Hard Limit Research Articles

Hard limits to cognitive flexibility: ants can learn to ignore but not avoid pheromone trails.

ABSTRACTLearning allows animals to respond to changes in their environment within their lifespan. However, many responses to the environment are innate, and need not be learned. Depending on the level of cognitive flexibility an animal shows, such responses can either be modified by learning or not. Many ants deposit pheromone trails to resources, and innately follow such trails. Here, we investigated cognitive flexibility in the ant Lasius niger by asking whether ants can overcome their innate tendency and learn to avoid conspecific pheromone trails when these predict a negative stimulus. Ants were allowed to repeatedly visit a Y-maze, one arm of which was marked with a strong but realistic pheromone trail and led to a punishment (electric shock and/or quinine solution), and the other arm of which was unmarked and led to a 1 mol l−1 sucrose reward. After ca. 10 trials, ants stopped relying on the pheromone trail, but even after 25 exposures they failed to improve beyond chance levels. However, the ants did not choose randomly: rather, most ants began to favour just one side of the Y-maze, a strategy which resulted in more efficient food retrieval over time, when compared with the first visits. Even when trained in a go/no-go paradigm which precludes side bias development, ants failed to learn to avoid a pheromone trail. These results show rapid learning flexibility towards an innate social signal, but also demonstrate a rarely seen hard limit to this flexibility.

Climate Change Adaptation on Small Island States: An Assessment of Limits and Constraints

Small Island States (SIDS) are among the nations most exposed to climate change (CC) and are characterised by a high degree of vulnerability. Their unique nature means there is a need for more studies focused on the limits to CC adaptation on such fragile nations, particularly regarding their problems and constraints. This paper addressed a perceived need for research into the limitations of adaptation on SIDS, focusing on the many unique restrictions. To this end, the study identified and described the adaptation limits they have by using a review of the literature and an analysis of case studies from a sample of five SIDS in the Caribbean and Pacific regions (Barbados, Trinidad and Tobago, Cook Islands, Fiji, Solomon Islands and Tonga). This research’s findings showed that an adaptable SIDS is characterised by awareness of various values, appreciation and understanding of a diversity of impacts and vulnerabilities, and acceptance of certain losses through change. The implications of this paper are two-fold. It explains why island nations continue to suffer from the impacts of CC and suggest some of the means via which adequate policies may support SIDS in their efforts to cope with the threats associated with a changing climate. This study concluded that, despite the technological and ecological limits (hard limits) affecting natural systems, adaptation to CC is limited by such complex forces and societal factors (soft limits) that more adequate adaptation strategies could overcome.

Review of Racially Equitable Admissions Practices in STEM Doctoral Programs

This study reviews literature on racially equitable admissions practices relevant to graduate programs in STEM. Graduate Record Exam (GRE) scores correlate more strongly with race, gender, and socioeconomic status than performance metrics for research during or after graduate school. Structural changes to admissions processes that can improve equity of admissions decisions and reduce correlations between admissions decisions and demographic data include using holistic review or composite scores that quantize more components of an application, removing hard limits on course requirements, admitting students as a cohort instead of to individual faculty sponsors, and diversifying admissions committees. Some alternative scoring methods attempt to measure personality traits, but performing these measurements during admissions may present difficulties. Bridge programs—whether they are implemented as collaborations with a minority-serving institution, a personalized educational program for each student admitted to a program, or a stand-alone program before the doctoral degree program—may be able to improve both recruitment and retention of students with underrepresented racial and ethnic identities in their field of study. Finally, financial barriers to applications can disproportionately affect underrepresented applicants due to systemic racism. We end with recommendations for graduate programs to improve equity in admissions processes.

Simulation of Vehicle Steering Angle and Lateral Acceleration in Mitigating Potential Run-Off-Road Crashes


 
 
 This paper proposes and presents the preliminary results of an integrated safety warning system for road vehicles based on lateral g-force monitoring. The proposed system issues a warning to the driver when the lateral load increases above a threshold level thereby reducing the risk of run-off-road crashes and loss of control. From the vehicles dynamic model, soft and hard speed limits are obtained. When the soft limit is breached, it warns the driver using a LED, and when the hard limit is breached, assistive braking is activated. Simulations were conducted to obtain the safe vehicle speed for various steering angle to ensure the lateral g-force remains in the range of 0.7 and 0.9. Vehicle parameters of the Proton Saga 1.3L were used for simulation. Simulation runs to study the effects of changes in steering-wheel angle and vehicle speed on the variation of lateral acceleration was performed, based on which a safe speed is obtained. Simulations were also conducted on both banked and unbanked roads. The results of this study would help in the design of a working prototype of the safety system.
 
