Joint Stiffness Parameters Research Articles

Uncertainty in Distinct Element Modeling of Freestanding Structures Considering Stiffness Parameters

ABSTRACT Numerical studies on freestanding structures typically leverage the Distinct Element Method where the response is dictated by joints; however, the response carries significant uncertainty due to joint stiffness parameters. This study aims to quantify the influence of joint stiffness parameters on the global response of freestanding structures subjected to seismic excitation in a probabilistic formulation. By using a range of joint stiffness values with rectangular rigid blocks of different aspect ratios, the impact of joint stiffness parameters is evaluated against a large suite of historical ground motions. Results indicate that increasing contact stiffness results in increased stability against overturning.

Two Parameter Model for Joint Stiffness Identification in a Single Lap Jointed Structural Assemblies Using Sub-Structure Synthesis

Many mechanical structures are composed of substructures connected together by joints such as bolts, welds, rivets etc. The added flexibility introduced by the joint to the structure has significant effect on the static and dynamic behavior of the structure and must be accurately modeled. This fact clearly indicates that the analyses will not properly model the dynamic behavior of the structure if the joints cannot be accurately identified. This has prompted the researchers to include the effect of joint stiffness in the structural model and to estimate the stiffness parameters using inverse dynamics. In the present work, single lap joint has been modeled as a pair of translational and rotational springs and frequency equation of the overall system has been developed using sub-structure synthesis. The validity of the proposed method is demonstrated; only first few natural frequencies of the composite system is required to estimate unknown joint stiffness parameters.

Influence of joint flexibility on buckling analysis of free–free beams

Abstract In this work, an application of two noded beam finite element methodology, which is demonstrated in the previous research work for vibration analysis of beam with a flexible joint problem, has been further extended here to investigate the buckling behaviour of free–free beam subjected to an in-plane compressive load. Joint is modelled as rotational spring, where the rotational spring stiffness governs the behaviour of the flexible joint. Variation of first five non-dimensional buckling loads of free–free beam with reference to the joint location as well as joint stiffness parameters are briefly presented. It is understood that looseness of the joint can significantly influence the buckling behaviour of free–free beam and plays an important role in accurately determining the buckling behaviour of jointed beams subjected to an in-plane compressive loads.

Seismic vulnerability assessment of free-standing massive masonry columns by the 3D Discrete Element Method

Free-standing masonry column is a recurrent typology of built cultural heritage. Usually raised without seismic design, columns are subject to intense rocking and overturning under strong ground motions. In this paper, a strategy to assess their seismic vulnerability using the 3-D Discrete Element Method is proposed. It includes calibrating the elastic parameters (joint stiffness parameters) using ambient vibration test data and developing fragility curves from a large set of time history analysis results. This procedure was verified for four selected water towers, a representative archaeological monument in Pompeii, Italy. Based on preliminary structural analyses, the water towers were modeled as rigid monolithic blocks. The outcomes of modal analysis were used to calibrate the input elastic parameters, starting from a range of values given by analytical calculations derived by literature. Artificial earthquakes with peak ground acceleration levels ranging from 0.167 g to 0.803 g were implemented as ground motions. Collapses and non-collapses configurations were subdivided through a threshold concerning the maximum allowable rotation of the blocks. Fragility curves were statistically developed; these revealed the overturning probabilities of the investigated structures and also enabled to conduct a parametric study of the Rayleigh damping constants.

Simultaneous identification of joint stiffness, kinematic and hand-eye parameters of measurement system integrated with serial robot and 3D camera

Purpose This paper aims to propose a measurement principle and a calibration method of measurement system integrated with serial robot and 3D camera to identify its parameters conveniently and achieve high measurement accuracy. Design/methodology/approach A stiffness and kinematic measurement principle of the integrated system is proposed, which considers the influence of robot weight and load weight on measurement accuracy. Then an error model is derived based on the principle that the coordinate of sphere center is invariant, which can simultaneously identify the parameters of joint stiffness, kinematic and hand-eye relationship. Further, considering the errors of the parameters to be calibrated and the measurement error of 3D camera, a method to generate calibration observation data is proposed to validate both calibration accuracy and parameter identification accuracy of calibration method. Findings Comparative simulations and experiments of conventional kinematic calibration method and the stiffness and kinematic calibration method proposed in this paper are conducted. The results of the simulations show that the proposed method is more accurate, and the identified values of angle parameters in modified Denavit and Hartenberg model are closer to their real values. Compared with the conventional calibration method in experiments, the proposed method decreases the maximum and mean errors by 19.9% and 13.4%, respectively. Originality/value A new measurement principle and a novel calibration method are proposed. The proposed method can simultaneously identify joint stiffness, kinematic and hand-eye parameters and obtain not only higher measurement accuracy but also higher parameter identification accuracy, which is suitable for on-site calibration.

