Attachment Stiffness Research Articles

Effect of mounting and fluid parameters on dynamic damping characteristics of a hydraulic damper

A mathematical model describing the dynamic damping characteristics (DDC) of a high-speed rail hydraulic damper is built and experimentally validated, followed parameter effect simulation on DDC of the damper is conducted by using the model. Simulation results show that the change of rubber attachment stiffness and entrapped air ratio would have apparent influence on Force-displacement characteristics of the damper, i.e., increasing of rubber attachment stiffness k0 would increase the main damping indices, but increasing of entrapped air ratio ε would decrease all of the dynamic damping characteristic indices apparently, so this implies that air entrapment of the fluid would apparently decrease the elasticity modulus of the fluid and weaken the dynamic characteristics of the damper. The established damper model and obtained result would be useful in the context of high-speed train research and design.

Investigating the Effects of Strapping Pressure on Human-Robot Interface Dynamics Using a Soft Robotic Cuff

Physical human-robot interfaces are a challenging aspect of exoskeleton design, mainly due to the fact that interfaces tend to migrate relatively to the body leading to discomfort and power losses. Therefore, the key is to develop interfaces that optimize attachment stiffness, i.e., reduce relative motion, without compromising comfort. To that end, we propose a method to obtain the optimal attachment pressure in terms of connection stiffness and comfort. The method is based on a soft robotic interface capable of actively controlling strapping pressure which is coupled to a cobot. Hereby the effects of strapping pressure on energetic losses, connection stiffness, and perceived comfort are analyzed. Results indicate a trade-off between connection stiffness and perceived comfort for this type of interface. An optimal strapping pressure was found in the 50 to 80 mmHg range. Connection stiffness was found to increase linearly over a pressure range from 0 mmHg (stiffness of 1139 N/m) to 100 mmHg (stiffness of 2232 N/m). And energetic losses were reduced by 42% by increasing connection stiffness. This research highlights the importance of strapping pressure when attaching an exoskeleton to a human and introduces a new adaptive interface to improve the coupling from an exoskeleton to an individual.

Parametric vibration analysis and validation for a novel portable hexapod machine tool attached to surfaces with unequal stiffness

The Free-Leg Hexapod (Free-Hex) machine tool is an advancement from the conventional Stewart platform, which has the fixed base platform removed to enable the limbs to be attached to a wider range of surfaces (e.g. non-flat and curved). However, in some scenarios (e.g. in-situ repair of industrial installations), the limbs of the Free-Hex need to be attached to the surfaces with unequal stiffness, which brings the challenge of predicting the dynamics of the system for conducting machining operations under dynamically stable conditions. In this paper, after introducing the attachment stiffness (i.e. feet attached to the environment with different materials) in the conventional dynamic model of the parallel manipulator, the dynamic behavior of the Free-Hex machine tools devoted to the in-situ operation environments was studied. Then, the experimental validation was conducted to prove the dynamic model developed in this paper. It was found that the errors of the proposed model are under 6% (i.e. 5.1% at symmetrical limb configuration and 5.8% at the arbitrary configuration) when the limbs are attached to the surfaces with unequal stiffness. Further, by applying the validated model, the dynamic performance of the Free-Hex with a wider range of attachment stiffness was analyzed. Overall, it was found that the attachment stiffness has a remarkable influence on the natural frequencies of the machine tool (e.g. the frequency of mode 4 at the symmetrical configuration is increased by 36.8% when the attachment stiffness of one limb changes from 0 to 1e+12 N/m). Thus, the work discussed in this paper can be utilized to avoid the dynamically unstable configurations of parallel kinematic machine tools (e.g. Free-Hex) when mounting on the surface with unequal stiffness in the in-situ operation environments.

Lightweight Body-In-White Design Driven by Optimization Technology

Structural optimization technologies are increasingly applied to lightweight automotive structural design. In a body-in-white (BIW) development project, optimization technologies are applied in three steps that are seamlessly integrated into engineering practice. By following a design-driven approach, an innovative design that was 50.65 kg lighter than the base design was achieved. First, topology optimization was used to investigate the optimal material distribution in a given design space, and helped in extracting the main load path, while considering crashworthiness, NVH, and static stiffness performance requirements. In the second step, the effective design of a beam cross-section and feasible joints was carried out based on optimization technology and expert knowledge. In this step, the deep involvement of manufacturing process experts on stamping and joint connection feasibility analysis increased the chance of success remarkably. The sensitivity analysis, topography optimization, and gauge optimization of the created draft 3D BIW frame comprised the last step before finalizing the initial design. The new design was validated by full load case simulations, including full vehicle crashworthiness analysis, mode frequency, and body mount point dynamic stiffness analysis, and BIW stiffness and component attachment stiffness analysis. The validation results indicate that the new BIW design achieves the performance goal, while being 13.3% lighter than the base design.

