Xiu Zhang

The University of Hong Kong, Hong Kong, Hong Kong

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Publications (20)23.73 Total impact

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    Yanpu Zhao, W. N. Fu, Xiu Zhang, S.L. Ho
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    ABSTRACT: In the process of finding the optimal solution by parameter sweeping analysis for optimal shape design problems, the silhouette of the device to be optimized usually needs to be changed many times. To avoid the tedious and time-consuming mesh regeneration process when the shape parameters change, a parameterized mesh technique is adopted in this paper. In the proposed method, the new coordinates of the nodes and their displacement vectors can be obtained instantly because all nodes are associated with the design parameters. To eliminate inverted elements in case of large shape deformation, a smart edge swapping technique is proposed. The edge swapping process is based on the nodal displacement vectors and the edge facing the displacement vector of the node is to be swapped. The parameterized mesh technique is fully described and examples are also given to showcase the effectiveness of the proposed method.
    IEEE Transactions on Magnetics 05/2013; · 1.42 Impact Factor
  • Xiu Zhang, H. L. Li, S. L. Ho, W. N. Fu
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    ABSTRACT: A multi-slice finite element model for wireless magnetic resonant energy transfer systems with circular spiral tape coils is presented. As the system is constructed using distributive capacitance, it is difficult to determine the parameters in the equivalent circuits analytically. To address the aforementioned problem, a multi-slice axisymmetric finite element method (FEM) is applied to analyze this wireless power transfer system. The merits of the proposed method are that three-dimensional FEM is not required and yet the distributive capacitances among the coils can be fully included in the proposed two-dimensional model. Compared with normal axisymmetric FEM without multi-slices, the proposed method can include the effect of distributive capacitance to produce more precise solutions. The proposed method is validated against experimental data.
    IEEE Transactions on Magnetics 05/2013; 49(5):1857-1860. · 1.42 Impact Factor
  • S. L. Ho, Xiu Zhang, W. N. Fu
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    ABSTRACT: Applications of magnetic-resonant wireless power transfer technology have been extensively researched because of its high efficiency, security and convenience. To recharge batteries, the induced ac voltage in the receiver coil of wireless systems must be rectified to dc, hence the power transfer systems are inevitably nonlinear. In this paper, a novel approach based on time-domain finite element method (FEM) is presented to analyze the nonlinear system in order to obtain frequency-sweeping solutions quickly. At the first step of the proposed method, the wireless power transfer system, excluding the rectifier circuit, will be equivalent to a voltage source and a general impedance at different frequencies. The equivalent circuit is then connected to the rectifier circuit. Numerical example being reported in this paper indicates that the results obtained using the proposed methodology are consistent with those obtained using traditional time-domain FEM method. If solutions at different operating frequencies are required, the proposed method can save a lot of computation time, because one just needs to solve the field problem using the time-domain FEM twice. From the numerical example on the performance analysis of the wireless power transfer system being studied, the computing time of the proposed time-domain method is only 2% of that required if traditional time-domain FEM method is used.
    IEEE Transactions on Magnetics 05/2013; 49(5):1781-1784. · 1.42 Impact Factor
  • Xiu Zhang, S. L. Ho, W. N. Fu
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    ABSTRACT: A hybrid optimal design strategy for wireless magnetic-resonant charger of deep brain stimulation devices is presented. It is proposed that a differential evolution algorithm with discrete variables (turn numbers of coils) and constrains (induced current and voltage in the load loop) is used to design the wireless power transfer system. The variables which normally include the sizes of the load coil, receiver coil, transmitter coil, source coil and capacitances are analyzed in the optimization study. Analytical formulas are embedded in the numerical optimization to speed up the convergence of the searching process. The designed receiver can receive enough power to recharge a 3.7 V circular button-type nickel-metal hydride rechargeable battery which can be implanted into the patients' skull. The performance of the designed system has been verified experimentally.
    IEEE Transactions on Magnetics 05/2013; 49(5):2145-2148. · 1.42 Impact Factor
  • W. N. Fu, Xiu Zhang, S. L. Ho
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    ABSTRACT: A novel and general method to quickly obtain frequency-domain solutions based on one or a few time-domain solutions is presented. The time-domain solution of electromagnetic field is found by using either two-dimensional (2-D) or three-dimensional (3-D) finite-element methods (FEM). The time-domain FEM is first used to obtain a solution for step function excitations by taking into account all the necessary initial conditions. Then, based on time-domain convolution theorem and the principle of integration by parts, a numerical algorithm to find the solutions of any excitations, without additional FEM computation, is deduced. A typical, interesting and meaningful application is to obtain the frequency-domain solutions when sweeping the operating frequency over a wide range. In this paper, a problem involving the finding of the resonant frequency of a copper loop connected with a pair of parallel plates using 3-D FEM is used as an illustrative example. The numerical experiment shows that the proposed method can reduce the computing time effectively and readily as it is easy to adjust the time step size in time-domain FEM.