 

Throughput analysis of 2-D OCDMA/pure ALOHA networks with access control and two user classes of variable length for multimedia traffic

In a previous research, the throughput of a 2-D optical code-division multiple-access (OCDMA)/unslotted ALOHA (U-ALOHA)/channel load sensing protocol network using an optical hard limiter and channel code was analyzed. This scheme assumed a fixed message length and one user class. However, the current and future themes of networks is multimedia traffic with variable message length and two different user classes of real-time and non-real-time. In this paper, we propose a 2-D OCDMA/U-ALOHA network with access control and two user classes of variable message length. We assume that the number of fixed-length packets in a message is geometrically distributed and perform access control by assigning two user classes with different access permission probabilities. The numerical results show the high priority user class (e.g. real-time data traffic) can maintain maximum system throughput under the heavy load at the expense of low priority user class (e.g. non-real-time data traffic). The proposed network protocol could obtain 100 Gbps and be a promising alternative for local area networks (LANs) and broadband optical access networks for larger capacity in response to fast growing of multimedia data traffic.

Beyond the tracked line of sight - Evaluation of the peripheral usable field of view in a simulator setting.

Combining advanced gaze tracking systems with the latest vehicle environment sensors opens up new fields of applications for driver assistance. Gaze tracking enables researchers to determine the location of a fixation, and under consideration of the visual saliency of the scene, to predict visual perception of objects. The perceptual limits, for stimulus identification, found in literature have mostly been determined in laboratory conditions using isolated stimuli, with a fixed gaze point, on a single screen with limited coverage of the field of view. The found limits are usually reported as hard limits. Such commonly used limits are therefore not applicable to settings with a wide field of view, natural viewing behavior and multi-stimuli. As handling of sudden, potentially critical driving maneuvers heavily relies on peripheral vision, the peripheral limits for feature perception need to be included in the determined perceptual limits. To analyze the human visual perception of different, simultaneously occurring, object changes (shape, color, movement) we conducted a study with 50 participants, in a driving simulator and we propose a novel way to determine perceptual limits, which is more applicable to driving scenarios.

The multi-commodity network flow problem with soft transit time constraints: Application to liner shipping

The multi-commodity network flow problem (MCNF) consists in routing a set of commodities through a capacitated network at minimum cost and is relevant for routing containers in liner shipping networks. As commodity transit times are often a critical factor, the literature has introduced hard limits on commodity transit times. In practical contexts, however, these hard limits may fail to provide sufficient flexibility since routes with even tiny delays would be discarded. Motivated by a major liner shipping operator, we study an MCNF generalization where transit time restrictions are modeled as soft constraints, in which delays are discouraged using penalty functions of transit time. Similarly, early commodity arrivals can receive a discount in cost. We derive properties that distinguish this model from other MCNF variants and adapt a column generation procedure to efficiently solve it. Extensive numerical experiments conducted on realistic liner shipping instances reveal that the explicit consideration of penalty functions can lead to significant cost reductions compared to hard transit time deadlines. Moreover, the penalties can be used to steer the flow towards slower or faster configurations, resulting in a potential increase in operational costs, which generates a trade-off that we quantify under varying penalty functions.

Three Simple Properties Explain Protein Stability Change upon Mutation.

Accurate prediction of protein stability upon mutation enables rational engineering of new proteins and insights into protein evolution and monogenetic diseases caused by single-point amino acid substitutions. Many tools have been developed to this aim, ranging from energy-based models to machine-learning methods that use large amounts of experimental data. However, as the methods become more complex, the interpretation of the chemistry underlying the protein stability effects becomes obscure. It is thus of interest to identify the simplest prediction model that retains complete amino acid specific interpretation; for a given number of input descriptors, we expect such a model to be almost universal. In this study, we identify such a limiting model, SimBa, a simple multilinear regression model trained on a substitution-type-balanced experimental data set. The model accounts only for the solvent accessibility of the site, volume difference, and polarity difference caused by mutation. Our results show that this very simple and directly applicable model performs comparably to other much more complex, widely used protein stability prediction methods. This suggests that a hard limit of ∼1 kcal/mol numerical accuracy and an R ∼ 0.5 trend accuracy exists and that new features, such as account of unfolded states, water colocalization, and amino acid correlations, are required to improve accuracy to, e.g., 1/2 kcal/mol.

