Predictive Control of Autonomous Kites in Tow Test Experiments

In this paper we present a model-based control approach for autonomous flight of kites for wind power generation. Predictive models are considered to compensate for delay in the kite dynamics. We apply Model Predictive Control (MPC), with the objective of guiding the kite to follow a figure-of-eight trajectory, in the outer loop of a two level control cascade. The tracking capabilities of the inner-loop controller depend on the operating conditions and are assessed via a frequency domain robustness analysis. We take the limitations of the inner tracking controller into account by encoding them as optimisation constraints in the outer MPC. The method is validated on a kite system in tow test experiments.Comment: The paper has been accepted for publication in the IEEE Control Systems Letters and is subject to IEEE Control Systems Society copyright. Upon publication, the copy of record will be available at http://ieeexplore.ieee.or

State Estimation for Kite Power Systems with Delayed Sensor Measurements

We present a novel estimation approach for airborne wind energy systems with ground-based control and energy generation. The estimator fuses measurements from an inertial measurement unit attached to a tethered wing and position measurements from a camera as well as line angle sensors in an unscented Kalman filter. We have developed a novel kinematic description for tethered wings to specifically address tether dynamics. The presented approach simultaneously estimates feedback variables for a flight controller as well as model parameters, such as a time-varying delay. We demonstrate the performance of the estimator for experimental flight data and compare it to a state-of-the-art estimator based on inertial measurements

Early nutrition causes persistent effects on pheasant morphology

Differences in growth conditions during early ontogeny have been suggested to cause permanent effects on the morphology and quality of birds. Yearly variation in growth conditions could thus result in morphological and quality differences between cohorts. In this study, we investigated the effect of small differences in the dietary protein content of captive ring-necked pheasants (Phasianus colchicus) during their first 8 wk posthatching. An experimental increase of the proportion of dietary protein during the first 3 wk of life accelerated growth, whereas a similar manipulation during the following 5 wk had only a limited effect. Compensatory growth during the postexperimental period equalized the size of chicks from different experimental treatments. However, a difference in tarsus length resulting from experimental treatment during the first 3 wk remained into adulthood. Furthermore, the protein content of the diet during the first 3 wk had an effect on the degree of fluctuating asymmetry in tarsus length, suggesting persistent effects on the quality of birds. The results of this study may explain size differences between cohorts that exist in pheasants and may also provide a link between the use of pesticides in agriculture and population effects on pheasants

Egg yolk androgen levels increase with breeding density in the European Starling, Sturnus vulgaris

1. High breeding density can cause elevated plasma androgen levels in adult birds. Since maternal androgens are deposited into egg yolk, high breeding density may result in elevated yolk androgen levels as well. 2. The relationship between breeding density and yolk androgen levels was examined in the European Starling, Sturnus vulgaris. The concentration and total content of yolk androstenedione and yolk testosterone were measured in eggs from 24 clutches distributed across nine different colonies of nestboxes. 3. Yolk androstenedione and testosterone levels were significantly higher in colonies where a greater proportion of nestboxes had active nests. 4. Yolk testosterone levels were significantly higher, and yolk androstenedione levels were marginally higher, in colonies with a greater absolute number of active nests. 5. Yolk androgen levels were not related to the number of active nests in adjacent nestboxes. 6. We conclude that female starlings nesting in colonies with higher breeding densities transfer more androgen to their eggs. 7. This relationship may be mediated by increased interfemale aggression, particularly towards floater females searching for mates or nests to brood parasitize, in high-density colonies. Such a relationship between maternal environment and maternal yolk androgens may represent adaptive maternal modification of offspring phenotype or a non-adaptive physiological constraint which females cannot avoid

Gain curves in depletable food patches: A test of five models with European starlings

A forager's gain curve, the cumulative number of prey harvested from a patch as a function of time spent in the patch, influences optimal patch departure rules and interpretations of patch use data. We describe models of five different search strategies that yield different gain curves. Hence they would influence a forager's decision for patch departure differently and, consequently, how researchers should interpret patch residence times and giving-up densities. However, the models are virtually impossible to separate based on data of the gain curves per se. Therefore, we develop a series of diagnostic tests that can be used to discriminate among models. These tests consider how the instantaneous harvest rate within patches depends on initial (IPD) and current prey density (CPD) and search time. We applied these tests to data collected from European starlings (Sturnus vulgaris) foraging in experimental food patches of known initial prey density. The starlings' harvest rate increased with CPD, an indication of diminishing returns. However, a given CPD yielded a lower instantaneous intake rate the higher the IPD. Thus, the two models most commonly assumed in foraging studies, systematic and random search, can be unequivocally rejected. Instead, we found support for a new model, negative stirring, in which the starlings spoil their own future foraging returns by aggregating the remaining prey items as they search

Turnover and nestedness drive plant diversity benefits of organic farming from local to landscape scales

Biodiversity-benefits of organic farming have mostly been documented at the field scale. However, these benefits from organic farming to species diversity may not propagate to larger scales because variation in the management of different crop types and seminatural habitats in conventional farms might allow species to cope with intensive crop management. We studied flowering plant communities using a spatially replicated design in different habitats (cereal, ley and seminatural grasslands) in organic and conventional farms, distributed along a gradient in proportion of seminatural grasslands. We developed a novel method to compare the rates of species turnover within and between habitats, and between the total species pools in the two farming systems. We found that the intrahabitat species turnover did not differ between organic and conventional farms, but that organic farms had a significantly higher interhabitat turnover of flowering plant species compared with conventional ones. This was mainly driven by herbicide-sensitive species in cereal fields in organic farms, as these contained 2.5 times more species exclusive to cereal fields compared with conventional farms. The farm-scale species richness of flowering plants was higher in organic compared with conventional farms, but only in simple landscapes. At the interfarm level, we found that 36% of species were shared between the two farming systems, 37% were specific to organic farms whereas 27% were specific to conventional ones. Therefore, our results suggest that that both community nestedness and species turnover drive changes in species composition between the two farming systems. These large-scale shifts in species composition were driven by both species-specific herbicide and nitrogen sensitivity of plants. Our study demonstrates that organic farming should foster a diversity of flowering plant species from local to landscape scales, by promoting unique sets of arable-adapted species that are scarce in conventional systems. In terms of biodiversity conservation, our results call for promoting organic farming over large spatial extents, especially in simple landscapes, where such transitions would benefit plant diversity most.Peer reviewe

Darwinian Selection and Non-existence of Nash Equilibria

We study selection acting on phenotype in a collection of agents playing local games lacking Nash equilibria. After each cycle one of the agents losing most games is replaced by a new agent with new random strategy and game partner. The network generated can be considered critical in the sense that the lifetimes of the agents is power law distributed. The longest surviving agents are those with the lowest absolute score per time step. The emergent ecology is characterized by a broad range of behaviors. Nevertheless, the agents tend to be similar to their opponents in terms of performance.Comment: 4 pages, 5 figure