Contact Compliance Research Articles

On the dynamic Coulomb's frictional contact problem for thermo‐piezoelectric materials

AbstractIn this work, we study, from a variational point of view, a dynamic contact problem between a thermo‐piezoelectric body and a thermally conductive foundation. The normal compliance contact condition and Coulomb's friction law are employed to model the contact. We provide existence and uniqueness results of a weak solution to the model using adequate auxiliary problems, an abstract result on nonlinear first‐order evolution inequalities and Banach fixed point argument. Finally, the continuous dependence of the solution on the surface traction force and the surface electrical charge is studied.

Synergizing Structural Stiffness Regulation with Compliance Contact Stiffness: Bioinspired Soft Stimuli‐Responsive Materials Design for Soft Machines

Stiffness regulation strategies endow soft machines with stronger functionality to cope with diverse application requirements, for example manipulating heavy items by improving structural stiffness. However, most programmable stiffness strategies usually struggle to preserve the inherent compliant interaction capabilities following an enhancement in structural stiffness. In this study, inspired by the musculocutaneous system, we propose a soft stimuli‐responsive material (SRM) by combining shape memory alloy into compliant materials. By characterizing the mechanical performance, the flexural modulus increases from 6.6 to 142.4 MPa under the action of active stimuli, crossing two orders of magnitude, while Young's modulus stays at 2.2 MPa during programming structural stiffness. This comparative result indicates that our SRMs can keep a lower contact stiffness for compliant interaction although structural stiffness increases. Then, we develop three diverse soft machines to show the application potential of this smart material, such as robotic grippers, wearable devices, and deployable mechanisms. By applying our materials, these machines possess stronger load‐bearing capabilities. Meanwhile, these demonstrations also illustrate the efficacy of this paradigm in regulating the structural stiffness of soft machines while maintaining their compliant interaction capabilities.

Skin-Inspired All-Natural Biogel for Bioadhesive Interface.

Natural material-based hydrogels are considered ideal candidates for constructing robust bio-interfaces due to their environmentally sustainable nature and biocompatibility. However, these hydrogels often encounter limitations such as weak mechanical strength, low water resistance, and poor ionic conductivity. Here, inspired by the role of natural moisturizing factor (NMF) in skin, a straightforward yet versatile strategy is proposed for fabricating all-natural ionic biogels that exhibit high resilience, ionic conductivity, resistance to dehydration, and complete degradability, without necessitating any chemical modification. A well-balanced combination of gelatin and sodium pyrrolidone carboxylic acid (an NMF compound) gives rise to a significant enhancement in the mechanical strength, ionic conductivity, and water retention capacity of the biogel compared to pure gelatin hydrogel. The biogel manifests temperature-controlled reversible fluid-gel transition properties attributed to the triple-helix junctions of gelatin, which enables in situ gelation on diverse substrates, thereby ensuring conformal contact and dynamic compliance with curved surfaces. Due to its salutary properties, the biogel can serve as an effective and biocompatible interface for high-quality and long-term electrophysiological signal recording. These findings provide a general and scalable approach for designing natural material-based hydrogels with tailored functionalities to meet diverse application needs.

On‐Skin Paintable Water‐Resistant Biohydrogel for Wearable Bioelectronics

AbstractTo achieve accurate monitoring of bioelectrical signals, it is essential to use customizable bioelectrodes that can self‐adapt to the skin's surface topography. Ion‐conducting hydrogel has received significant attention in this field due to its softness, adhesion, and skin‐like mechanical properties. However, these bioelectrodes currently suffer from degradation of adhesion, electrical conductivity, and skin‐compliance when exposed to aqueous environments. This significantly limits the application of bioelectrodes. Herein, a customizable biohydrogel that can be applied on skin or fabric by solvent volatilization for liquid ink‐gel film conversion is reported. The biohydrogel's distinct characteristic of transitioning between a liquid and hydrogel phase establishes superb conformal contact and dynamic compliance with the epidermis. This effectively eliminates motion artifacts and results in lower contact impedance and noise in both static and dynamic states when compared to existing bioelectrodes. The biohydrogel is applied to the cotton fabric to create electrocardiogram (ECG) monitoring garments. These garments enable the acquisition of ECG signals with high accuracy in aqueous environment for over 72 h. Besides, the biohydrogel‐based garments outperform the commercial gel electrodes by 83.5% in signal‐to‐noise ratio. Additionally, the cotton/biohydrogel electrode facilitates multi‐channel, high‐fidelity recording of ECG signals, enabling high‐performance capture and classification of ECG waveforms across multiple channels.

