Natural frequency analysis and experiment for 3SPS+1PS parallel hip joint manipulator based on rigid-flexible coupling theory

Songtao Wang,Gang Cheng,Xihui Chen,Jianhua Yang 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.3

For analyzing natural frequency of a 3SPS+1PS parallel hip joint manipulator, a rigid-flexible coupling theory is proposed. 3SPS denotes that the PHJM has three legs and each leg type is SPS. 1PS denotes that the PHJM has one constrained leg and the constrained leg type is PS. The rigid finite element method and flexible finite element method are the base of the rigid-flexible coupling theory. Firstly, the basic element models of the PHJM are established based on the rigid-flexible coupling analysis. Secondly, the elastic dynamic models of the flexible legs are established based on the Lagrange equation and flexible finite method. Finally, the rigid-flexible coupling model of the PHJM is assembled by building the relationship between the rigid and flexible elements. Based on the rigid-flexible model, the natural frequency of the PHJM is analyzed. In addition, the effect parameters of the natural frequency are analyzed. The analysis results of the natural frequency show that the natural frequency is symmetric distribution in the working space, and the minimum value of the 1 st order natural frequency is 2.3 Hz. The operating frequency (1 Hz) is much lower than the 1 st order natural frequency, so the PHJM will not generate resonance and can operate stably. The analysis results of the parameter show that the PHJM has the minimal sensitivity with the effect parameter E and has the maximal sensitivity with the effect parameter H, so the length of the middle leg has the biggest impact on the natural frequency. Natural frequency analysis results have been checked by the hammer test, which verifies the feasibility of the rigid-flexible coupling theory. At the same time, the regression analysis is carried out based on the hammer test results. Using the rigid-flexible coupling theory to solve the natural frequency of the parallel manipulator, the physical meaning is explicit and the modeling process is clear, so the rigid-flexible coupling theory is a universal method.

Frequency error based identification of cracks in beam-like structures

Wenguang Liu,Mark E. Barkey 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.10

A crack identification method of a single edge cracked beam-like structure by the use of a frequency error function is presented in this paper. First, the dynamic theory of Euler-Bernoulli beams was employed to derive the equation of the natural frequency for a single edge cracked cantilever beam-like structure. Subsequently, the cracked section of the beam was simulated by a torsional spring. The flexibility model of the torsional spring due to the crack was estimated by fracture mechanics and energy theory. Thereafter, a function model was proposed for crack identification by using the error between the measured natural frequencies and the predicted natural frequencies. In this manner, the crack depth and crack position can be determined when the total error reaches a minimum value. Finally, the accuracy of the natural frequency equation and the viabilty of the crack identification method were verified in the case studies by the measured natural frequencies from the literature. Results indicate that the first two predicted natural frequencies are in good agreement with the measured ones. However, the third predicted natural frequency is smaller than the measured natural frequency. In the case of small measured frequency errors, the predicted crack parameters are in good agreement with the measured crack parameters. However, in the case of large measured frequency errors, the predicted crack parameters only give roughly estimated results.

콘크리트 댐체의 고유진동수 측정과 변화거동

홍웅기(Woong-Gi Hong),이광재(Kwang-Jae Lee),진영문(Young-Moon Jin),송정언(Jeong-Un Song) 한국엔터테인먼트산업학회 2015 한국엔터테인먼트산업학회논문지 Vol.9 No.2

본 연구는 환경공해진동을 평가하기 위한 기초자료를 얻기 위하여 수행되었다. 콘크리트 댐체에서 고유진동수를 측정하여 다음과 같은 결론을 얻었다. 댐체의 양 끝단에서 저주파수 대역으로 측정되었고, 특히 E 측점과 J 측점 사이에서 주파수 대역의 편차가 크고 랜덤하게 나타났다. 이는 바람의 영향과 수압, 댐체 중앙 부근으로 갈수록 고유진동수가 다른 측점보다 더욱 크게 나타나는 특성 때문이라 판단된다. 변위가속도 또한 고유진동수와 유사한 경향을 보였다. 외부에서 전달되는 외력의 강제진동수는 댐체의 고유진동수 영역보다 항상 작은 값으로 산출되었으며, 주파수 대역은 대부분 충격성 저주파영역을 나타냈다. 이러한 전달 과정에서 매질자체가 갖는 고유특성이나 수진점의 고유진동수, 또는 실시간 주파수분석 등을 고려한 환경공해진동의 정량적인 진동영향평가 기법이 필요하며, 아울러 고유진동수를 고려한 예측모델을 구성하기 위하여 구조체의 기초물성 및 기여도를 분석하고, 계측 데이터의 오차분석, 표준편차 및 보정에 의한 모델의 타당성 평가와 검증 과정이 수행될 때 환경공해진동의 정량적인 평가가 확립될 수 있을 것으로 사료된다. The purpose of this study is to obtain basic data for assessment the vibration as one of the environmental pollution. Natural frequencies in free field were measured as follows: At both ends of the dam crest, the natural frequency was measured with a relatively low frequency. Particularly, the deviation of frequency domain at E and J M.Ps were large and determined randomly. Because toward the central part of the dam crest, the effects of wind, water pressure, and natural frequencies are more large compared to the other M.Ps. Additionally, acceleration values were also measured similar to the trend of natural frequency. Vibration impact assessment that considered the natural characteristics of the medium in the delivery process, natural frequency of vibration receptor and the real-time frequency analysis reached the point of need. This is intended to identify vibration amplification by the resonance of frequency and additional vibration energy. And in order to configure the prediction model which considering the characteristics of frequency, the basic physical properties and contributiveness to the general structure must be analyzed. Feasibility assessment and verification of the model using standard deviation, correction and error analysis of measurement data must be carried out. As a result, the quantitative assessment of the vibration as one of the environmental pollution can be expected to be established.

