植物根区电阻抗成像系统设计与实验

为了能及时地监测植物根区的状态,设计了一种32电极的电阻抗成像系统,包括硬件构成和图像重构系统,以数字合成技术（DDS）产生正弦波激励信号,采用相邻激励模式,以乘法器对测量信号和参考信号进行模拟解调,得到被解调信号相对参考信号的实部信息和虚部信息,计算信号的幅值、相位和实部、虚部,基于成像正问题和逆问题算法,以Matlab及开源套件EIDORS予以实现。设计了专用电源,将220V交流电源经整流滤波、线性稳压后为系统提供多种正负电源。实验测量了系统的信噪比、通道一致性等参数,验证了实验平台的可靠性,并在盛有盐水的容器中进行了实时电阻抗系统成像实验,对绝缘材料和胡萝卜介质检测,结果表明本系统可有效辨别被测物,并具有很好的分辨率和重复性。

Development and Experiment on Electrical Impedance Tomography System for Plant Root Zone

To monitor the state of the plant root zone in real time, a 32-electrode electrical impedance tomography system and image reconstruction method were developed. Direct digital synthesis (DDS) generated sine wave signals, which was converted into an excitation signal by a voltage controlled current source (VCCS). The channel switching circuit injected the excitation signal into the electrode at the edge of the tank to be measured in an adjacent excitation mode. The circuit board and the electrodes were connected with shielded wires to reduce noise. Voltage measurement data were amplified by the instrumentation amplifier and demodulated by an analog multiplier, and then filtered and collected by A/D into the main control unit. Finally, the signal was sent to the host computer through the RS485 bus protocol, and then the data was preprocessed, reconstructed, and analyzed. The system incorporated the following novel features. The current source of the EIT system was a dual op-amp voltage controlled current source composed of a programmable gain instrumentation amplifier and an op-amp. Digital isolator isolated the measurement circuit and the excitation circuit. The measurement circuit channel switch adopted analog multiplexers MAX306. The decoder and the transistor drove the relay to switch the excitation signal channel and control the order of the excitation current injected into the electrode. Direct digital synthesis, channel switching circuit, programmable gain instrumentation amplifier, and other programmable devices were controlled by the main control unit. The multiplier circuit was used to demodulate the measurement signal and the reference signal, and the real part information and imaginary part information of the demodulated signal relative to the reference signal can be obtained to calculate the amplitude, phase, real and imaginary parts of the signal. Matlab and the open-source suite EIDORS were used to realize the reconstructed image based on imaging forward and inverse problem algorithms. In addition, a power supply circuit was designed to provide a variety of positive and negative power supplies for the EIT system after rectifying, filtering, and linearly stabilizing the 220V AC power supplied. In the experiment, parameters such as system signal-noise ratio (SNR) and channel consistency measured to verify the reliability of the EIT system. Experiment with real-time electrical impedance system imaging in a tank containing saltwater. And the real-time imaging function of the electrical impedance system was tested in a tank filled with saltwater. In the experiment of insulating material and carrot medium, the result showed that this system can effectively distinguish the measured object, and had good resolution and repeatability.