Exploration of swimming performance for a biomimetic multi-joint robotic fish with a compliant passive joint

In this paper, a novel compliant joint with two identical torsion springs is proposed for a biomimetic multi-joint robotic fish, which enables imitatation of the swimming behavior of live fish. More importantly, a dynamic model based on the Lagrangian dynamic method is developed to explore the compliant passive mechanism. In the dynamic modeling, a simplified Morrison equation is utilized to analyze the hydrodynamic forces. Further, the parameter identification technique is employed to estimate numerous hydrodynamic parameters. The extensive experimental data with different situations match well with the simulation results, which verifies the effectiveness of the obtained dynamic model. Finally, motivated by the requirement for performance optimization, we firstly take advantage of a dynamic model to investigate the effect of joint stiffness and control parameters on the swimming speed and energy efficiency of a biomimetic multi-joint robotic fish. The results reveal that phase difference plays a primary role in improving efficiency and the compliant joint presents a more significant role in performance improvement when a smaller phase difference is given. Namely, at the largest actuation frequency, the maximum improvement of energy efficiency is obtained and surprisingly approximates 89%. Additionally, the maximum improvement in maximum swimming speed is about 0.19 body lengths per second. These findings demonstrate the potential of compliance in optimizing joint design and locomotion control for better performance.

New Method and Portable Measurement Device for the Calibration of Industrial Robots

This paper presents an automated calibration method for industrial robots, based on the use of (1) a novel, low-cost, wireless, 3D measuring device mounted on the robot end-effector and (2) a portable 3D ball artifact fixed with respect to the robot base. The new device, called TriCal, is essentially a fixture holding three digital indicators (plunger style), the axes of which are orthogonal and intersect at one point, considered to be the robot tool center point (TCP). The artifact contains four 1-inch datum balls, each mounted on a stem, with precisely known relative positions measured on a Coordinate Measuring Machine (CMM). The measurement procedure with the TriCal is fully automated and consists of the robot moving its end-effector in such as a way as to perfectly align its TCP with the center of each of the four datum balls, with multiple end-effector orientations. The calibration method and hardware were tested on a six-axis industrial robot (KUKA KR6 R700 sixx). The calibration model included all kinematic and joint stiffness parameters, which were identified using the least-squares method. The efficiency of the new calibration system was validated by measuring the accuracy of the robot after calibration in 500 nearly random end-effector poses using a laser tracker. The same validation was performed after the robot was calibrated using measurements from the laser tracker only. Results show that both measurement methods lead to similar accuracy improvements, with the TriCal yielding maximum position errors of 0.624 mm and mean position errors of 0.326 mm.

A new control method of flexible-joint manipulator with harmonic drive

Considerable elasticity and nonlinear friction caused by harmonic transmission challenge the performance of flexible-joint manipulators. The uncertain dynamics of manipulator and the inadequate measurable states also limit the controller design. A new control method is proposed to address these problems, achieving the precise motion control of the flexible-joint manipulator. The method consists of three cascaded controllers: an adaptive controller, a torque-tracking controller, and a motor controller. The adaptive controller was adopted to generate the desired torque ensuring the robustness for uncertain dynamics. The torque-tracking controller derived the position compensation for motor control according to the torque error. As the elastic torque is under control, the vibration caused by harmonic drive can be eliminated. The motor was controlled based on poles-assignment method and friction compensation. The Kalman observer based on the Brownian motion model observed both velocity and the high-order derivatives of torque sensing. The stability of the control method was strictly proved. Calibration was performed on each joint to obtain the required joint stiffness and motor friction parameters. The control method was verified on a single joint and the frequency response of the system was obtained. The results show that the controller has good performance. The controller was realized on the self-developed seven-degree-of-freedom manipulator. The results reveal that the controller has high-precision tracking performance.

