On Time Optimization of Centroidal Momentum Dynamics

Recently, the centroidal momentum dynamics has received substantial attention to plan dynamically consistent motions for robots with arms and legs in multi-contact scenarios. However, it is also non convex which renders any optimization approach difficult and timing is usually kept fixed in most trajectory optimization techniques to not introduce additional non convexities to the problem. But this can limit the versatility of the algorithms. In our previous work, we proposed a convex relaxation of the problem that allowed to efficiently compute momentum trajectories and contact forces. However, our approach could not minimize a desired angular momentum objective which seriously limited its applicability. Noticing that the non-convexity introduced by the time variables is of similar nature as the centroidal dynamics one, we propose two convex relaxations to the problem based on trust regions and soft constraints. The resulting approaches can compute time-optimized dynamically consistent trajectories sufficiently fast to make the approach realtime capable. The performance of the algorithm is demonstrated in several multi-contact scenarios for a humanoid robot. In particular, we show that the proposed convex relaxation of the original problem finds solutions that are consistent with the original non-convex problem and illustrate how timing optimization allows to find motion plans that would be difficult to plan with fixed timing.Comment: 7 pages, 4 figures, ICRA 201

Domanda potenziale e rischio traffico/introiti nel Project Financing di un’infrastruttura viaria a pedaggio

Il Project Financing di un’infrastruttura viaria a pedaggio pone come elemento centrale di valutazione della convenienza e finanziabilità dell’opera la capacità di generare, come introiti da pedaggio, i flussi di cassa che costituiscono la garanzia primaria per i finanziatori privati. L’articolo pone in evidenza la centralità della previsione della domanda di traffico e degli introiti attesi sull'infrastruttura, esplorando gli strumenti e le tecniche modellistiche che consentono di stimarne i valori potenziali e, sulla base di questi, di formulare la decisione di investimento. La discussione chiarisce, tuttavia, come i tradizionali modelli di previsione della domanda potenziale di traffico siano inevitabilmente affetti da quote non eliminabili d’incertezza, legati essenzialmente alla variabilità degli input. Tutto ciò si traduce in elementi non trascurabili di rischio, che mal si coniugano con gli obiettivi di convenienza e redditività che l’investitore si pone nei confronti del progetto. L’articolo propone all’attenzione del lettore un approccio quantitativo all’analisi di rischio che consente di definire statisticamente, ossia individuandone la distribuzione di probabilità, la domanda e gli introiti potenziali, superando la percezione deterministica delle valutazioni, e riportandole nel contesto probabilistico che più propriamente la descrive. La discussione elaborata chiarisce l’operatività di questo approccio e ne evidenzia l’utilità nella valutazione della sostenibilità dell’iniziativa e nella formulazione della scelte di investimento. Essa risulta, altresì, utile a comprendere l’importanza di tale approccio, in termini di oggettivazione e trasparenza, nella strutturazione dei rapporti reciproci e nella definizione delle relative garanzie tra i soggetti pubblici e privati che intervengono

Energy balance on a wheat crop in the southeast of the Buenos Aires province

Contar con valores observados de radiación neta, de flujos de calor sensible y latente en el aire, y del flujo de calor en el suelo para diversos usos de suelo permite ajustar y validar modelos atmosféricos. En este trabajo se presentan dichos flujos estimados sobre una cobertura de trigo en el sudeste de la región pampeana argentina. El análisis de la información obtenida muestra que tanto los flujos turbulentos de calor sensible y latente, como el flujo de calor en el suelo presentan una variabilidad asociada tanto a las condiciones atmosféricas como al estado de desarrollo del cultivo. Estos comportamientos están principalmente determinados por la disponibilidad de energía en el sistema modelado a través de la radiación neta, y también por la variabilidad de la estructura de la cobertura causada por variaciones en los estadios fenológicos de la misma. Los resultados mostraron una correlación entre valores máximos del flujo de cantidad de movimiento y calor latente. Durante las fases vegetativas y de oración el flujo de calor latente domina ampliamente la partición de energía, mientras que durante el llenado y maduración de frutos su rol es menor, y cobran mayor importancia los flujos de calor sensible y en el suelo.Having observational values of net radiation flux, sensible and latent heat flux in the air, and soil heat flux for varied land uses allows us to adjust and validate atmospheric models. This paper presents the estimated results of the aforementioned fluxes for a surface having a wheat crop, based on micrometeorological data observed in the south-east area of the Pampas region in Argentina. The analysis of the obtained information shows that sensible and latent heat turbulent fluxes and soil heat flux have a variability which is associated with both atmospheric conditions and development stage of the crop. These are mainly determined by energy availability in the modelled system through net radiation, and also with the canopy structure variability caused by the various crop phenological stages. Findings show a correlation between the maximum values of momentum flux and latent heat flux. During the vegetative and flowering stages the latent heat flux dominates energy partitioning, while during filling and ripening fruit its role is smaller, and become more important the sensible heat and soil fluxes.Fil: Righetti, Silvina Andrea. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina; Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria; Argentina;Fil: Gassmann, María Isabel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina; Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria; Argentina