Dynamic behaviour of a high-speed train hydraulic yaw damper

ABSTRACTThis paper focuses on revealing the dynamic behaviour of a hydraulic yaw damper under very small excitation conditions. First, the measured yaw damper movement is presented when a train experiences unstable motions. It shows that the yaw damper is characterized by very small harmonic movement between 0.5 and 2 mm. Following this, a simplified physical model of the yaw damper is developed which has the ability to reproduce its dynamic performance in the range of operating conditions, and then suitably validated with experimental results. At last, the dynamic behaviour of the yaw damper under very small amplitudes is investigated by comparing with its static behaviour, and the dynamic stiffness and damping in terms of key parameters are studied. It is concluded that there is a great difference in the damper performance between dynamic and static conditions which is caused by the internal damper flexibility under small amplitudes. The percentage of entrapped air in oil, rubber attachment stiffness, and leakage flow have a great effect on the dynamic behaviour of the yaw damper related to the dynamic stiffness and damping. The effect is even more remarkable for smaller amplitudes regarding the dissolved air in oil. Oil leakage has a greater impact on dynamic damping than dynamic stiffness. The series stiffness of the yaw damper is mainly provided by the spring effect of the oil when the rubber attachment stiffness reached a certain limit, and an additional increase in rubber attachment stiffness becomes useless to further enhance the overall stiffness of the damper.

Irreversible passive energy transfer of an immersed beam subjected to a sinusoidal flow via local nonlinear attachment

Passive vibration control of immersed structures in deep water subjected to the external fluid flow has rarely been studied. With the aim of passively absorbing, vibration of a doubly fixed beam under a sinusoidal flow using a nonlinear attachment is studied in this paper. The beam is modeled using the Euler-Bernoulli beam theory. The essentially nonlinear attachment has purely cubic stiffness and linear damping. Required conditions for occurring relaxation oscillations, Hopf and saddle-node bifurcations in the system response are studied. Analytical and numerical approaches are used to analyze the steady-state responses of the considered model. In addition, the influence of the location, the damping and the stiffness of the nonlinear absorber and external load are investigated on the dynamical behavior of the system. Furthermore, influence of the attachment stiffness on quasi-periodic motion boundaries surveyed. It is shown that in the presence of the nonlinear energy pumping the amplitude of the system response decreases significantly and as the attachment location approaches the beam supports the occurrence of the relaxation oscillations decreases and saddle-node bifurcations take place in the system response. The presented results and the proposed modeling approach can be used to design and enhance the performance of nonlinear vibration absorbers for subsea structures.

Design and operating mode analysis of hybrid actuation system based on EHA/SHA

By analysing the structure and working principle of hybrid actuation system (HAS) which consists of a servo-hydraulic actuator (SHA) and an electro-hydrostatic actuator (EHA), the overall mathematical model with dissimilar redundancy conforms is established, which considers various influencing factors, such as air load and attachment stiffness. Furthermore, the mechanism of dynamic force fighting is expounded. In addition, a new control strategy which uses the trajectory generator and weight coefficient theory is proposed to restrain the force fighting. The modelling and simulation analysis between the new control strategy and a conventional proportional–integral–derivative control strategy is carried out as well. Based on the above researches, quantitative analysis is carried out on the procedure of operating mode switching, which is from active/active mode to active/passive mode. The results indicate that the position/velocity trade-off control strategy has a good control performance, and effectively restrains the static force fighting as well as dynamic force fighting. Besides that, it can also shorten the system response time. The HAS can still meet the actuating requirements after the operating mode switches to active/passive mode. The actuator in passive state generates a certain static error by the reason of tracking the position of the control surface.

