Contact Stiffness Research Articles

Active Compliance Control of a Position-Controlled Industrial Robot for Simulating Space Operations

An industrial robot with a six-axis force/torque sensor is usually used to produce a zero-gravity environment for testing space robotic operations. However, using traditional force control methods, such as admittance control, causes position-controlled industrial robots to undergo from force divergence owing to intrinsic time delay. In this paper, a new force control method is proposed to eliminate the force divergence. A hardware-in-the-loop (HIL) simulator with an industrial robot is first presented. The free-floating satellite dynamics and the motion mapping from the satellites to simulator are both established. Thus, the effects of measurement delay and dynamic response delay on contact velocity and force are investigated. After that, a real-time estimation method for contact stiffness and damping is proposed based on the adaptive Kalman filter. The measurement delay is compensated by a phase lead model. Moreover, the identified contact parameters are adopted to modify contact forces, and thus the dynamics response delay can be compensated for. Finally, a co-simulation and experiments were conducted to verify the force control method. The results show that contact stiffness and damping could be identified exactly and that the simulation divergence could be prevented. This paper proposes an active compliance control method that can deal with force constrained tasks of a position-controlled robot in unknown environments.

Dynamic response of vehicle-track interaction at the bolted rail joint in the presence of the polygonal wheel

Rail joints are the weakest link in the railway track structure due to the presence of discontinuities in the track stiffness and wheel-rail contact geometry. This paper presents the dynamic response of vehicle-track interaction at the bolted rail joint (BRJ) in the presence of polygonal wheel wear. A three-dimensional coupled vehicle-track dynamic model was established using the finite element method (FEM) and the multi-body system (MBS) approach, in which the flexibilities of the wheelset and track are taken into account. The proposed dynamic model is validated by the comparisons of field measurement and calculated vertical axle-box acceleration in time and frequency domains. The effects of the flexible wheelset in the presence of polygonal wear on the vehicle-track dynamic interaction at the BRJs are illustrated through comparisons with those obtained using without polygonal wheel model. The results show that the polygonal wheel wear and the rail joint irregularities can induce high-magnitude vertical wheel-rail contact force, and exacerbate the vibration responses of the axle box, the wheelset, and the rails. The simulation results also indicate that the wheelset bending modes, the P2 resonance due to rail joint excitation, and the track vibration modes have significant effects on the vertical vehicle-track dynamic interaction at the supported and suspended BRJs.

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.

Modelling Approaches of Wear-Based Surface Development and Their Experimental Validation

Surface topography has a significant influence on the friction behaviour in lubricated contacts. During running-in, the surface topography is continuously changed. The surface structure influences the contact stiffness (asperity contact pressure) as well as the microhydrodynamics (flow factors). In this study, different models for wear simulation of real rough surfaces were created in Matlab© (MathWorks, Natick, MA) and Abaqus© (ABAQUS Inc., Palo Alto, CA, USA) using the Usersubroutine Umeshmotion. The arithmetic mean height Sa(wh), the maximum height Sz(wh), as well as the asperity contact pressure pasp(h,wh) as a function of the wear height (wh) are used to characterise the surface for the respective wear state. The surface characteristics obtained from the simulations are validated with parameters from experiments. The aim of this study was to create a simulation methodology for mapping surface development during the running-in process. The results show, that the qualitative course of the surface parameters can be reproduced with the applied simulation methodology. Compared to the experiments, the rough surfaces are flattened faster. By adapting the simulation results in postprocessing, good agreements with the experiments can be achieved.

FEATURE-ORIENTED PROCESS OPTIMIZATIONS

The example of structural and parametric optimization of the functionally-oriented technological processmachining of mold’s parts is described in the article. The most essential criteria of optimization arewearproofness, fatigue strength, contact stiffness and corrosion resistance of production. In the articlethe brought methodology over of researches and algorithm of optimization.

Long-Term In Vitro Assessment of Biodegradable Radiopaque Composites for Fiducial Marker Fabrication.

