• Journal of the Korean Society for Precision Engineering
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• v.34 no.2
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• pp.115-123
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• 2017

This paper presents the design and the control performance of a novel dynamic compliant-arm support with parallel elastic actuators that was developed to assist with the daily living activities of those whose arms are compromised by muscular disease or the aging process. The parallel elastic-arm support consists of a compliant mechanism with combined passive and active components for human interaction and to reach the user's desired positions. The achievement of these tasks requires impedance control, which can change the virtual stiffness, damping coefficients, and equilibrium points of the system; however, the desired-position tracking by the impedance control is limited when the end-effector weight varies according to the equipping of diverse objects. A prompt algorithm regarding weight calibration and friction compensation is adopted to overcome this problem. A result comparison shows that, by accurately assessing the desired workspace, the proposed algorithm is more effective for the accomplishment of the desired activities.

• Advances in aircraft and spacecraft science
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• v.10 no.3
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• pp.257-279
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• 2023

This manuscript is dedicated to deriving the closed form solutions of free vibration of viscoelastic nanobeam embedded in an elastic medium using nonlocal differential Eringen elasticity theory that not considered before. The kinematic displacements of Euler-Bernoulli and Timoshenko theories are developed to consider the thin nanobeam structure (i.e., zero shear strain/stress) and moderated thick nanobeam (with constant shear strain/stress). To consider the internal damping viscoelastic effect of the structure, Kelvin/Voigt constitutive relation is proposed. The perforation geometry is intended by uniform symmetric squared holes arranged array with equal space. The partial differential equations of motion and boundary conditions of viscoelastic perforated nonlocal nanobeam with elastic foundation are derived by Hamilton principle. Closed form solutions of damped and natural frequencies are evaluated explicitly and verified with prestigious studies. Parametric studies are performed to signify the impact of elastic foundation parameters, viscoelastic coefficients, nanoscale, supporting boundary conditions, and perforation geometry on the dynamic behavior. The closed form solutions can be implemented in the analysis of viscoelastic NEMS/MEMS with perforations and embedded in elastic medium.

• Ocean Systems Engineering
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• v.13 no.4
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• pp.401-421
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• 2023

Reducing hawser line tensions and dynamic responses to a certain level is of paramount importance as the hawser lines provide important structural linkage between 2 body TLP-TAD system. The objective of this paper is to demonstrate how MR Damper can be utilized to achieve this. Hydrodynamic coefficients and wave forces for two bodies including second-order effects are obtained by 3D diffraction/radiation panel program by potential theory. Then, multi-hull-riser-mooring-hawser fully-coupled time-domain dynamic simulation program is applied to solve the complex two-body system's dynamics with the Magneto-Rheological (MR) Damper modeled on one end of hawser. Since the damping level of MR Damper can be changed by inputting different electric currents, various simulations are conducted for various electric currents. The results show the reductions in maximum hawser tensions with MR Damper even for passive control cases. The results also show that the hawser tensions and MR Damper strokes are affected not only by input electric currents but also by initial mooring design. Further optimization of hawser design with MR Damper can be done by active MR-Damper control with changing electric currents, which is the subject of the next study.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.17 no.3
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• pp.138-148
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• 2005

Dynamic response of floating structures such as floating body and floating bridges subject to wave load is to be calculated in frequency domain. Added mass coefficient, damping coefficient and wave exciting force are obtained numerically from frequency domain formulation of linear potential theory and boundary element method for a floating body which is partially submerged into water and subjected to wave force. Next, the equation of motion for the dynamic behavior of a floating structure which is supported by the floating bodies and modeled with finite elements is written in frequency domain. hker a hemisphere is analyzed and compared with the published references as examples of floating bodies, the hydrodynamic coefficients for a pontoon type floating body which supports a floating bridge are determined. The dynamic response of the floating bridge subject to design wave load can be solved using the coefficients obtained for the pontoons and the results are plotted in the frequency domain. It can be seen from the example analysis that although the peak frequency of the incoming wave spectrum is near the natural frequency of the bridge, the response of the bridge is not amplified due to the effect that the peak frequency of wave exciting force is away from the natural frequency of the bridge.

• Journal of the Korean Society for Precision Engineering
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• v.28 no.3
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• pp.292-300
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• 2011

Control systems in machinery equipment provide correction signals to motion units in order to reduce or cancel out the mismatches between sensor feedback signals and command or desired values. In this paper, we introduce a simulator for control characteristics of machinery equipment. The purpose of the simulator development is to provide mechanical system designers with the ability to estimate how much dynamic performance can be achieved from their design parameters and selected devices at the designing phase. The simulator has a database for commercial parts, so that the designers can choose appropriate components for servo controllers, motors, motor drives, and guide ways, etc. and then tune governing parameters such as controller gains and friction coefficients. The simulator simulates the closed-loop control system which is built and parameter-tuned by the designer and shows dynamic responses of the control system. The simulator treats the moving table as a 6 degrees-of-freedom rigid body and considers the motion guide blocks stiffness, damping and their locations as well as sensor locations. The simulator has been under development for one and a half years and has a few years to go before the public release. The primary achievements and features will be presented in this paper.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.3
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• pp.855-864
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• 2013

