Circular Track Research Articles

Influence of task complexity on mechanical efficiency and propulsion technique during learning of hand rim wheelchair propulsion

Objective: To investigate the consequence of task complexity on gross mechanical efficiency and propulsion technique during the learning process of hand rim wheelchair propulsion. Methods: Three groups of unimpaired subjects ( N = 10 each) received a 3-week wheelchair practice period (3 week −1, nine practice trials) with different levels of complexity, i.e. propelling a stationary wheelchair ergometer, wheelchair propulsion on a motor-driven treadmill or at a circular wheelchair track. During practice trials 1 and 9, gross mechanical efficiency and propulsion technique variables (work per cycle, cycle frequency, push and cycle time, effective force) were measured. Results: Using multi-level regression analysis, no differences in the development over time in mechanical efficiency and propulsion technique could be discerned between the three conditions of task complexity. Only the percentage push time during the cycle decreased significantly more in the group that practiced on the ergometer compared to the treadmill-practice group. For all three groups a change over time was shown for cycle frequency, push time and cycle time. Discussion: Under the current experimental conditions, task complexity does not have an influence on gross mechanical efficiency and propulsion technique during the learning process of wheelchair propulsion. The 3-week practice period had a favorable practice effect on timing regardless of the task complexity.

Hippocampal place cells show increased sensitivity to changes in the local environment following prefrontal cortex lesions.

It has been proposed that the prefrontal cortex modulates neural activity in posterior cortex via inhibitory mechanisms. As a result, damage to the former area may produce disinhibition in posterior regions and increase sensitivity to extraneous information. This hypothesis was investigated by examining how prefrontal cortex lesions affected the firing of hippocampal place cells in freely moving rats. In experiment 1, the positional firing of lesion-group cells was altered to a greater extent than that of control-group cells when objects were introduced into the recording environment. This suggested that place cell firing was overly influenced by local cues in the prefrontal-lesioned animals. In experiment 2 place cells were recorded while rats foraged on a circular track with access to both local and distal multimodal cues. Although the position of place fields in lesion-group cells was not excessively tied to local cues, a greater proportion of the fields lost their spatial selectivity following a rotation of these cues. The cue-related effects were associated with larger extracellular action-potential amplitudes and a greater incidence of burst-firing in lesion-group cells. This finding is consistent with the hypothesis that lesions of the prefrontal cortex result in a disinhibition of posterior cortex.

Accuracy of non-differential GPS for the determination of speed over ground

Accurate determination of speed is important in many studies of human and animal locomotion. Some global positioning system (GPS) receivers can data log instantaneous speed. The speed accuracy of these systems is, however, unclear with manufacturers reporting velocity accuracies of 0.1–0.2 ms −1. This study set out to trial non-differential GPS as a means of determining speed under real-life conditions. A bicycle was ridden around a running track and a custom-made bicycle speedometer was calibrated. Additional experiments were performed around circular tracks of known circumference and along a straight road. Instantaneous speed was determined simultaneously by the custom speedometer and a data logging helmet-mounted GPS receiver. GPS speed was compared to speedometer speed. The effect on speed accuracy of satellite number; changing satellite geometry, achieved through shielding the GPS antenna; speed; horizontal dilution of precision and cyclist position on a straight or a bend, was evaluated. The relative contribution of each variable to overall speed accuracy was determined by ANOVA. The speed determined by the GPS receiver was within 0.2 ms −1 of the true speed measured for 45% of the values with a further 19% lying within 0.4 ms −1 ( n=5060). The accuracy of speed determination was preserved even when the positional data were degraded due to poor satellite number or geometry. GPS data loggers are therefore accurate for the determination of speed over-ground in biomechanical and energetic studies performed on relatively straight courses. Errors increase on circular paths, especially those with small radii of curvature, due to a tendency to underestimate speed.

Patterning of circular structure arrays with interference lithography

We have developed an interference lithography method that enables the production of circular periodic patterns. The range of accessible patterns includes concentric circular rings, spiral- and spoke-like patterns, and dots positioned periodically on concentric circular tracks. The method is based on the use of transmission diffraction gratings to create circular beams with wave fronts that are not accessible with reflective or refractive optics. Diffraction gratings with equally spaced concentric circles are used to obtain diverging or converging conical waves with radial phase variation and spiral-like gratings are used to obtain waves with the phase varying in both radial and azimuthal directions. We have experimentally demonstrated the method with electron-beam written diffraction gratings, which we illuminated with a coherent laser beam to form the interference patterns. Many different designs are possible that bring two, three, or more beams together to form a desired interference pattern. The technique should in general be useful in applications requiring circular periodic patterns, which may include production of patterned magnetic media, spoke patterns for angular encoders, and photonic crystals with circular symmetry.

