The Method Of Wristband Pulse Wave Detection Based On Bio-impedance

With the rapid development of science and technology and the increasing demand,wearable biological signal detection method has been given more and more attention.Wearable medical devices can be mainly divided into three sections: sensor part,data transmission part and user application part.The sensor,which is used to pick-up the physiological signals of human,is the most crucial part for the design of wearable product,and is also the focus and the difficulty in the recent research in the field of biomedical engineering.Continuous monitoring of pulse waves is of great significance for the evaluation of human health.At present,there are three common methods for pulse wave detection,including photoplethysmography(PPG),piezoelectric sensor,and bio-impedance.But the PPG method is commonly measured on the finger tips,and the piezoelectric method has a high demand for stable and continuous pressure.So the two methods are not suitable for wearable applications.Impedance measurement method is simple and convenient,while this method generally needs to overcome the influence of basic impedance and skin-electrode-contact impedance,and it needs a wide dynamic range,which is difficult to realize.In order to overcome the difficulties of the impedance measurement method for the continuous monitoring of pulse waves,wrist impedance pulse wave measurement method was proposed in this research.Bridge method is well known for the measurement of small resistance.This research proposed DAC auto balancing bridge method,which was based on the blood impedance and the surrounding tissue impedance model,to measure wrist impedance pulse wave.In this study,the relationship between the changes of the pulse wave impedance and the amplitude of the output signal is analyzed by theoretical derivation,and also the balancing condition of the bridge,the principle of reducing the static basic impedance and the skin-electrode-contact impedance during measuring impedance pulse wave.The oversampling and fast digital phase-locked algorithm was used to detect the impedance of pulse wave.The digital demodulation of the impedance pulse wave signal was realized in this research.The effective resolution of ADC was also improved.Integrated with the measurement method of electrodes with different materials,the conductive rubber ribbon was used as the electrode for measuring the pulse wave.The experiment proved the comfort and feasibility of conductive rubber as wrist electrode.And we also estimated its ability to reduce the influence of the motion artifact.On the basis of designing the hardware circuit and programing software algorithm,this research used PSoC3 series CY8C3866 chip as the development board.We realized the wrist pulse impedance measurement and analyzed the measurement results of impedance pulse wave before and after bridge balance.The result demonstrated the design of this research can reduce the influence of individual differences,and improve the accuracy of the measurement system.The method proposed here provides valuable experience for wearable portable devices with micro power consumption,low cost and small size.