Contact Stability Research Articles

Dynamics and stability of rigid rotors levitated by passive cylinder-magnet bearings and driven/supported axially by pointwise contact clutch

A stable rotor—supported laterally by passive magnetic bearings and longitudinally by magnetic forces and a clutch—loses suddenly its contact to the clutch and executes abruptly longitudinal movements away from its original equilibrium position as a result of small increases in angular velocity. Such an abrupt unstable behaviour and its reasons are thoroughly theoretically as well as experimentally investigated in this work. In this context, this paper gives theoretical as well as experimental contributions to the problem of two dimensional passive magnetic levitation and one dimensional pointwise contact stability dictated by mechanical–magnetic interaction. Load capacity and stiffness of passive multicylinder magnetic bearings (MCMB) are thoroughly investigated using two theoretical approaches followed by experimental validation. The contact dynamics between the clutch and the rotor supported by MCMB using several configurations of magnet distribution are described based on an accurate nonlinear model able to reliably reproduce the rotor-bearing dynamic behaviour. Such investigations lead to: (a) clear physical explanation about the reasons for the rotor's unstable behaviour, losing its contact to the clutch and (b) an accurate prediction of the threshold of stability based on the nonlinear rotor-bearing model, i.e. maximum angular velocity before the rotor misses its contact to the clutch as a function of rotor, bearing and clutch design parameters.

Enhancement of anatase functionalization and photocatalytic self-cleaning properties of keratins by microwave-generated plasma afterglow

A microwave-generated plasma afterglow (MWGPA) treatment was applied to keratin fibers to improve their adhesion to anatase nanocrystals by modifying their surface chemical and physical structure. The induced photocatalytic self-cleaning property has significantly been improved by 70% as evidenced by complete stain decomposition in 6h only. The effects of gas mixtures, gas flow, treatment distance, treatment duration and power flow of the MWGPA treatment on keratin's surface were investigated. The MWGPA-induced alteration of keratin's surface structure resulted in an increase in the uptake of anatase crystals and thus enhanced photocatalytic self-cleaning activities. The contact angle, absorption time, deposition behavior, adhesion and stability of anatase coating of keratin fibers are discussed. The anatase uptake and coating uniformity were characterized quantitatively using Time-of-Flight Secondary Ion Mass Spectrometry. This efficient and low energy plasma surface activation approach demonstrated potential toward the practical application of anatase in keratinous materials.

Robust stability of teleoperation systems with time delay: a new approach.

In this paper, we propose an approach to the control of linear teleoperation systems under time delays. Unlike traditional delay-robust control systems that guarantee passive communication channel through the transmission of wave variables, the new approach uses the concept of absolute stability for the physically expressive Lawrence's four-channel structure for transmitting the standard power variables, i.e., force and position. By incorporating kinesthetic performance requirements, we derive an absolutely stable four-channel controller that is transparent when time delay is negligible. Experimentally, the study evaluates and compares the performance of the proposed controller with that of a benchmark wave variable-based controller. The results indicate contact stability for large delays, a lack of position drift, and improved position and force tracking in both the free motion and rigid contact regimes for small delays.

Electric Contact Stability and Wear Durability at a Nanoscale Sliding Electric Contact

Electric Contact Stability and Wear Durability at a Nanoscale Sliding Electric Contact

Development and Evaluation of Mechanical Regulator of Dynamical Interference in Human-Robot Cooperative Task System

This paper presents development and evaluation of a mechanical regulator and a robot motion control scheme for a human-robot cooperative task system. There are contact stability problems that a control system becomes unstable when the robot using impedance control comes into contact with a stiff environment in human-robot interactions. The proposed robot motion control system enables stable contact with the stiff environment by the proposed regulator. The proposed regulator is developed to adjust the dynamical interference between object inherent dynamics and robot dynamics, and it provides dynamics transparency to a human operator in human-robot dynamics cooperation. The regulator is composed of mechanical load adjuster to transmit the human force to the robot and motion measuring system that detects relative positional deviation data between the human arm tip and a tip of the robot. To confirm the stability of the proposed control system, stability analysis simulations and verification experiments using an arm robot are performed. A human-robot cooperative experiment shows that effectiveness of the proposed system for increasing the stability and the dynamics transparency in human-robot physical interactions.

