the potential to relieve drivers of mental as well as physical workload. The results of previous research however, have raised some confusing issues about the specific effects of Adaptive Cruise Control (ACC) on driver mental workload (MWL)--some studies report reduced MWL compared to manual driving, while others find no effect. Two hypotheses are proposed in an attempt to explain these discrepancies: (a) that any potential MWL reductions due to ACC could be masked by the overriding influence of steering demand; or (b) that the tasks designed in some experiments do not exploit the adaptive nature of the ACC system, therefore precluding any potential benefits. Two related experiments were designed to test these hypotheses. It was found that the main reason for the discrepant findings was the nature of the driving task chosen--constant-speed tasks do not realise the mental workload benefits of ACC. Future researchers using ACC devices are advised to use variable-speed tasks to ensure that all aspects of device functionality are covered.     

Identifying the task variables that influence perceived object assembly complexity

There is a general lack of understanding as to what issues affect assembly task performance when using diagrammatic instructions because few of the task variables contributing to assembly complexity have been identified. Using a task analysis of a range of self-assembly products, seven task variables hypothesized to predict assembly complexity were identified and studied in the instruction comprehension phase of assembly. Experiment 1 took nine real world assembly instructions and described each in terms of the seven task variables. Seventy-two participants gave a subjective rating of assembly difficulty for each assembly, showing a clear relationship between the task variables and perceived assembly difficulty. As real world assemblies provide little control a second experiment used an orthogonal design to systematically vary the values of each of the assembly task variables in 16 abstract assemblies. Forty-two participants compared the 16 assembly instructions to a final assembly. There was a clear relationship between the task variables and the time taken to view the instructions. Further, it was found that it is possible to predict the complexity of assembly tasks based upon the levels of the task variables identified. The task variables identified are a significant step towards identifying the factors that influence assembly complexity, together with providing progress towards a tool for predicting assembly complexity.     

An adaptive mesh refinement of quadrilateral finite element meshes based upon an a posteriori error estimation of quantities of interest: modal response

Modal analysis is commonly performed in a vehicle development process to assess dynamic responses of structure designs. This paper presents an adaptive quadrilateral refinement process for modal analysis of elastic shells based upon a posteriori error estimation in natural frequencies. The process provides engineers with an estimation of their modal analysis quality and an effective adaptive refinement tool for quadrilateral meshes. The effectiveness of the process is demonstrated on the eigenvalue analyses of two numerical examples, a shock tower cap and a roof structure. It shows that the solution error in the frequency of interest is effectively reduced through the adaptive refinement process, and the resulting frequency of interest converges to the solution of a very fine model.     

Model Checking

Die Gewährleistung der korrekten Funktionsweise von Hard- und Software ist ein entscheidender Faktor bei der heutigen Systementwicklung. Dies trifft ganz besonders auf das Gebiet der sog. sicherheitskritischen Systeme zu, bei dem ein Systemversagen Menschenleben gefährden kann.     

Blind separation of positive sources by globally convergent gradient search

The instantaneous noise-free linear mixing model in independent component analysis is largely a solved problem under the usual assumption of independent nongaussian sources and full column rank mixing matrix. However, with some prior information on the sources, like positivity, new analysis and perhaps simplified solution methods may yet become possible. In this letter, we consider the task of independent component analysis when the independent sources are known to be nonnegative and well grounded, which means that they have a nonzero pdf in the region of zero. It can be shown that in this case, the solution method is basically very simple: an orthogonal rotation of the whitened observation vector into nonnegative outputs will give a positive permutation of the original sources. We propose a cost function whose minimum coincides with nonnegativity and derive the gradient algorithm under the whitening constraint, under which the separating matrix is orthogonal. We further prove that in the Stiefel manifold of orthogonal matrices, the cost function is a Lyapunov function for the matrix gradient flow, implying global convergence. Thus, this algorithm is guaranteed to find the nonnegative well-grounded independent sources. The analysis is complemented by a numerical simulation, which illustrates the algorithm.