Variable Time-Step Physics Engine with Continuous Compliance Contact Model for Optimal Robotic Grinding Trajectory Planning.

In the transition from virtual environments to real-world applications, the role of physics engines is crucial for accurately emulating and representing systems. To address the prevalent issue of inaccurate simulations, this paper introduces a novel physics engine uniquely designed with a compliant contact model designed for robotic grinding. It features continuous and variable time-step simulations, emphasizing accurate contact force calculations during object collision. Firstly, the engine derives dynamic equations considering spring stiffness, damping coefficients, coefficients of restitution, and external forces. This facilitates the effective determination of dynamic parameters such as contact force, acceleration, velocity, and position throughout penetration processes continuously. Secondly, the approach utilizes effective inertia in developing the contact model, which is designed for multi-jointed robots through pose transformation. The proposed physics engine effectively captures energy conversion in scenarios with convex contact surface shapes through the application of spring dampers during collisions. Finally, the reliability of the contact solver in the simulation was verified through bouncing ball experiments and robotic grinding experiments under different coefficients of restitution. These experiments effectively recorded the continuous variations in parameters, such as contact force, verifying the integral stability of the system. In summary, this article advances physics engine technology beyond current geometrically constrained contact solutions, enhancing the accuracy of simulations and modeling in virtual environments. This is particularly significant in scenarios wherein there are constant changes in the outside world, such as robotic grinding tasks.

Damped Normal Compliance (DNC) and the restitution coefficient

This work introduces and studies a novel normal compliance contact condition with damping, the Damped Normal Compliance (DNC), which is more realistic than the usual normal compliance condition that is often used in modeling contact between solids. The condition is applied in a model for the contact of a rigid mass with a reactive obstacle and allows for energy dissipation during contact. We analyze the model and show that the condition is of mathematical, as well as applied, interest. Then, we establish its relationship with the so-called “coefficient of restitution,” e , and show that the concept is not well defined, since e depends on the system parameters, applied force, and initial data. The various theoretical results are depicted using computer simulations.

Sliding friction of a pillar array interface: part I.

Biology is replete with examples, at length scales ranging from the molecular (ligand-receptor binding) to the mesoscopic scale (wing arresting structures on dragonflies) where shape-complementary surfaces are used to control interfacial mechanical properties such as adhesion, friction, and contact compliance. Related bio-inspired and biomimetic structures have been used to achieve unique interfacial properties such as friction and adhesion enhancement, directional and switchable properties. The ability to tune friction by altering surface structures offers advantages in various fields, such as soft robotics and tire manufacturing. Here, we present a study of friction between polydimethylsiloxane (PDMS) samples with surfaces patterned with pillar-arrays. When brought in contact with each other the two samples spontaneously produce a Moiré pattern that can also be represented as an array of interfacial dislocations that depends on interfacial misorientation and lattice spacing. Misorientation alone produces an array of screw dislocations, while lattice mismatch alone produces an array of edge dislocations. Relative sliding motion is accompanied by interfacial glide of these patterns. The frictional force resisting dislocation glide arises from periodic single pillar-pillar contact and sliding. We study the behavior of pillar-pillar contact with larger (millimeter scale) pillar samples. Inter-pillar interaction measurements are combined with a geometric model for relative sliding to calculate frictional stress that is in good agreement with experiments.