Biodegradable polymer-based composite materials may be successfully utilised to fabricate fiducial markers (FMs), which are intended to precisely label tumour margins during image-guided surgery or radiotherapy. However, due to matrix degradability, the stability of the functional properties of FMs depends on the chosen polymer. Thus, this study aimed to investigate novel radiopaque composites which varied in the polymeric matrix-polycaprolactone (PCL), poly(L-lactide-co-caprolactone) (P[LAcoCL]) with two molar ratios (70:30 and 85:15), and poly(L-lactide-co-glycolide) (with molar ratio 82:18). The radiopaque component of the materials was a mixture of barium sulphate and hydroxyapatite. The changes in water contact angle, stiffness, and radiopacity occurring during the 24-week-long degradation experiment were examined for the first time. This study comprehensively analyses the microstructural causes of composites behaviour within degradation experiments using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), gel permitted chromatography (GPC), and scanning electron microscopy (SEM). The obtained results suggest that the utilized biodegradable matrix plays an essential role in radiopaque composite properties and stability thereof. This long-term in vitro assessment enabled a comparison of the materials and aided in choosing the most favourable composite for FMs' fabrication.

Modeling of normal contact stiffness for surface with machining textures and analysis of its influencing factors

A new contact stiffness model for the rough surface with different machining textures is established. The studied surface consists of regular surface machining texture in the global scale and stochastic micro-asperities in the local scale. The proposed model improves upon previous work that considers only the effect of stochastic asperities and completely ignores that of the machining textures generated by different machining methods. The analytical expression to calculate the contact stiffness of the studied textured surface is derived and numerical results are obtained via the finite element method. A high-precision test bench to measure the interface contact stiffness of the workpiece with different surface textures is designed. The measured results and simulated results are directly compared to demonstrate the validity of the proposed method. The effects of different texture types and surface roughness values on the contact stiffness of the proposed textured surface are analyzed. The proposed method provides a research basis for the accurate calculation of nonlinear contact stiffness and dynamic characteristics of the surface with complex texture topography.

Simulation and experimental investigation of the surgical needle deflection model during the rotational and steady insertion process.

Needle insertion is executed in numerous medical and brachytherapy events. Exact needle insertion into inhomogeneous soft biological tissue is of useful importance due to its significance in clinical diagnosis (especially percutaneous) and treatments. The surgical needles used in such processes can deflect during the percutaneous process. Needle deflecting which affects needle - soft tissue interface and needle controllability have a crucial role in establishment precision. In this paper, we have analyzed a mechanics-based model both rotational and non-rotational needle insertion, and studied the deflection phenomenon in both insertion cases, we validated it with a real-time nonlinear Dassault Systèmes® ABAQUS simulation model. For definite contact force, the maximum the contact stiffness was, the minimum it inserted, the cohesive surface model was used to investigate the needle insertion analysis, where the fracture point was defined by a failure strain and with the help of the in, the fully failed components would be removed. Using living tissue comparable PVA gel materials, the needle insertion force model is developed from insertion experimentations with the help of two different processes (rotational and non-rotational needle insertion). In a rotational needle, deflection is less than in a non-rotational needle. The preliminary insertion was observed in the rotational needle at 1.261 mm (experiment), and 1.538 mm (simulation), and for non-rotational needle insertion, the initial insertion was noticed at 1.756 mm (experiment) and 1.982 mm (simulation). The main aim of this study is to navigate the surgical needle in an accurate way to reduce the erroneousness for a clinical diagnosis like anesthesia, brachytherapy, biopsy, and modern microsurgery operation.

Normal contact stiffness model for fractal surfaces considering scale dependence and friction behavior

The normal contact stiffness on rough surfaces has a significant impact on the interfacial dynamic characteristics of mechanical joints. Based on the fractal geometry theory, this work improves the contact stiffness modeling by considering the complete contact characteristics of multiple asperities and frictional factor. First, to include the property that the critical contact areas of asperities are scale-dependent, the model for the contact stiffness is formed on each scale and the relationship between total stiffness and load on rough surfaces is obtained by the summation of all length scale asperities. Second, friction factor is taken into account in the revised normal contact stiffness model, where a contact friction coefficient is introduced into the equation to incorporate the effects of friction force. Moreover, the influences of fractal dimension D and fractal roughness G on normal contact stiffness and friction coefficient are investigated. Through the comparison of numerical and experimental results, the revised model is shown to be more reasonable and presents higher consistency of the stiffness versus load curve than the original model. It is therefore concluded that a complete and more accurate model for normal contact stiffness is proposed in this work with precise modeling of scale-dependent contact characteristics and friction behavior taken into account, which is critical for exact estimation of rigidity of contact surfaces in industrial applications.

The Microstructures and Characteristics of NiO Films: Effects of Substrate Temperature.

