Low Structural Stiffness Research Articles

Vibration serviceability assessment of ribbon-shaped large-span footbridge at high altitudes under wind-pedestrians coupling effects

Large-span footbridges have low structural stiffness, light deadweight and small damping and thus are prone to significant vibration under the wind-pedestrian coupling effect. Fast and accurate calculation of structural dynamic response is the key to vibration serviceability assessment of footbridges. This study proposed a method of vibration serviceability assessment considering wind-pedestrian coupling for the ribbon-shaped large-span footbridge at high altitudes. The simulated crowd load and wind load were obtained through theoretical derivation and wind tunnel tests. A refined numerical model was developed to analyze the dynamic response of a large-span footbridge under wind-pedestrian coupling. The peak acceleration of the footbridge under different cases was statistically analyzed for an empirical formula, and the serviceability assessment of the footbridge was realized based on the fitting formula. The results revealed that the dynamic response of the footbridge under wind-pedestrian coupling was larger than that under one type of load alone. The dynamic response under the coupling effect was not equal to the linear superposition of the dynamic response in the case of one load. The closer the excitation frequency is to the natural frequency of the structure, the larger the dynamic response of the structure is, and the superposition of multiple modes occurs.

Experimental investigation on the machining performance of honeycomb seal structures by using electrochemical discharge machining

Aerospace metal honeycomb seal structures are an essential component of aircraft engines, and their manufacturing has been at the forefront of industrial focus. Due to the low structural stiffness and high-strength materials, it has become one of the complex components that are difficult to machine. To achieve precision manufacturing of honeycomb seal structures, this study proposes the use of electrochemical discharge machining (ECDM) for machining honeycomb seal structures, which has the high efficiency of electric discharge machining (EDM) and superior surface quality of electrochemical machining (ECM). The experiment results explored the influence of different experimental variables on surface quality, energy distribution relation and material removal rate (MRR). Moreover, in order to predict the machined depth, the mathematical relation between different variables and machined depth was established, which can be expressed as h = 1.407c0.1135u0.3102v-0.391, and a better machining outcome was achieved at 1 wt%, 15 V, and 22 mm/min with an optimal η2 of 0.36. Finally, this study elucidates the material removal mechanism of honeycomb seal structures in ECDM, which demonstrates that ECDM has a high application value in the manufacturing of honeycomb seal structures.

Robot base position and spacecraft cabin angle optimization via homogeneous stiffness domain index with nonlinear stiffness characteristics

The use of mobile robots for machining large components has received considerable research interest for the application of industrial robots in the machinery manufacturing sector. However, the low structural stiffness of industrial robots can result in poor machining quality under the action of cutting forces. Therefore, this paper proposes a simultaneous optimization method the mobile robot base position and cabin angle using homogeneous stiffness domain (HSD) index for large spacecraft cabins. First, a nonlinear joint stiffness model that considers the gravity compensator mechanism is established to describe the stiffness characteristics of heavy-duty robots more accurately. Subsequently, a HSD index is proposed to evaluate the overall stiffness values and stiffness fluctuation for all robot postures in the machining program. An optimization model is then established based on the HSD under the constraints of machining accessibility, joint angle limitation and singularity. The optimal base position and cabin angle are determined simultaneously using the sparrow search algorithm. Finally, simulation and milling experiments are used to demonstrate that the optimization method proposed in this paper can effectively improve the machining quality.

Bio-Inspired Space Robotic Control Compared to Alternatives.

Controlling robots in space with necessarily low material and structural stiffness is quite challenging at least in part due to the resulting very low structural resonant frequencies or natural vibration. The frequencies are sometimes so low that the very act of controlling the robot with medium or high bandwidth controllers leads to excitation of resonant vibrations in the robot appendages. Biomimetics or biomimicry emulates models, systems, and elements of nature for solving such complex problems. Recent seminal publications have re-introduced the viability of optimal command shaping, and one recent instantiation mimics baseball pitching to propose control of highly flexible space robots. The readership will find a perhaps dizzying array of thirteen decently performing alternatives in the literature but could be left bereft selecting a method(s) deemed to be best suited for a particular application. Bio-inspired control of space robotics is presented in a quite substantial (perhaps not comprehensive) comparison, and the conclusions of this study indicate the three top performing methods based on minimizing control effort (i.e., fuel) usage, tracking error mean, and tracking error deviation, where 96%, 119%, and 80% performance improvement, respectively, are achieved.