A Variable Impedance Skill Learning Algorithm Based on Kernelized Movement Primitives

This paper proposes a novel learning from demonstrations (LfD) method based on kernelized movement primitives (KMP). The original KMP algorithm is excellent at generalizing and handling high-dimensional inputs, but it is slightly inadequate in reproduction accuracy. To address this issue, we make two improvements to the KMP algorithm. Firstly, a multivariate Gaussian process (MV-GP) is employed to model the reference trajectory, which preliminarily improves the reproduction accuracy of KMP. Secondly, an optimization problem is formulated to learn the hyperparameters of the kernel function in KMP, which reduces the dependence on experience. We also propose a novel variable impedance control (VIC) approach to trade off contact compliance against tracking accuracy by utilizing the probabilistic properties of the KMP. Comparative simulations and experiments are conducted to validate the proposed LfD algorithm.

Pre-Exposure Prophylaxis and HIV Prevention Among Key Populations in Nigeria.

The discovery of innovative pharmacological preventative measures in Human Immunodeficiency Virus transmission has boosted optimism in the successful control of HIV/AIDS with the objective of eradication and the end of the epidemic. Hence, assessing the effect of pre-exposure prophylaxis (PrEP) on HIV prevention among key populations (KPs) is imperative in advancing the conversation on ending HIV/AIDs. This was a cross-sectional study using data from KPs (female sex workers (FSWs), men who have sex with men (MSM), and people who inject drugs (PWIDs) enrolled in Heartland Alliance Ltd/GTE One Stop Shops (OSS) between May 2019 and April 2023 in the six selected Nigerian states (Lagos, Bayelsa, Cross River, Akwa Ibom, Jigawa, and Niger). Data were cleaned in a spreadsheet and analyzed using IBM-SPSS version 28.0. Descriptive statistics, including frequency and percentage calculations, were conducted to analyze the dataset. Additionally, cross-tabulation analysis was performed, with a significance level set at 0.05, to explore relationships between variables. The study population comprised 13,580 participants, with 56.1% female. In all, 43.9% were aged 18-35 and 97.3% were single. Among the three KPs examined, FSW was the largest subgroup (50.8%), followed by MSM (28.1%) and PWID (21.1%). Most participants had been on PrEP for 0-3 months (87.8%), and PrEP was initiated for 81.6% due to high-risk sexual behaviors and 9.9% for serodiscordant relationship reasons. Most of them were enrolled in community healthcare settings (97.0%). The results showed that 99.9% remained HIV negative, while only two (0.01%) were seroconverted while on PrEP. The HIV-positive cases did not complete one-month PreP treatment, comprising an FSW and a PWID. Recent HIV contact or poor compliance with PrEP medication are two possible causes of seroconversion. The findings underscore the significance of integrating PrEP into a comprehensive HIV prevention approach, including newer molecules that will improve adherence and the necessity of ongoing monitoring and support for PrEP users. With these insights, there can be an advocacy for promoting PrEP among the KPs as a vital component of Nigeria's HIV prevention strategy.

First contact investigations and compliance to treatment in patients with uveitis.

To ascertain the pattern of investigations at first contact in uveitic patients and evaluate compliance to treatment. An observational study comprised of 201 uveitic patients presenting for the first time to our centre from January 2019 to June 2020. Detailed information regarding systemic investigations undertaken by specialists at the time of first contact and the cost of these investigations were reviewed on the first visit to our centre. Compliance with the treatment was determined and reasons behind non-compliance were evaluated on the first follow-up in patients who had no improvement in clinical signs and symptoms. The mean age of the study group was 35.35±14.1y and gender composition was 59.7% males and 40.3% females. Anterior uveitis was observed in 45.3% of patients, intermediate uveitis in 31.8% of patients, posterior uveitis in 14.9% of patients and panuveitis in 8.0% of patients. Association with a systemic disease was evident in 17.9% of patients. When compared with standard guidelines and uveitis patterns, systemic investigations were identified to be relevant only in 38.3% of patients. Non-compliance to treatment was documented in 22.4% of patients. Common reasons for non-compliance were inadequate counselling by the treating physician about treatment in 26.7% of patients and a busy schedule at work/school in 22.2% of patients. Significant number of investigations performed at first contact is found to be contrary to standard guidelines and are not contributory to the care. About a quarter of patients in this study are found to be non-compliant with the treatment. Compliance is more challenging to achieve in school-going children and working adults. The availability of comprehensive, periodically updated, evidence-based guidelines on the role of investigations and the use of trained counsellors may help to channelize proper evaluation and improve compliance to treatment, respectively, in patients with uveitis.

