This paper carries out a comprehensive and systematic comparison study on the kinematic performance of four six degrees of freedom(6-DOF)parallel mechanisms with different topologies,i.e.,6-UPS,3-(2-UPR)U,3-(2-UCR)U,a...This paper carries out a comprehensive and systematic comparison study on the kinematic performance of four six degrees of freedom(6-DOF)parallel mechanisms with different topologies,i.e.,6-UPS,3-(2-UPR)U,3-(2-UCR)U,and 3-R(2-RPR)U.The research begins by elaborating in detail the similarities and differences among these four parallel mechanisms.By standardizing the definition of the coordinate system for each mechanism,the inverse kinematics and the Jacobian matrix of these four mechanisms are systematically derived.Employing a set of motion/force transmission indices,which are directly obtained from the Jacobian matrix,the kinematic performances of the four mechanisms are thoroughly analyzed and compared within the given workspaces while maintaining the same dimensional parameters for all cases.The comparison study of these four parallel mechanisms extends beyond local transmission indices to also include global transmission indices,covering both position and orientation workspaces,as well as assessments at both the local and global workspace levels.This comprehensive approach ensures a detailed and fair evaluation of their respective kinematic capabilities.The results indicate that the comprehensive kinematic performances of the four parallel mechanisms are similar,and providing a solid theoretical foundation for innovative design and practical guidance for selecting optimal architectures based on specific application requirements.展开更多
According to the 2017 Summary Conference of China National Textile and Apparel Council(CNTAC)held on January 22nd to 23rd,in the first 11 months of 2017,the overall operation of China’s textile industry showed a stea...According to the 2017 Summary Conference of China National Textile and Apparel Council(CNTAC)held on January 22nd to 23rd,in the first 11 months of 2017,the overall operation of China’s textile industry showed a steady growth rate,stable domestic demand growth。展开更多
Negative step response experimental method is used in wrist force sensor＇s dynamic performance calibration. The exciting manner of negative step response method is the same as wrist force sensor＇s load in working. T...Negative step response experimental method is used in wrist force sensor＇s dynamic performance calibration. The exciting manner of negative step response method is the same as wrist force sensor＇s load in working. This experimental method needn＇t special experiment equipments. Experiment＇s dynamic repeatability is good. So wrist force sensor＇s dynamic performance is suitable to be calibrated by negative step response method. A new correlation wavelet transfer method is studied. By wavelet transfer method, the signal is decomposed into two dimensional spaces of time-frequency. So the problem of negative step exciting energy concentrating in the low frequency band is solved. Correlation wavelet transfer doesn＇t require that wavelet primary function be orthogonal and needn＇t wavelet reconstruction. So analyzing efficiency is high. An experimental bench is designed and manufactured to load the wrist force sensor orthogonal excitation force/moment. A piezoelectric force sensor is used to setup soft trigger and calculate the value of negative step excitation. A wrist force sensor is calibrated. The pulse response function is calculated after negative step excitation and step response have been transformed to positive step excitation and step response. The pulse response function is transferred to frequency response function. The wrist force sensor＇s dynamic characteristics are identified by the frequency response function.展开更多
This paper presents a study on the motion response of a tension-leg platform(TLP) under first-and second-order wave forces, including the mean-drift force, difference and sum-frequency forces. The second-order wave fo...This paper presents a study on the motion response of a tension-leg platform(TLP) under first-and second-order wave forces, including the mean-drift force, difference and sum-frequency forces. The second-order wave force is calculated using the full-field quadratic transfer function(QTF). The coupled effect of the horizontal motions, such as surge, sway and yaw motions, and the set-down motion are taken into consideration by the nonlinear restoring matrix. The time-domain analysis with 50-yr random sea state is performed. A comparison of the results of different case studies is made to assess the influence of second-order wave force on the motions of the platform. The analysis shows that the second-order wave force has a major impact on motions of the TLP. The second-order difference-frequency wave force has an obvious influence on the low-frequency motions of surge and sway, and also will induce a large set-down motion which is an important part of heave motion. Besides, the second-order sum-frequency force will induce a set of high-frequency motions of roll and pitch. However, little influence of second-order wave force is found on the yaw motion.展开更多