Влияние осевой деформативности стыков на результаты статического расчета несущей системы УИКСС

The article is devoted to the problem of static calculation of the UIKSS system (universal industrial frame construction system), which is a precast reinforced concrete frame with girder-free non-accumulating floors, taking into account the axial deformability of plug joints between columns and slabs. The principle of modeling the frame in the LIRA-SAPR software package is considered and illustrated with an example, taking into account the joint stiffness parameters determined by the author’s method developed on the basis of numerical and experimental studies. The comparison and analysis of the obtained forces and displacements with and without taking into account the deformability of plug joints of columns with floor slabs is presented.

The parameter identification model considering both geometric parameters and joint stiffness

PurposeBoth geometric and non-geometric parameters have noticeable influence on the absolute positional accuracy of 6-dof articulated industrial robot. This paper aims to enhance it and improve the applicability in the field of flexible assembling processing and parts fabrication by developing a more practical parameter identification model.Design/methodology/approachThe model is developed by considering both geometric parameters and joint stiffness; geometric parameters contain 27 parameters and the parallelism problem between axes 2 and 3 is involved by introducing a new parameter. The joint stiffness, as the non-geometric parameter considered in this paper, is considered by regarding the industrial robot as a rigid linkage and flexible joint model and adds six parameters. The model is formulated as the form of error via linearization.FindingsThe performance of the proposed model is validated by an experiment which is developed on KUKA KR500-3 robot. An experiment is implemented by measuring 20 positions in the work space of this robot, obtaining least-square solution of measured positions by the software MATLAB and comparing the result with the solution without considering joint stiffness. It illustrates that the identification model considering both joint stiffness and geometric parameters can modify the theoretical position of robots more accurately, where the error is within 0.5 mm in this case, and the volatility is also reduced.Originality/valueA new parameter identification model is proposed and verified. According to the experimental result, the absolute positional accuracy can be remarkably enhanced and the stability of the results can be improved, which provide more accurate parameter identification for calibration and further application.

Visual perception of joint stiffness from multijoint motion.

Humans have an astonishing ability to extract hidden information from the movements of others. For example, even with limited kinematic information, humans can distinguish between biological and nonbiological motion, identify the age and gender of a human demonstrator, and recognize what action a human demonstrator is performing. It is unknown, however, whether they can also estimate hidden mechanical properties of another's limbs simply by observing their motions. Strictly speaking, identifying an object's mechanical properties, such as stiffness, requires contact. With only motion information, unambiguous measurements of stiffness are fundamentally impossible, since the same limb motion can be generated with an infinite number of stiffness values. However, we show that humans can readily estimate the stiffness of a simulated limb from its motion. In three experiments, we found that participants linearly increased their rating of arm stiffness as joint stiffness parameters in the arm controller increased. This was remarkable since there was no physical contact with the simulated limb. Moreover, participants had no explicit knowledge of how the simulated arm was controlled. To successfully map nontrivial changes in multijoint motion to changes in arm stiffness, participants likely drew on prior knowledge of human neuromotor control. Having an internal representation consistent with the behavior of the controller used to drive the simulated arm implies that this control policy competently captures key features of veridical biological control. Finding that humans can extract latent features of neuromotor control from kinematics also provides new insight into how humans interpret the motor actions of others. NEW & NOTEWORTHY Humans can visually perceive another's overt motion, but it is unknown whether they can also perceive the hidden dynamic properties of another's limbs from their motions. Here, we show that humans can correctly infer changes in limb stiffness from nontrivial changes in multijoint limb motion without force information or explicit knowledge of the underlying limb controller. Our findings suggest that humans presume others control motor behavior in such a way that limb stiffness influences motion.

