High-gain PI-like control of Euler–Lagrange mechanical systems: Simulations and experiments in 2DoF robotic manipulators

Euler–Lagrange mechanical systems usually possess nonlinear models, which are subject to uncertainties and unknown disturbances. In this article, a robust but smooth controller is proposed, relying on a high-gain proportional–integral structure that renders an accurate tracking, even in the case of uncertain effects. The tracking performance can be improved by adjusting a single parameter, deriving into a prescribed stability regime, where the tracking errors are locally uniformly ultimately bounded, and where the controller depends on a change of coordinates that integrates the proportional, integral, and derivative information of the tracking error. The advantages of this proposal become evident without considering an intricate implementation procedure. Numerical results are provided to highlight the reliability of the proposed method.

From giant clumps to clouds – I. The impact of gas fraction evolution on the stability of galactic discs

ABSTRACT The morphology of gas-rich disc galaxies at redshift $\sim 1\!-\!3$ is dominated by a few massive clumps. The process of formation or assembly of these clumps and their relation to molecular clouds in contemporary spiral galaxies are still unknown. Using simulations of isolated disc galaxies, we study how the structure of the interstellar medium and the stability regime of the discs change when varying the gas fraction. In all galaxies, the stellar component is the main driver of instabilities. However, the molecular gas plays a non-negligible role in the interclump medium of gas-rich cases, and thus in the assembly of the massive clumps. At scales smaller than a few 100 pc, the Toomre-like disc instabilities are replaced by another regime, especially in the gas-rich galaxies. We find that galaxies at low gas fraction (10 per cent) stand apart from discs with more gas, which all share similar properties in virtually all aspects we explore. For gas fractions below $\approx 20{{\ \rm per\ cent}}$, the clump-scale regime of instabilities disappears, leaving only the large-scale disc-driven regime. Associating the change of gas fraction to the cosmic evolution of galaxies, this transition marks the end of the clumpy phase of disc galaxies, and allows for the onset of spiral structures, as commonly found in the local Universe.

Comparison of stabilizing mechanisms between discrete- and continuous-time consumer-resource models

There is rich literature on using continuous-time and discrete-time models for studying population dynamics of consumer-resource interactions. The continuous-time framework is generally used to model populations with overlapping generations and all year-round reproduction. In contrast, discrete-time models are more suited for populations with non-overlapping generations that reproduce in a discrete pulse determined by season. Inspired by the Nicholson-Bailey/Lotka-Volterra modeling formalisms in discrete-time/continuous-time, respectively, we consider host-parasitoid interactions with an arbitrary parasitoid attack rate that is a function of both the host and parasitoid population densities. We characterize and compare stability regimes in both modeling frameworks for analogous host reproduction and attack rates. Our analysis shows that a Type II functional response is stabilizing in both modeling frameworks only when combined with other mechanisms, such as mutual interference between parasitoids. This stability regime related to a Type II functional response is smaller in the discrete-time framework compared to continuous-time, and shrinks with increasing host reproduction. A Type III functional response is by itself stabilizing, but the extent of attack-rate acceleration needed is much higher in the discrete-time framework, and its stability regime expands with increasing host reproduction. Finally, our results show that while mutual parasitoid interference can stabilize population dynamics, cooperation between parasitoids to handle hosts is destabilizing in both frameworks. However, a combination of a Type III functional response together with parasitoid cooperation can create stability. In summary, our comparative analysis systematically characterizes diverse ecological processes driving stable population dynamics in discrete-time and continuous-time consumer-resource models.

Dielectric Elastomer as a New Material for Electrostatically Actuated Microbeams: Stability Analysis

Due to the interesting properties such as light weight and high deformation ability, dielectric elastomer (DE) resonators can be good alternatives for conventional silicon resonant beams used in micro-electro-mechanical systems (MEMS). This paper proposes a modeling in which a pre-stretched clamped-clamped DE-based microbeam oscillating above the ground substrate is subjected to an external electrostatic pressure. Using a DE-based beam affects the total rigidity of the system, which may lead to an anticipated saddle-node or pitchfork bifurcation. Hence, the present study tries to analyze the effects of DE properties on changing the stability regime of DE-based microbeams under electrostatic actuation. The stability of the system has been investigated using an eigen-value form of the problem. The effects of DE properties including pre-stress, relative permittivity and voltage value across the electrodes on pull-in or divergence instability as well as the frequency response of the system have been investigated. Moreover, the critical values of the DE voltage as a booster of instability occurrence have been obtained in either the presence or absence of the direct current (DC) voltage. It has been found that the pre-stress and appropriate DE permittivity can provide a needed magnitude of the DE actuating voltage to alter the resonance frequency and stability positions of the structure.

Boundary Layer Stability Regime at DACCIWA Site Using Gradient Richardson Number

Meteorological data including air temperature and wind speed which were collected from DACCIWA measurement site at a tropical agricultural field site in Ile-Ife (7.55oE, 4.56oE), south-western Nigeria have been used to classify boundary layer stability regimes using gradient Richardson number. Three categories were considered to deduce the pattern of stability conditions namely stable, unstable and neutral conditions for 3-hourly intervals at 0.00, 03.00, 06.00, 09.00, 12.00, 15.00, 18.00 and 21.00 hours from 15th June to 31st July 2016. The data were sampled every 1sec and stored subsequently as 10 minutes averages for all the measured parameters. The data was further reduced to 30 minutes averages for easy analysis and manipulation in the calculation of gradient Richardson number used for boundary layer stability regime characterization. The results showed that the month of June 2016 had prevalence of stable regime from 0:00 – 6:00 am and 6:00 pm; 9:00 am was predominantly neutral and shared similar pattern with 9:00 pm. Unstable regime was slightly observed at 12:00 pm and majorly observed at 3:00 pm. The month of July had a little shift from what was observed in the month of June. Predominance of neutral conditions was observed from 9:00 pm to 9:00 am; Hours of 12:00 – 3:00 pm were dominated by unstable regime while 6:00 pm was dominated by stable regime.

Effect of Quarls on the Blowout Stability and Emission of Pollutants of a Liquid-Fueled Swirl Burner

Stringent emissions standards for NOx and carbon monoxide (CO) prompt lean combustor development. With this motivation, combustion stability issues emerge since the desired operating point approaches the lean blowout limit. In this paper, an atmospheric, 15 kW lean premixed prevaporizing-type swirl burner, equipped with a plain jet airblast atomizer, was investigated at various atomizing pressures and combustion air flow rates, using quarls from 0 deg to 60 deg in 15 deg steps. Both the 15 deg and the 30 deg quarls provided a 42% higher lean blowout stability on average in terms of mean mixing tube discharge velocity, compared to the baseline burner. However, the superior stability regime was encumbered by a rapidly increasing CO emission. In parallel, the NOx emission vanished due to the more dilution air and incomplete combustion. The 60 deg quarl provided a moderately extended blowout stability limitation, while the NOx emission slightly increased and the CO emission reduced compared to the baseline burner.