Whirl flutter optimisation-based solution of twin turboprop aircraft using a full-span model

Whirl flutter is a specific type of flutter instability, relevant for turboprop aircraft, caused by the effect of rotating parts as a propeller or a gas-turbine engine rotor. The proposed optimisation-based analytical procedure is used to determine the critical values of the engine attachment stiffness parameters for the preselected flutter speed. For the half-span model, two design variables are used. The objective function is defined as the minimization of the engine vibration mode frequency sum. Design constraints keep the engine frequency ratio and the flutter stability at the selected velocity. However, application of a full-span model is necessary in some cases. In this case, special models of both symmetric and antisymmetric engine vibrations and four design variables must be used. Design constraints maintain the pitch mode frequency ratio, the yaw mode frequency ratio and the critical mode frequency ratio. Critical modes are dependent on the relation between the rotational direction of both propellers (identical or inverse). A flutter design constraint is applied as well. The described methodology is demonstrated on the application example of a twin-engine commuter aircraft. Demonstrated cases include symmetrical revolutions of propellers for both identical and inverse directions of rotation, cases of single engine failure and single propeller feathering, and finally, cases of unsymmetrical revolutions including the reduced and increased revolutions of a single propeller, for both identical and inverse directions of rotation.

Prediction of the in-service performance of a railway hydraulic damper

In this study, an improved physical parametric model with key in-service parameters was established and experimentally validated for a high-speed railway hydraulic damper. A subtle variable oil property model was built and coupled into the full model to address the dynamic flow losses and the relief-valve system dynamics. Experiments were conducted to evaluate the accuracy and robustness of the full damper model and simulation, which determined the damping characteristics over an extremely wide range of excitation speeds. Further simulations with in-service conditions and excitations were performed using the validated model, and the results revealed that improper key in-service parameters, such as improper rubber attachment stiffness, entrained air ratios and small mounting clearances, can greatly degrade the damping capability of a hydraulic damper. The obtained physical model includes all the influential factors that have an impact on the damping characteristics, so it will serve as a useful basic theory in the prediction of in-service performance, optimal specification and product design optimization of hydraulic dampers for modern high-speed rail vehicles.

Probabilistic evaluation of the material properties of the in vivo subject-specific articular surface using a computational model.

This article used probabilistic analysis to evaluate material properties of the in vivo subject-specific tibiofemoral (TF) joint model. Sensitivity analysis, based on a Monte Carlo (MC) method, was performed using a subject-specific finite element (FE) model generated from in vivo computed tomography (CT) and magnetic resonance imaging (MRI) data, subjected to two different loading conditions. Specifically, the effects of inherent uncertainty in ligament stiffness, horn attachment stiffness, and articular surface material properties were assessed using multifactorial global sensitivity analysis. The MRI images were taken before and after axial compression, and when the flexion condition had been maintained at up to 90 degree flexion in the subject-specific knee joint. The loading conditions of the probabilistic subject-specific FE model (axial compression and 90 degree flexion) were similar to the MRI acquisition setup. We were able to detect the influence of material parameters while maintaining the potential effect of parametric interactions. Throughout the in silico property optimization, a subject-specific FE model was used and less sensitive parameters were eliminated in the global sensitivity method. Soft tissue material properties were estimated using an optimization procedure that involved the minimization of the differences between the kinematics predicted by the subject-specific model and those obtained through in vivo subject-specific data. The results of this approach suggest that the articular surface mechanical properties could be found by using in vivo measurements, which clarifies the valuable tool for future subject-specific studies related to TF joint scaffolds, allografts and biologics. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1390-1400, 2017.

Parameters for Modeling Stranded Cables as Structural Beams

This paper presents a method for determining the effective homogenous beam parameters for stranded cables made up of non-homogenous wires, as well as characterization of the attachment method commonly used for cable harnesses on space structures. There is not yet a predictive model for quantifying the structural impact of cable harnesses on space flight structures, and towards this goal, the authors aim to predict cable resonance behavior from basic cable measurements. Cables can be modeled as shear beams, but the shear beam model assumes a homogenous, isotropic material, which a stranded cable is not. Thus, the cable-beam model requires both knowledge of the cable constraints and calculation of effectively homogenous properties, including density, area, bending stiffness, and modulus of rigidity to predict the natural frequencies of the cable. Through a combination of measurement and correction factors, upper and lower bounds for effective cable properties and attachment stiffness are calculated and shown to be effective in a cable-beam model for natural frequency prediction. Although the cables investigated are spaceflight cables, the method can be applied to any stranded cable for which the constituent material properties can be determined.

Local stiffness control for reducing vehicle interior noise by using FRF-based synthesis method

This paper explores the feasibility of estimating the interior noise of a vehicle compartment when the attachment stiffness (i.e., the local stiffness of the mounting point at which operational force is applied) is altered. In order to change the local stiffness of the vehicle body, it was assumed that the local stiffness could be changed by attaching some additional systems to the vehicle body. The frequency response function (FRF)-based synthesis method is used herein to estimate the change in the local stiffness of the modified vehicle body. In addition, the change in the noise transfer function (NTF) can also be estimated using this method. Since the operational force transmitted from the source excitation to the body attachment is required to predict the interior noise of the vehicle body in the operational condition, a transfer path analysis (TPA) technique was performed. Various vehicles and additional systems were utilized to estimate the dynamic properties of the modified vehicle body. Results indicated that the synthesis technique is an appropriate method to estimate the change in the local stiffness and NTF of the modified vehicle body. The results also show that the interior noise of the body can be estimated accurately by using the synthesis technique and TPA procedure when the vehicle body is locally modified.