This paper presents a novel 1T2R three degrees of freedom redundantly actuated and overconstrained ■parallel machining head(■denotes the active prismatic joint), which can construct 5-axis hybrid machine to complete...This paper presents a novel 1T2R three degrees of freedom redundantly actuated and overconstrained ■parallel machining head(■denotes the active prismatic joint), which can construct 5-axis hybrid machine to complete high speed freedom surface milling for large complex structural components in aerospace. Firstly, based on the screw theory, the mobility of the proposed parallel manipulator is briefly analysed. Secondly, the kinematic inverse position and the parasitic motion of the parallel manipulator are explicitly expressed. Furthermore, motion-force transmission performance evaluation indices are derived in detail via an alternative approach based on the screw theory. More importantly, a simple method for quickly solving the maximum virtual power coefficient is proposed, and the motion-force transmission performance evaluation index is greatly improved. To evaluate the kinematic performance, its workspace is calculated. With numerical examples, performance distribution atlases of the manipulator are depicted visually. The corresponding results illustrate that the proposed parallel manipulator has better orientation workspace and superior motion-force transmission performance than the 2 PRU-PRS parallel manipulator, which proves the validity and applicability of applying this manipulator as a machining head.展开更多
Forced-air convection cooling of high-power electronic devices is widely used, but it has a problem that a rise in temperature of the air used to cool the upstream devices decreases the cooling capa-bility for the dow...Forced-air convection cooling of high-power electronic devices is widely used, but it has a problem that a rise in temperature of the air used to cool the upstream devices decreases the cooling capa-bility for the downstream devices. In this study we made an experimental apparatus including a memory card array and measured the effect of the rise in temperature of the air on the heat transfer coefficient of the memory cards that were downstream in the air flow. Using these mea-surements, we devised a simple calculation model, called the thermal diffusion layer model, to calculate the heat transfer coefficient of multiple rows of memory cards. The rise in temperature of downstream memory cards due to higher temperature air can be evaluated with a parameter representing the delay of thermal mixing for air. The heat transfer coefficient calculated with the thermal diffusion layer model agreed with our experimental results.展开更多
Forced convection cooling of fins on a high-temperature wall has been used to cool high-power electronic devices. We numerically calculated and experimentally measured the forced convection heat transfer coefficient a...Forced convection cooling of fins on a high-temperature wall has been used to cool high-power electronic devices. We numerically calculated and experimentally measured the forced convection heat transfer coefficient and pressure drop of a diamond-shaped fin-array with water flow in this study, which had been reported to have a self-induced flip-flop flow phenomenon. Although the flip-flop flow phenomenon occurred in calculations, it was not observed in experiments. The heat transfer and pressure drop of the diamond-shaped fin-array could be estimated with equations for turbulent flow in tubes.展开更多
Numerous types of floating breakwaters have been proposed,tested and commercialized in the past decades.The majority of these breakwaters are made of solid bodies;hence,they are relatively bulky and are not readily to...Numerous types of floating breakwaters have been proposed,tested and commercialized in the past decades.The majority of these breakwaters are made of solid bodies;hence,they are relatively bulky and are not readily to be rapidly installed at the targeted sites when immediate wave protection of the coastal and offshore facilities is needed.Furthermore,the application of these hard floating structures at the recreational beaches is rather unlikely due to potential deadly marine traffic collision.To overcome these problems,a flexible air-filled wave attenuator(AFWA)has been developed in the present study.This floating breakwater is made of flexible waterproof membrane materials.The main body consists of a rectangular air-filled prism and is ballasted by sandbags located around the floating module.The objective of this study is to evaluate the wave transmission,wave reflection,energy dissipation,motion responses and mooring forces of the AFWA under the random wave actions using physical modelling.The test model located in a 20 m long wave flume was subjected to a range of wave heights and periods.The wave profiles in the vicinity of the test model were measured using wave probes for determination of wave transmission,reflection and energy loss coefficients.The motion responses in terms of heave,surge and pitch,and wave forces acting on the mooring lines were measured using a motion tracking system and load cells,respectively.The experimental results reveal that the AFWA is effective in attenuating up to 95%in the incoming wave height and has low-wave-reflection properties,which is commendable for floating breakwaters.展开更多
基金Supported by National Natural Science Foundation of China(Grant Nos.52325501,U24B2047).
