Research Of Wireless Powering Techniquesfor Micro Devices In Gastrointestinal Tract Inspecting

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.