Optical limiting and speckle of low power continuous wave laser beams using nonlinear scattering in photorefractive Zr: LiNbO3 crystals

The optical limiting caused by nonlinear scattering in the photorefractive Zr-doped LiNbO3 crystals was studied for lower-power laser beams with wavelengths of 514.5 and 532 nm. The measurements of speckle evolution indicate that the samples with 0.625 ≤ [Zr] ≤ 1.25 mol% exhibit the very large gain factor for holographic amplification of noise gratings. Based on the direct measurements of the output power depending on the input one, we conclude that these samples behaved as hard optical limiters for the laser beams with a low incident power. Thus, these crystals find their potential application in the optical limiting devices.

Comprehensive analysis of pulsed plasma nitriding preconditions on the fatigue behavior of AISI 304 austenitic stainless steel

This study aims to draw an exact boundary for microstructural and mechanical behaviors in terms of pulsed plasma nitriding conditions. The pulsed plasma nitriding treatment was applied to AISI 304 austenitic stainless steel at different temperatures and durations. Results reveal that nitriding depth increased as process temperature and duration increase. The nitriding depth remarkably increased at 475°C for 8 h and at 550°C for 4 h. An austenite structure was transformed into a metastable nitrogen-oversaturated body-centered tetragonal expanded austenite (S-phase) during low-temperature plasma nitriding. The S-phase was converted to CrN precipitation at 475°C for 8 h and at 550°C for 4 h. Surface hardness and fatigue limit increased through plasma nitriding regardless of process conditions. The best surface hardness and fatigue limit were obtained at 550°C for 4 h because of the occurrence of CrN precipitation.

Vertical cavity capacitive transducer.

The capacitive micromachined ultrasonic transducer (CMUT) has inherent advantages, such as larger bandwidth and monolithic integration capability with electronics, when compared to the piezoelectric transducer. The most significant shortcoming of the device is the trade-off between input and output sensitivities. Adequate receive sensitivity requires an electric field intensity on the order of 105 V/m, which can be achieved by sub-micron gap heights. However, a small gap limits the device stroke and, consequently, the maximum output pressure. This paper addresses this problem by proposing a CMUT with a vertical cavity. The membrane of the device has a piston part that is surrounded by the sidewalls of a vertical cylinder formed in the substrate. The fringing electric field pulls the piston in the vertical direction; hence, the gap height remains fixed, which alleviates the hard limit on device stroke. The performance of the proposed device is compared to that of the conventional CMUT by theoretical and analytical methods, and a micro-fabrication method is devised. Additionally, a millimeter-scale device has been manufactured and tested as a proof of concept.

The smallest eigenvalue distribution of the Jacobi unitary ensembles

In the hard edge scaling limit of the Jacobi unitary ensemble generated by the weight xα(1 − x)β, x ∈ [0, 1], α, β > −1, the probability that all eigenvalues of Hermitian matrices from this ensemble lie in the interval [t, 1] is given by the Fredholm determinant of the Bessel kernel. We derive the constant in the asymptotics of this Bessel kernel determinant. A specialization of the results gives the constant in the asymptotics of the probability that the interval (− a, a), a > 0 is free of eigenvalues in the Jacobi unitary ensemble with the symmetric weight (1 − x2)β, x ∈ [− 1, 1].

Complex Dynamics and Hard Limiter Control of a Fractional-Order Buck-Boost System

Chaos and control analysis for the fractional-order nonlinear circuits is a recent hot topic. In this study, a fractional-order model is deduced from a Buck-Boost converter, and its discrete solution is obtained based on the Adomian decomposition method (ADM). Chaotic dynamic characteristics of the fractional-order system are investigated by the bifurcation diagram, 0-1 test, spectral entropy (SE) algorithm, and NIST test. Meanwhile, the control of the fractional-order Buck-Boost model is discussed through two different ways, namely, the intensity feedback and the hard limiter control. Specifically, the hard limiter control can be realized using a current limiter in the circuit, where the current limiter device is applied to control the branch current. The results show that the proposed fractional-order system has complex dynamic behaviors and potential application values in the engineering field.