Latest Studies on Metal-Molecule-Metal Junctions

As promising building blocks for molecular electronics, organic molecules have attracted intense research interest. Metal-molecule-metal junctions are often used as testbeds for studying organic molecules' charge transport properties. In this article, fabrication methods, nanoscalability and addressability of these junctions are reviewed. Fabrication approaches are classified into soft contact, scanning probe microscopy, against-nanowire, crossed-wire, shadow angle evaporation and nanopore junctions. The effects of preparation method on the junction charge transport properties are systematically discussed. In general, the scanning tunneling microscopy technique is suitable for fast screening of molecular conductance, but cannot address junction that limits their in-situ temperature-dependent characterizations. The nanopore junction guarantees good control over the device size and the intrinsic contact stability, however, the nature of the electrode-molecule interface is not well understood. Shadow angle evaporation and soft contact techniques can effectively reduce the possibility of device short circuiting; however, the electrode dimensions limit potential applications. The against-nanowire method provides an easy way to fabricate addressable junctions, and if combined with the crossed-wire procedure may have potential for fabrication and three-dimensional integration of molecular junctions.

Cutting Edge: Tumor-Targeting Antibodies Enhance NKG2D-Mediated NK Cell Cytotoxicity by Stabilizing NK Cell–Tumor Cell Interactions

Monoclonal antibodies represent a promising approach to fight a variety of tumors, but their mode of action remains to be fully understood. NK cells can recognize Ab-coated targets, as well as stress ligands, on tumor cells. In this study, we investigated how NK cells integrate both kinds of activating signals. NK cell-mediated killing was maximal with the combined recognition of NKG2D ligands and Ab; surprisingly, only NKG2D engagement substantially enhanced degranulation. Conversely, Ab recognition by NK cells uniquely increased contact stability with tumor cells. Furthermore, using intravital imaging of solid tumors, we showed that Ab recognition favored prolonged interactions between NK cells and targets. Altogether, our results demonstrate that NK cell-mediated killing can be differentially regulated at the level of degranulation and contact stability by distinct activating receptors. Thus, complementary signals mediated by recognition of stress ligands and tumor-specific Abs may contribute to the efficacy of NK cells during mAb therapy.

Smooth ceramic titanium nitride contacts on AlGaN/GaN-heterostructures

Two contact systems on AlGaN/GaN-heterostructures were investigated with respect to their electrical performance and surface morphology. The first system is a standard Ti/Al-based multilayer contact system, generally exhibiting an excellent electrical performance. However strong surface roughening and degradation after annealing due to high diffusion rates of aluminum and gold often occur, which in turn affect the contact stability and reliability. Thus, a contact system based on sputtered titanium nitride was studied. TiN is well known as a thermally stable material with a good conductivity, but is used only indirectly via thermal transformation of thin Ti layers in typical group III nitride circuits. Using the TiN contact system, we were able to achieve a competitive electrical performance with a minimum value of contact resistivity of 1.8 × 10−5 Ω cm2 at an annealing temperature of 850 °C for 45 s. Most importantly, the surface roughness was greatly reduced (maximum rms values of 1.8 nm). The contact formation via rapid thermal processing was studied using Auger electron depth profiling.

