Indirect Field Oriented Control of Induction Motors is Robustly Globally Stable

Field orientation, in one of its many forms, is an established control method for high dynamic performance AC drives. In particular, for induction motors, indirect fieldoriented control is a simple and highly reliable scheme which has become the de facto industry standard. In spite of its widespread popularity no rigorous stability proof for this controller was available in the literature. In a recent paper (Ortega et al, 1995) [Ortega, R., D. Taoutaou, R. Rabinovici and J. P. Vilain (1995). On field oriented and passivity-based control of induction motors: downward compatibility. In Proc. IFAC NOLCOS Conf., Tahoe City, CA.] we have shown that, in speed regulation tasks with constant load torque and current-fed machines, indirect field-oriented control is globally asymptotically stable provided the motor rotor resistance is exactly known. It is well known that this parameter is subject to significant changes during the machine operation, hence the question of the robustness of this stability result remained to be established. In this paper we provide some answers to this question. First, we use basic input-output theory to derive sufficient conditions on the motor and controller parameters for global boundedness of all solutions. Then, we give necessary and sufficient conditions for the uniqueness of the equilibrium point of the (nonlinear) closed loop, which interestingly enough allows for a 200% error in the rotor resistance estimate. Finally, we give conditions on the motor and controller parameters, and the speed and rotor flux norm reference values that insure (global or local) asymptotic stability or instability of the equilibrium. This analysis is based on a nonlinear change of coordinates and classical Lyapunov stability theory

Social Robots as Language Tutors:Challenges and Opportunities

In this paper we highlight several challenges we encountered while developing an Intelligent Tutoring System. Most importantly, technical limitations are currently standing in the way of the robot's ability to behave fully autonomously, and there is a need for methods and best practices from the field of human-computer interaction to ensure that user experience goals related to the quality of the holistic experience of interacting with a robot are set, and subsequently met. We also identify opportunities in the form of a modular (technical) architecture, and the implementation of a human-centered design process by including this discipline as one of the core components when setting up a project in the field of human-robot interaction

Explaining nascent entrepreneurship across countries

This paper aims at explaining cross-country variation in nascent entrepreneurship. Regression analysis is applied using various explanatory variables derived from three different approaches. We make use of the Global Entrepreneurship Monitor database, including nascent entrepreneurship rates for 36 countries in 2002 as well as variables from standardized national statistics. The first approach relates the level of entrepreneurship of a country to its level of economic development. We found evidence for a U-shaped relationship. The second approach deals with a regime switch where the innovative advantage moves from large, established enterprises to small and new firms, because new technologies have reduced the importance of scale economies in many sectors. The third approach assumes that nascent entrepreneurship depends upon aggregate conditions such as technology, demography, culture and institutions, influencing opportunities, resources, skills and preferences. Several indicators of these aggregate conditions are found to correlate with nascent entrepreneurship. A full model combining the three approaches includes a U-shaped relationship with per capita income as well as with Porter's innovative capacity index, in addition to effects of social security expenditure (-) and the total business ownership rate (+). Finally, a (former) communist-country dummy plays an important role.

Milk fatty acids in relation to feeding practices on Dutch organic farms

Various recent studies have indicated positive interactions between organic animal production and poly-unsaturated fatty acids, particularly CLA and omega-3. In this paper, we present the monitoring results of 15 organic farms concerning the rela-tionship between feeding practices and fatty acid pattern. The results indicate a strong negative effect of maize and grass silage and concentrate feeding on CLA levels in milk fat. On the other hand, feeding fresh grass has a positive effect. Omega-3 levels in milk fat seem less related to feed characteristics: similar relations to those with CLA seem to exist, but feed composition explains the variation in omega-3 levels only to a limited extent, while there is a large farm influence. This might be related to the energy balance of the cows. A comparison of measured CLA and omega-3 levels in organic milk with common levels in conventional milk shows a small difference during the stable period. Effects of adding oil - as a possibility to improve the level of CLA and omega-3 in a way that is organically acceptable - are explored, but effects remain small

Generating mass and topological terms to the antisymmetric tensor matter field by Higgs mechanism

The interaction between the complex antisymmetric tensor matter field and a scalar field is constructed. We analyze the Higgs mechanism and show the generation of mass and topological terms by spontaneous symmetry breaking.Comment: Accepted for publication in Phys. Lett.

SPECIES IDENTITY IMPORTANT TO ACHIEVE BENEFITS OF DIVERSE GRASSLAND MIXTURES

In semi-natural grasslands, plant diversity has shown to correlate with increased primary production and reduced weed invasion. If such positive effects also apply to low input agricultural grasslands, the resource-use efficiency could be improved. To investigate whether other grassland species and diverse grassland mixtures could be more productive, a grassland on a shallow sandy soil in the south of the Netherlands was sown in April 2009 with seven non-leguminous grassland mixtures. Though some sown species vanished quickly, diverse mixtures proved less susceptible for invading species three years after sowing (p<0.05). Yields in 2010 and 2011 were 11% higher for mixtures with two functional groups of grassland species compared with mixtures with only one functional group (p<0.05). A further increase in species diversity did not further increase production though overyielding was significant, transgressive overyielding was not apparent, mainly due to the strong effect of one specific functional group: mixtures containing tussock grass had a 14% higher production than mixtures without tussock grass. Tussock grass (mainly Dactylis glomerata) also had a major effect on botanical composition: Trifolium repens did invade into mixtures without tussock grass to a much higher extent (19% of the ground cover) than into mixtures with tussock grass (6%; p<0.01). However, total ground cover by invading species was mainly influenced by the number of functional groups present in the sown mixture. The results suggest that diverse grassland mixtures can improve agricultural production and reduce the susceptibility of grass sods for invading species, but that the specific composition of the mixtures has a strong influence

Tachyons, Supertubes and Brane/Anti-Brane Systems

We find supertubes with arbitrary (and not necessarily planar) cross section; the stability against the D2-brane tension is due to a compensation by the local momentum generated by Born-Infeld fields. Stability against long-range supergravity forces is also established. We find the corresponding solutions of the infinite-N M(atrix) model. The supersymmetric D2/anti-D2 system is a special case of the general supertube, and we show that there are no open-string tachyons in this system via a computation of the open-string one-loop vacuum energy.Comment: 1+23 pages, 2 figures, LaTeX. V2, 1+28 pages: Further generalization to non-planar cross-sections and addition of an entirely new section with the explicit supergravity solutions. V3, 1+30 pages: Bound on the angular momentum added, other minor changes in Section

Why UX Research Matters for HRI: The Case of Tablets as Mediators

Many human-robot interaction systems involve a third component: a tablet, which can either be separate or integrated in the robot (as is the case in SoftBank Robotics' Pepper robot). Such a tablet can be used, for instance, to present information to the human user or to gain control over the robot's complex surroundings, by introducing a virtual environment as a substitute for interactions that would normally happen in the physical world. While such a tablet can potentially have a big impact on the usability of the entire system and affect the interaction between human and robot, it is often not explicitly included when evaluating the user experience of human-robot interaction. This paper describes a case study where three evaluation methods were combined in order to get a comprehensive overview of the user experience of an Intelligent Tutoring System (ITS), consisting of a robot and a tablet. The results show several major usability issues with the virtual environment, which could have affected the experience of interacting with the robot. This underlines the importance of including not only the robot itself, but also any other interaction mediators in an iterative design process