The influence of the substrate temperature on the structural, surface morphological, optical and nanomechanical properties of NiO films deposited on glass substrates using radio-frequency magnetron sputtering was examined by X-ray diffraction (XRD), atomic force microscopy (AFM), UV-Visible spectroscopy and nanoindentation, respectively. The results indicate that the substrate temperature exhibits significant influences on both the grain texturing orientation and surface morphology of the films. Namely, the dominant crystallographic orientation of the films switches from (111) to (200) accompanied by progressively roughening of the surface when the substrate temperature is increased from 300 °C to 500 °C. The average transmittance of the NiO films was also found to vary in the range of 60-85% in the visible wavelength region, depending on the substrate temperature and wavelength. In addition, the optical band gap calculated from the Tauc plot showed an increasing trend from 3.18 eV to 3.56 eV with increasing substrate temperature. Both the hardness and Young's modulus of NiO films were obtained by means of the nanoindentation continuous contact stiffness measurements mode. Moreover, the contact angle between the water droplet and film surface also indicated an intimate correlation between the surface energy, hence the wettability, of the film and substrate temperature.

Dynamics analysis of width-varying microcantilevers: Interplay between eigenfrequencies, contact stiffness and interaction forces

In this study, the resonance-frequency dependence and modal sensitivity of the flexural vibration modes of overhang/T-shaped microcantilevers to the interaction force and surface stiffness variations were analysed, and a closed-form expression was derived. The Euler–Bernoulli beam theory was used to develop the overhang/T-shaped models, and a characteristic formula representing the dependence of cantilever frequencies on the overhang dimensions was obtained. The results of the derived expression were analysed using numerical simulations to investigate and compare the effects of the overhang width, length, repulsive and attractive forces, and surface contact stiffness on the flexural mode of overhang/T-shaped cantilevers. Furthermore, a closed-form expression for the modal sensitivities of the width-varying cantilever was derived, and the modal sensitivities were compared using numerical simulations. Finally, the effects of the interaction forces and contact stiffness on the frequency response and sensitivities of different cantilevers based on their stiffness and geometrical parameters were verified experimentally. This study can open new paths for designing, fabricating, and using width-varying cantilevers in sensing and imaging applications, particularly for cantilever array systems.

Structural and mechanical characterization of as-cast CuAlBe alloy

Copper-Beryllium (Cu-Be) materials are known as non-sparking elements and are used for tools in gaseous rich atmospheres. In this article we present few preliminary results from the obtaining and analyze of a new chemical composition CuAlBe alloy. The experimental alloys were cast in a vacuum induction furnace from CuBe master alloy and high purity aluminium using a graphite crucible and a metal casting mold. The alloys obtained were analyzed by chemical composition homogenization point of view (energy dispersive spectroscopy – EDS), microstructure (optical – MO and electronic microscopy - SEM) and mechanical properties (microhardness, Young modulus and contact stiffness). The addition of beryllium present an increasing of hardness effect collaborated with an increase of friction coefficient.

Characterization of power-law constitutive relationship of nickel-based single crystal superalloys under different loading rates by nanoindentation with different types of indenters

This work attempts to explore a method to characterize the effect of loading rate on the power-law constitutive parameters of nickel-based single crystal superalloys (NBSX) by nanoindentation, which avoids the influence of indentation sinking-in or piling-up and indentation size effects. Specifically, combing the reduced modulus obtained from a cylindrical flat punch indentation test with the indentation contact stiffness obtained from two pyramidal indentation tests with different equivalent half cone angles, the actual indentation projected contact area of two pyramidal indenters can be acquired. Based on the actual projected contact area and the intrinsic hardness results calculated by Nix-Gao model, the corrected indentation load-depth curves from two pyramidal indenters are constructed. The elastic modulus can be known from the reduced modulus in advance, and then according to the equivalent half cone angle of the pyramidal indenters and the corrected indentation load-depth curves, the representative strain and stress of two pyramidal indenters are obtained, respectively. The power-law constitutive parameters of NBSX along [001] orientation under different loading rates are accomplished by inverse analyses of the representative stress and strain of two pyramidal indenters, and verified by the test results of a pyramidal indenter with another equivalent half cone angle. Results demonstrate the effectiveness of the explored method.

Establishment of discrete element flexible model of the tiller taro plant and clamping and pulling experiment.