Flexible electrode belt for EIT using nanofiber web dry electrodes

Efficient connection of multiple electrodes to the body for impedance measurement and voltage monitoring applications is of critical importance to measurement quality and practicality. Electrical impedance tomography (EIT) experiments have generally required a cumbersome procedure to attach the multiple electrodes needed in EIT. Once placed, these electrodes must then maintain good contact with the skin during measurements that may last several hours. There is usually also the need to manage the wires that run between the electrodes and the EIT system. These problems become more severe as the number of electrodes increases, and may limit the practicality and portability of this imaging method. There have been several trials describing human–electrode interfaces using configurations such as electrode belts, helmets or rings. In this paper, we describe an electrode belt we developed for long-term EIT monitoring of human lung ventilation. The belt included 16 embossed electrodes that were designed to make good contact with the skin. The electrodes were fabricated using an Ag-plated PVDF nanofiber web and metallic threads. A large contact area and padding were used behind each electrode to improve subject comfort and reduce contact impedances. The electrodes were incorporated, equally spaced, into an elasticated fabric belt. We tested the electrode belt in conjunction with the KHU Mark1 multi-frequency EIT system, and demonstrate time-difference images of phantoms and human subjects during normal breathing and running. We found that the Ag-plated PVDF nanofiber web electrodes were suitable for long-term measurement because of their flexibility and durability. Moreover, the contact impedance and stability of the Ag-plated PVDF nanofiber web electrodes were found to be comparable to similarly tested Ag/AgCl electrodes.

An integrated model of active glenohumeral stability

We present the first model of the glenohumeral joint implementing active muscle driven humeral positioning and stabilization without a priori constraints on glenohumeral kinematics. Previously established methods were used to predetermine the path, activation timing and resultant force contribution of 27 individual muscle segments at any given joint position. Artificial boundary conditions were applied in a three-dimensional finite element model of the joint and progressively released until the humeral head was completely free to rotate and translate within the fixed glenoid according to the compressive component of the predetermined resultant force. The shear component was then added such that no boundary conditions other than muscular force were applied. The framework was exploited to simulate elevation as a composite of instantaneous positions and theoretically demonstrate that joint stability can be achieved exclusively through muscular activity. Predicted muscle moment arms, muscle activation timing, humeral head translations, joint contact forces and stability ratio were comparable with existing experimental and in vivo data. This framework could be valuable for subject specific modeling and may be used to address clinical hypotheses related to shoulder joint stability that cannot be pursued using simplified modeling approaches.

Measurement of grinding temperatures using a foil/workpiece thermocouple

The high temperature generated in abrasive processes is the main factor responsible for thermal damage to a ground surface. It can be predicted through the thermal balance of the heat fluxes in the process. Such predictions can be experimentally verified using a foil/workpiece thermocouple. To estimate the thermal behaviour of such a sensor, it was dynamically calibrated with a laser beam to measure its frequency response. It was found that the response of the sensor has a time constant dependant on the thermal load and cannot be modelled by a simple first order function. In the calibration conditions used, the sensor is fast enough to measure the surface temperature with a time constant less than 100μs. A high frequency acquisition system allows the signal to be measured at the local grit scale so that the activity of grits and the contact stability between the foil and the workpiece during grinding can be studied. Using the peak temperature and the local cooling after a peak, suitably processed, a local background temperature can be defined. It is shown that this background temperature can be evaluated more accurately by matching the global cooling curve to a finite element solution. The temperatures obtained from the local minima of the local diffusive cooling curve agree better with measured results than a temperature obtained by low pass filtering, which can overestimate the background temperature and so the partition ratio.

Contact instabilities of anisotropic and inhomogeneous soft elastic films

Anisotropy plays important roles in various biological phenomena such as adhesion of geckos and grasshoppers enabled by the attachment pods having hierarchical structures like thin longitudinal setae connected with threads mimicked by anisotropic films. We study the contact instability of a transversely isotropic thin elastic film when it comes in contact proximity of another surface. In the present study we investigate the contact stability of a thin incompressible transversely isotropic film by performing linear stability analysis. Based on the linear stability analysis, we show that an approaching contactor renders the film unstable. The critical wavelength of the instability is a function of the total film thickness and the ratio of the Young's modulus in the longitudinal direction and the shear modulus in the plane containing the longitudinal axis. We also analyze the stability of a thin gradient film that is elastically inhomogeneous across its thickness. Compared to a homogeneous elastic film, it becomes unstable with a longer wavelength when the film becomes softer in going from the surface to the substrate.

