On relative degree,chattering and fractal nature of parasitic dynamics in sliding mode control

With respect to relative degree and chattering in sliding mode (SM) control systems, the notion of fractal dynamics is introduced, and a conjecture is formulated that the character of parasitic dynamics of real control systems is fractal. A model of fractal dynamics is proposed. The characteristics of fractal dynamics are studied in the frequency and time domains. It is shown that with fractal parasitic dynamics SM control systems will always feature chattering and non-ideal closed-loop performance. An example of analysis is provided.     

Finding exact periodic solutions in Lure systems through periodic signal mapping

A solution of the periodic problem in a nonlinear system comprising a single-valued symmetric nonlinearity (Lure system) and linear dynamics is presented. The solution is designed as an iterative application of Periodic Signal Mapping to refine the approximate solution obtained through the describing function method. The algorithm is based upon the transformation of the original nonlinear system into an equivalent nonlinear system for which the filtering hypothesis is satisfied exactly, and for that reason the latter being suitable for application of the developed algorithm. The solution sought for is a fixed point of the periodic signal mapping. Conditions of asymptotic convergence of the proposed algorithm are given. The proposed approach is illustrated by two examples of analysis of periodic motions in a relay feedback system and chattering in a terminal sliding mode.     

Protein identification and localization using mass spectrometry and staining tests in cross-sections of polychrome samples

The identification and localization of the proteinaceous binders are essential issues in studies of painting materials and techniques, for further proposing valid restoration and conservation treatments of the painted or polychrome works of art. The challenge for analytical chemists and conservation scientists is the availability of methods able to simultaneously identify and map the presence of the binders in the multilayered structure of a sample and the possibility to use a very low amount of sample from the studied art object (considering also the criteria of minimum sampling). These methods should be fast, reproducible in different artefacts and in case of mixture of protein-based binders with other non-proteinaceous constituents (oils, resins, waxes, gums etc.) and also economical (both in terms of materials and time consume). In this context, the present paper proposes an innovative protocol of investigation using two complementary techniques – Matrix-Assisted Laser Desorption/Ionisation – Time of Flight Mass Spectrometry (MALDI-TOF MS) and staining tests (one visible and one fluorescent stain) assisted by Optical Microscopy (OM) on cross-section of samples – for the simultaneous identification and mapping of protein – and oil-based binders in paint materials. The novelty is based on the use of MALDI-TOF MS on cross-sections of paints together with a fluorescent stain for protein identification and mapping (mainly used in the area of proteomics) complementing the use of a traditional visible stain for oil-based material identification. The protocol was successfully applied on several samples taken from a Czech medieval polychrome sculpture, entitled “The Mourning of Jesus Christ” (16th century) belonging to the Moravian Gallery (Brno).     

The Tyranny of the Empty Frame

This study explores the notion of the “tyranny of the empty frame” within the online departments of the two leading Slovenian newspapers, Delo and Dnevnik, where online journalists—newsworkers with little or no training or experience in photojournalism—are required to provide each news item with at least one photograph. By adopting newsroom observation and in-depth interviews with online journalists working for Delo and Dnevnik, we investigate paradoxes associated with this imperative. Despite often being faced with a desperate search for “any image,” online journalists at both institutions are reluctant to use citizen-produced photographs—their use appears to be an aberration rather than a norm. Instead, they frequently resort to using what they refer to as a “symbolic photograph” to overcome the lack of visual material originating from the reported event. This move from indexical–iconic nexus to metaphoric visual communication signals challenges to the dominant paradigm of press photography. At the same time, we suggest that we should not view the phenomena of citizen photojournalism as a radical break with the past but as the latest development in a series of interrelated processes, intellectual projects of modernity, such as ocularcentrism, journalism, capitalism and the nation state.     

Frequency domain precision analysis and design of sliding mode observers

Estimation precision and bandwidth of sliding mode (SM) observers are analyzed in the frequency domain for different settings of the observer design parameters. It was shown previously that the SM observer could be analyzed as a relay feedback-feedforward system. It is feedback with respect to the measured variable of the system being observed, and feedforward with respect to the control applied to the system being observed. This approach is now further extended to analysis of effects of design parameter change on observer performance. An example of SM observer design for estimation of DC motor speed from the measurements of armature current is considered in the paper. The input-output properties of observer dynamics are analyzed with the use of the locus of a perturbed relay system (LPRS) method.     

Why Johnny struggles when familiar concepts are taken to a new mathematical domain: towards a polysemous approach

This article is concerned with cognitive aspects of students’ struggles in situations in which familiar concepts are reconsidered in a new mathematical domain. Examples of such cross-curricular concepts are divisibility in the domain of integers and in the domain of polynomials, multiplication in the domain of numbers and in the domain of vectors, and roots in the domain of reals and in the domain of complex numbers. The article introduces a polysemous approach for structuring students’ concept images in these situations. Post-exchanges from an online forum and excerpts from an interview were analyzed for illustrating the potential of the approach for indicating possible sources of students’ misconceptions and meta-ways of thinking that might make students aware of their mistakes.     

Biomechanical jumping differences among elite female handball players with and without previous anterior cruciate ligament reconstruction: a novel inertial sensor unit study

Persistent biomechanical and jumping capacity alterations have been observed among female athletes who have sustained anterior cruciate ligament (ACL) injuries. The purpose of this study was to examine if biomechanical jumping differences persist among a cohort of elite female handball players with previous ACL reconstruction several years after return to top-level competition. In order to achieve this goal, a direct mechanics simplified analysis by using a single Inertial Sensor Unit (IU) was used. Twenty-one elite female (6 anterior cruciate ligament reconstructed and 15 uninjured control players) handball players were recruited and evaluated 6.0 ± 3.5 years after surgical anterior cruciate ligament reconstruction. Bilateral and unilateral vertical jumps were performed to evaluate the functional performance and a single inertial sensor unit was employed in order to collect 3D acceleration and 3D orientation data. Previously ACL-reconstructed analysed athletes demonstrated significant (p < 0.05) alterations in relation to the three-dimensional axis (X–Y–Z) supported accelerations and differing jump phase durations, including jumping performance values, in both bilateral and unilateral jumping manoeuvres several years after ACL reconstruction. Identification of the encountered deficits through the use of an IU devise could provide clinicians with a new reliable tool for movement analysis in a clinical setting.     

Examining school-related delinquencies,extracurricular activities,and grades in adolescents

Using the theoretical framework of risk and resiliency, the study estimated the contribution of five school-related delinquencies (behaviour problems, schoolwork problems, out-of-school suspension, in-school suspension and grade retention) and seven extracurricular activities (arts, sports, clubs, tutoring, volunteering, religious involvement and scouts) on the report card grades of students. An adolescent sample (N?=?2070) from the US National Household Education Survey was examined. A three-step hierarchical multiple regression found that the protective effects of extracurricular activities remained predictive of better grades, even after controlling for the risks of school-related delinquencies. The extracurricular activity of arts best predicted better grades, but the school-related delinquency of schoolwork problems best predicted worse grades. The discussion focuses on the implications of increasing resilience by promoting extracurricular involvement and combating the risk of school delinquencies.