The taro harvesting process is affected by a complex system composed of particle mechanics system and multi-body dynamics system. The discrete element method(DEM) can effectively solve the nonlinear problem of the interaction between harvesting components and working materials. Therefore, the discrete element model of taro tiller plants is of great importance for taro harvesting. This paper proposes a simulation method to establish a discrete element flexible plant model and dynamic clamping and pulling process of taro tiller plant. Discrete Element models of taro corm and flexible tiller petiole and leaf were established using DEM method, and the discrete element flexible model of the taro plant was established. Taro clamping and pulling force testing platform was designed and built. The single factor and Plackett-Burman experiments were used to determine the simulation parameters and optimize the taro plant model by taking the correlation coefficient of clamping force and correlation coefficient of pulling force collected from the simulation and the bench experiment as the experiment index. The parameter calibration results of discrete element model of taro plant are as follows: petiole-petiole method/tangential contact stiffness was 8.15×109 N·m-3, and normal/tangential critical stress was 6.65×106 Pa. The contact stiffness of pseudostem- corm method was 1.22×109 N·m-3, the critical stress of normal/tangential was 1.18×105 Pa, and the energy of soil surface was 4.15×106J·m-3. When the pulling speed is 0.1, 0.2, 0.3, 0.4 and 0.5 m·s-1, the correlation coefficients between the simulation experiment and the bench experiment are 0.812, 0.850, 0.770, 0.697 and 0.652, respectively. The average value of correlation coefficient is 0.756, indicating that the simulated discrete element plant model is close to the real plant model. The discrete element model of taro plant established in this paper has high reliability. The final purpose of this paper is to provide a model reference for the design and optimization of taro harvester by discrete element method.

Suppression of lateral vibration in a shafting system via an auxiliary active magnetic bearing with position control

The bearings in a shafting system are important paths for the lateral vibration of the shaft to transmit to the hull structure and induce underwater sound. An auxiliary active magnetic bearing for suppressing the lateral vibration at the stern support of a shafting system is investigated. The active magnetic bearing situated close to the stern bearing generates electromagnetic forces to support the shaft, which decreases the contact stiffness as well as the friction between the shaft and the stern bearing and accordingly reduces the stern support acceleration. A dynamic model is established using the Timoshenko beam theory and Hamilton’s principle and the performance of the active magnetic bearing in vibration suppression of the shafting system is evaluated with position control. An experimental system is also built to verify the effectiveness of the active magnetic bearing. Numerical and experimental results have shown that the active magnetic bearing is able to suppress the stern support acceleration responses after the shaft is suspended by the active magnetic bearing. Moreover, the orbit of the shaft center is controlled as well.

Research on Natural Frequency of Piezoelectric Pressure Sensor Affected by Preload

The natural frequency of piezoelectric pressure sensors is seriously affected by preload, but there is currently a lack of relevant quantitative analysis, resulting in the lack of a corresponding theoretical basis for sensor design and packaging. In order to solve the above problems, based on the fractal theory, the relationship between the preload and the normal contact stiffness under different surface topography and element materials is deduced and simply verified by finite element simulation. Based on the sensor structure and size, a dynamics model is established, and the relationship between the preload and the natural frequency is obtained. Shock tube calibration experiments with different preload are used to verify the correctness of the relationship. The experimental results are similar to the theoretical calculations, proving the correctness and feasibility of the theory. The theoretical calculation results can be used for the selection of the preload and the estimation of the natural frequency of the actual piezoelectric pressure sensor, which is of great significance for the mass production of piezoelectric pressure sensors.

Numerical simulation of rock mass blasting vibration using particle flow code and particle expansion loading algorithm

In the numerical analysis of rock mass blasting using the particle expansion loading algorithm in the PFC method, the size of the blasting funnel is usually consistent with the experiment, but the blasting seismic wave produced is quite different from the reality. By reasonably determining the micro-parameters that affect the simulation effect of blasting vibration, the particle expansion loading algorithm can better simulate the actual situation of the blasting effect of the rock mass. The studied rock's density, uniaxial compressive strength, and elastic modulus are 2500 kg/m3, 80.1 MPa, and 65 GPa, respectively. Firstly, based on the theory of the PFC method and the law of energy conservation, the attenuation process of the seismic waves was controlled by the critical damping ratio and contact stiffness of the model. Then, the rock blasting numerical simulation is carried out using the particle expansion loading algorithm. The critical damping ratio mainly affects particle vibration's kinetic energy attenuation rate. At the same time, the contact stiffness can control both the peak velocity and the duration of blasting vibration. Finally, the field blasting experiment verifies the rationality of the quantitative relationship between the critical damping ratio and contact stiffness and the specific parameters of blasting seismic waves in the simulation of rock blasting vibration. It is verified that the particle expansion loading algorithm in the PFC method can not only simulate the blasting funnel forming well but also simulate the blasting vibration effect in the outlying area, which provides a more reliable numerical simulation method for the simulation of rock blasting.