Extended Finite Element Method (XFEM) Model for the Damage Mechanisms Present in Joints Bonded Using Adhesives Doped with Inorganic Fillers

The use of adhesive bonding in diverse industries such as the automotive and aerospace sectors has grown considerably. In structural construction, adhesive joints provide a unique combination of low structural weight, high strength and stiffness, combined with a relatively simple and easily automated manufacturing method, characteristics that are ideal for the development of modern and highly efficient vehicles. In these applications, ensuring that the failure mode of a bonded joint is cohesive rather than adhesive is important since this failure mode is more controlled and easier to model and to predict. This work presents a numerical technique that enables the precise prediction of the bonded joint’s behavior regarding not only its failure mode, but also the joint’s strength, when inorganic fillers are added to the adhesive. To that end, hollow glass particles were introduced into an epoxy adhesive in different amounts, and a numerical study was carried out to simulate their influence on single lap joint specimens. The numerical results were compared against experimental ones, not only in terms of joint strength, but also their failure pattern. The neat adhesive, which showed 9% and 20% variations in terms of failure load and displacement, respectively. However, looking at the doped configurations, these presented smaller variations of about 2% and 10% for each respective variable. In all cases, by adding glass beads, crack initiation tended to change from adhesive to cohesive but with lower strength and ductility, correctly modeling the general experimental behavior as intended.

Structural Assessment of Endodontic Files via Finite Element Analysis

A methodology for the structural assessment of Nickel-Titanium (Ni-Ti) endodontic files and a novel approach to predict their fatigue behavior using finite element method (FEM) were proposed. ProTaper-Universal F1 and F2 endodontic files were selected due to availability of extensive test data needed for the validation of the methodology. Bending and torsional loadings were analyzed since these provide essential data for the structural integrity assessment for the endodontic files. High-definition FEM models and their computationally efficient idealized versions were developed. The results for the bending and torsional stiffness of the F1 endodontic file agreed with the literature data validating the proposed methodology. Hysteresis energy density was shown to give promising results as a predictor of low cycle fatigue failure. The predictions with the idealized models matched those of the high-definition models, justifying the proposed idealizations. The validated models demonstrated that F2 has 60% higher bending and torsion resistance and 7% higher hysteresis energy density per cycle with respect to F1, leading to the conclusion that F1 has a lower structural stiffness but a longer fatigue life as compared to F2. In summary, the developed methodology allows for the structural and durability evaluation of various design parameters for Ni-Ti endodontic files.

Vibration Analysis and Diagnostics of a High Speed Centrifuge for Fast Reactor Fuel Reprocessing Applications

This study aims at understanding the cause of vibrations observed during the operation of high speed centrifuges in a radioactive environment and materializing ways to mitigate these vibrations to ensure dynamic stability of centrifuges during operation. For this, a condition-based monitoring study was carried out for the centrifuge and its supporting structure using dynamic co-relation study in frequency domain during the operation of centrifuge at its operating speed of 19[Formula: see text]700[Formula: see text]rpm. Dynamic characteristics of the support structure were analyzed using standard finite element code. It was observed that high vibration amplitudes in the original support structure, before modification, owed to its low structural stiffness. Hence, structural modifications were carried out to reduce the dynamic crossover response of the support structure at the critical speeds and operating speeds of centrifuge. It was found that structural modification resulted in considerable reduction in the vibration amplitude of the support structure, compared to its initial configuration.

