15,763 research outputs found
Fault tolerant control of a quadrotor using L-1 adaptive control
Purpose â The growing use of small unmanned rotorcraft in civilian applications means that safe operation is increasingly important. The purpose of this paper is to investigate the fault tolerant properties to faults in the actuators of an L1 adaptive controller for a quadrotor vehicle.
Design/methodology/approach â L1 adaptive control provides fast adaptation along with decoupling between adaptation and robustness. This makes the approach a suitable candidate for fault tolerant control of quadrotor and other multirotor vehicles. In the paper, the design of an L1 adaptive controller is presented. The controller is compared to a fixed-gain LQR controller.
Findings â The L1 adaptive controller is shown to have improved performance when subject to actuator faults, and a higher range of actuator fault tolerance.
Research limitations/implications â The control scheme is tested in simulation of a simple model that ignores aerodynamic and gyroscopic effects. Hence for further work, testing with a more complete model is recommended followed by implementation on an actual platform and flight test. The effect of sensor noise should also be considered along with investigation into the influence of wind disturbances and tolerance to sensor failures. Furthermore, quadrotors cannot tolerate total failure of a rotor without loss of control of one of the degrees of freedom, this aspect requires further investigation.
Practical implications â Applying the L1 adaptive controller to a hexrotor or octorotor would increase the reliability of such vehicles without recourse to methods that require fault detection schemes and control reallocation as well as providing tolerance to a total loss of a rotor.
Social implications â In order for quadrotors and other similar unmanned air vehicles to undertake many proposed roles, a high level of safety is required. Hence the controllers should be fault tolerant.
Originality/value â Fault tolerance to partial actuator/effector faults is demonstrated using an L1 adaptive controller
Obtaining Stiffness Exponents from Bond-diluted Lattice Spin Glasses
Recently, a method has been proposed to obtain accurate predictions for
low-temperature properties of lattice spin glasses that is practical even above
the upper critical dimension, . This method is based on the observation
that bond-dilution enables the numerical treatment of larger lattices, and that
the subsequent combination of such data at various bond densities into a
finite-size scaling Ansatz produces more robust scaling behavior. In the
present study we test the potential of such a procedure, in particular, to
obtain the stiffness exponent for the hierarchical Migdal-Kadanoff lattice.
Critical exponents for this model are known with great accuracy and any
simulations can be executed to very large lattice sizes at almost any bond
density, effecting a insightful comparison that highlights the advantages -- as
well as the weaknesses -- of this method. These insights are applied to the
Edwards-Anderson model in with Gaussian bonds.Comment: corrected version, 10 pages, RevTex4, 12 ps-figures included; related
papers available a http://www.physics.emory.edu/faculty/boettcher
âWe could end up in a lot of troubleâ Teachersâ communications with young children about mental health
Purpose â Stigma towards people with mental health problems is a significant problem and appears trenchant despite recent anti-stigma campaigns. Attitudes develop in young children, and may be stronger and less malleable in adolescence. Early intervention may be important for mental health education and stigma prevention. Theory, evidence and practical considerations suggest that teachersâ involvement is key. By exploring communication about mental health between teachers and young children, it will be possible to elaborate how stigma develops and may be ameliorated. This study explored teachersâ accounts of this communication and the factors that influence it.
Methodology â Semi-structured interviews with fifteen primary school teachers were transcribed and analysed using a grounded theory approach.
Findings â Discussions about mental health were largely absent from the classroom, due to teachersâ anxiety. Teachers felt the need to protect children from exposure to people with mental health problems and even from information about the topic, believed they lacked the necessary expertise, worried that such discussions were outside their remit, and were anxious about parentsâ reactions.
Originality/Value â This was the first study to interview teachers on this topic and suggests that a significant opportunity to address stigma is being missed. Teachersâ silence may reinforce that mental health problems are taboo, and prevent children from developing knowledge and a language to talk about mental health. The inclusion of teachers in early mental health education is more sustainable and could promote more inclusive attitudes, especially if supported by educational policy and curriculum.