Framework for automatic contact detection in a multibody system

The mechanical behavior of multibody systems can be modeled with several numerical methods. One can mention the Multibody Dynamics (MBD), the Finite Element Method (FEM), the Discrete Element Method (DEM) among others, which have the common feature of solving for the dynamics of a system of rigid/flexible bodies. In this context, the mechanical contact interaction between bodies has to be considered in the model. First, one needs to adopt a strategy to seek for contact occurrence and, when it is detected, to evaluate and include contributions into the mathematical model. However, handling contacts in such models is not a straightforward task, as they can be numerous (depending on the number of bodies considered in the model) and complex, due to the particularities in the geometric shape that each body presents, influencing in contact detection and in its inclusion, as a contribution into the model.In this work, we propose an integrated framework for handling and managing the automatic contact detection between bodies within a whole complex multibody system. Both rigid and flexible bodies can be considered. Strategies such as MBD, FEM and DEM can be employed together with the proposed strategy. Pointwise contact interactions are modeled based on the geometric description of each body’s boundary, with the aid of the master–master contact formulation.A hierarchy of bounding volumes for the collision detection is employed, ruling the creation of a contact–candidates pairs list, for which the Local Contact Problem (LCP) is solved — as a detailed determination for the contact pointwise location on local surface parameterizations, representing bodies’ boundaries. As the contact interface, we employ a particular hybrid law that can represent a classical linear contact stiffness, Hertzian contact and other possibilities, together with a barrier to avoid penetration between bodies.We show applications of the technique involving general rigid polyhedra in contact with beam finite elements — as a FEM/DEM coupling, such as rolling rigid bodies, as a classical multibody dynamics application. The herein proposed strategy can be incorporated using distinct time-integration solvers, such as it is independent on MBD/FEM/DEM particularities.

Numerical study of bladed structures with geometric and contact nonlinearities

This paper deals with the reduced order modeling of turbomachine bladed structures accounting for both geometric nonlinearities and nonlinear blade-tips/casing contacts in a numerically efficient way. A recently derived methodology, based on the concept of modal derivatives, is first used to study the contact interactions of a single rotating blade impacting a rigid casing. In this reduction procedure, nonlinear internal forces due to large displacements are evaluated using the stiffness evaluation procedure and contact is numerically handled with Lagrange multipliers. In-depth analyses, including clearance consumption computations and frequency analyses with a continuation procedure, are performed to understand and characterize the combined influence of contact and geometric nonlinearities on the blade’s dynamics. The methodology is then generalized to full bladed disks using Component Mode Synthesis techniques with fixed interfaces. Each sector of the cyclically symmetric structure is projected onto a reduction basis composed of Craig–Bampton modes and a selection of their modal derivatives. A second reduction allows reducing the cyclic boundary degrees-of-freedom. The methodology is first assessed using a harmonic excitation at blade-tips, without contact. When applied to the bladed disk subjected to contact interactions, the methodology allows identifying the main interaction speeds that can be detrimental for the engine integrity. Through this work, the numerical strategy is applied on an open industrial compressor bladed disk model based on the NASA rotor 37 in order to promote the reproducibility of results.

Finite Element Analysis of Nanoindentation Responses in Bi2Se3 Thin Films

In this study, the nanoindentation responses of Bi2Se3 thin film were quantitatively analyzed and simulated by using the finite element method (FEM). The hardness and Young’s modulus of Bi2Se3 thin films were experimentally determined using the continuous contact stiffness measurements option built into a Berkovich nanoindenter. Concurrently, FEM was conducted to establish a model describing the contact mechanics at the film/substrate interface, which was then used to reproduce the nanoindentation load-depth and hardness-depth curves. As such, the appropriate material parameters were obtained by correlating the FEM results with the corresponding experimental load-displacement curves. Moreover, the detailed nanoindentation-induced stress distribution in the vicinity around the interface of Bi2Se3 thin film and c-plane sapphires was mapped by FEM simulation for three different indenters, namely, the Berkovich, spherical and flat punch indenters. The results indicated that the nanoindentation-induced stress distribution at the film/substrate interface is indeed strongly dependent on the indenter’s geometric shape.