Contact Force Variation Research Articles

Influence of altered gait patterns on the hip joint contact forces

Children who exhibit gait deviations often present a range of bone deformities, particularly at the proximal femur. Altered gait may affect bone growth and lead to deformities by exerting abnormal stresses on the developing bones. The objective of this study was to calculate variations in the hip joint contact forces with different gait patterns. Muscle and hip joint contact forces of four children with different walking characteristics were calculated using an inverse dynamic analysis and a static optimisation algorithm. Kinematic and kinetic analyses were based on a generic musculoskeletal model scaled down to accommodate the dimensions of each child. Results showed that for all the children with altered gaits both the orientation and magnitude of the hip joint contact force deviated from normal. The child with the most severe gait deviations had hip joint contact forces 30% greater than normal, most likely due to the increase in muscle forces required to sustain his crouched stance. Determining how altered gait affects joint loading may help in planning treatment strategies to preserve correct loading on the bone from a young age.

간격이 있는 탄성 보 위를 고속 주행하는 바퀴의 Hertz 접촉을 고려한 동역학적 해석

With the local Hertz deformation on the contact point, the dynamic contact between a high-speed wheel and an elastic beam having a gap is numerically analyzed by solving the whole equations of motion of the wheel and the beam subjected to the contact condition. For the stability of the time integration the velocity and acceleration constraints as well as the displacement constraint are imposed on the contact point. Especially the acceleration contact condition on the gap is formulated, and it is demonstrated that the contact force variation computed by the velocity contact constraint or by the acceleration contact constraint agrees well with that computed by the displacement contact constraint. The numerical examples show that, when the wheel passes on the gap, the solution is governed by the stiffness of the local Hertzian deformation.

Impact modelling and analysis of the compliant legged robots

The dynamics of the compliant legged robots are commonly characterized by the spring loaded inverted pendulum model. In this article, a new model is presented for the impact of the compliant legged robots. Compared with the spring loaded inverted pendulum model, the new model provides a means to take into account the factors such as the sliding of the terminal, the vibrations of the leg with distributed compliance and variation of the nonlinear contact force. The effectiveness of the model is validated by the finite element program LS-DYNA from the velocity and energy point of view. Based on the method used, the influences of the parameters, such as the incident angle, width, thickness and Young’s modulus of the flexible leg on the dynamic characteristics of the robot, are analysed. It is found that the post-impact vertical velocity of the robot decreases with the increasing incident angles, and increases approximately linearly with the width and thickness, while the variation of Young’s modulus of the flexible leg has little effects on it. The same is true of the energy loss ratio during the impact. All the parameters investigated are more intuitive than the stiffness described in the spring loaded inverted pendulum model and can be utilized directly for the optimization design and locomotion control of the compliant legged robots. In addition, the vibrations and multiple impacts can be predicted by the proposed model to reduce the energy loss.

Dynamic contact analysis of a tensioned beam with a moving mass–spring system

The dynamic contact problem of a tensioned beam with clamped-pinned ends is analyzed when the beam contacts a moving mass–spring system. The contact and contact loss conditions are expressed in terms of constraint equations after considering the dynamic contact between the beam and the moving mass. Using these constraints and equations of motion for the beam and moving mass, dynamic contact equations are derived and then discretized using the finite element method, which is based on the Lagrange multiplier method. The time responses for the contact forces are computed from these discretized equations. The contact force variations and contact loss are investigated for the variations of the moving mass velocity, the beam tension, the moving mass, and the stiffness of the moving mass–spring system. In addition, the possibility of contact loss and safe contact conditions between the moving mass and the tensioned beam are also studied.