Effects of age-related gait changes on the biomechanics of slips and falls

A laboratory study was conducted to examine gait changes associated with aging and the effect of these changes on initiation of slips and frequency of falls utilizing newly defined biomechanical parameters of slips and falls. Twenty-eight participants from two age groups (young and old) walked around a circular track at a comfortable pace wearing a safety harness. A slippery floor surface was placed on the walking track over the force plate at random time intervals without the participants' awareness. Synchronized kinetic and kinematic measurements were obtained on both slippery and non-slippery walking surfaces. The results indicated that older participants' horizontal heel contact velocity was significantly faster, step length was significantly shorter, and transitional acceleration of the whole body centre-of-mass (COM) was significantly slower than younger participants. Older participants' initial friction demand, as measured by required coefficient of friction (RCOF), was not significantly different than their younger counterparts. Additionally, older participants slipped longer and faster, and fell more often than younger participants. A comparison of horizontal heel contact velocity for participants who fell with participants who did not fall indicated that, in general, fallers' horizontal heel contact velocity was faster than non-fallers. However, a comparison of RCOF for participants who fell with participants who did not fall suggested that RCOF was not a totally deterministic factor influencing actual fall events. These findings suggest that gait changes associated with aging (especially higher horizontal heel contact velocity and slower transition of the whole body COM) affect initiation of slip-induced falls.

Instrumentation of the DNER Circular Track

ABSTRACT This paper presents the instrumentation used on the Experimental Circular Track of Brazil's National Highway Department (DNER—Departamento Nacional de Estradas de Rodagem), with the purpose of studying the use of concrete (whitetopping) to resurface flexible pavements. This instrumentation was made up of six strain gauges which were imbedded in the pavement layers. The results obtained from the sensor measurements were then compared against values calculated using the computer programs: ELSYM5, DIPLOMAT, ILSL2 and KENSLABS. These comparisons yielded similar results, especially against those of programs ELSYM5 and DIPLOMAT (with full friction).

Micromagnetic simulation of thermal ripple in thin films: “Roller-coaster” visualization

We present simulations of pulse-induced magnetization-rotation experiments in Permalloy. These lead to temporary domain formation (“thermal ripple”) and help to explain the time dependence of experimental results. To understand and visualize the motion, we find it very useful to exploit a mathematical isomorphism of this problem (in the limit Ms≫Hpulse) to the problem of a massive particle on a circular track (“roller coaster”). The height (gravitational potential energy) of this track is proportional to the Stoner–Wohlfarth energy. The fact that the resulting “precession” is really oscillation in a plane, and the fact that this oscillation overshoots the minimum-energy configuration (the inertia effect) are much more intuitive in the roller coaster picture than in the conventional “M precesses about the effective field” picture. Animated simulations of this behavior are available on the web (http://bama.ua.edu/∼visscher/mumag/).

유체 봉입 마운트의 동적 특성화를 위한 집중질량 요소를 갖는 기계적 모형의 문제점 파악과 실험 방법 개선을 통한 수력학적 모형의 타당성 확인

Hydraulic mounts show strong1y frequency-dependent stiffness and damping characteristics in low frequency range, which result from so called inertia track dynamics. A lumped mass has been incorporated in several mechanical models of the literature to take the inertia effect of the fluid in the track into consideration. Although complex s%illness by the mechanical model showed good agreements with the measured values, there exists a critical pitfall. In this paper, the shortcomings of mechanical models with lumped mass for hydraulic founts are clearly identified by illustrating actual measurements of the stiffness parameters for a hydraulic mount. It is conclusively discussed that the inertia effect of the fluid flow through the circular track is significant but latent. As an alternative to the mechanical model, a hydraulic model is claimed to be used for further dynamic analysis of engine/mount system or whole car system.

Wear coefficient equation for aluminium-based matrix composites against steel disc

A new formulation of the wear coefficient was developed and tested experimentally. Two different types of pin-on-disc wear tests were conducted using three commercial, A6061 aluminum-based metal–matrix composites (MMCs). One type of test resulted in a spiral track and the other a circular track. Hardened tool steel discs were used as the sliding counterface for MMC pins having 10, 15 and 20% alumina reinforcements. A new wear equation was derived and shown to be a better predictor of steady-state wear coefficients. It is based on an exponential transient wear volume equation and Archard’s equation.