Investigation of Aeroelasticity Effect on Missile Separation from the Internal Bay

There is a strong aerodynamic interference when launching the missile in the embedded mode. During the separation process, the carrier aircraft safety may be threatened due to large slenderness ratio, low structural stiffness, and aeroelasticity effects of the missile. The present study simulates missile separation in the presence of the aeroelasticity effects based on the computational fluid dynamics (CFD), rigid body dynamics (RBD), and computational structure dynamics (CSD) coupling method. A hybrid dynamic grid method consisting of the mixed overset unstructured grid and deformation grid is utilized. In order to verify the accuracy of the coupled numerical method, store separation from a wing and AGARD 445.6 wing flutter are first simulated as two standard test cases. The verification results imply that the present coupled numerical method is reliable and capable in simulation of the aeroelastic effect in missile separation. The influence of aeroelasticity on the separation trajectory of a missile from the internal bay is systematically studied at different states. Numerical results show that aeroelasticity substantially affects the missile angular displacement, while it has a slight impact on the linear displacement of the center of mass. Mach number and flight altitude are two important flight parameters that characterize the aeroelasticity effect on missile separation from the internal bay.

Model Predictive Control Applied to Different Time-Scale Dynamics of Flexible Joint Robots

Modern Lightweight robots are constructed to be collaborative, which often results in a low structural stiffness compared to conventional rigid robots. Therefore, the controller must be able to handle the dynamic oscillatory effect mainly due to the intrinsic joint elasticity. Singular perturbation theory makes it possible to decompose the flexible joint dynamics into fast and slow subsystems. This model separation provides additional features to incorporate future knowledge of the joint-level dynamical behavior within the controller design using the Model Predictive Control (MPC) technique. In this study, different architectures are considered that combine the method of Singular Perturbation and MPC. For Singular Perturbation, the parameters that influence the validity of using this technique to control a flexible-joint robot are investigated. Furthermore, limits on the input constraints for the future trajectory are considered with MPC. The position control performance and robustness against external forces of each architecture are validated experimentally for a flexible joint robot. The experimental validation shows superior performance in practice for the presented MPC framework, especially respecting the actuator torque limits.

Variable Stiffness Identification and Configuration Optimization of Industrial Robots for Machining Tasks

Industrial robots are increasingly being used in machining tasks because of their high flexibility and intelligence. However, the low structural stiffness of a robot significantly affects its positional accuracy and the machining quality of its operation equipment. Studying robot stiffness characteristics and optimization methods is an effective method of improving the stiffness performance of a robot. Accordingly, aiming at the poor accuracy of stiffness modeling caused by approximating the stiffness of each joint as a constant, a variable stiffness identification method is proposed based on space gridding. Subsequently, a task-oriented axial stiffness evaluation index is proposed to quantitatively assess the stiffness performance in the machining direction. In addition, by analyzing the redundant kinematic characteristics of the robot machining system, a configuration optimization method is further developed to maximize the index. For numerous points or trajectory-processing tasks, a configuration smoothing strategy is proposed to rapidly acquire optimized configurations. Finally, experiments on a KR500 robot were conducted to verify the feasibility and validity of the proposed stiffness identification and configuration optimization methods.

Dynamic modeling and vibration prediction of an industrial robot in manufacturing

Industrial robots have been widely used in modern manufacturing with the benefit of their flexibility, cost efficiency, and multi-functionality. However, when the industrial robot is applied in high material removal rate operations, e.g., drilling and milling, even if a little machining excitation may result in significant vibration due to its relatively low structural stiffness. This can lead to poor surface quality and precision of machined parts. The objective of this paper is to present a dynamic modeling and vibration prediction method of an industrial robot subjected to machining forces. First, the robot is modeled as a multibody system consisting of body elements and hinge elements using the transfer matrix method for multibody systems. Second, the transfer matrices and the transfer equations of all the robotic elements are established, followed by the deduction of the overall transfer matrix and the overall transfer equation of the robot. Then, the natural modes of the robot are calculated by constructing and solving its characteristic equation, and the vibration response is obtained by using the principle of modal superposition. Finally, the numerical simulations, hammer excitation test, and milling experiment are carried out, and the results demonstrate the effectiveness of the presented modeling and analysis method for the machining vibration prediction of an industrial robot. This paper offers effective guidance for machining vibration calculation and suppression of an industrial robot in manufacturing.

