摘要
能量供给是人体消化道微型诊查装置(以下简称:微诊查装置)首要的和最基本的问题。要使目前的胶囊内窥镜具备主动运动功能,并且能以较高帧率、较清晰地获取消化道图像信息,对能量的需求非常大,以纽扣电池供电的方式已无法满足需要。
基于电磁感应的无线供能技术已经在经皮能量传输中得到广泛应用,可采用此技术为微诊查装置无线供能。采用电磁感应方式供能时,体外发射线圈产生交变磁场,体内接收线圈感应产生能量,两线圈通过电磁耦合实现能量的无线传输。与经皮能量传输不同的是:在微诊查装置的无线供能中,体内、外线圈之间的距离较远;体内接收线圈的空间尺寸有限,且在人体消化道内的位置、姿态均不确定。这些因素导致了能量传输效率较低、且不稳定。要传输足够的能量,发射线圈产生的电磁场必须足够强,而这势必对人体组织产生不利影响。所以,微诊查装置无线供能研究要解决的核心问题是能量传输的稳定性、效率和人体安全性。
微诊查装置被吞服后,在人体消化道内的位置和姿态均不确定。为了使微诊查装置在姿态改变时仍能获得较稳定能量,研究了姿态任意变换情况下获得较稳定能量的方法,设计了三维正交接收线圈和整流后再并联的接收电路,在微诊查装置的姿态改变时,可合成输出较稳定能量。为了使微诊查装置在位置改变时仍能获得较稳定能量,研制了亥姆霍兹结构的发射线圈。研究表明,采用亥姆霍兹发射线圈,可在人体消化道区域内产生较为均匀的电磁场,解决能量传输的位置稳定性问题。为了解决因发射线圈失谐而导致的传输效率下降问题,研究了改变逆变频率使发射线圈在新的频率下谐振和通过补偿电路维持发射线圈在原有频率下谐振两种方法的特点,得到了高品质因数耦合电路应通过维持发射回路和接收回路同频谐振来提高传输效率这一关键性结论,并对由开关电容组成的补偿电路进行了深入研究,以解决能量传输的频率稳定性问题。
限于人体生理特征,微诊查装置的空间尺寸严格限定,要为微诊查装置提供足够能量,提高传输效率是关键。本文从决定传输效率的耦合、发射、接收三个方面入手,研究了提高传输效率的措施和方法。针对发射、接收线圈之间距离较远,接收线圈的尺寸较小而导致的两线圈之间耦合度弱的特点,研究了发射、接收线圈与匹配电容组成的并-并、并-串、串-并、串-串四种链路的传输效率,得到了能量传输效率最大化的条件,指出了串联谐振电路和并联谐振电路的不同适用条件,并从根本上揭示了影响能量传输效率的因素:耦合系数和线圈品质因数。通过优化耦合电路的谐振频率,选择合适的线圈参数、磁芯材料、负载因子,使传输效率有了一定的提高。为了提高发射效率,设计了全桥逆变电路,并配置了阻尼吸收网络,使发射电路能可靠、高效地工作。为了提高接收效率,研究了几种常用整流电路的整流效率和接收功率,并综合考虑空间利用,为能量接收部分确定了合适的整流合成电路。为了给微诊查装置提供一个稳定的电压,设计了整流后两级转换电路。
采用电磁感应方式为体内的微诊查装置无线供能时,体外发射线圈产生的交变磁场势必对人体组织产生不利影响,必须对电磁场与人体组织的相互作用进行研究,并对其安全性进行评估,这是无线供能技术应用于人体的安全性前提。当前,国际上公认的电磁安全性标准主要有国际非电离辐射防护委员会的导则和IEEE C95.1-2005标准,人体电磁安全性衡量的主要指标为感应电流密度和比吸收率(SAR)。由于目前还不能通过实验方法对这两个电磁剂量学量进行直接测量,数值计算方法是当前学术界普遍采用的方法。本文通过有限积分法,在高精度人体电磁计算模型上求解了人体组织中的电流密度、SAR等电磁剂量。高精度人体电磁计算模型以美国可视人体项目的数据集为基础,应用半自动图像分割技术,重建了包含56种人体组织的高精度三维真实人体结构模型,其分辨率达0.33mm×0.33mm×2mm;通过对人体结构模型中每种组织电特性的指定,建立了人体电磁计算模型。在求解了电磁剂量学量,并对体外发射线圈产生的电磁场进行安全性评估的同时,本文还分析了发射电流、电磁场频率等因素对电磁剂量的影响,并根据传输效率,为无线供能选择了最优工作频率,使其在传输足够能量的同时,对人体的不利影响最小。
为了从整体上对以上的研究成果进行验证,本文还研制了一款功耗为90mW视频胶囊内窥镜,并进行了无线供能离体实验。实验结果表明,当发射功率为12.5W,在视频胶囊内窥镜的姿态、位置任意变化的情况下,均能传输出较清晰的视频图像,证明了本研究的有效性。
Power supply is the primary and basic issue for the micro devices in gastrointestinal tract inspecting. The power consuming will be much more when the wireless capsule endoscope (WCE) is equipped with active locomotion, or when to capture the gastrointestinal tract images more clearly with a higher frame rate. The tradition method using the button batteries as the power supply can’t offer enough power for the micro inspection devices any longer.