Dynamic Analysis on the Effect of Pavement’s Natural Frequency to Pavement Performance Using Finite Element Modelling

Julius Marvin Flores,Carlo Elipse,Hyun Jong Lee,Kideok Kim 한국도로학회 2018 한국도로학회 학술대회 발표논문 초록집 Vol.2018 No.05

Pavement performance usually depends on the pavement’s material property, traffic and environmental conditions. Current pavement design programs such as the Mechanistic Empirical Pavement Design Guide use these factors in assessing the pavement life and performance in terms of different distresses like rutting and fatigue cracking. Theoretically, the cracking and rutting behaviour of pavements are based on accumulated strains experienced by the pavement which is brought by the weight and loading speed of vehicles. A steady state loading device was used in the field to evaluate pavement deflection’s behaviour in varying loading frequencies. It was observed that the pavement deflection increases as the loading frequency also increases until it approaches a certain frequency wherein the deflection decreases thereafter. In this study, a three-dimensional finite element pavement model was established using ABAQUS wherein the effect of the vehicle’s loading frequencies was analysed. The calculated static deflection and stress from the finite element (FE) model were found to have good correlation with the KENPAVE measured deflection and stress. The deflections of different pavement conditions were further studied and analysed by generating several pavement geometries and strength from the FE model using a frequency sweep response analysis. It was found that the geometric condition and the current modulus of the pavement can amplify the pavement deflection by a factor, β, depending on the loading frequency. The peak deflection was found to be occurring when the loading frequency approaches one of the pavement’s natural frequencies. Based on the finding from this study, the natural frequency is an important factor to be considered in designing pavements. Further study is recommended to understand more on how to minimize the effect of natural frequency to pavement life.

Effect of prestressing on the first flexural natural frequency of beams

Jaiswal, O.R. Techno-Press 2008 Structural Engineering and Mechanics, An Int'l Jou Vol.28 No.5

In this paper the effect of prestressing force on the first flexural natural frequency of beams is studied. Finite element technique is used to model the beam-tendon system, and the prestressing force is applied in the form of initial tension in the tendon. It is shown that the effect of prestressing force on the first natural frequency depends on bonded and unbonded nature of the tendon, and also on the eccentricity of tendon. For the beams with bonded tendon, the prestressing force does not have any appreciable effect on the first flexural natural frequency. However, for the beams with unbonded tendon, the first natural frequency significantly changes with the prestressing force and eccentricity of the tendon. If the eccentricity of tendon is small, then the first natural frequency decreases with the prestressing force and if the eccentricity is large, then the first flexural natural frequency increases with the prestressing force. Results of the present study clearly indicate that the first natural frequency can not be used as an easy indicator for detecting the loss of prestressing force, as has been attempted in some of the past studies.

Effect of prestressing on the first flexural natural frequency of beams

O. R. Jaiswal 국제구조공학회 2008 Structural Engineering and Mechanics, An Int'l Jou Vol.28 No.5

In this paper the effect of prestressing force on the first flexural natural frequency of beams is studied. Finite element technique is used to model the beam-tendon system, and the prestressing force is applied in the form of initial tension in the tendon. It is shown that the effect of prestressing force on the first natural frequency depends on bonded and unbonded nature of the tendon, and also on the eccentricity of tendon. For the beams with bonded tendon, the prestressing force does not have any appreciable effect on the first flexural natural frequency. However, for the beams with unbonded tendon, the first natural frequency significantly changes with the prestressing force and eccentricity of the tendon. If the eccentricity of tendon is small, then the first natural frequency decreases with the prestressing force and if the eccentricity is large, then the first flexural natural frequency increases with the prestressing force. Results of the present study clearly indicate that the first natural frequency can not be used as an easy indicator for detecting the loss of prestressing force, as has been attempted in some of the past studies.