Late-time cosmic evolution of dust: solving the puzzle

ABSTRACT Dust is an essential ingredient of galaxies, determining the physical and chemical conditions in the interstellar medium. Several complementary observational evidences indicate that the cosmic dust mass density significantly drops from redshift z = 1 to z = 0. Clearly, and for the first time during cosmic evolution, dust must be destroyed more rapidly than it is formed. By considering the dust production/destruction processes acting in this cosmic time lapse, we find that the drop can be explained if dust is mainly destroyed by astration (49 per cent contribution in the fiducial case) and supernova (SN) shocks within galaxies (42 per cent). Our results further imply that on average each SN destroys only $M_{\mathrm{ d},\mathrm{ sn}} =0.45\, \mathrm{ M}_\odot$ of dust, i.e. 5–10 times less than usually assumed, with a hard upper limit of Md,sn < 3.0 M⊙ set by the available metal budget and maximal grain growth. The lower efficiency might be explained by effective shielding of dust against shock processing in pre-SN wind shells.

Pre-Load Studies on a 2-m Long Nb3Sn 11 T Model Magnet for the High Luminosity Upgrade of the LHC

The Large Hadron Collider luminosity upgrade requires the replacement of standard 8.3 T Nb-Ti main dipole magnets with pairs of shorter 11 T Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn dipole magnets. After the manufacturing and testing of several 2 m-long 11 T models, the full-length 11 T magnet series production is now ongoing. As a result of a degraded quench performance in several short models, attributed to excessive stress in the coils' midplanes, an upper hard limit for the coils' pre-stress was set to avoid any conductor degradation. With the aim of further investigating the influence of a lower pre-stress, an additional double-aperture short model was manufactured. The first aperture was assembled with the standard pre-stress target, in line with the series production. The second aperture was assembled with half of the standard pre-stress. Both apertures were then assembled into a 2-in-1 structure, but connected individually in order to power them independently to allow for comparison studies. Contrary to previously built 11 T magnets, the conductor type is also different as Powder-in-tube (PIT) cable was used, instead of standard Restacked-Rod-Process (RRP) cable. Additionally, the quench heaters for magnet protection were not impregnated as previously done, but placed on the outer radial surface of the coils, in agreement with the final decision for the 11 T series production. This paper describes the manufacturing and testing campaign results of this last 11 T short model.

“From a chef’s perspective or what I can sell on the menu?” Exploring culinary professionals’ attitudes toward specialty beef production and barriers to adoption

ABSTRACT This research aims to understand the attitudes and values of culinary ‘innovators’ already working with ‘local’ or ‘grass-fed/finished’ beef. Semi-structured interviews were conducted with specialty chefs and butchers in the Rocky Mountain Region based on the hypothesis that they operate as opinion leaders and understand that the benefits of, and barriers to, implementation of their current meat program is crucial for successfully introducing new products, such as Rocky Mountain Legume-Finished beef. Interviews revealed that professionals struggle with being ‘conscientious’ and ‘sustainable’ while still providing a consistent product for their customers. Although a few chefs spoke of setting ‘hard limits’ and ‘high standards’ for products, this did not emerge as a financial reality for other chefs or customers. In order for alternative sources of beef to be truly viable, beef production must address both environmental and economic concerns, while providing a consistent product in terms of taste and quality. Understanding the concerns of culinary professionals allows alternative ranching and agricultural production methods to be tailored as a viable for beef producers.

Manufacturing and Characterization of 3D Miniature Polymer Lattice Structures Using Fused Filament Fabrication.

The potential of additive manufacturing to produce architected lattice structures is remarkable, but restrictions imposed by manufacturing processes lead to practical limits on the form and dimension of structures that can be produced. In the present work, the capabilities of fused filament fabrication (FFF) to produce miniature lattices were explored, as they represent an inexpensive option for the production of polymer custom-made lattice structures. First, fused filament fabrication design guidelines were tested to assess their validity for miniature unit cells and lattice structures. The predictions were contrasted with the results of printing tests, showing some discrepancies between expected outcomes and resulting printed structures. It was possible to print functional 3D miniature open cell polymer lattice structures without support, even when some FFF guidelines were infringed, i.e., recommended minimum strut thickness and maximum overhang angle. Hence, a broad range of lattice structures with complex topologies are possible, beyond the cubic-type cell arrangements. Nevertheless, there are hard limits in 3D printing of miniature lattice structures. Strut thickness, length and orientation were identified as critical parameters in miniature lattice structures. Printed lattices that did not fully comply with FFF guidelines were capable of bearing compressive loads, even if surface quality and accuracy issues could not be fully resolved. Nevertheless, 3D printed FFF lattice structures could represent an improvement compared to other additive manufacturing processes, as they offer good control of cell geometry, and does not require additional post-processing.