Elastic properties of unconsolidated sandstones of interest for carbon storage

AbstractUnconsolidated sandstones are attractive targets for underground storage of carbon due to their high porosity and permeability. Monitoring of injection and movement of CO2 in such formations using elastic waves requires an understanding of the acoustic properties of the sandstone. Current approaches often use the so‐called soft‐sand model in which a Hertz–Mindlin model of the acoustic properties at high porosity is mixed with the acoustic properties of the mineral phase to predict the acoustic properties over the entire porosity range. Using well‐log data from two unconsolidated sand formations of interest for CO2 storage, we discuss the limitations of this model and provide an alternative approach in which the mechanical properties of grain contacts are obtained by inversion, and the properties of infill material lying within the pore space are estimated. The formations considered are the Paluxy Formation in Kemper County, Mississippi, and the Frio Formation near Houston, Texas. The ratio of the normal to shear compliance of the grain contacts is found to be significantly less than unity for both formations. This implies that the grain contacts are more compliant in shear than in compression. However, the grain contact compliance is higher and the ratio of the normal to shear compliance is lower for the Frio example than for the Paluxy case, and this may lead to sliding at grain contacts with low shear compliance and transport of grains during fluid flow, particularly if CO2 acts to weaken any cement that may be present at the grain contacts. Such transport was suggested by Al Hosni et al. in explaining why the magnitude of the time‐lapse effect due to the injection of CO2 at the Frio CO2 injection site is greater than predicted using conventional rock physics models. A simple model of the mechanical properties of infill material lying within the pore space suggests that the bulk and shear moduli of infill material in the Paluxy case are significantly higher than the Frio case, consistent with the lower grain contact compliance in the Paluxy case.

A Dual-Modal Hybrid Gripper with Wide Tunable Contact Stiffness Range and High Compliance for Adaptive and Wide-Range Grasping Objects with Diverse Fragilities.

The difficulties of traditional rigid/soft grippers in meeting the increasing performance expectations (e.g., high grasping adaptability and wide graspable objects range) of a single robotic gripper have given birth to numerous soft-rigid coupling grippers with promising performance. However, it is still hard for these hybrid grippers to adaptively grasp various objects with diverse fragilities intact, such as incense ash and orange, due to their limited contact stiffness adjustable range and compliance. To solve these challenging issues, herein, we propose a dual-modal hybrid gripper, whose fingers contain a detachable elastomer-coated flexible sheet that is restrained by a moving frame as a teardrop shape. The gripper's two modes switched by controlling the moving frame position can selectively highlight the low contact stiffness and excellent compliance of the teardrop-shaped flexible sheets and the high contact stiffness of the moving frames. Moreover, the contact stiffness of the teardrop-shaped sheets can be wide-range adjusted by online controlling the moving frame position and offline replacing the sheets with different thicknesses. The compliance of the teardrop-shaped sheets also proves to be excellent compared with an Ecoflex 10 fingertip with the same profile. Such a gripper with wide-range tunable contact stiffness and high compliance demonstrates excellent grasping adaptability (e.g., it can safely grasp several fragile strawberries with a maximum size difference of 18 mm, a strawberry with a left/right offset of 3 cm, and a strawberry in two different lying poses) and wide-range graspable objects (from 0.1 g super fragile cigarette ashes to 5.1 kg dumbbell).

Effectiveness of Friction Force Reduction in Sliding Motion Depending on the Frequency of Longitudinal Tangential Vibrations, Sliding Velocity and Normal Pressure

Abstract The article presents the results of experimental research and simulation analyses of the influence of slip velocity, normal pressures and vibration frequency on the effectiveness of friction force reduction carried out in sliding motion in the presence of forced tangential vibrations. In experimental studies, changes in the driving force were measured during the slip of the upper body over the vibrating lower body. The direction of these vibrations was parallel both to the contact plane and to the direction of movement of the shifted body. The simulation tests were carried out in the Matlab/Simulink environment through the use of numerical procedures that were specially created for this purpose. Dynamic friction models considering the tangential compliance of contact and the phenomenon of pre-sliding displacement were used for calculations. The paper presents the designated values of the so-called coefficient of average friction force reduction in sliding motion for the following friction pairs: steel C45–steel C45, steel C45–cast iron GGG40 and steel C45–polytetrafluoroethy-lene PTFE (Teflon). The results of numerical analyses were in good agreement with those of experimental tests. A significant dependence of the level of average friction force reduction on the frequency of forced vibrations, sliding velocity as well as the kind of sliding pair material, and normal pressures was shown.