Importance of the Wheel Vertical Dynamics in the Squeal Noise Mechanism on a Scaled Test Bench

This paper investigates the influence of the wheel vertical dynamics in the mechanism of squeal noise on a scaled test bench. To this purpose, sustained oscillations are first studied on a single degree of freedom oscillator, considering both a decreasing slope of the friction curve and a vertical excitation. Their relative importance to sustain the oscillations is discussed. Then, a mathematical model of a quarter scale test bench is developed in the frequency domain. Using this model, it is shown that the squeal noise resulting from the excitation of the bending modes of the wheel is sustained because these bending modes are associated with variations of the vertical contact force. Results are further confirmed by experiments conducted on a scaled test bench.

Importance of the Wheel Vertical Dynamics in the Squeal Noise Mechanism on a Scaled Test Bench

This paper investigates the influence of the wheel vertical dynamics in the mechanism of squeal noise on a scaled test bench. To this purpose, sustained oscillations are first studied on a single degree of freedom oscillator, considering both a decreasing slope of the friction curve and a vertical excitation. Their relative importance to sustain the oscillations is discussed. Then, a mathematical model of a quarter scale test bench is developed in the frequency domain. Using this model, it is shown that the squeal noise resulting from the excitation of the bending modes of the wheel is sustained because these bending modes are associated with variations of the vertical contact force. Results are further confirmed by experiments conducted on a scaled test bench.

Effect of impact velocity and specimen stiffness on contact forces in a weight-controlled chewing simulator

Objectives Chewing simulators are used for preclinical evaluation of newly developed dental restorative materials. To guarantee the independence of test conditions, contact forces during chewing simulation should be independent of the specimen. Because of its mode of operation, i.e., impact of an antagonist, this requirement is not met for a widely used chewing simulator (Willytec/SD Mechatronik, Feldkirchen-Westerham, Germany). This study was therefore intended to clarify the extent to which specimen stiffness affects maximum contact force at different impact velocities. Possible differences between the forces in the eight test chambers were also of interest. Methods From each of five dental materials differing in Young's modulus, eight cylindrical disks were manufactured and embedded in specimen holders. Alumina spheres were used as antagonists. During chewing simulations with different impact velocities and dental materials, vertical acceleration was recorded and contact forces were estimated on the basis of these measurements. Results Specimen stiffness and impact velocity had a substantial effect on maximum contact force. The force overshoot relative to the static load ranged from 4% for small specimen stiffness and low impact velocity to values greater than 200% for high specimen stiffness and high impact velocity. Large differences between the chambers were also detected. Significance Weight-controlled chewing simulations should be performed either with a low impact velocity or with a spring-damper system (placed between mass and specimen) which efficiently reduces the effects of contact force variation. Influence of specimen stiffness on contact forces must be considered at data interpretation.

유한 요소 해석 기법을 이용한 고속 철도 차량의 집전 성능 해석

본 논문에서는 상용 유한 요소 해석 프로그램인 SAMCEF 를 이용하여 고속 철도 차량의 집전성능을 예측할 수 있는 해석 모델을 개발하였다. 3 자유도 스프링-댐퍼-질량의 판토그래프 모델을 생성하였고, 실제 시스템과의 리셉턴스를 비교함으로써 신뢰성을 검증하였다. UIC 799 OR 기준에서 제시한 가선계의 이론적 파동전파 속도와 가선계 유한 요소 해석 모델에서 측정한 파동 전파 속도를 비교 하였다. 드로퍼의 길이를 조절하여 전차선의 중력에 의한 초기 처짐 현상을 구현하였다. 가선계와 판토그래프를 접촉 요소를 이용하여 연성하였으며, 판토그래프가 300 ㎞/h 및 370 ㎞/h 로 주행할 때의 접촉력 변화를 도출하였다. 접촉력의 평균, 표준편차, 최대 및 최소값 등을 분석함으로써 본 논문에서 제시한 해석모델의 유효성을 검증하였다.