Joint Stiffness Identification and Deformation Compensation of Serial Robots Based on Dual Quaternion Algebra

As the application of industrial robots is limited by low stiffness that causes low precision, a joint stiffness identification algorithm for serial robots is presented. In addition, a deformation compensation algorithm is proposed for the accuracy improvement. Both of these algorithms are formulated by dual quaternion algebra, which offers a compact, efficient, and singularity-free way in robot analysis. The joint stiffness identification algorithm is derived from stiffness modeling, which is the combination of the principle of virtual work and dual quaternion algebra. To validate the effectiveness of the proposed identification algorithm and deformation compensation algorithm, an experiment was conducted on a dual arm industrial robot SDA5F. The robot performed a drilling operation during the experiment, and the forces and torques that acted on the end-effector (EE) of both arms were measured in order to apply the deformation compensation algorithm. The results of the experiment show that the proposed identification algorithm is able to identify the joint stiffness parameters of serial industrial robots, and the deformation compensation algorithm can improve the accuracy of the position and orientation of the EE. Furthermore, the performance of the forces and torques that acted on the EE during the operation were improved as well.

Fatigue and fracture characterization of fiberglass grid-reinforced beam specimens using four-point bending notched beam fatigue test and digital image correlation technique

The purpose of this research is to develop an experimental and analytical framework for describing, modeling, and predicting the reflective cracking patterns and crack growth rates in GlasGrid®-reinforced asphalt pavements. In order to fulfill this objective, the effects of different interfacial conditions (mixture type, tack coat type, and grid opening size) on reflective cracking-related failure mechanisms and the fatigue and fracture characteristics of fiberglass grid-reinforced asphalt concrete beams were studied by means of four-point bending notched beam fatigue tests (NBFTs). Also, the digital image correlation technique was utilized to determine the lengths of the interfacial and vertical cracks during each test. The stress intensity factors (SIFs) at the crack tips were backcalculated using a finite element model and applying linear elastic fracture mechanic principles to the crack lengths. The local effect of the reinforcement on the stiffness of the system at a vertical crack-interface intersection or the resistance of the grid system to the deflection differential at the joint/crack (joint stiffness) for GlasGrid®-reinforced asphalt concrete beams was determined by implementing a joint stiffness parameter into the finite element code. The crack length and SIF test results were used to find the Paris’ law parameters for each interlayer condition. According to the NBFT results, GlasGrid® reinforcement retards reflective crack growth by stiffening the composite system and introducing joint stiffness. The results also show that the higher the bond strength and interlayer stiffness values, the greater the joint stiffness and retardation effects.

3-D nonlinear behavior of an obelisk subjected to the Lorca May 11, 2011 strong motion record

A series of very shallow earthquakes occurred in the late afternoon of May 11, 2011 near the city of Lorca and 55km from Murcia in Spain. The main shock (Mw 5.2) occurred at 17:47 (UTC), causing 9 deaths and over 250 injured with widespread damage and panic.The strong motion generated by the event was recorded in the town of Lorca near the epicenter. It was a short duration record with peak ground acceleration higher than 0.37g. In the vicinity of the recorded motion (360m), there is an obelisk composed of 11 frustums resting on a 1-m-high foundation with a total height of 5.71m. The general motion observed after the event was of the order of 1cm (translation) for most interfaces, while at the upper two, the motion was of rotation of 2° to 4° in the counterclockwise direction seen from above.In order to understand the behavior of such a structure, numerical simulations of the obelisk were performed with the discrete element code 3DEC, employing a rigid block model. The deformation is assumed to be concentrated at the joints between the blocks, governed in the linear range by the joint stiffness parameters, in the normal and shear directions. In the nonlinear range, a purely frictional behavior is adopted, without tensile strength. An elastic joint was introduced at the base of the model, simulating the deformability of the foundation. The global model deformability was calibrated by means of in situ dynamic characterization.Dynamic analyses were performed, taking into account the large displacement effects that govern the block rocking motion, applying the recorded seismic motion in the 3 directions. Parametric studies were undertaken to assess the influence of the various input parameters. The model results are compared with the observed permanent displacements of the structure.

Gender Differences in Limb and Joint Stiffness during the Fencing Lunge

The aim of the current investigation was to examine gender differences in limb and joint stiffness characteristics during the fencing lunge. Ten male and ten female fencers completed simulated lunge movements. Lower limb kinematics were collected using an eight camera optoelectric motion capture system which operated at 250 Hz. Measures of limb and joint stiffness were calculated as a function of limb length and joint moments divided by the extent of limb and joint excursion. Gender differences in limb joint stiffness parameters were examined statistically using independent samples t-tests. The results showed firstly that both limb (male = 64.22 ±19.12, female = 75.09 ±22.15 N.kg.m) and hip stiffness (male = 10.50 ±6.00, female = 25.89 ±15.01 Nm.kg.rad) were significantly greater in female fencers. In addition it was also demonstrated that knee moment (male = 1.64 ±0.23, female = 2.00 ±0.75 Nm.kg) was significantly larger in females. On the basis of these observations, the findings from the current investigation may provide further insight into the aetiology of the distinct injury patterns observed between genders in relation to fencing.