Keywords - school, young children, teachers, mental health education, stigma prevention, policy, grounded theory
Article Classification â Research Pape
Spin-one ferromagnets with single-ion anisotropy in a perpendicular external field
In this paper, the conventional Holstein-Primakoff method is generalized with
the help of the characteristic angle transformation [Lei Zhou and Ruibao Tao,
J. Phys. A {\bf 27} 5599 (1994)] for the spin-one magnetic systems with
single-ion anisotropies. We find that the weakness of the conventional method
for such systems can be overcome by the new approach. Two models will be
discussed to illuminate the main idea, which are the ``easy-plane" and the
``easy-axis" spin-one ferromagnet, respectively. Comparisons show that the
current approach can give reasonable ground state properties for the magnetic
system with ``easy-plane" anisotropy though the conventional method never can,
and can give a better representation than the conventional one for the magnetic
system with ``easy-axis" anisotropy though the latter is usually believed to be
a good approximation in such case. Study of the easy-plane model shows that
there is a phase transition induced by the external field, and the
low-temperature specific heat may have a peak as the field reaches the critical
value.Comment: Using LaTex. To be published in the September 1 issue of Physical
Review B (1996). Email address: [email protected]
Composting paper and grass clippings with anaerobically treated palm oil mill effluent
Purpose The purpose of this study is to investigate the composting performance of anaerobically treated palm oil mill effluent (AnPOME) mixed with paper and grass clippings. Methods Composting was conducted using a laboratory scale system for 40 days. Several parameters were determined: temperature, mass reduction, pH, electrical conductivity, colour, zeta potential, phytotoxicity and final compost nutrients. Results The moisture content and compost mass were reduced by 24 and 18 %, respectively. Both final compost pH value and electrical conductivity were found to increase in value. Colour (measured as PtCo) was not suitable as a maturity indicator. The negative zeta potential values decreased from â12.25 to â21.80 mV. The phytotoxicity of the compost mixture was found to decrease in value during the process and the final nutrient value of the compost indicates its suitability as a soil conditioner. Conclusions From this study, we conclude that the addition of paper and grass clippings can be a potential substrate to be composted with anaerobically treated palm oil mill effluent (AnPOME). The final compost produced is suitable for soil conditioner
Correlated radial velocity and X-ray variations in HD 154791/4U 1700+24
We present evidence for approximately 400-d variations in the radial velocity
of HD 154791 (V934 Her), the suggested optical counterpart of 4U 1700+24. The
variations are correlated with the previously reported approximately 400 d
variations in the X-ray flux of 4U 1700+24, which supports the association of
these two objects, as well as the identification of this system as the second
known X-ray binary in which a neutron star accretes from the wind of a red
giant. The HD 154791 radial velocity variations can be fit with an eccentric
orbit with period 404 +/- 3 d, amplitude K=0.75 +/- 0.12 km/s and eccentricity
e=0.26 +/- 0.15. There are also indications of variations on longer time scales
>~ 2000 d. We have re-examined all available ASM data following an unusually
large X-ray outburst in 1997-98, and confirm that the 1-d averaged 2-10 keV
X-ray flux from 4U 1700+24 is modulated with a period of 400 +/- 20 d. The mean
profile of the persistent X-ray variations was approximately sinusoidal, with
an amplitude of 0.108 +/- 0.012 ASM count/s (corresponding to 31% rms). The
epoch of X-ray maximum was approximately 40 d after the time of periastron
according to the eccentric orbital fit. If the 400 d oscillations from HD
154791/4U 1700+24 are due to orbital motion, then the system parameters are
probably close to those of the only other neutron-star symbiotic-like binary,
GX 1+4. We discuss the similarities and differences between these two systems.Comment: 6 pages, 2 figures; accepted by Ap
Correlated X-ray and Optical Variability in Mkn 509
We present results of a 3 year monitoring campaign of the Seyfert 1 galaxy
Markarian 509, using X-ray data from the Rossi X-ray Timing Explorer (RXTE) and
optical data taken by the SMARTS consortium. Both light curves show significant
variations, and are strongly correlated with the optical flux leading the X-ray
flux by 15 days. The X-ray power spectrum shows a steep high-frequency slope of