GPR performances for thickness calibration on road test sites

This paper presents ground penetrating radar (GPR) experiments conducted on a number of road test sites. In its capacity as a State civil engineering research organization, the Laboratoire Central des Ponts et Chaussées (LCPC) possesses large-scale testing facilities, such as a circular pavement fatigue test track, ideal for road-related research. This particular facility is composed of several sections of different known structures especially well-adapted for GPR investigations. Our study of GPR techniques made use of a commercial system for measuring asphalt layer thickness. Various road structures have been examined with a GSSI system. Cores have been drilled and permittivity measurements performed in the laboratory. The initial study reveals a comparison between thickness measured by coring and that calculated from both measured permittivities and GPR-selected times. A second study has consisted of avoiding calibration with core drilling, by means of recording GPR signals with a non-destructive testing technique on the multi-layer structures (i.e. common middle point). A numerical reconstruction has then been developed in order to calculate the thickness and velocity of each layer from experimental data. When compared with the cored samples, results show a sufficient level of accuracy in the first two or three layer thickness measurements to satisfy the needs of road facility managers.

Muscles in microgravity: from fibres to human motion

In simulated or actual microgravity, human and animal postural muscles undergo substantial atrophy: after about 270 days, the muscle mass attains a constant value of about 70% of the initial one. Most animal studies reported preferential atrophy of slow twitch fibres whose mechanical properties change towards the fast type. However, in humans, at the end of a 42-days bed rest study, a similar atrophy of slow and fast fibres was observed. After microgravity, the maximal force of several muscle groups showed a substantial decrease (6–25% of pre-flight values). The maximal power during very short “explosive” efforts of 0.25–0.30 s showed an even greater fall, being reduced to 65% after 1 month and to 45% (of pre-flight values) after 6 months. The maximal power developed during 6–7 s “all-out” bouts on an isokinetic cycloergometer was reduced to a lesser extent, attaining about 75% of pre-flight values, regardless of the flight duration. In these same subjects, the muscle mass of the lower limbs declined by only 9–13%. Thus, a substantial fraction of the observed decreases of maximal power is probably due to a deterioration of the motor co-ordination brought about by the absence of gravity. To prevent this substantial decay of maximal absolute power, we propose that explosive exercise be added to the daily in-flight training schedule. We also describe a system aimed at reducing cardiovascular deconditioning wherein gravity is simulated by the centrifugal acceleration generated by the motion of two counter rotating bicycles ridden by the astronauts on the inner wall of a cylindrical space module. Finally, cycling on circular or elliptical tracks may be useful to reduce cardiovascular deconditioning in permanently manned lunar bases. Indeed, on the curved parts of the path, a cyclist generates an outward acceleration vector ( a c). To counterbalance ac, the cyclist must lean inwards, so that the vectorial sum of a c plus the lunar gravity tends to the acceleration of gravity prevailing on Earth.

Distal landmarks and hippocampal place cells: effects of relative translation versus rotation.

Hippocampal neurons are selectively active when a rat occupies restricted locations in an environment. These place cells derive their specificity from a multitude of sources, including idiothetic cues and sensory input derived from both distal and local landmarks. Most experiments have attempted to dissociate the relative strengths and roles played by these sources by rotating one set against the other. Few studies have addressed the effects of relative translation of the local cue set versus salient distal landmarks. To address this question, ensembles of place cells were recorded as a rectangular or circular track was moved to different locations in a room with controlled visual landmarks. Place cells primarily maintained their firing fields relative to the track (i.e., occupying new locations relative to the distal landmarks), even though the track could occupy completely nonoverlapping regions of the room. When the distal landmarks were rotated around the circular track, however, the place fields rotated with the landmarks, demonstrating that the cues were perceptible to the rat. These results suggest that, under these conditions, the spatial tuning of place cells may derive from an interaction between local and idiothetic cues, which define the precise firing locations of the cells and the relationships between them, and distal landmarks, which set the orientation of the ensemble representation relative to the external environment.