Contact Dynamics Analysis and Stability Evaluation for Lunar Lander with 4 Legs

Contact Dynamics Analysis and Stability Evaluation for Lunar Lander with 4 Legs

Electric contact stability of anti-wear probes

For the practical realization of precise processes or devices utilizing scanning nanoprobes, not only improvement in the wear resistance of the probe tip but also a stable electric contact at the sliding probe electrode is required. To meet both these requirements, the authors developed an anti-wear probe having a supporting Si tip and a metal electrode, both of which slide on the substrate simultaneously. Probes with various electrode materials were evaluated to investigate the key factors of the material choice for maintaining good electric contact over a long sliding distance. The results show that the optimal management of material hardness, surface roughness, and probe contact force was important to realize an optimum performance from the probe.

Laminin 511 partners with laminin 332 to mediate directional migration of Madin–Darby canine kidney epithelial cells

Sustained directional migration of epithelial cells is essential for regeneration of injured epithelia. Front-rear polarity of migrating cells is determined by local activation of a signaling network involving Cdc42 and other factors in response to spatial cues from the environment, the nature of which are obscure. We examined the roles of laminin (LM)-511 and LM-332, two structurally different laminin isoforms, in the migration of Madin-Darby canine kidney cells by suppressing expression of their α subunits using RNA interference. We determined that knockdown of LM-511 inhibits directional migration and destabilizes cell-cell contacts, in part by disturbing the localization and activity of the polarization machinery. Suppression of integrin α3, a laminin receptor subunit, in cells synthesizing normal amounts of both laminins has a similar effect as knockdown of LM-511. Surprisingly, simultaneous suppression of both laminin α5 and laminin α3 restores directional migration and cell-cell contact stability, suggesting that cells recognize a haptotactic gradient formed by a combination of laminins.

The Dimeric Structure of the Cpn60.2 Chaperonin of Mycobacterium tuberculosis at 2.8 Å Reveals Possible Modes of Function

Mycobacterium tuberculosis expresses two proteins (Cpn60.1 and Cpn60.2) that belong to the chaperonin (Cpn) family of heat shock proteins. Studies have shown that the two proteins have different functional roles in the bacterial life cycle and that Cpn60.2 is essential for cell viability and may be involved in M. tuberculosis pathogenicity. Cpn60.2 does not form a tetradecameric double ring, which is typical of other Cpns. We have determined the crystal structure of recombinant Cpn60.2 to 2.8 Å resolution by molecular replacement; the asymmetric unit (AU) contains a dimer, which is consistent with size-exclusion high-performance liquid chromatography and dynamic light-scattering measurements of the soluble recombinant protein. However, we suggest that the actual Cpn60.2 dimer may be different from that identified within the AU on the basis of surface contact stability, solvation free-energy gain, and functional aspects. Unlike the dimer found in the AU, which is formed through apical domain interactions, the dimeric form we propose here provides a free apical domain that is required for normal chaperone activity and may be involved in M. tuberculosis association with macrophages and arthrosclerosis plaque formation. Here we describe in detail the structural aspects that lead to Cpn60.2 dimer formation and prevent the formation of heptameric rings and tetradecameric double rings.

Immobilized Pool of NCAM180 in the Postsynaptic Membrane Is Homeostatically Replenished by the Flux of NCAM180 from Extrasynaptic Regions

Homeostatic mechanisms maintaining high levels of adhesion molecules in synapses over prolonged periods of time remain incompletely understood. We used fluorescence recovery after photobleaching experiments to analyze the steady state turnover of the immobile pool of green fluorescent protein-labeled NCAM180, the largest postsynaptically accumulating isoform of the neural cell adhesion molecule (NCAM). We show that there is a continuous flux of NCAM180 to the postsynaptic membrane from nonsynaptic regions of dendrites by diffusion. In the postsynaptic membrane, the newly delivered NCAM180 slowly intermixes with the immobilized pool of NCAM180. Preferential immobilization and accumulation of NCAM180 in the postsynaptic membrane is reduced after disruption of the association of NCAM180 with the spectrin cytoskeleton and in the absence of the homophilic interactions of NCAM180 in synapses. Our observations indicate that the homophilic interactions and binding to the cytoskeleton promote immobilization of NCAM180 and its accumulation in the postsynaptic membrane. Flux of NCAM180 from extrasynaptic regions and its slow intermixture with the immobile pool of NCAM180 in the postsynaptic membrane may be important for the continuous homeostatic replenishment of NCAM180 protein at synaptic contacts without compromising the long term synaptic contact stability.