Real-Time Laser Tracker Compensation of Robotic Drilling and Machining

Due to their flexibility, low cost and large working volume, 6-axis articulated industrial robots are increasingly being used for drilling, trimming and machining operations, especially in aerospace manufacturing. However, producing high quality components has demonstrated to be difficult, as a result of the inherent problems of robots, including low structural stiffness and low positional accuracy. These limit robotic machining to non-critical components and parts with low accuracy and surface finish requirements. Studies have been carried out to improve robotic machine capability, specifically positioning accuracy and vibration reduction. This study includes the description of the hardware, software and methodologies developed to compensate robot path errors in real time using a single three-degrees-of-freedom (DOF) laser tracker, as well as the experimental results with and without compensation. Performance tests conducted include ballbar dynamic path accuracy test, a series of drilling case studies and a machining test. The results demonstrate major improvements in path accuracy, hole position accuracy and hole quality, as well as increases in accuracy of a machined aluminum part.

Comparison of 2 single incision slings on the vagina in an ovine model

Stress urinary incontinence carries a significant healthcare burden for women worldwide. Single incision slings are minimally invasive mesh devices designed to treat stress urinary incontinence. For prolapse repair, meshes with higher porosity and lower structural stiffness have been associated with improved outcomes. In this study, we compared the higher stiffness, lower porosity Altis sling with the lower stiffness, higher porosity Solyx sling in an ovine model. We hypothesized that SIS-B would have a negative impact on the host response. A total of Altis and Solyx single incision slings were implanted suburethrally into sheep according to the manufacturer's instructions on minimal tension. The mesh-urethral-vaginal complex and adjacent ungrafted vagina (no mesh control) were harvested en bloc at 3 months. Masson's trichrome and picrosirius red staining of 6 μm thin sections was performed to measure interfiber distance and tissue integration. Smooth muscle contractility to a 120 mM KCl stimulus was performed in an organ bath to measure myofiber-driven contractions. Standard biochemical assays were used to quantify glycosaminoglycan, total collagen, and elastin content, and collagen subtypes. Bending stiffness was performed in response to a uniaxial force to define susceptibility to folding/buckling. Statistical analysis was performed using Mann-Whitney, Gabriel's pairwise post hoc, Wilcoxon matched-pairs, and chi-square tests. The animals had similar ages (3-5 years), parity (multiparous), and weights (45-72 kg). Trichrome cross sections showed that the Altis sling buckled in a "C" or "S" shape in most samples (8 of 11), whereas buckling after Solyx sling implantation was observed in only a single sample (1 of 13; P=.004). Tissue integration, as measured by the presence of collagen or smooth muscle between the mesh fibers on trichrome 4× imaging, was increased in samples implanted with the Solyx sling compared with the Altis sling (P<.05). Total collagen content decreased significantly with both products when compared with the ungrafted vagina consistent with stress shielding. There was no difference in the 2 groups with regard to glycosaminoglycan or elastin content. The Altis sling mesh tissue complex demonstrated significantly higher amounts of both collagen types I and III than the Solyx sling-implanted tissue and the ungrafted control. Smooth muscle contractility in response to 120 mM KCl was decreased after implantation of both slings compared with the sham (P=.011 and P<.01), with no difference between mesh types (P=.099). Bending stiffness in the Altis sling was more than 4 times lower than in the Solyx, indicating an increased propensity to buckle (0.0186 vs 0.0883). The structurally stiffer Altis sling had decreased tissue integration and increased propensity to buckle after implantation. Increased collagen types I and III after the implantation of this device suggests that these changes may be associated with a fibrotic response. In contrast, the Solyx sling largely maintained a flat configuration and had improved tissue integration. The deformation of the Altis sling is not an intended effect and is likely caused by its lower bending stiffness. Both meshes induced a decrease in collagen content and smooth muscle contractility similar to previous findings for prolapse meshes and consistent with stress shielding. The long-term impact of buckling warrants further investigation.

