Research And Implementation Of Human Impedance Measurement System Based On FPGA

The cell fluid of the human body is composed of intracellular fluid and extracellular fluid,which maintain dynamic balance in a healthy state.When the body suffers from certain diseases,such as kidney disease,there is an imbalance in the amount of water in the fluid inside and outside the cells,resulting in dehydration or edema.Bioelectrical impedance technology is a technology that uses electrical method to measure human moisture to extract bioelectrical impedance information of human body and reflect physiological or pathological information of human body according to these information.Human impedance is measured by applying safety excitation current to human body through constant current source,and collecting voltage and current signals reflected from human body to calculate human impedance value.This method is non-invasive,harmless,simple and cheap,so it has been widely used in the medical field.In the measurement process needs to use the microcontroller to generate signals and data processing,and based on the FPGA platform for this research and implementation is to take advantage of the FPGA can process data in parallel,in the signal processing at the same time,can work with other logic circuits or multipliers,so as to shorten the running time.In the current research on human impedance measurement system,when the system works in high frequency band,such as more than 500 KHz,the measurement results still have great distortion.In this paper,the hardware circuit and software algorithm are studied and designed to solve this problem,so as to optimize its measurement accuracy in high frequency range.The main work contents are as follows:(1)This paper summarizes the research method of human impedance measurement system,starting from the principle of bioelectrical impedance,describes the theoretical support of its research method.After the analysis of freiberga equivalent circuit model,Siedbaum equivalent circuit model and Bigmeyer equivalent circuit model,according to the characteristics of the system designed in this paper,the equivalent circuit model is determined,which makes the system more convenient to calculate the capacitance of cell membrane and the resistance of the fluid inside and outside the cell.The four-electrode measurement method is defined,which uses the differential input method to collect the reflected voltage signal of human body,improves the signal precision,removes the common error interference,and makes the system have stronger anti-interference ability.(2)According to the resistance and capacitance characteristics of human cells,the current excitation signal in sweep mode is designed to effectively shorten the measurement time and avoid the amplitude attenuation caused by the high working frequency of the system.The frequency range of the sweep signal is 1KHz ? 1MHz.(3)The hardware circuit of the system is designed,including constant current source circuit,voltage acquisition circuit,current acquisition circuit and linear amplifier circuit and other modules,and the parameters of the circuit are simulated and compared.Finally,the output impedance of constant current source is kept above352 K ?.The measurement result distortion caused by the decrease of output impedance of constant current source under high frequency signal is solved.By adding an operational amplifier at the feedback input end of the circuit to reduce the feedback end shunt,and adding a shielded drive circuit at the output end,so that in the following board level debugging,the output linear error of constant current source circuit is less than 2.8%,that is,in practical application,the circuit has good anti-interference ability.(4)Design the software part of the system.Including asynchronous FIFO design,data transmission module design based on UART protocol and phase sensitive detection algorithm design.In the phase sensitive detection algorithm design part,the algorithm is theoretically deduced and simulated.The simulation results show that the maximum error between the preset phase difference and the calculated phase difference is 0.013°,and the maximum error between the preset amplitude and the calculated amplitude is 2.27%.It can be seen that the algorithm has high precision in calculating the amplitude and phase of complex impedance signals.After the algorithm is implemented in FPGA,the amplitude and phase results obtained are transmitted to PC through UART module.(5)In this paper,the circuit and algorithm designed for the overall system verification.In terms of the system performance and precision testing,a circuit model with known impedance value is tested.The experimental results show that the average amplitude error and phase error of the system are 1.15% and 0.30° respectively.Then three volunteers of similar age and body shape were measured for human impedance,and the human impedance values at each frequency were calculated.