    IEEE Transactions on Magnetics 04/2013; 49(4):1284-1289. · 1.42 Impact Factor
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    ABSTRACT: Optimal design problems of electromagnetic devices are generally multimodal, nondifferentiable, and constrained. This makes metaheuristic algorithm a good choice for solving such problems. In this paper, a newly developed metaheuristic algorithm is presented to address the aforementioned issues. The proposed algorithm is based on the paradigm of artificial bee colony (ABC). A drawback of the original ABC algorithm is because its solution variation is only 1-D, as this decreases its convergence speed. In this paper, a one-position inheritance scheme is proposed to alleviate this drawback. An opposite directional (OD) search is also proposed to accelerate the convergence of the ABC algorithm. The novel algorithm is applied to both TEAM Workshop problem 22 and a loudspeaker design problem. Both discrete and continuous cases of problem 22 are tested. The effectiveness and efficiency of the proposed algorithm are demonstrated by comparing its performance with those of the original ABC, an improved ABC known as Gaussian ABC, and differential evolution algorithms.
    IEEE Transactions on Magnetics 01/2013; 49(8):4811-4816. · 1.42 Impact Factor
  • W. N. Fu, Xiu Zhang, S. L. Ho
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    ABSTRACT: A fast algorithm for finding the frequency-domain solutions of electromagnetic field computation using time-domain finite-element method (FEM) is presented. Base solutions, which are time-domain solutions when the excitations are substituted by step functions, are computed first using time-stepping FEM in time domain. Then, based on these base solutions in time domain, all solutions in frequency domain at different operating frequencies can be easily and quickly obtained by simple addition and subtraction operations. It can be seen, from a numerical experiment on the performance analysis of a magnetic-resonant wireless power transfer system, that the computing time of the proposed time-domain method is less than 5% of that required if conventional frequency-domain method is used. The proposed method can be generalized and extended to any linear systems.
    IEEE Transactions on Magnetics 01/2013; 49(1):530-535. · 1.42 Impact Factor
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    ABSTRACT: Magnetic resonant wireless power transfer (MRWPT) is a developing technology to transfer power over a relatively long distance. It offers a promising solution in avoiding costly and risky battery replacement surgery in bio-implantable devices. One of the obstacles of the application of this transfer technique is that its efficiency is not satisfactory and the design method has not been presented systemically. In this paper, a design method of MRPWT system with a novel hybrid resonator for deep brain stimulation (DBS) device is proposed. A new formula to determine the diameter of the resonators according to the power transfer distance is presented. The merit of the proposed design is that the transmitter coil of the MRPWT system is modulated precisely with improved magnetic coupling towards the target coil while minimizing the power loss in the coils; hence the power transfer efficiency can be improved. Experiment is carried out to verify the validity and effectiveness of the proposed design method.
    IEEE Transactions on Magnetics 11/2012; 48(11):4518-4521. · 1.42 Impact Factor
  • Xiu Zhang, S. L. Ho, W. N. Fu
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    ABSTRACT: The technology of wireless power transfer has made a great contribution to the quality of life of many people, especially to those relying on implantable devices in the past decades. In order to improve the performance of wireless power transfer systems, a dual-layer nested optimization method using differential evolution (DE) algorithm is proposed in this paper. In contrast to previous methods to derive the compensated capacitance, this proposed method determines the capacitance values using optimization method. By testing a prototype, it is found that the proposed design can have much higher power transfer efficiency when compared to that of traditional method. The dual-layer nested optimization method, based on the design constrains, is also used to determine the parameters of the coils to obtain the maximum power transfer efficiency of the whole system. The result is validated using finite element method (FEM).
    IEEE Transactions on Magnetics 11/2012; 48(11):4511-4514. · 1.42 Impact Factor
  • Xiu Zhang, S. L. Ho, W. N. Fu
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    ABSTRACT: Wireless power transfer systems are finding increasing applications which can enhance the living standard of the general public. Typical examples are wireless charging of mobile phone batteries and car batteries. Thanks to the advent of power electronics in the past few decades, some wireless charging systems are now emerging in the commercial sector. However, the transfer efficiency and transfer distance of these chargers are limiting the technology to a few specific applications only. In order to increase the transfer distance, some researchers have successfully proposed to use relay coils to improve their wireless power transfer efficiency. In this paper, the role of relay resonator is examined critically. The reported findings indicate that it is not always desirable to have relay coils in wireless charger as the relay resonator may increase, or sometimes reduce, the overall power transfer efficiency. The findings also reveal that there is an optimal position for the designer to locate the relay resonator to maximize the power transfer efficiency.
    IEEE Transactions on Magnetics 11/2012; 48(11):4026-4029. · 1.42 Impact Factor