Modeling and Analysis of Fixel Designs for Micromanufacturing Active Fixturing

This paper presents an investigation of fixel design alternatives for active (dynamic) fixturing to be incorporated into mesoscale manufacturing systems. Using simple compliant mechanisms and components, fixels exhibiting mechanically adjustable stiffness characteristics are achievable. Via manual or automated stiffness adjustments, these fixels provide functionality for enabling greater control of the dynamic response of the workpiece subject to vibrations and/or variations in contact forces at the tool-workpiece-fixture interface. To quantify the fixel functionality, this paper presents theoretical models of the stiffness characteristics expressed as a function of the mechanical variable(s), thus forming a basis for exploring the adjustability in stiffness achievable for each fixel design. Also presented are results of the dynamic behavior of the active fixturing implemented in a milling process based on a “regenerative force, dynamic deflection model” augmented with the active fixturing variable stiffness model and inclusion of tool runout. These simulation results indicate the expected performance of the active fixturing upon its implementation in actual fixturing for the creation of micron features on micro- and macroparts.

Effect of back pressure on impact and compression-after-impact characteristics of composites

Low velocity impact and compression-after-impact characteristics of a typical plain weave E-glass/epoxy composite are studied experimentally. Atmospheric pressure was maintained on the top surface and different pressures were applied on the rear side during impact experiments. Pressure on the rear side of the impacted plate is referred to as back pressure in further discussion. Effect of back pressure on the impact behavior is studied. It is observed that the variation in peak contact force and maximum central deflection are governed by two opposing phenomena. The parameters influencing the opposing phenomena are: induced curvature because of back pressure, effective pre-stressing and effective thickness. The incident impact energy was the same in all the experiments. Post-impact compressive strength was also investigated.

Integrated Simulation between Flexible Body of Catenary and Active Control Pantograph for Contact Force Variation Control

Railway transport has been developed for a variety of requirements with a diversity of studies and technologies in recent years. In particular, the intercity railway transport that can be operated at speed of more than 350 km/h is the goal for the railway industry. Due to vibration and drag forces at high speed, contact force variation occurs between pantograph and catenary. This variation also causes instability in the pantograph and catenary interaction. In this study, multibody dynamics analysis is used to model the catenary. The integration of the catenary model and the pantograph model in the simulation flow produces contact force variations. A sinusoidal feed forward force and a simple feedback control force are applied to control the wave-like contact force fluctuations by means of active dampers. Evaluation of the combination of active control forces will produce optimized forces that may be able to maintain, thus improve the contact force variations.

Analysis of Contact Force Variation between Contact Wire and Pantograph Based on Multibody Dynamics

In recent years, high speed railway vehicle technologies have been studied and investigated in order to develop their performances with the objectives of improving riding comfort, noise reduction and high efficiency from environmental perspective. One of the essential performances is current collection stability. Operating at high speed conditions, the current collection system which consists of contact wire and pantograph suffers from contact force variation. In order to reduce such variation, pantograph design should be comprised by active control thus realizing stability. Numerical analysis can be applied to model the current collection and in particular simulate the contact force variation itself. In this study, Finite Element Method and Absolute Nodal Coordinate Formulation are used to model the wire. A free vibration experiment that focuses on the tension of wire is performed to validate the numerical models. The validated result from the experiment demonstrates the accuracy of the numerical analysis in modeling the wire. The techniques are then applied to model the contact wire for the contact force investigation. The result indicated the availability of pantograph design via simulation to achieve steady contact force.

A novel study of head motion hysteresis issues in contact probe recording systems

Contact-based recording on ferroelectric media and polymer media have been proposed and investigated for high-density probe storage. To achieve fast data rate, it is necessary to perform reading or writing with multiple probe heads simultaneously. However, localized variations in tribological behavior at the contact interface between the probes and recording media and variation in normal contact force would result in variation in friction and stiction over the interface and therefore amongst individual heads in the probe head array. This causes variation in the hysteretic motion of the heads over the array, which in turn results in off-track motion during track-positioning and timing errors during scanning. In this paper a novel method of modeling these effects is developed to study the effect of relative head-motion hysteresis (RHMH) on timing-error during data read–write and off-track errors during seek-settle. A mathematical model of RHMH during scanning along the bit-wise direction is developed based on the idea of stochastic averaging. A transient response model is similarly developed for estimating hysteresis effects during seek-settle. These models help to understand parametric dependencies of RHMH and can be used in designing the probe heads, the probe–media interface (PMI) and the probe system in general. Further several schemes involving modulation of the normal force at the PMI are proposed to mitigate RHMH and their benefits are analyzed using the models described in this paper.