Effect of occlusal contact stability on the jaw closing point during tapping movements

PurposeWe studied the relationship between tapping point reproducibility and stability of occlusal contacts at maximum intercuspation. MethodsTapping movements of 12 adult volunteers who had dentition with natural teeth were recorded, and distances between the tapping point (TP) and the intercuspal position (ICP) at the incisal point were calculated. Occlusal contacts at the ICP of individual subjects were also evaluated with black-colored silicone impression material. The correlation between TP–ICP distance and occlusal contact stability was studied. ResultsTP–ICP distance exhibited negative correlations with the total number of teeth showing occlusal contact at the ICP. Standard deviations of TP–ICP distance also negatively correlated with the extension of occlusal contact area over dentition. ConclusionsThis finding indicates that occlusal contacts at the ICP affect the kinematic behavior of tapping movements. The results of this study also suggest that jaw movement data may provide useful clinical information for the evaluation of occlusal contact at ICP.

Effect of meniscus replacement fixation technique on restoration of knee contact mechanics and stability.

The menisci are important biomechanical components of the knee. We developed and validated a finite element model of meniscal replacement to assess the effect of surgical fixation technique on contact behavior and knee stability. The geometry of femoral and tibial articular cartilage and menisci was segmented from magnetic resonance images of a normal cadaver knee using MIMICS (Materialise, Leuven, Belgium). A finite element mesh was generated using HyperWorks (Altair Inc, Santa Ana, CA). A finite element solver (Abaqus v6.9, Simulia, Providence, RI) was used to compute contact area and stresses under axial loading and to assess stability (reaction force generated during anteroposterior translation of the femur). The natural and surgical attachments of the meniscal horns and peripheral rim were simulated using springs. After total meniscectomy, femoral contact area decreased by 26% with a concomitant increase in average contact stresses (36%) and peak contact stresses (33%). Replacing the meniscus without suturing the horns did little to restore femoral contact area. Suturing the horns increased contact area and reduced peak contact stresses. Increasing suture stiffness correlated with increased meniscal contact stresses as a greater proportion of tibiofemoral load was transferred to the meniscus. A small incremental benefit was seen of simulated bone plug fixation over the suture construct with the highest stiffness (50 N/mm). Suturing the rim did little to change contact conditions. The nominal anteroposterior stiffness reduced by 3.1 N/mm after meniscectomy. In contrast to contact area and stress, stiffness of the horn fixation sutures had a smaller effect on anteroposterior stability. On the other hand suturing the rim of the meniscus affected anteroposterior stability to a much larger degree. This model emphasizes the importance of the meniscus in knee biomechanics. Appropriate meniscal replacement fixation techniques are likely to be critical to the clinical success of meniscal replacement. While contact conditions are mainly sensitive to meniscus horn fixation, the stability of the knee under anteroposterior shear loads appeared to be more sensitive to meniscal rim fixation. This model may also be useful in predicting the effect of biomaterial mechanical properties and meniscal replacement shape on knee contact conditions.

Synthesis of control law considering wheel–ground interaction and contact stability of autonomous mobile robot

SUMMARYWe proposed a new method of mobile robot motion synthesis. A dynamical model describing the slip phenomenon taking into account the wheel–ground interaction was derived. The novelty of this work stems from the assumption that the slip already exists and the wheel motion pattern must reduce it, not exceeding the acceleration and torque limits. Moreover, a slip estimation method is proposed by introducing a critical friction coefficient. The theoretical considerations are confirmed by simulation and experiment. This research was performed in the framework of the PROTEUS project aiming at the development of autonomous robots for inspection and exploration. These robots will move in natural terrain.