In-Bed Biosignal Acquisition From Conventional Differential Pressure Sensor Based on Thermal Flow Principle

We propose a non-invasive pneumatic biosensing method that measures the pressure on a tube placed under a bed cushion. Although similar methods use microphones for pressure sensing, the high sensitivity of microphones may cause problems such as saturation, slow recovery from saturation, narrow dynamic range, and low signal-to-noise (S/N) ratio. However, the proposed method uses a differential pressure sensor based on the thermal flow principle which has been developed and commercialized recently. We derive theoretical models of both microphone and thermal flow measurements to show that the proposed method can solve the problems of methods based on microphones. Basic experimental results validate both models providing the time constant (65 s), saturation range (-80-+40 Pa), noise level (4.02 Pa), dynamic range (32 dB) and S/N ratio (21 dB) of the low-frequency microphone developed for biosignal measurement and the superior time constant (0.012 s), saturation range (-500-+500 Pa), noise level (0.82 Pa), dynamic range (62 dB) and S/N ratio (56 dB) of the proposed method based on a differential pressure sensor. Furthermore, the effective S/N ratio in the measurement of heart beat signal was also compared between the low-frequency microphone and the proposed method. The former showed S/N ratio (22.1 dB), and the latter showed S/N ratio (28.1dB).