Errores en contenidos de Regresión y Correlación Lineal por parte de estudiantes universitarios

El estudio de los errores que manifiestan los estudiantes en su proceso de aprendizaje, provee información valiosa de cómo construyen el conocimiento estadístico y ofrece una oportunidad para mejorar los procesos de enseñanza y de aprendizaje. En el presente trabajo se llevo a cabo un estudio descriptivo de los errores detectados al evaluar los bloques temáticos de regresión y correlación lineal en estudiantes universitarios. Para el análisis y clasificación de las respuestas de los estudiantes (referidas a intervalo de confianza, planteamiento de hipótesis, selección del estadístico de prueba, distribución del estimador, reglas de decisión, entre otros) se aplicaron técnicas cualitativas y cuantitativas. Análisis descriptivo univariado (promedios, medianas, cuantiles y medidas de variabilidad, entre otros). Para el análisis comparativo entre variables categóricas se confeccionaron tablas de contingencia empleando la prueba de Chi cuadrado. Mediante el análisis de contenido de entrevistas, se indagó sobre la opinión de los estudiantes acerca de los factores que pudieron contribuir al error. A partir de ello se identificaron los errores en contenidos conceptuales y procedimentales. Se analizó la influencia de variables socio-demográficas (edad, sexo, tenencia de hijos y trabajo), de desempeño académico y de dedicación al estudio (número de materias aprobadas, notas obtenidos en evaluaciones de Estadística I, primer parcial de Estadística II, cantidad de veces de cursado de Estadística II y nivel de asistencia a clases teóricas y prácticas). Los resultados indicaron que los estudiantes presentan confusión entre conceptos claves como parámetro y estimador o regresión y correlación; dificultad para interpretar tanto las hipótesis como las conclusiones en el contexto de los problemas planteados y en el reconocimiento de los supuestos implícitos en cada metodología estadística. Además, los estudiantes indicaron el alto grado de complejidad, no sólo debido a la cantidad de conceptos y fórmulas que incluyen, sino a la relación entre los conceptos involucrados.Fil: Righetti, Andrea. Universidad Nacional de Córdoba. Facultad de Ciencias Económicas. Instituto de Estadística y Demografía; Argentina.Otras Ciencias de la Educació

Errores detectados en estudiantes universitarios al desarrollar pruebas de asociación estadística

En el presente artículo se indican los errores más frecuentes detectados al evaluar pruebas de asociación estadística, tanto a nivel conceptual como procedimental en los bloques temáticos de regresión, correlación y tablas de contingencia. Se define una tipología de errores según conceptos clave y se analizan por género, edad, grado de desempeño académico y nivel de asistencia a clases. A partir de un análisis multivariado se determinan tres perfiles de estudiantes. Mediante el análisis de contenido de entrevistas, se indaga sobre los errores detectados y la opinión de los estudiantes acerca de los factores que pueden contribuir en los errores. Finalmente se presentan estrategias alternativas de enseñanza como propuesta superadora de los errores detectados. De esta manera se intenta contribuir a una mejor comprensión de los conceptos de asociación estadística por parte de los estudiantes universitarios. PALABRAS CLAVE:Regresión – Correlación – Tablas de Contingencia – Errores Conceptuales y Procedimentales

R134a and its low GWP substitutes R1234yf and R1234ze(E) condensation inside a 4mm horizontal smooth tube

The substitution of HFC134a with low GWP refrigerants is one of the most important challenge for refrigeration and air conditioning. The possible substitutes include natural refrigerants, such as HC600 (Butane) and HC600a (Isobutane), and also synthetic refrigerants, such as HFO1234yf and HFO1234ze(E). The HC refrigerants exhibit very low GWP, 3 and 4 HC600a and HC600 respectively, good thermodynamic and transport properties, and pressure and volumetric performance very similar to HFC134a. The major drawback of HC refrigerants is their high flammability, being classified in class A3 according to ASHRAE classification. Also the HFO refrigerants present a mild flammability, being classified in class A2L. In fact it is very difficult to found low GWP substitutes for traditional HFC refrigerants with no flammability, as a weak chemical stability and / or a big chemical reactivity are presuppositions for low GWP. Both HFO1234yf and HFO1234ze(E) seem to be very promising as substitute for HFC134a, showing a GWP lower than 1 together with pressure and volumetric properties closely near to those of HFC134a. This paper presents the comparative analysis of HFC134a HFO1234yf and HFO1234ze(E)during saturated condensation inside a 4 mm horizontal smooth tube. The experimental tests were carried out at three different saturation temperatures (30, 35, and 40 °C) at decreasing vapour quality up to sub-cooled liquid condition, to evaluate the specific contribution of refrigerant mass flux, mean vapour quality, and saturation temperature. The refrigerant mass flux ranges from 100 to 600 kg m-2 s-1. The experimental measurements were reported in term of condensation heat transfer coefficients and frictional pressure drops and plotted in non-dimensional co-ordinates showing the heat transfer factor and the friction factor against the equivalent Reynolds number. A transition point from gravity dominated and forced convection condensation was found for an equivalent Reynolds number around 10000-20000. HFO1234yf and HFO1234ze(E) exhibit heat transfer coefficients and pressure drops similar to HFC134a and they seem to be valuable long term low GWP substitutes for HFC134a. The experimental heat transfer coefficients in the forced convection condensation regime were very well predicted by the Akers et al. (1959) model, whereas the Friedel (1979) correlation was able to reproduce the frictional pressure drop data both in gravity dominated and forced convection condensation regimes