文摘This paper carries out a comprehensive and systematic comparison study on the kinematic performance of four six degrees of freedom(6-DOF)parallel mechanisms with different topologies,i.e.,6-UPS,3-(2-UPR)U,3-(2-UCR)U,and 3-R(2-RPR)U.The research begins by elaborating in detail the similarities and differences among these four parallel mechanisms.By standardizing the definition of the coordinate system for each mechanism,the inverse kinematics and the Jacobian matrix of these four mechanisms are systematically derived.Employing a set of motion/force transmission indices,which are directly obtained from the Jacobian matrix,the kinematic performances of the four mechanisms are thoroughly analyzed and compared within the given workspaces while maintaining the same dimensional parameters for all cases.The comparison study of these four parallel mechanisms extends beyond local transmission indices to also include global transmission indices,covering both position and orientation workspaces,as well as assessments at both the local and global workspace levels.This comprehensive approach ensures a detailed and fair evaluation of their respective kinematic capabilities.The results indicate that the comprehensive kinematic performances of the four parallel mechanisms are similar,and providing a solid theoretical foundation for innovative design and practical guidance for selecting optimal architectures based on specific application requirements.
文摘According to the 2017 Summary Conference of China National Textile and Apparel Council(CNTAC)held on January 22nd to 23rd,in the first 11 months of 2017,the overall operation of China’s textile industry showed a steady growth rate,stable domestic demand growth。
基金National Hi-tech Research and Development Program of China(863 Program,No.2001AA42330).
文摘Negative step response experimental method is used in wrist force sensor＇s dynamic performance calibration. The exciting manner of negative step response method is the same as wrist force sensor＇s load in working. This experimental method needn＇t special experiment equipments. Experiment＇s dynamic repeatability is good. So wrist force sensor＇s dynamic performance is suitable to be calibrated by negative step response method. A new correlation wavelet transfer method is studied. By wavelet transfer method, the signal is decomposed into two dimensional spaces of time-frequency. So the problem of negative step exciting energy concentrating in the low frequency band is solved. Correlation wavelet transfer doesn＇t require that wavelet primary function be orthogonal and needn＇t wavelet reconstruction. So analyzing efficiency is high. An experimental bench is designed and manufactured to load the wrist force sensor orthogonal excitation force/moment. A piezoelectric force sensor is used to setup soft trigger and calculate the value of negative step excitation. A wrist force sensor is calibrated. The pulse response function is calculated after negative step excitation and step response have been transformed to positive step excitation and step response. The pulse response function is transferred to frequency response function. The wrist force sensor＇s dynamic characteristics are identified by the frequency response function.
基金supported by the National Natural Science Foundation of China(Nos.51239008 and 51279130)
文摘This paper presents a study on the motion response of a tension-leg platform(TLP) under first-and second-order wave forces, including the mean-drift force, difference and sum-frequency forces. The second-order wave force is calculated using the full-field quadratic transfer function(QTF). The coupled effect of the horizontal motions, such as surge, sway and yaw motions, and the set-down motion are taken into consideration by the nonlinear restoring matrix. The time-domain analysis with 50-yr random sea state is performed. A comparison of the results of different case studies is made to assess the influence of second-order wave force on the motions of the platform. The analysis shows that the second-order wave force has a major impact on motions of the TLP. The second-order difference-frequency wave force has an obvious influence on the low-frequency motions of surge and sway, and also will induce a large set-down motion which is an important part of heave motion. Besides, the second-order sum-frequency force will induce a set of high-frequency motions of roll and pitch. However, little influence of second-order wave force is found on the yaw motion.