The dynamic characteristics of natural rubber bearings, which are used as isolator, are dependent on the main rubber's dynamic behaviors and nonlinear properties. Rubber materials tend to undergo an aging process under the influence of mechanical or environmental factors, so they inevitably end up facing damage. A main cause of aging like this is known to be oxidization, which occurs through the heat of reaction at high temperatures. Accordingly, in this study an accelerated thermal aging test was carried out in order to compare the characteristic values of the bearings before and after thermal aging occurs. As a result of this experiment, it was found that a thermal aging phenomenon could have some effects on shear stiffness, energy absorption, and equivalent damping coefficients of the bearings. Furthermore, a deterioration in the dynamic properties of the natural rubber bearings caused by the thermal aging was applied to an actual bridge and then the effects of such thermal aging on the seismic performance of the bridge were also compared and analyzed based on numerical analysis. As a result of this analysis, it was found that the changes in the basic properties of the natural rubber bearings caused by the thermal aging bring only a minor effect on the seismic performance of bridges.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.32 no.2A
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• pp.109-116
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• 2012

The dynamic properties of lead rubber bearings, which are used as isolator, are dependent on the main rubber's dynamic behaviors and nonlinear qualities. Rubber materials tend to undergo an aging process under the influence of mechanical or environmental factors, so they can end up inevitably facing damage. A main cause of such aging is known to be oxidization, which occurs through the heat of reaction at high temperatures. Accordingly, in this study an accelerated thermal aging test was carried out in order to compare the characteristic values of the bearings with each other before and after thermal aging occurs. As a result of this experiment, it was found that a thermal aging phenomenon could have an effect on shear stiffness, energy absorption, and equivalent damping coefficients. Furthermore, a decline in the dynamic properties of the lead rubber bearings by means of the thermal aging process was applied to an actual bridge and the effects of such thermal aging on the seismic performance of the bridge were also compared and analyzed based on numerical analysis. As a result of this analysis, it was found that the changes in the basic properties of the lead rubber bearings have a minor effect on the seismic performance of bridges.

• Journal of the Earthquake Engineering Society of Korea
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• v.8 no.2
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• pp.27-33
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• 2004

Dynamic response analysis is conducted for a floating bridge subjected to multiple support earthquake excitation. The floating bridge used in this study is supported by discrete floating pontoons and horizontal pretension cables supported at both ends of the bridge. The bridge is modeled with finite elements and the hydrodynamic added mass and added damping due to the surrounding fluid around pontoons are obtained using boundary elements. During the analysis the concept of retardation function is utilized to consider the frequency dependency of the hydrodynamic coefficients. Multiple support excitation is introduced at both ends of the bridge and the time history response is compared to that of a simultaneous excitation. The results show that the multiple support excitation yields larger values in some responses. for example in cable tensions. than the sumultaneous excitation.

• Journal of Ocean Engineering and Technology
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• v.30 no.2
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• pp.84-90
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• 2016

In this study, a six degree-of-freedom motion analysis of a wind-wave hybrid platform equipped with numerous wave energy converters (WECs) was carried out. To examine the effect of the WECs on the platform, an analysis of one-way coupling was carried out, which only considered the power take-off (PTO) damping of the static WECs on the platform. The equation of motion of a floating platform with mooring lines in the time domain was established, and the responses of the one-way coupled platform were then compared with the case of a platform without any coupling effects from the WECs. The hydrodynamic coefficients and wave exciting forces were obtained from the 3D diffraction/radiation pre-processor code WAMIT based on the boundary element method. Then, an analysis of the dynamic responses of the floating platform with or without the WEC effect in the time domain was carried out. All of the dynamics of a floating platform with multiple wind turbines were obtained by coupling FAST and CHARM3D in the time domain, which was further extended to include additional coupled dynamics for multiple turbines. The analysis showed that the PTO damping effect on platform motions was negligible, but coupled effects between multiple WECs and the platform may differentiate the heave, roll, and pitch platform motions from the one without any effects induced by WECs.

• Journal of the Earthquake Engineering Society of Korea
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• v.5 no.4
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• pp.27-38
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• 2001

A mechanical lumped parameter model is proposed for the dynamic modeling of a semi-infinite reservoir. A semi-analytic transmitting boundary is derived for a semi-infinite 2-D reservoir of constant depth. The characteristics of the solution are examined in both frequency and time domains. Mass, damping and spring coefficients of the mechanical model are obtained to preserve the major features of the solution such as eigenfrequencies and the shapes of Bessel functions that appear as kernels in the convolution integrals. The lumped parameter model in its final form consists of two masses, a spring and two dampers for each eigenfrequency. Application examples demonstrated that the new lumped parameter model could be used for the time domain analysis of dam-reservoir systems.