R1233zd(E) and R245fa Flow Boiling Heat Transfer and Pressure Drop inside a 4.2 mm ID Microfin Tube

This paper presents R1233d(E) and R245fa flow boiling heat transfer and pressure drop measurements inside a mini microfin tube having internal diameter at the fin tip equal to 4.2 mm, 40 fins 0.15 mm high, and a helix angle of 18°. The tube was brazed inside a copper plate and electrically heated from the bottom. Sixteen T-type thermocouples were located in the copper plate to monitor the wall temperature. The experimental measurements were carried out at a constant mean saturation temperature of 30 °C, by varying the refrigerant mass velocity between 100 kg m-2 s-1 and 300 kg m-2 s-1, the vapour quality, and the heat flux from 15 to 90 kW m-2. The experimental results are here presented in terms of two-phase heat transfer coefficient, onset dryout vapour quality, and frictional pressure drop. Moreover, R1233zd(E) has been proposed as low GWP alternative to R245fa. In this paper, the two fluids performance is compare under the same working conditions and commented. Finally, the experimental measurements were used to assess a few selected models for boiling heat transfer coefficient and frictional pressure drop estimations available in the open literature for microfin tubes

Multi-contact Stochastic Predictive Control for Legged Robots with Contact Locations Uncertainty

Trajectory optimization under uncertainties is a challenging problem for robots in contact with the environment. Such uncertainties are inevitable due to estimation errors, control imperfections, and model mismatches between planning models used for control and the real robot dynamics. This induces control policies that could violate the contact location constraints by making contact at unintended locations, and as a consequence leading to unsafe motion plans. This work addresses the problem of robust kino-dynamic whole-body trajectory optimization using stochastic nonlinear model predictive control (SNMPC) by considering additive uncertainties on the model dynamics subject to contact location chance-constraints as a function of robot's full kinematics. We demonstrate the benefit of using SNMPC over classic nonlinear MPC (NMPC) for whole-body trajectory optimization in terms of contact location constraint satisfaction (safety). We run extensive Monte-Carlo simulations for a quadruped robot performing agile trotting and bounding motions over small stepping stones, where contact location satisfaction becomes critical. Our results show that SNMPC is able to perform all motions safely with 100% success rate, while NMPC failed 48.3% of all motions

Biopolyesters and bio based additives based blends and composites for application in packaging and agriculture

The utilization of “bio-polymers” for the production of “bio-plastic” is worldwide an assessed priority with the aim of reducing dependence from petro sources, and handle the concern for disposal of waste generated from not degradable plastics Biobased polyesters such as Polyhydroxyalkanoates (PHAs) and Polylactide (PLA) are promising biobased, compostable polymer suitable for replacing petro-derived polymer for several single use applications but are addressed even for durable materials requiring more demanding technical performances [1,2], in particular in terms of mechanical properties and stability. Consequently a better knowledge of the crystallization behavior of PLA [3] and PHAs, and its effects on the mechanical properties is crucial in order to extend bio polyesters industrial applications, and even for optimization of polymeric matrices to be further used for biocomposite or active packaging production. In the present study we have addressed the use of biobased biodegradable reactive plasticizers for production of PLA based films by blow moulding [4], and the production of biocomposites with either PLA or PHA based polymeric matrices and natural fibres such as wood, bran, and Posidonia oceanica [5]. Those studies were inserted in the activites of Regional project PHA (Project POR FESR 2014-2020) addressing production of pots and items degradable even in soil and marine water, and of the European Union’s Horizon2020, Project AGRIMAX GA: n° 720719, addressing valorization of agriculture biomass (tomato, olive, potato, bran) for different ranges of applications including biocomposites. Pla based films were produced by use of functional plasticizers derived from soy bean oils, or from cardanol, with properties comparable to polypropylene or high density polyethylene. Biocomposites were produced with either PLA or PHA polymeric matrices with up to 30% by weight of natural fibres