Non-linear parametric modelling of a high-speed rail hydraulic yaw damper with series clearance and stiffness

At high operational speeds, the train system becomes very sensitive to parameter variations of components. Therefore, it is imperative to incorporate more accurate component models in the vehicle dynamics studies. This study addresses a more subtle and comprehensive non-linear parametric model of a high-speed rail hydraulic yaw damper. A new concept of a hydraulic yaw damper model is suggested, in which the small mounting clearance, the series stiffness, and the viscous damping are built in. The series stiffness is the tandem result of the dynamic oil stiffness, the rubber attachment stiffness, and the mounting seat stiffness. A dynamic oil property model is established and coupled to the entire modelling process, in which the density, the dynamic viscosity, the volumetric elastic modulus, and the stiffness of the oil are all changeable in terms of the instantaneous working pressure, the oil temperature, and the entrapped air ratio of the oil. The dynamic flow loss and the valve system dynamics are also incorporated. Experiments validated that the established non-linear parametric model is accurate and robust in predicting the damping characteristics within an extremely wide speed range. The validated damper model was then successfully applied to a thorough parameter sensitivity analysis and damping nature prediction under practical, in-service conditions. The established damper model couples all the main influential factors that are not or are insufficiently considered in normal-speed problems; thus, it will be more accurate and appropriate for furthering high-speed problem studies.

Influence of Attachment Pressure and Kinematic Configuration on pHRI with Wearable Robots

The goal of this paper is to show the influence of exoskeleton attachment, such as the pressure on the fixation cuffs and alignment of the robot joint to the human joint, on subjective and objective performance metrics (i.e. comfort, mental load, interface forces, tracking error and available workspace) during a typical physical human‐robot interaction (pHRI) experiment. A mathematical model of a single degree of freedom interaction between humans and a wearable robot is presented and used to explain the causes and characteristics of interface forces between the two. The pHRI model parameters (real joint offsets, attachment stiffness) are estimated from experimental interface force measurements acquired during tests with 14 subjects. Insights gained by the model allow optimisation of the exoskeleton kinematics. This paper shows that offsets of more than ±10 cm exist between human and robot axes of rotation, even if a well‐designed exoskeleton is aligned properly before motion. Such offsets can create interface loads of up to 200 N and 1.5 Nm in the absence of actuation. The optimal attachment pressure is determined to be 20 mmHg and the attachment stiffness is about 300 N/m. Inclusion of passive compensation joints in the exoskeleton is shown to lower the interaction forces significantly, which enables a more ergonomic pHRI.

Influence of Attachment Pressure and Kinematic Configuration on pHRI with Wearable Robots

The goal of this paper is to show the influence of exoskeleton attachment, such as the pressure on the fixation cuffs and alignment of the robot joint to the human joint, on subjective and objective performance metrics (i.e. comfort, mental load, interface forces, tracking error and available workspace) during a typical physical human-robot interaction (pHRI) experiment. A mathematical model of a single degree of freedom interaction between humans and a wearable robot is presented and used to explain the causes and characteristics of interface forces between the two. The pHRI model parameters (real joint offsets, attachment stiffness) are estimated from experimental interface force measurements acquired during tests with 14 subjects. Insights gained by the model allow optimisation of the exoskeleton kinematics. This paper shows that offsets of more than ±10 cm exist between human and robot axes of rotation, even if a well-designed exoskeleton is aligned properly before motion. Such offsets can create interface loads of up to 200 N and 1.5 Nm in the absence of actuation. The optimal attachment pressure is determined to be 20 mmHg and the attachment stiffness is about 300 N/m. Inclusion of passive compensation joints in the exoskeleton is shown to lower the interaction forces significantly, which enables a more ergonomic pHRI.