An efficient detailed layer model for prediction of separator damage in a Li-Ion pouch cell exposed to transverse compression

Li-ion pouch cells can be understood as thin-layer laminates. Pressure loads normal to flat electrodes can cause failure or thermal runaway. However, detailed models for investigating transverse pressure loads in which each layer is resolved are computationally intensive. This paper outlines two approaches for time-efficient yet accurate detailed explicit finite element models of a cell. Both approaches are characterised by the fact that they are divided into in-plane and out-of-plane behaviour. The modelling of interlaminar contacts is completely omitted, to prevent parasitic contact compliance and efforts for contact handling. A discrete element formulation is used for a computationally efficient simulation of transverse loads. This prevents a reduction of the critical time step, which can result from transversal deformations. The difference between the two modelling approaches lies in the modelling of the out-of-plane behaviour. In the first approach, the behaviour in the out-of-plane direction is generated by characterising each component separately. The second approach assumes homogenous behaviour in the thickness direction. Both approaches rely on characterization data derived from component and cell tests. The cell models are validated against cell tests. Numerical results obtained with the two modelling approaches differ only slightly from the validation data.

Analysis of a doubly-history dependent variational–hemivariational inequality arising in adhesive contact problem

This paper is devoted to studying a dynamic adhesive contact model where the body is composed of a viscoelastic material with long memory. The adhesion process is modeled by a bonding field on part of the contact surface and on which the contact and the friction are modeled by nonmonotone Clarke subdifferential conditions with adhesion. Meanwhile, on another part of the contact surface, the contact is comprised of a normal compliance contact condition while the memory effects of the obstacle are considered, the friction is modeled by a version of Coulomb's law of dry friction and the friction bound is dependent on the normal stress. The variational form of the model generates a system of a doubly-history dependent variational–hemivariational inequality as well as a differential equation. Then, on the basis of the proof of abstract variational–hemivariational inequality, the existence and uniqueness results of the contact problem are established.

The effect of contact compliance of sliding pair on friction force reduction at longitudinal tangential vibrations

The main purpose of this work was to experimentally test the impact of modification to the tangential contact stiffness of the friction pair, caused by changing the kind of material, roughness of the contact surfaces and surface pressures, on the level of friction force reduction in the sliding motion, under the influence of high-frequency longitudinal tangential vibrations introduced into the contact area. The tests were carried out on an original test stand for three kinds of friction pair materials, such as: steel C45, cast iron GGG40 and polytetrafluoroethylene PTFE – Teflon, working in homogeneous connections: steel – steel, cast iron – cast iron and Teflon – Teflon and in mixed connections: steel – Teflon, steel – cast iron and cast iron – Teflon. For each friction pair, at defined values of normal pressures, the compliance characteristics of the contact in the tangential direction were determined. On their basis, as a function of normal pressures, values of the tangential contact stiffness coefficient were calculated. Then, also as a function of normal pressures, experimental tests were carried out on changes in the friction force in sliding motion occurring in the contact under the influence of longitudinal tangential vibrations. The tests were carried out for different speeds of the drive, with fixed vibration parameters, such as amplitude and frequency. On the basis of obtained results of the dependence of contact compliance on the kind of material of the friction pair, contact surface roughness and surface pressures, as well as on the dependence of friction force changes under the influence of tangential vibrations on the same factors, the relationship between the level of friction force reduction under the influence of these vibrations and contact compliance was examined.