Study on numerical simulation methods of the railway vehicle‐track dynamic interaction

AbstractA very efficient numerical simulation method of the railway vehicle–track dynamic interaction is described. When a vehicle runs at high speed on the railway track, contact forces between a wheel and a rail vary dynamically due to the profile irregularities existing on the surface of the rail. A large variation of contact forces causes undesired deteriorations of a track and its substructures. Therefore these dynamic contact forces are of main concern of the railway engineers. However it is very difficult to measure such dynamic contact forces directly. So it is important to develop an appropriate numerical simulation model and identify structural factors having a large influence on the variation of contact forces.When a contact force is expressed by the linearized Hertzian contact spring model, the equation of motions of the system is expressed as a second–order linear time–variant differential equation which has a time–dependent stiffness coefficient. Applying a well–known Newmark direct integration method, a numerical simulation is reduced to solving iteratively a time–variant, large–scale sparse, symmetric positive–definite linear system.In this study, by defining a special vector named a contact point one, it is shown that this time–variant stiffness coefficient can be expressed simply as a product of the contact point vector and its transpose and so the Sherman–Morrison–Woodbury formula applied for updating the inverse of the coefficient matrix. As a result, the execution of numerical simulation can be carried out very efficiently. A comparison of the computational time is given. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Differential wear and plastic deformation as causes of squat at track local stiffness change combined with other track short defects

Squats have become a major problem in the track of many railways. In the quest for the root causes of squats, it is observed that they are occasionally found at locations of track stiffness changes such as at fish-plated insulated joints and at switches and crossings. Obviously, there should be other factors in the track, which, together with the stiffness change, have played important roles otherwise there will be squats at all such locations. A validated hybrid multibody-finite element model of vehicle–track vertical interaction is extended to simulate the frictional dynamic rolling contact at a fish-plated insulated joint in order to identify such factors. Elastic–plastic rail material property is taken into account. It is found that it is track short defect in the preload condition of the bolts and the contact between the fishplates and the rail head, which together with the stiffness change, causes large normal and longitudinal contact force variation at the fishplate end so that differential wear and differential plastic deformation may accumulate at a fixed location. With proper wavelength, the accumulated rail top geometry deviation may grow into a squat. The significance of the present work lies in that other track short defects such as damaged and improper railpads and fastening, and ballast voids may also have such effects, which may be responsible for a large portion of the many squats in the tracks. This gives the direction for further work.

Physiological Basis of Limb-Impedance Modulation During Free and Constrained Movements

Arm stiffness is a critical factor underlying stable interactions with the environment. When the hand moves freely through space, a stiff limb would most effectively maintain the hand on the desired path in the face of external perturbations. Conversely, when constrained by a rigid surface, a compliant limb would allow the surface to guide the hand while minimizing variations in contact forces. We aimed to identify the physiological basis of stiffness adaptation for these two classes of movement. Stiffness can be regulated by two mechanisms: coactivation of antagonistic muscles and modulation of reflex gains. We hypothesized that subjects would select high stiffness (high coactivation and/or reflex gains) in free space and high compliance (low coactivation and reflex gains) for constrained movements. We measured EMG and the H-reflex during constrained and unconstrained movement of the wrist. As predicted, subjects coactivated antagonist muscles more when performing the unconstrained movement. Contrary to our hypothesis, however, H-reflex amplitude was higher for the constrained movement despite the a priori preference for lower reflex gains in this situation. In addition, the H-reflex depended on the task and the net force exerted by the limb on the environment, rather than showing a simple dependence on the level of muscle activation. Thus stiffness seems to increase in free space compared with constrained motion through the use of coactivation, whereas spinal loop gains are adjusted to better regulate the influence of afferences on the ongoing movement. These observations support the hypothesis of movement programming in terms of impedance.