Pavement Surface Macrotexture Measurement and Applications

Different techniques for measuring pavement surface macrotexture and their application in pavement management are discussed. The main applications of surface macrotexture are to measure the frictional properties of the pavement surface and to detect hot-mix asphalt (HMA) construction segregation or nonuniformity. Since surface macro-texture can be measured quite efficiently using noncontact technologies and provides important information regarding pavement safety and HMA construction quality, this parameter may be included in the quality assurance or control procedures. Correlations between different measuring devices were investigated utilizing different HMA wearing surfaces. Excellent correlation was found between the circular track meter and sand patch measurements. In addition, the macrotexture determined using a laser profiler correlates well with that determined with sand patch measurements. Consistent with previous studies, it was found that the skid number gradient with speed is inversely proportional to the pavement macrotexture. However, there was a noticeable difference in speed dependency when smooth and ribbed tires were used. Oscillations in the percent normalized gradient with time due to seasonal variations were also observed. Macrotexture measurements hold great promise as tools to detect and quantify segregation for quality assurance purposes. A standard construction specification was proposed in a recent NCHRP study. However, the equation proposed for computing the nonsegregated estimated (mean) texture depth could not be applied to the mixes studied. An alternative equation has been proposed, which estimates the surface macrotexture using the mix nominal maximum size and voids in the mineral aggregate. The study was based on the mixes used at the Virginia Smart Road. Further investigation using other mixes is recommended.

EFFECTS OF GRADATION OF HOT-MIXED ASPHALT CONCRETE AND SECOND ROLLING ON PAVEMENT MACROTEXTURE

The profile of twenty four types of asphalt concrete pavement was measured by Circular Track Meter. It was shown that the ratio of Mean Profile Depth and Root Mean Square of the profile indicated the type of macrotexture, that is, positive or negative texture. Eighteen types of hot-mix asphalt concrete with various gradations were compacted in the laboratory using a newly developed compaction method that simulates the rolling that occurs in the field during construction. It was found that the ratio of the weight of aggregate retained 4.75mm sieve and passing 0.15mm sieve (R4.75/P0.15) has a large effect on the macrotexture. Secondary rolling with a rubber-tired roller tends to change the texture to a positive value when R4. 75/P0. 15 is small.

General Spatial Curve Joint for Rail Guided Vehicles: Kinematics and Dynamics

In the framework of multibody dynamics for rail-guided vehicleapplications, a new kinematic constraint is proposed, whichenforces that a point of a body follows a reference path while thebody maintains a prescribed orientation relative to a Frenet frameassociated to the spatial track curve. Depending on the specificapplication, the tracks of rail-guided vehicle are described byanalytical line segments or by parametric curves. For railway andlight track vehicles, the nominal geometry of the track isgenerally done by putting together straight and circular tracksegments, interconnected by transition track segments that ensurethe continuity of the first and second derivatives of the railwayin the transition points. For other applications, the definitionof the track is done using parametric curves that interpolate agiven number of control points. In both cases, the completecharacterization of the tracks also requires the definition of thecant angle variation, which is done with respect to the osculatingplane associated to the curve. The track models for multibodyanalysis must be in the form of parameterized curves, where thenominal geometry is obtained as a function of a parameterassociated to the curve length. The descriptions adopted hereensure, not only that the type of continuity of the original trackdefinition is maintained, but also that no unwanted deviationsfrom the nominal track geometry are observed, which can beperceived in the dynamic analysis as track perturbations. In thiswork different types of track geometric descriptions arediscussed. The application of cubic splines, to interpolate a setof points used to describe the track geometry, leads to undesiredoscillations in the model. The parameterization of analyticalsegments of straight, circular and cubic polynomial track segmentsdoes not introduce such oscillations on the track geometry but itis rather complex for the description of railways with largeslopes or with vertical curves. Splines with tension minimize theundesired oscillations of the interpolated curve that describesthe railway track nominal geometry, but the curve segmentparameters are not proportional to the length of the track. It isproposed here that the nominal geometry of the track is describedby a discreet number of points, which are organized in a tabularmanner as function of a parameter that is the length of the trackmeasured from its origin to a given point. For each entry, thetable also includes the vectors defining the Frenet frames and thederivatives required by the track constraint. The multibody codeinterpolates such table to obtain all required geometriccharacteristics of the track. With applications to a rollercoaster, the suitability of this description is discussed in termsof the choice of original parametric curves used to construct thetable, the size of the length parameter step adopted for the tableand the efficiency of the computer implementation of theformulation.

Instrumentation of the DNER Circular Track

ABSTRACT This paper presents the instrumentation used on the Experimental Circular Track of Brazil's National Highway Department (DNER—Departamento Nacional de Estradas de Rodagem), with the purpose of studying the use of concrete (whitetopping) to resurface flexible pavements. This instrumentation was made up of six strain gauges which were imbedded in the pavement layers. The results obtained from the sensor measurements were then compared against values calculated using the computer programs: ELSYM5, DIPLOMAT, ILSL2 and KENSLABS. These comparisons yielded similar results, especially against those of programs ELSYM5 and DIPLOMAT (with full friction).