Flutter/Limit Cycle Oscillation Analysis and Experiment for Wing-Store Model

A delta wing experimental model with an external store has been designed and tested in the Duke University wind tunnel. The wing structure is modeled theoretically by using von Karman plate theory that allows for geometric strain-displacement nonlinearities in the plate wing structure. A component modal analysis is used to derive the full structural equations of motion for the wing/store combination system. A three-dimensional time domain vortex lattice aerodynamic model including a reduced-order model aerodynamic technique and a slender-body aerodynamic theory for the store are also used to investigate the nonlinear aeroelastic system. The effects of the store pitch stiffness (attachment stiffness), the span location of store, and the store aerodynamics on the critical flutter velocity and limit cycle oscillations (LCO) are discussed. The correlations between the theory and experiment are good for both the critical flutter velocity and frequency but not good for the LCO amplitude, especially when the store is located near the wing tip. The theoretical structural model needs to be improved to determine LCO response, and improved results are shown in the companion paper as obtained with a higher-order structural model.

Aspects on damper-attachment compliance

This paper investigates how attachment compliance coming from mounting bushings or brackets affects damper efficiency. Analyses with a simple mass-spring-damper system show that compliance in damper-attachment points reduces the damper efficiency. If however vibration isolation of the mass is considered, it may be seen that compliance increases low frequency vibrations but reduces high frequency vibrations. Through analyses of this system, the relative damping ratio is studied as a function of excitation frequency and attachment stiffness. Numerical values of typical damper-attachment stiffness in heavy vehicles are furthermore obtained from both static finite element (FE) analysis of the chassis frame and from dynamic FE analysis of a tractor. The effect damper-attachment compliance has on vehicle behaviour is finally quantified with MBS simulations of a tractor semi trailer combination. It is found that attachment stiffness should be considered when simulating load cases containing high frequency inputs.

서스펜션 성능 확보를 위한 고강성 차페 개발 프로세스 연구

This paper describes the development process of high stiffness body for ride and handling performance. High stiffness and light weight vehicle is a major target in the refinement of Passenger cars to meet customers' contradictable requirements between ride and handling performance and fuel economy This paper describes the analysis approach process for high stiffness body through the data level of body stiffness. According to the frequency band. we can suggest the design guideline about lg cornering static stiffness, torsional and lateral stiffness, body attachment stiffness. The ride and handling characteristic of a vehicle Is significantly affected by vibration transferred to the body through the chassis mounting points from front and rear suspension. It is known that body attachment stiffness is an important factor of ride and handling performance improvement. And high stiffness helps to improve the flexibility of bushing rate tuning between handling and road noise. It makes possible to design the good handling performance vehicle and save vehicles to be used in tests by using mother car at initial design stage. These improvements can lead to shortening the time needed to develop better vehicles.

A study on the body attachment stiffness for the road noise

The ride and noise characteristics of a vehicle are significantly affected by the vibration transferred to the body through the chassis mounting points in the engine and suspension. It is known that body attachment stiffness is an important factor of idle noise and road noise for NVH performance improvement. The body attachment stiffness serves as a route design aimed at isolating the vibration generated inside the car due to the exciting force of the engine or road. The test result of the body attachment stiffness is shown in the FRF curve data; the stiffness level and sensitive frequency band are recorded by the data distribution. The stiffness data is used for analyzing the parts that fail to meet the target stiffness at a pertinent frequency band. The analysis shows that the target frequency band is between 200 and 500 Hz. As a result of the comparison in a mounted suspension, the analysis data is comparable to the test data. From these results, there is a general agreement between the predicted and measured responses. This procedure makes it possible to find the weak points before a proto car is produced, and to suggest proper design guidelines in order to improve the stiffness of the body structure.

Weld magnification factors for semi-elliptical surface cracks in fillet welded T-butt joint models

The weld magnification factor method has been widely used in the determination of the stress intensity factor (SIF) for weld-toe cracks in welded structural components. Weld magnification factors Mkare normally derived from two-dimensional crack models with fillet weld profiles to take account of the effect of weld-notch stress concentration at the deepest point of the crack front. This paper presents a detailed three-dimensional analysis of weld-toe surface cracks in fillet welded T-butt joint models using the finite element method. Effects of the weld notch and the welded attachment stiffness on the SIFs of the weld-toe surface cracks have been studied quantitatively. Weld magnification factors applying to the whole surface crack fronts have been estimated. Numerical results show two contradictory effects; that the effect of weld notch increases SIF values throughout the shallow surface crack fronts which are in the region of notch stress concentration, while the effect of local structural constraint reduces the SIF values. The increase in the SIF values mainly depends upon the relative crack front depth and the decrease in the SIF values mainly depends upon the crack shape aspect ratio for a specific weld profile. Both effects on the weld magnification factors can be estimated separately. A simple approach for deriving the weld magnification factors for various weld-toe surface crack problems is proposed for engineering applications.