Performance Evaluations of the 3<sup>rd</sup> Rail Type Power Collector for Rubber-Tired Vehicles

This study was aimed to verify the performance and stability of the 3rd rail type power collector for rubber-tired vehicles. Performance (power interruptions), durability (stresses), stability (vibrations and contact force variations) of the developed power collector were measured and analyzed, while the rubber-tired vehicle ran at various speeds on the test track(about 2.4km).

Use of active steering in railway bogies to reduce rail corrugation on curves

The aim of the current paper is to assess the ability of active steering systems to prevent the development of rail corrugation in curves. For that, the corrugation growth rates obtained for a passive vehicle are compared with those obtained for active vehicles (in the current work two strategies for active steering are described and implemented: relative angle control and yaw moment control). The procedure for the calculation of the corrugation growth is described: first the quasi-static creepages produced during curve negotiation are calculated by means of a railway simulator program (SIMPACK) in different cases. Second, these obtained creepages are introduced in a non-linear corrugation model that evaluates dynamic variations in normal and tangential wheel rail contact forces. Finally, on the basis of these forces, the evolution of the rail profile is determined. The paper includes results of the selected control strategies that show the potentiality of active steering systems in reducing corrugation growth rates.

Elastic-Plastic Finite-Element Analysis of Repeated, Two-Dimensional Wheel-Rail Rolling Contact under Time-Dependent Load

A study was performed using a finite-element model to obtain stresses, strains, and deformations for repeated, two-dimensional rolling contact of a locomotive driving wheel and a rail under time-dependent load. An advanced cyclic plasticity model was used with a commercial finite element code via a material subroutine. The time-dependent load was considered a harmonic variation of the wheel-rail normal contact force. The normal contact pressure was assumed to follow the Hertzian distribution and the tangential force followed the Carter distribution. A wavy profile is formed on the running surface of the rail subjected to the harmonic variation of the normal (vertical) contact force. The developed wavelength of the profile corresponds to the frequency of the normal contact force for the actual train speed. The creepage or rolling-sliding condition plays an important role in the residual strains and deformations, but its influence on the residual stresses is insignificant. The residual stresses at the surface decrease with increasing rolling passes and gradually tend to stabilize. The residual strains and surface displacements increase with increasing rolling cycle, but the increases in residual strain and surface displacement per rolling pass (ratchetting rate) decay. The residual stresses, strains, and deformations near the wave trough of the residual wavy deformation are larger than those near the wave crest. For any given creepage including zero value, when the number of rolling passes increases, the surface depth of the wavy-deformed surface increases but the ratchetting rate decays. The results are useful in investigating the influence of plastic deformation on rail corrugation.

Analysis and Optimal Design of Fixture Clamping Sequence

Abstract Considering the great impacts of the application sequence of multiclamps on the workpiece machining accuracy, this paper analyzes and optimizes clamping sequence. A new methodology that takes into account the varying contact forces and friction force during clamping is presented for the first time. A new analysis model is established to capture the effect of clamping sequence on contact force distributions and workpiece machining accuracy. It reveals that the historical accumulation of clamping steps influences heavily the final distribution of contact forces in the workpiece-fixture system. Therefore, the present contact forces in each clamping step are solved incrementally in terms of contact forces of the precedent step by means of the principle of the total complementary energy. Furthermore, based on the fact that the variation of contact forces from one step to another results in different workpiece deformations and position, a novel design model is formulated to select optimally the clamping sequence in order to minimize the workpiece deformation and position errors. Workpieces of low stiffness and high stiffness are investigated separately in order to simplify the modeling of clamping sequence optimization. Some numerical tests are finally demonstrated to validate the proposed model and method. Computational results are discussed and compared with experimental results available in the reference.