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    ABSTRACT: With the advent of electronic technology, researchers are devoting increasing interests to wireless power transfer methods. Generally finite-element method (FEM) is a powerful tool for numerical simulation of such systems. Due to the relatively high operating frequency, the presence of inductance and distributed capacitance in the system as well as eddy current and displacement current become two essential issues in the study of electromagnetic field distribution in wireless power transfer systems. One of the major problems to be addressed is the gap between low and high frequencies as conventional low frequency methods cannot include displacement current, while common high frequency methods focus mainly on the ultrahigh frequencies, such as in antenna studies. In this paper, a 3-D FEM including displacement current is developed to model wireless power transfer system to include displacement current and eddy current and close the gap of high and low frequency modeling methods. No frequency sweeping, which is required in conventional studies, is necessary in the proposed algorithm. Hence the computing time of the proposed algorithm requires only 1.5% of the normal time domain FEM method and this is a significant accomplishment for the electromagnetic community.
    IEEE Transactions on Magnetics 10/2012; · 1.42 Impact Factor
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    ABSTRACT: A novel error estimator for adaptive mesh refinement in finite element analysis (FEA) of magnetic field using quadratic finite elements is presented. The method uses a novel heuristic a posteriori error estimator, which is easy to compute and simple to implement, as an indicator of the numerical errors of the computed solution. The proposed error estimator is the L2 norm of the difference between the computed quadratic finite element solution and the interpolated linear solution. Throughout the time-stepping process for problems excited by periodic sinusoidal excitations, this error estimator can also be used to efficiently compute the numerical error at each time step and guide the adaptive mesh refinement in transient FEA. A multi-time-step adaptive mesh refinement method is also proposed in this paper for transient problems. The proposed method does not need to interpolate the solution from old mesh to a new adaptively refined mesh in transient FEA and hence there is no interpolation error. The effectiveness of the proposed error estimator is illustrated through several numerical examples being reported in this paper.
    IEEE Transactions on Magnetics 10/2012; · 1.42 Impact Factor
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    Yanpu Zhao, Xiu Zhang, S L Ho, W N Fu
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    ABSTRACT: A numerical method for analyzing eddy current magnetic field in high-speed moving conductors using local discontinuous Galerkin (LDG) method is presented. A typical numerical example is chosen to illustrate the accuracy and nonoscillatory nature of the method for a convection dominated case. The simulation results using the proposed algorithm are validated using standard finite element method (FEM). As an application of LDG method in engineering problems, the TEAM workshop problem 9 is studied and solved by the proposed method with second order rectangular discontinuous elements. Numerical results show that the method can solve eddy current problems with high speed moving conductors accurately.
    IEEE Transactions on Magnetics 02/2012; · 1.42 Impact Factor
  • Xiu Zhang, S. L. Ho, W. N. Fu
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    ABSTRACT: An emerging technology of wireless power transfer technique based on electromagnetic resonant coupling, also known as witricity, has been drawing a lot of attention from academia as well as from practitioners since it was reported in 2007 by a group of researchers from the Massachusetts Institute of Technology (MIT). In this paper, a prototype with square planar spiral structure based on witricity is proposed. To compute the resonant frequency, an equivalent circuit model is presented and simulated. The impedance matrix is computed by using a formula method and a 3-D finite-element method (FEM) of eddy-current magnetic field. The results indicate that the numerical method has a better accuracy. In order to reduce the resonant frequency, different conditions are analyzed quantitatively to study the relationship between the parameters and the relation of the prototypes with their resonant frequencies. The findings are found to offer a solid foundation for the optimization of witricity prototypes.
    IEEE Transactions on Magnetics 01/2011; 47:3200-3203. · 1.42 Impact Factor
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    ABSTRACT: Magnetic stimulation has become a very topical research area in bioelectromagnetics studies. One of the major problems in understanding the observed changes in biological systems resulting from exposures to magnetic fields is the relationship of biological changes to physical processes. It is well known that acupuncture has many varied physiologic effects on the functional systems of human subjects. However, there are still many unknown questions or issues pertinent to acupuncture which are yet to be answered. The combination of acupuncture with magnetic stimulation is, for example, an important aspect worthy of thorough clinical studies. In this paper, the effects for magnetic stimulation on acupoints are discussed in three ways: physical analysis based on electromagnetism; medical treatment analysis based on Chinese medical theory of acupuncture; experimental observations on the electroencephalographic (EEG) waveforms as a result of magnetic stimulation on some acupoints of the human subjects participating in this study.
    IEEE Transactions on Applied Superconductivity 07/2010; · 1.20 Impact Factor