Surface Texture Measurements of Crack Surface to Establish Joint Shear Stiffness

Pavement design has evolved from a purely empirical approach to a more mechanistic-based approach. One of the primary distresses considered when designing jointed plain concrete pavements is faulting. The development of faulting is largely a function of the load transfer efficiency at the joints. For undoweled joints, the load is primarily transferred through the aggregate interlock provided by the texture resulting from the meander in the transverse crack propagating off from the sawed joint. This paper shows how volumetric surface texture, a measurement of the surface texture at the crack face, can be used for this purpose. A method is presented for estimating the joint stiffness parameter, Jagg, required for the faulting prediction model incorporated into the AASHTOWare Pavement ME Design software based on the measured surface texture. This method allows the effects of aggregate size, type, and quantity to be captured and joint stiffness to be quantified when faulting is being predicted.

Finite Element Model Updating of Machine-Tool Spindle Systems

The unknown joint dynamics are the main obstacle that limits the accuracy of the finite element (FE) model of a machine-tool spindle assembly. In this paper, an FE model updating method is proposed to assist industrial engineers in achieving a reliable model that can accurately represent the dynamic characteristics of machine-tool spindle systems. In the proposed FE model updating procedure, the iterative algorithm based on frequency response functions (FRFs) is applied. The joint stiffness parameters are identified through the iteration process, while the FE model is updated simultaneously. The proposed method was applied to update an existing coupled model of a machine-tool spindle system. The experimental results show that the identified joint stiffness parameters are acceptable and the dynamic behavior of the spindle mounted in the machine tool column is predicted reliably.

Minimum Vibration Mechanism Design Via Convex Programming

One means of designing reduced vibration mechanisms is to ensure that the mechanism’s natural frequency be sufficiently greater than the driving frequencies of the actuators. In this paper we consider the problem of determining a mechanism’s mass, inertial, and joint stiffness parameters so as to maximize the lowest natural frequency of the mechanism. We show that this leads to a convex programming problem, which is characterized by a global optima that can be found with efficient interior point algorithms. Several case studies involving open and closed chain mechanisms demonstrate the viability of the design methodology.

위치 정밀도 향상을 위한 관절강성 파라미터 포함 로봇 캘리브레이션

This paper presents a new robot calibration algorithm with joint stiffness parameters for the enhanced positioning accuracy of industrial robot manipulators. This work is towards on-going development of an industrial robot calibration software which is able to identify both the kinematic and non-kinematic robot parameters. In this paper, the conventional kinematic calibration and its important considerations are briefly described first. Then, a new robot calibration algorithm which simultaneously identifies both the kinematic and joint stiffness parameters is presented and explained through a computer simulation with a 2 DOF manipulator. Finally, the developed algorithm is implemented to Hyundai HX165 robot and its resulting improvement of the positioning accuracy is addressed.

Parametric Equations Using Generic Representation of Joint Stiffness

For most design and recertification work on tubular structures, it is not practical to determine the joint hot spot stresses experimentally or using detailed finite element analysis. Therefore, parametric equations were developed in the past for various joint geometries to relate stress concentration factors around the joints to basic joint geometrical parameters. Such parametric equations have limited applications to reinforced joints because the nature of reinforcement varies and may be difficult to represent by a generic set of geometric parameters. In this paper, a new method is introduced to include local joint stiffness in the parametric equations represented by a generic characteristic of the joint determined by modal analysis. The parametric equations produced in this paper can be applied to any reinforced T-joint regardless of the nature of reinforcement. This is especially useful in those cases where the exact nature of reinforcement is not known, for example, hidden in the interior or deteriorated through age or fabrication error. The dimensionless joint stiffness parameter can be calculated by a simple modal test and a beam model without needing to know the nature and details of the reinforcement.