-2.0, breaking to a slope of -1.0 at at timescale of 34 days. The lag from
optical to X-ray emission is most likely caused by variations in the accretion
disk propagating inward.Comment: 13 pages, 3 figures. Accepted for publication in the Astrophysical
Journa
Simulating the Impact of the Global Economic Crisis and Policy Responses on Children in Ghana
Like many countries in sub-Saharan Africa, Ghana is experiencing the impact of the global crisis and the uncertain economic outlook. Indeed, as Ghanaâs economy is among the most open in Africa, it is expected that the country has been and will continue to be severely affected by the crisis, although strong export prices of its main exports (gold and cocoa) may at least partially counteract the effects associated with the crisis. The main goal of this paper is to understand the potential impacts of the 2008/9 global crisis on different dimensions of child poverty (monetary, hunger, school participation, child labour and access to health services) in Ghana and to support the policy-maker in designing the most appropriate policy response to counteract the negative effects of the crisis. As timely data are not available, a combined macro-micro economic model to predict the impact of the global crisis on children was developed. Simulations suggest that the financial crisis would increase monetary poverty and hunger across all regions of Ghana, eroding many of the gains made over the past few years. Indeed, in comparison with the year preceding the crisis, instead of a reduction of four percentage points in child monetary poverty in 2011 predicted in the absence of crisis, the simulations indicate a 6.6 percentage point increase, with a continuous increasing pattern over the period of study. The global crisis is also predicted to severely deepen hunger among children, which is simulated to increase up to 6.6 percentage points in 2011 beginning with a sharp increase already in 2009. For both monetary poverty and hunger, the impact of the crisis differs across all regions, with the Eastern, Volta and Greater Accra regions predicted to be the most affected. Childrenâs participation in schooling and labour, as well as their access to health services, are forecast to be much less affected by the crisis, although it is found to reverse predicted increases in enrolment and health access (with substitution toward more modern types of health services) and forecasted reductions in child labour. Finally, alternative policy options have been simulated: a cash transfer programme targeted to poor children is found to be generally more effective in protecting children than food subsidies. Indeed, with a total budget equivalent to 1% of 2008 GDP, a cash transfer â equivalent to an individual annual amount of 19.8 Cedis â would cut the predicted increase in monetary poverty by over two percentage points in 2011. Although Ghana might be in a position to rapidly implement a cash transfer programme building on the existing Livelihood Empowerment against Poverty (LEAP) programme, other interventions (or mix of policies) might be more cost-effective in the short run. A combination of a universal or regionally targeted cash transfer programmes for children aged 0 to 5 years old, together with a school-feeding programme in poorer regions, might represent an effective way to intervene quickly to improve child well-being.Global economic crisis, child poverty, hunger, education, child labour, health, West and Central Africa, Ghana, social protection
Distance Oracles for Time-Dependent Networks
We present the first approximate distance oracle for sparse directed networks
with time-dependent arc-travel-times determined by continuous, piecewise
linear, positive functions possessing the FIFO property.
Our approach precomputes approximate distance summaries from
selected landmark vertices to all other vertices in the network. Our oracle
uses subquadratic space and time preprocessing, and provides two sublinear-time
query algorithms that deliver constant and approximate
shortest-travel-times, respectively, for arbitrary origin-destination pairs in
the network, for any constant . Our oracle is based only on
the sparsity of the network, along with two quite natural assumptions about
travel-time functions which allow the smooth transition towards asymmetric and
time-dependent distance metrics.Comment: A preliminary version appeared as Technical Report ECOMPASS-TR-025 of
EU funded research project eCOMPASS (http://www.ecompass-project.eu/). An
extended abstract also appeared in the 41st International Colloquium on
Automata, Languages, and Programming (ICALP 2014, track-A
- âŠ