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    ABSTRACT: A large number of studies reveal that repetitive TMS (rTMS) can have strong influences on cortical functions, even though the effect is very dependent on the stimulation parameters. At the same time, it is commonly acknowledged that acupuncture plays an important role in traditional Chinese medicine, and it is indeed receiving more and more attentions from the world. It is likely that a combination of acupuncture and magnetic stimulation which is non-invasive, painless and secure could give rise to treatments with substantial medical significance. This study discusses (a) the difference of EEG with pre- and post-stimulations at two different acupoints including Zusanli (St36) and Hegu (LI4) with three different frequencies (0.5 Hz, 1 Hz and 3 Hz) based on complexity analysis; (b) the character of the somatosensory evoked potentials(SEPs) in response to acupoint stimulation (Hegu) and mock point stimulation. The result indicates that different acupoints can induce different brain activity under the same stimulation correlated with the physical properties of the acupoints; and P150 can be observed after acupoint stimulation whereas mock point stimulation does not produce the same effect.
    IEEE Transactions on Applied Superconductivity 07/2010; · 1.20 Impact Factor
  • Xiu Zhang, S. L. Ho, W. N. Fu
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    ABSTRACT: As an emerging technology of wireless power transfer, an electromagnetic resonant coupling technique, also known as witricity, has been widely discussed. In this paper, a prototype with square planar spiral structure based on witricity is proposed. To compute the resonant frequency, an equivalent circuit model is presented. The inductance is calculated based on Wheeler formula. The accuracy of the equivalent circuit model is verified both experimentally and numerically using Ansoft HFSS which is a three-dimensional (3-D) finite element (FE) software. The developed equivalent model is used to optimize the design of the coils to realize resonance at the lowest resonanting frequency.
    01/2010;
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    ABSTRACT: Magnetic stimulation has become a very topical research area in brain studies because of its non-invasiveness, painlessness and security. It is well known that acupuncture has many varied physiologic effects on a number of functional systems of human subjects. In this paper, the EEG is recorded while the acupoint of Zusanli (St36) is stimulated by excitations of the same magnitude at different frequencies (0.5 Hz, 1 Hz, 3 Hz). Differences of EEG on the pre- and post-stimulation of the acupoint with magnetic field based on complexity analysis are reported. The data are analyzed with sample entropy (SampEn) and illustrated using brain information mapping (BIM). The results indicate that the SampEn of all the data can be used to distinguish the magnetically stimulated states from the normal state. It is also observed that while all frequencies can induce increases of SampEn over the whole brain, with the most obvious changes being found in the temporal lobe. This observation is consistent with the responses of human to acupuncture. The change of SampEn has significant statistical significances with a stimulation at 3 Hz, even though the effect is less obvious with other frequencies, when compared with the brain activities in normal state (p<0.05).
    IEEE Transactions on Magnetics 11/2009; · 1.42 Impact Factor
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    ABSTRACT: Transcranial magnetic stimulation (TMS) is a powerful, non-invasive tool for investigating functions in the brain. The target inside the head is stimulated with eddy currents induced in the tissues by the time-varying magnetic field. TMS has been used in several applications in medical and clinical research which include brain mapping, treatment of mood disorder and schizophrenia, treatment of epilepsy, treatment of chronic pain and so on. The stimulation effect can be affected by the stimulation intensity. For coils with the same shape, different winding methods make the coil have different stimulation intensity. In this paper, three different methods for winding circular coils are discussed. The electromagnetic fields induced by the three different circular coils were analyzed. The results show that the circular coil with the pancake coil winding has the strongest stimulation intensity.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 02/2008; 2008:4270-3.
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    ABSTRACT: To investigate the relationship of electroencephalographic activities and transcutaneous magnetic stimulation at acupoints. Three healthy male volunteers participated in this study. 64-channel electroencephalogram was recorded and its power spectrum was analyzed during sham magnetic stimulation and during real magnetic stimulation. Slow-wave components increase and fast-wave components decrease during magnetic stimulation at SHENMEN point. This result is consistent with the effect of acupuncture at SHENMEN point. The spectrum of EEG suggests magnetic stimulation at acupoint has some effect on brain activities. For the magnetic stimulation and acupuncture have the same effect on brain activities, the action mechanism of magnetic stimulation at acupoints might be similar to that of acupuncture.
    Noninvasive Functional Source Imaging of the Brain and Heart and the International Conference on Functional Biomedical Imaging, 2007. NFSI-ICFBI 2007. Joint Meeting of the 6th International Symposium on; 11/2007

Publication Stats

19 Citations
23.73 Total Impact Points

Institutions

  • 2012–2013
    • The University of Hong Kong
      • Department of Electrical and Electronic Engineering
      Hong Kong, Hong Kong
    • The Hong Kong Polytechnic University
      • Department of Electrical Engineering
      Hong Kong, Hong Kong
  • 2008
    • Hebei University of Technology
      • Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability
      Ho-pei-ts’un, Beijing, China