A framework for accuracy enhancement in milling thin-walled narrow-vane turbine impeller of NiAl-based superalloy

Impeller with narrow-vane is a typical thin-walled structural part of the turbine engine. The high-accuracy requirement for the impeller is difficult to achieve in the machining process due to its low structural stiffness and narrow machining domain. This paper proposes a framework for enhancing the milling accuracy of the thin-walled narrow-vane turbine impeller, which is made of NiAl-based superalloy. The proposed framework consists of a machinability study of the impeller material, a machining deformation analysis, and an error compensation. By studying the milling force and tool wear behavior experimentally, the machinability study yielded optimized process parameters for machining NiAl-based superalloy. A cantilever beam–based tool deformation model and a finite element analysis method model were developed respectively to analyze and predict the deformation of the milling tool in machining the impeller. A flexible and iterative compensation method was studied for decreasing the machining error when milling the impeller. The effectiveness of the proposed framework has been validated experimentally. The results show that the milling accuracy of the turbine impeller has been enhanced significantly.

Adaptive Variable Stiffness Particle Phalange for Robust and Durable Robotic Grasping.

Grasping is an important characteristic of robots in interacting with humans and the environment. Due to the inherent compliance of soft grippers, they can easily adapt to novel objects and operate safely in a human-centered environment. However, soft hands suffer from poor grasping robustness and operation durability, especially for heavy objects or objects with sharp spikes, mainly due to their fragile material and low structural stiffness of the soft actuators. Thus, the widespread use of soft hands in daily applications is still limited. Existing works have shown a promising direction to enhance grasping performance by solving the contradiction between inherent compliance/adaptability and loading capacity. It is known that the stiffness of the robotic phalange is highly related to the performance of robotic hands. In this article, we propose a novel variable stiffness particle phalange, called VSPP here. The proposed VSPP exhibits variable stiffness characteristics without the need for dedicated actuation by utilizing passive particle jamming resulted from forces in interacting with the environment. The VSPP can cooperate with any kind of actuators, soft or rigid, to function as a compliant and robust robotic hand. A prototype robotic hand based on VSPP could maintain reliable grasping even when pierced by sharp objects such as a needle, a cactus, and a durian. This durability is effective both in air and underwater, thus presents new possibilities for the soft robotic hand to work in a harsh environment. The inherent multidirectional compliance of the VSPP makes safety in human/robot interaction guaranteed. The design and modeling presented in this research will provide useful guidance in VSPP applications. A prototype gripper, VSPP-3, composed of three 2-segments VSPP fingers and pneumatic joints, has been built for demonstrations in reliable and robust grasping of daily objects. The sample grasping has shown that the proposed VSPP has great potential for a robust and durable soft robotic hand or gripper design.

Design and experiments on an inflatable link robot with a built-in vision sensor

Soft robots and manipulators with inflatable links possess inherent safety characteristics that make them a very interesting choice for tasks requiring human-robot interaction. However, they may face important challenges in their performance due to low structural stiffness. Accurate positioning of the end effector may prove a difficult task due to limitations in gravity compensation, or due to the occurrence of oscillations during motion, especially in the absence of precise information about the payload that is being manipulated. In this paper a novel approach to the design and control of a robot with an inflatable link is proposed, where the link assumes the triple role of compliant structural element, touch sensor, and active mechanism to adjust the performance of vibration control algorithms. For this, a vision sensor is placed inside the link to provide information about the structural deformation. The sensor is used to measure the tip displacement, and also to detect contact of the link surface with the surrounding environment. The concept was implemented on a two-joint rigid manipulator with a single inflatable link. Experiments on vibration control and contact detection of the inflatable link are reported where the control system was able to significantly reduce tip oscillations, including when a payload was added to the tip. The robot was also able to make binary detection of contact between the inflatable link and the user, and change its operation mode accordingly.