基金supported by the Fundamental Research Funds for the Central Universities (Nos. 2018JBZ007, 2018YJS136 and 2017YJS158)China Scholarship Council (CSC) (No. 201807090079)National Natural Science Foundation of China (NSFC) (No. 51675037)
文摘This paper presents a novel 1T2R three degrees of freedom redundantly actuated and overconstrained ■parallel machining head(■denotes the active prismatic joint), which can construct 5-axis hybrid machine to complete high speed freedom surface milling for large complex structural components in aerospace. Firstly, based on the screw theory, the mobility of the proposed parallel manipulator is briefly analysed. Secondly, the kinematic inverse position and the parasitic motion of the parallel manipulator are explicitly expressed. Furthermore, motion-force transmission performance evaluation indices are derived in detail via an alternative approach based on the screw theory. More importantly, a simple method for quickly solving the maximum virtual power coefficient is proposed, and the motion-force transmission performance evaluation index is greatly improved. To evaluate the kinematic performance, its workspace is calculated. With numerical examples, performance distribution atlases of the manipulator are depicted visually. The corresponding results illustrate that the proposed parallel manipulator has better orientation workspace and superior motion-force transmission performance than the 2 PRU-PRS parallel manipulator, which proves the validity and applicability of applying this manipulator as a machining head.
文摘Forced-air convection cooling of high-power electronic devices is widely used, but it has a problem that a rise in temperature of the air used to cool the upstream devices decreases the cooling capa-bility for the downstream devices. In this study we made an experimental apparatus including a memory card array and measured the effect of the rise in temperature of the air on the heat transfer coefficient of the memory cards that were downstream in the air flow. Using these mea-surements, we devised a simple calculation model, called the thermal diffusion layer model, to calculate the heat transfer coefficient of multiple rows of memory cards. The rise in temperature of downstream memory cards due to higher temperature air can be evaluated with a parameter representing the delay of thermal mixing for air. The heat transfer coefficient calculated with the thermal diffusion layer model agreed with our experimental results.
文摘Forced convection cooling of fins on a high-temperature wall has been used to cool high-power electronic devices. We numerically calculated and experimentally measured the forced convection heat transfer coefficient and pressure drop of a diamond-shaped fin-array with water flow in this study, which had been reported to have a self-induced flip-flop flow phenomenon. Although the flip-flop flow phenomenon occurred in calculations, it was not observed in experiments. The heat transfer and pressure drop of the diamond-shaped fin-array could be estimated with equations for turbulent flow in tubes.
基金the Project by Yayasan Universiti Teknologi PETRONAS(No.0153AA-E95)。
文摘Numerous types of floating breakwaters have been proposed,tested and commercialized in the past decades.The majority of these breakwaters are made of solid bodies;hence,they are relatively bulky and are not readily to be rapidly installed at the targeted sites when immediate wave protection of the coastal and offshore facilities is needed.Furthermore,the application of these hard floating structures at the recreational beaches is rather unlikely due to potential deadly marine traffic collision.To overcome these problems,a flexible air-filled wave attenuator(AFWA)has been developed in the present study.This floating breakwater is made of flexible waterproof membrane materials.The main body consists of a rectangular air-filled prism and is ballasted by sandbags located around the floating module.The objective of this study is to evaluate the wave transmission,wave reflection,energy dissipation,motion responses and mooring forces of the AFWA under the random wave actions using physical modelling.The test model located in a 20 m long wave flume was subjected to a range of wave heights and periods.The wave profiles in the vicinity of the test model were measured using wave probes for determination of wave transmission,reflection and energy loss coefficients.The motion responses in terms of heave,surge and pitch,and wave forces acting on the mooring lines were measured using a motion tracking system and load cells,respectively.The experimental results reveal that the AFWA is effective in attenuating up to 95%in the incoming wave height and has low-wave-reflection properties,which is commendable for floating breakwaters.
