Stabilization of carrier-frequency servomechanisms. II. Design formulae for proportional-derivative networks     

Andrew Sobczyk 《Journal of The Franklin Institute》1948,246(2):95-121

In an alternating current servomechanism, the error is proportional to the modulation envelope of a modulated-carrier error signal. It is shown in part I that for stability and fidelity of the servo, it is highly desirable that the effect of the controller includes a proportional-derivative action on the modulation envelope. This action may be obtained with various forms of RC networks, including the parallel “T,” bridge “T,” and Wien Bridge forms.This part contains detailed design procedures and tables of values for the various types of proportional-derivative networks. Several forms of parallel “T” networks arise from the fact that there are five independent time constants in the network, while in order to realize the desired transfer characteristic it is necessary to impose only four conditions. It is indicated how the remaining degree of freedom may be used to obtain the most suitable input and output impedances for the source and load impedances with which the parallel “T” is to be used. The derivations for the parallel “T” formulae are given in an Appendix.Tolerance requirements on the components of parallel “T” and bridge “T” networks are derived. If ±1 per cent components are used at 60 cycles, the resonant frequency will lie between 56.4 and 63.6 cycles, and the notch width (rejection band width) will be within ±0.99 cps. of the correct value. In order to guarantee that the phase shift at 60 cycles is within ±10°, the percentage deviation of each part must be less than ($\text{9.0}\text{T}{}_{\mathrm{d}}\text{ω}{}_0$), where ω₀ is the carrier angular frequency, T_d the derivative time constant.  相似文献   

A Personnel Management Case Study in a Canadian National Sport Organisation     

Samuel Y. Todd  Damon P.S. Andrew  Sara E. Sowieta 《Sport Management Review》2009,12(1):49-56

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Cost Versus Enrollment Bubbles     

Richard K. Vedder  Andrew Gillen 《Academic Questions》2011,24(3):282-290

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The professor as an artist,a scientist,and an entrepreneur     

Andrew F. Sikula  John P. Sikula 《The Teacher Educator》2013,48(4):15-19

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Parental choice in Scotland     

Frank Echols  Andrew McPherson  J. Douglas Willms 《教育政策杂志》2013,28(3):207-222

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Book Reviews: Steven Spielberg's America by Frederick Wasser     

Andrew Gordon 《Journal of American culture (Malden, Mass. : 2003)》2010,33(3):255-257

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Should we be encouraging pupils to ask more questions?     

Andrew Whittaker 《Educational studies》2012,38(5):587-591

This article investigates the issue of secondary school pupils asking questions. This is an important topic on which very little has been published to date. The article reviews the current literature, which almost exclusively reports the lack of student initiated, content related questioning in classrooms. A small study is described that challenges this view, finding a significantly greater level of student participation, a high percentage of inquiry driven questions and little reluctance on the part of pupils to participate actively in lessons. However, when these questioning patterns, which appear to indicate exemplary lessons on paper, are examined in terms of pupil learning, it is found that large numbers of student questions can lead to severe disruption unless adequately controlled. For effective learning to take place, it is a vital that the learner has the opportunity to ask questions of the teacher. The vast bulk of the literature explores this topic through quantitative analysis of questioning patterns and suggested strategies for increasing the quality and frequency of pupil’s questions. This article reviews this research and explores the issue further through classroom observations, pupil questionnaires and interviews with teachers. The findings are compared with the current paradigm and the issue is discussed in terms of impact on pupil learning.  相似文献   

Good practice for enhancing the engagement and success of commencing students     

Karen J. Nelson  Carole Quinn  Andrew Marrington  John A. Clarke 《Higher Education》2012,63(1):83-96

There is widespread recognition that higher education institutions (HEIs) must actively support commencing students to ensure equity in access to the opportunities afforded by higher education. This role is particularly critical for students who because of educational, cultural or financial disadvantage or because they are members of social groups currently under-represented in higher education, may require additional transitional support to “level the playing field.” The challenge faced by HEIs is to provide this “support” in a way that is integrated into regular teaching and learning practices and reaches all commencing students. The Student Success Program (SSP) is an intervention in operation at the Queensland University of Technology (QUT) designed to identify and support those students deemed to be at risk of disengaging from their learning and their institution. Two sets of evidence of the impact of the SSP are presented: First, its expansion (a) from a one-faculty pilot project (Nelson et al. in Stud Learn Eval Innov Dev 6:1–15, 2009) to all faculties and (b) into a variety of applications mirroring the student life cycle; and second, an evaluation of the impact of the SSP on students exposed to it. The outcomes suggest that: the SSP is an example of good practice that can be successfully applied to a variety of learning contexts and student enrolment situations; and the impact of the intervention on student persistence is sustained for at least 12 months and positively influences student retention. It is claimed that the good practice evidenced by the SSP is dependent on its integration into the broader First Year Experience Program at QUT as an example of transition pedagogy in action.  相似文献   

Moment transfer factors for column-supported cast-in-situ hollow core slabs     

Hai-tao Li  Andrew John Deeks  Li-xin Liu  Dong-sheng Huang  Xiao-zu Su 《浙江大学学报(A卷英文版)》2012,13(3):165-173

Hollow core slabs are becoming of increasing interest as the construction industry attempts to minimise the impact of its activities on the environment. By forming voids in the interior of a concrete slab, the amount of concrete used can be reduced without significantly altering the capacity of the structure. In this study, we examined the inner force transfer mechanism of a column-supported cast-in-situ hollow core slab using finite element analysis. Both a hollow core slab and the corresponding solid slab were analysed using ANSYS and the results were compared. The orientation of the tube fillers causes the stiffness of the hollow slab to be orthotropic, potentially changing the distribution of load carried in the two orthogonal directions. Both the cross-section’s moments in the column strip and near the columns in the hollow core slab become larger than that in the solid floor. As well, the cross-section’s stiffness along the tube arrangement direction is larger than that of the radial cross-section, which causes the direction along the hole of the hollow core slab to carry more moment than the radial direction. The conversion factors of the two directions are proposed from the comparison for four typical areas of the hollow core slab, as are the moment distribution coefficients.  相似文献   

Sport,physical activity and well-being: an objectivist account     

Andrew Bloodworth  Mike McNamee  Richard Bailey 《Sport, Education and Society》2013,18(4):497-514

It is widely maintained that sport and physical activities contribute to the development of young people's well-being. Others argue that sports’ contribution to good living is so strong that it is even thought to be a human right. Typically, however, the value of physical activity and sport to our well-being is conceptualized and researched within a subjectivist framework. We reject this framework on three grounds: (1) its impermanence; (2) its hedonistic shallowness; and (3) its epistemological inadequacy. In contrast, we argue that the value of sports and physical activities ought to be situated in fundamental arguments about the necessary conditions for human flourishing. According to this objectivist view, there are certain constituents of a good life without which human flourishing becomes impossible. We argue that sports and physical activities offer distinctive ways to help realize these objective constituents. It follows that, to the extent to which certain sections of society are deprived of opportunities to engage in sport and physical activity, or are offered diminished provision thereof, they thereby suffer a deficit in well-being.  相似文献