Recubrimiento de <i>Stellite 6</i> sobre acero inoxidable realizado con láser de CO<sub>2</sub> para válvulas de escape de motores diesel

To reduce the recovery or the replacement costs of diesel engine exhaust valves, they are manufactured with an economic base material, and a coating which is deposited on the seat valve in order to reach high hardness and good impact, corrosion and high temperature wear resistance (>550 °C) and without lubrication. In this work, appropriate laser cladding parameters have been determined to obtain Stellite 6 coatings over AISI 304 steel (as plane test specimens) and SAE EV8 steel (as valves) substrates. One and two superimposed tracks were deposited on the seat valves, and modifying the laser power as a function of the rotated angle at the beginning and the end of the circular tracks, pores and cracks have been minimized and the thickness of the track were made uniform. Hardness, dilution and final microstructure of the different coatings have been analysed. A 10 % dilution and 550 HV in the tracks over plane test specimens was observed, while valves with one track showed 25 % and 430 HV respectively. With two superimposed tracks the hardness was up to 470 HV in the upper track.

Constant-curl Laplacian equation: a new approach for the analysis of flows around bodies

Constant-curl Laplacian equation is a new approach to study the behaviour of flows around bodies that do not present boundary-layer separation. It owns the simplicity and linearity of a potential analysis but also includes the rotational effects induced by a rotational displacement of the body such as turbine blades, manoeuvring aircrafts, etc. It assumes that this rotation is constant so that complex curved trajectories should be treated approximating the original path by a series of circular tracks. The idea is based upon a kinematic scheme that states the nullity of the velocity field Laplacian, imposing simultaneously the conditions of incompressibility and constant curl for velocity. We use 9-nodes isoparametric finite elements interpolating velocities and imposing the constant-curl condition by a modified penalty method and incompressibility by a classical one. The numerical results were compared satisfactorily against experimental data for aerodynamic forces and wake constitution on wind-turbine blades.

Relationship between Transitional Acceleration of the Whole Body Center-Of-Mass and Friction Demand Characteristic during Gait

A laboratory study was conducted to examine the gait changes associated with aging and the effect of these changes on initiation of slips, initial friction demand, and slip distance utilizing newly defined biomechanical parameters of slips and falls. Twenty-eight subjects from two age groups (young and old) walked around a circular track at a comfortable pace wearing a safety harness. Slippery floor surface was automatically placed on the walking track over force platforms at “unexpected” random time intervals utilizing remote controlled floor changer. Synchronized kinetic and kinematic measures were obtained on both slippery and non-slippery floor surfaces. The results indicated that older subjects step length was significantly shorter, and transitional acceleration of the whole body center-of-mass (COM) was significantly slower than their younger counterparts. Older subjects initial friction demand as measured by required coefficient friction (RCOF) was not significantly different than their younger counterparts. Additionally, older subjects slipped longer and faster than younger subjects. Bivariate correlation analyses suggested that transitional acceleration of the whole body COM was significantly related to friction demand characteristic (e.g., RCOF). These findings suggest that gait changes associated with aging (especially slower transition of the whole body COM) may affect initiation of slip-induced falls among older adults.

Dynamic Interactions between Local Surface Cues, Distal Landmarks, and Intrinsic Circuitry in Hippocampal Place Cells

A number of computational models of hippocampal place cells incorporate attractor neural network architecture to simulate key findings in the place cell literature, including the properties of pattern completion, firing in the absence of visual input, and nonlinear responses to environmental manipulations. To test for evidence of attractor dynamics, ensembles of place cells were recorded using multiple-tetrode techniques. After many days of experience in an environment with salient local surface cues on a circular track and salient distal landmarks on the wall, the local surface cues were rotated as a set in opposition to the distal landmarks. The amount of mismatch between the local and distal sets of cues varied from 45 to 180°. If place cells were parts of strong attractors, then their place fields should follow either the local cues or the distal cues as an integrated ensemble. Instead, in single recording sessions, some place cells were controlled by the distal landmarks, other cells were controlled by the local cues, and other cells became silent or gained new fields. In some cases, individual place fields split in half, following both the local and distal cues. If place cells are indeed parts of attractor networks in the hippocampus, then the attractors may be weak relative to the inputs from external sources, such as representations of the sensory environment and representations of heading direction, in a familiar, well explored environment.