High-resolution MEMS inertial sensor combining large-displacement buckling behaviour with integrated capacitive readout

Commercially available gravimeters and seismometers can be used for measuring Earth’s acceleration at resolution levels in the order of {mathrm{ng}}/sqrt {mathrm{Hz}} (where g represents earth’s gravity) but they are typically high-cost and bulky. In this work the design of a bulk micromachined MEMS device exploiting non-linear buckling behaviour is described, aiming for {mathrm{ng}}/sqrt {mathrm{Hz}} resolution by maximising mechanical and capacitive sensitivity. High mechanical sensitivity is obtained through low structural stiffness. Near-zero stiffness is achieved through geometric design and large deformation into a region where the mechanism is statically balanced or neutrally stable. Moreover, the device has an integrated capacitive comb transducer and makes use of a high-resolution impedance readout ASIC. The sensitivity from displacement to a change in capacitance was maximised within the design and process boundaries given, by making use of a trench isolation technique and exploiting the large-displacement behaviour of the device. The measurement results demonstrate that the resonance frequency can be tuned from 8.7 Hz–18.7 Hz, depending on the process parameters and the tilt of the device. In this system, which combines an integrated capacitive transducer with a sensitivity of 2.55 aF/nm and an impedance readout chip, the theoretically achievable system resolution equals 17.02 {mathrm{ng}}/sqrt {mathrm{Hz}}. The small size of the device and the use of integrated readout electronics allow for a wide range of practical applications for data collection aimed at the internet of things.

Effect of structural stiffness on impact-sliding wear behavior of aluminium alloy

Impact-sliding wear is a typical form of composite wear and occurs in many industrial equipment. The effects of structural stiffness on the impact-sliding wear damage and mechanisms of aluminum alloy were investigated using a bespoke impact-sliding device. The damage evolution and mechanisms of the impact-sliding wear for three different structural stiffness conditions were analyzed. Results showed that the main damage zone gradually shifted along the sliding direction with the increase in the structural stiffness. The wear damage of the material was higher for the low and high structural stiffness conditions and resulted in a very rough surface. However, for the medium structural stiffness, the impact-sliding wear exhibited a slow and linear increase and relatively smooth wear scars were observed. Moreover, the main wear mechanism transformed from delamination to ploughing as the structural stiffness increased.

A methodology for design and fatigue analysis of power cables for wave energy converters

The recent development of subsea power cables for various offshore marine renewable energy technologies has identified the need for new cables that have low structural stiffness properties. This type of cable is referred to as dynamic cable because of its high bending flexibility compared to static cables. The current study presents a cable design model and simulation models that were developed for the design and fatigue analysis of dynamic cables. These models were applied on a subsea dynamic power cable with a design that is suitable for a floating point-absorbing wave energy converter (WEC), where the cable must withstand cyclic loads imposed by the motions of the WEC, the waves and the ocean currents. The cable design model is presented with its detailed design and dimensioning methodology for cables with multiorder helical structures, with respect to desired (target) mechanical properties. The cable design model is verified against a verification study in the literature. A simulation model of a fatigue test rig for accelerated rotational bending is presented. The results from the numerical simulations and the subsequent fatigue analyses are compared against results from experiments using the test rig. The influence of the dynamic effects and mechanical properties on the fatigue life of the cable is discussed. This study contributes to a better understanding of the fatigue failure mechanisms of the cable, and it also highlights the importance of further development of numerical models.

Predicting Path Inaccuracies in Robot-based Machining Operations Using Inverse Kinematics

Machine tools are dominating the machining sector because of their high reliability in process accuracy. On the other hand, industrial robots provide an opportunity for use in applications with fewer process requirements. They offer a high cost-benefit ratio in comparison to conventional machine tools, due to their low investment costs and high flexibility. Low absolute accuracy, low structural stiffness as well as missing consistent process planning tools – designed for industrial robots – prevent an industrial breakthrough. In particular, process planning tools provide a lot of potential for compensating process inaccuracies. This paper presents an approach for predicting path inaccuracies of machining operations using industrial robots. Using the inverse kinematics, rotatory axes’ progressions are calculated for predicting critical switching movements that could lead to high path deviations. On this basis, process parameters like the clamping position or motion strategies will be optimized in order to reduce the number of switching points.