Wireless power supply technology based on inductive coupling will be one of the suitable choices as it has already verified in transcutaneous energy transmission (TET) system. In the inductive coupling power supply, The power transmitter outside generates an alternating magnetic field through the transmitting coil, the power receiver inside gathers the power through the receiving coil, Energy is transferred between the transmitting and receiving coils by inductive coupling. Unlike the TET, in the power supply for the micro devices in gastrointestinal tract inspecting, the distance between the transmitting and receiving coil is large, size of receiver coil is strictly limited, and move important is that the micro devices moves freely in the gastrointestinal tract, leading to its position and orientation changing constantly during normal operation. All these factors will lead to insatiably and low power transmission efficiency of power transmission. So, the key problems of wireless power transmission for micro devices are stability, efficiency and human tissues safety.
The orientation and location of micro devices are always indetermination when it continues the natural way down the gastrointestinal track. For the orientation varying, methods and designs of stably power transmission were study, a three-dimensional (3-D) receiving coil and a circuit of parallel connection after full-bridge rectifying were put forward, the receiving coil and the circuit can composing a stable power even the micro device’s orientation is varying. For the location varying, a Helmholtz transmitting coil as the primary outside was designed and manufactured. Our research show that the Helmholtz coil can generated a uniform alternating magnetic field covering whole of the alimentary tract, and the receiving coil can receive stable energy regardless of micro device’s position relative to the generated magnetic field. To deal with the decreasing of transmission efficiency due to the detuning of transmitting coil, two methods were proposed and studied: one is to alter the transmitting frequency and make the transmitting coil resonate at a new frequency, and the other is to maintain the transmitting coil resonate at the fixed frequency which is identify with the transmitting coil by compensation. The key conclusion that the transmitting and receiving coil with high Q should be resonate at the same frequency for the transmission efficiency improving was conclude. In the application of this conclusion, a feed back control circuit using the switching capacitor as the compensation was propose, the control circuit can maintain the transmitter resonates at the fixed frequency and the frequency stability of power transmission may be resolved.
Due to the physiological characteristics of human body, the size of the micro devices in gastrointestinal tract inspecting is strictly limited. In order to transfer enough power to micro devices, the transmission efficiency improving is very important. In this paper, we handle the transmission efficiency through three aspects: coupling, transmitter and receiver, and the study on the coupling efficiency is the key. For the feature of weak coupling between the transmitting and receiving coil due to the distance between them and size of receiving coil, the transmission efficiency of four circuit topologies was deduced and concluded the maximizing conditions. The serial resonant circuit and parallel resonant circuit should be used according to the application situation. The essential factors effecting power transmission efficiency, which are coupling coefficient and coils’quality factor, was revealed, and the transmission efficiency was improved by the optimizing of frequency, selection of coil parameters and using of ferrite core. To the transmitter, a full-bridge inverter was proposed and a snubber network used to reduce overvoltage and switching losses of MOSFETs was adopted, thus, the transmitter can work effectively and stably. To the receiver, the efficiency and output power of several rectifiers were studied; the full-bridge rectifier was selected as the power receiver in the view of space utilization. A converter circuit using DC-DC and Ldo (Low drop-out) was also designed, which can supply a much stable power to micro devices while the converter efficiency is still high.
The patient may suffer from the electromagnetic field produced by transmitting coil when using the wireless power transmission as the power supply. Researches were carried out on the interference between the electromagnetic field and human tissues, and adverse of electromagnetic wave on the human body was evaluated. Currently, there have existed two main stands on the human tissues exposed to the electromagnetic wave. One is Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields defined by International Commission on Non Ionizing Radiation Protection (ICNIRP), and the other is the IEEE C95.1 standard. In these stands, current density and SAR are the primary dosimeter parameters for low-frequency magnetic fields evaluation. However, these parameters data can be hardly acquired by experimentation. Numerical electromagnetism provides a new way for biological dosimetry and it is a widely used method currently. In this thesis, the current density and SAR in human body caused by transmitting coil were calculated using finite integral method based on a high resolution human electromagnetic calculation mode. The human calculation mode, which consists of 56 kinds of tissues or organs, was constructed based on VHP dataset through semiautomatic image segmentation technique. The resolution of the model is 0.33mm in transverse section and 2mm between slices, after the dielectric properties of every tissue in the model were assigned, an electromagnetic calculation model was achieved. In this study, the influencing factors, such as transmission frequency and transmission current on the current density and SAR were also discussed. Combined with the transmission efficiency, an optimized resonance frequency was concluded, which make the power transmit at a high efficiency while the adverse affection of electromagnetic field on the human body is small.
In order to verify the practical performance of wireless power transmission designed by the above study, a video capsule endoscope based on a wireless power supplying was designed and manufacture. In the vitro experiments, it shows that the video capsule endoscope, consuming a power of 90 mW, can transmit a satisfying video when its position and orientation are varying and when the transmitting power is 12.5w. The experiments have shown the validity of this research.
引文
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