Clarifying the definition of ‘transonic’ screw dislocations

ABSTRACT A number of recent Molecular Dynamics (MD) simulations have demonstrated that screw dislocations in face centred cubic (fcc) metals can achieve stable steady state motion above the lowest shear wave speed () which is parallel to their direction of motion (often referred to as transonic motion). This is in direct contrast to classical continuum analyses which predict a divergence in the elastic energy of the host material at a crystal geometry dependent ‘critical’ velocity . Within this work, we first demonstrate through analytic analyses that the elastic energy of the host material diverges at a dislocation velocity () which is greater than , i.e. . We argue that it is this latter derived velocity () which separates ‘subsonic’ and ‘supersonic’ regimes of dislocation motion in the analytic solution. In addition to our analyses, we also present a comprehensive suite of MD simulation results of steady state screw dislocation motion for a range of stresses and several cubic metals at both cryogenic and room temperatures. At room temperature, both our independent MD simulations and the earlier works find stable screw dislocation motion only below our derived . Nonetheless, in real-world polycrystalline materials cannot be interpreted as a hard limit for subsonic dislocation motion. In fact, at very low temperatures our MD simulations of Cu at 10 Kelvin confirm a recent claim in the literature that true ‘supersonic’ screw dislocations with dislocation velocities are possible at very low temperatures.

On the efficacy of SoC-preconditioning on the utilization of battery packs in Electric Vehicles

During the last decade Electric Vehicles (EVs) surged in popularity. However, their mass adoption is slowed by the limited capacity of their Energy Storage System (ESS). Lithium Ion (Li-Ion) technology has established itself as the de-facto standard for mobile applications, though its energy density and cost put a hard limit on the maximum size of viable EV battery packs. Maximizing its utilization therefore becomes of central importance. To efficiently use a battery pack over its entire lifetime, the State of Health (SoH) of the cells needs to be taken into account. In this paper, we propose a novel preconditioning algorithm to minimize the time an EV is connected to the charging station. Our proposed approach uses existing Active Cell Balancing (ACB) hardware of the battery pack to precondition the State of Charge (SoC) of cells such that all cells reach the top SoC threshold at the same time without requiring an additional balancing phase during charging. This is done by considering the individual cells’ SoH to precondition them for achieving an equal time to charge fully. Applying the same approach for discharging, we can extend the driving range of an EV, which otherwise is limited by the cell with the lowest SoC in the pack. Our analysis of various usage scenarios shows that our proposed preconditioning algorithm increases the usable energy of the battery pack by up to 1.8% compared to conventional balancing algorithms while effectively halving the time connected to a charging station, all without requiring any additional hardware components.

Pilot Tone Power Limits of Brillouin Amplified Carrier Recovery for Optical Communications

The benefit of Brillouin amplification for optical carrier recovery from pilot tones is evaluated in application to coherent signal detection without a conventional local oscillator (LO) laser. The focus being on the narrow gain bandwidth suppressing broadband spectral noise power relative to the pilot tone. Calculations indicate its use at the receiver with realistic 30 dB gain and 30 MHz gain bandwidth permit lowering the minimum pilot tone to signal power ratio (PSR) for low distortion signal demodulation by up to ≈24 dB in the case of a 10 GHz wide signal channel, and even larger for broader channels. The ultimate limit being its ratio to the narrowness of the Brillouin gain bandwidth. Low noise Brillouin gain is also accounted for from characterization by a coherent receiver and digital signal processing approach for an optical fiber used as the gain medium in experiments. Its use in optical carrier recovery from a 3 channel WDM 48 Gb/s 64-QAM signal with polarization multiplexed pilot tones demonstrates improvement consistent with predictions in reducing the impact of pilot tone noise to near that for a conventional LO. The estimated low PSR limit for a bit error rate below the hard decision FEC limit was extrapolated as ≈-38 dB. This rose to ≈-30 dB with inclusion of 80 km link transmission due to the restricted signal launch power for avoiding nonlinear distortion. The insight highlights the wide-ranging capabilities for enhancing performance in advanced coherent communications.