공진회피를 위한 철도차량의 고유진동수 해석 및 측정에 관한 연구

홍도관(Hong, Do-Kwan),정재부(Jeong, Jae-Boo),정승욱(Jung, Seung-Wook),김경배(Kim, Gyeong-Bae),안찬우(Ahn, Chan-Woo) 한국소음진동공학회 2012 한국소음진동공학회 논문집 Vol.22 No.8

This paper deals with the natural frequency analysis and two experiments to evaluate first twisting and bending natural frequency of railway vehicle. The KS R 9228 testing method is generally performed as pseudo FRF(frequency response function) which is widely used by two accelerometers. The exciting method is utilized using the load weight(1 ton release). The modal testing is used to verify KS R 9228 testing result and the natural frequency analysis result. The first twisting and bending natural frequency should be above 10 Hz by resonance which is mostly generated between bogie and vehicle frame exciting low frequency. The first twisting and bending natural frequency of railway vehicle are successfully verified between analysis and test.

Steering Wheel Core 진동해석을 통한 최적화 설계에 대한 고찰

채천우(Chunwoo Chae),박동진(Dongjin Park),홍찬호(Chanho Hong),남병찬(ByungChan Nam) 한국자동차공학회 2011 한국자동차공학회 학술대회 및 전시회 Vol.2011 No.11

With rapid change of automotive market, multi-function and complexity in limited shape, cost reduction and weight lightening become more and more popular topic in these days. So it is necessary to study on design optimization. Steering Wheel is one of the parts that drivers touch very often in a car and its main function is steering. Especially, steering wheel vibration and noise becomes one of important quality evaluation criterion. Steering Wheel needs to meet basic strength, resonance by low natural frequency, natural frequency that meets more than optimum level and avoids fatigue fracture. Though steering wheel weight will be increased and natural frequency will be decreased relatively, designers often add rib shape on weak part for strength reinforcement and optimum natural frequency. So those methods do not work effectively. This study focus on the method that improves both of strength and natural frequency by change of geometric moment of inertia through changing cross section shape of spoke (2~4 spoke) and keeping mass at same time.Model of steering wheel core for study was simplified and every model’s rim and base keep its own shape and changed only cross section shape of spoke. Natural frequency of steering wheel core was checked by Nastran FX that used as study solver (Mesh size: 2㎜, node qty: 390,000 ~ 430,000EA, element qty 240,000 ~ 250,000). The study was conducted at same condition of armature mass by every spoke quantity. It is possible to confirm the correlation between cross section size change and its natural frequency of steering wheel core through natural frequency study and it is also possible to apply cross section size change on stress concentrated point occurred for strength reinforcement. As result, it can be judged that cross section change in same mass works on strength reinforcement, high natural frequency and cost reduction.

Workspace and stability analysis of a 6-DOF cable-driven parallel robot using frequency-based variable constraints

허준무,박방주,박종오,김창세,정진우,박경수 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.3

Because of lots of advantages such as a large workspace, high dynamics and high payload capacity, CDPRs have been widely used in various applications. In this study, kinematics and dynamics of CDPRs was derived for considering dynamic characteristics of CDPRs. This research considered two effective excitation frequencies which are the vibration induced by frame structural characteristics and acceleration profiles. Also, two main frequencies were considered as the variable constraints. They are the fundamental frequencies of each cable and the whole system. By considering them, the wrench-feasible workspace was determined and analyzed at conditions of various accelerations. In comparing the fixed and the frequency-based variable constraints, the size of the workspace was decreased slightly. However, the frequency-based variable constraint method raised stiffness of cables and end-effector. The stability of cables and end-effector was raised. Especially, the stability of cables was highly improved. Finally, the variable constraints method was verified with experiments. This method leads to high stability system.

강재 풍력 터빈 타워의 고유진동수 특성 분석

이윤우,최준호,장민서,강성용,강영종 한국복합신소재구조학회 2014 복합신소재구조학회 학술발표회 Vol.2014 No.04

Natural frequency characteristic of Wind turbine tower is important for designing of tower due to guarantee of structural safety of tower. In GL specification, natural frequency of tower should be designed by consideration of blade rotational frequency. Natural frequency characteristic of tower could be changed by mass ratio of RNA-tower, modeling method of blade and angle of blade in idling condition. In this research, natural frequency of tower is analysed by ABAQUS and compared it result according to tower dimension.