Influence of contact compliance of parts and the ratio of hardness of their materials on the load capacity of a bolted joint

The influence of the contact compliance of the joint of parts under different conditions of deformation of a rough surface, as well as the ratio of the hardness of the materials of the parts on the load capacity of a pre-tightened bolted joint, is considered. Keywords: surface roughness, penetration, flattening, material hardness ratio, plastic hardness. matlin@vstu.ru, kazankin_vmn@mail.ru

Model Predictive Variable Impedance Control of Manipulators for Adaptive Precision-Compliance Tradeoff

Impedance control (IC) is widely used in contact-rich manipulator tasks since it provides manipulators with both operation precision and contact compliance, and trades them off by impedance parameters. However, fixed impedance parameters limit the applicability of IC in complex tasks during which the focus on the operation precision or the contact compliance is variable, such as physical human–robot collaboration and complex assembly tasks. This article presents model predictive variable impedance control approaches for adaptive precision-compliance tradeoff to satisfy variable task requirements. Specifically, we establish a novel impedance model, which transforms the variable impedance law design problem into a control law design problem, and allows us to consider novel impedance constraints that can determine manipulators' extreme precision and compliance properties. According to whether state constraints are further considered, one-step (OS) and multistep (MS) model predictive control approaches are proposed to solve these transformed control problems, and the corresponding optimization problem of the OS MPC can be established as a QP problem due to the special form of the novel impedance model. A tank-based approach is further proposed to correct the variable impedance laws for the system passivity concern. Various comparative experiments are conducted to validate the effectiveness of the presented approaches.

Evaluation, calibration, and modal analysis for determination of contact stiffness between workpiece and components of milling fixture

The stability of workpiece inside fixture influences the dynamics of machining. Although, the workpiece is uniquely and deterministically positioned and grasped by the fixture, it undergoes small displacements due to the contact compliance resulting from the frictional slip under the external loads and vibrations. The frictional slip of workpiece, along or around the main axes relative to the fixture components is a major cause of instability and the main element influencing the contact stiffness between the workpiece and the fixture. In the present study, a procedure has been developed to evaluate the contact stiffness by employing mathematical and experimental tools. The grasp matrices defined in analytical model were obtained by a combination of calibration and measurement of forces, displacements, and friction coefficient. The contact stiffness was also determined by modal analysis of the assembly of workpiece and fixture and the results were compared. It was evidenced that the dry friction existing in the contact areas between the workpiece and fixture elements could lead to the stick-slip instability, detrimental to the dynamics of machining. The main contribution and novelty of the present study can be outlined, as follows. The contact stiffness between workpiece and fixture was modeled and measured. The frictional slip was modeled and measured in addition to the physical compliances. Dry friction between workpiece and fixture was detected as a source of stick-slip. Procedures were developed to calibrate and measure all forces and displacements. Modal analysis was done as a complementary tool to verify the results.

Knowledge and Compliance among Contact Lens Wearers Living in Kuala Lumpur, Malaysia: A Cross-Sectional Study.

Many factors determine the success of wearing contact lens, including knowledge and compliance towards lens care. This study aimed to evaluate the level of knowledge and compliance between two groups of adult contact lens wearers in Kuala Lumpur, Malaysia. A total of 60 participants aged 18-30 years old volunteered to participate in this study (30 participants prescribed contact lenses at the Universiti Kebangsaan Malaysia [UKM] optometry clinic and 30 participants fitted at private practices). Participants were interviewed using a structured questionnaire consisting of nine questions on basic knowledge related to lens wear and 13 questions on compliance, categorised into questions on the cleaning process, disinfection, accessories care and replacement schedule. All participants wore disposable contact lens 53.3% (n = 32) participants preferred using monthly disposable lenses. Both groups had the same level of knowledge, except knowledge of the effects of makeup (P < 0.000) and duration of the solution used for cleaning (P < 0.010), showing lack of knowledge on contact lenses among participants in private practices group. There were no significant differences between the groups in terms of the level of compliance with the cleaning process (P = 0.830), disinfection (P = 0.725), accessories care (P = 0.865) and replacement schedule (P = 0.699). Participants from UKM optometry clinic had better knowledge on wearing contact lens; moreover, both groups had good compliance towards lens care. Contact lens practitioners should provide all the necessary information to contact lens wearers so that they are equipped to handle contact lenses correctly, which would minimise the risk of eye complications.