Estimation of Respiratory States Based on a Measurement Model of Airflow Characteristics in Powered Air-Purifying Respirators Using Differential Pressure and Pulse Width Modulation Control Signals—In the Development of a Public-Oriented Powered Air-P

Fluid dynamics modeling was conducted for the supply unit of a Powered Air-Purifying Respirator (PAPR) consisting of a nonwoven fabric filter and a pump, as well as for the exhaust filter (nonwoven fabric). The supply flow rate <b><i>Q</i>₁</b> was modeled as a function of the differential pressure <b>Δ<i>P</i></b> and the duty value <b>d</b> of the PWM control under a constant pump voltage of <b><i>V</i></b> = 12.0 [V]. In contrast, the exhaust flow rate <b><i>Q</i>₂</b> was modeled solely as a function of <b>Δ<i>P</i></b>. To simulate the pressurized hood compartment of the PAPR, a pressure buffer and a connected "respiratory airflow simulator" (a piston-cylinder mechanism) were developed. The supply unit and exhaust filter were connected to this pressure buffer, and simulated respiratory flow was introduced as an external disturbance flow. Under these conditions, it was demonstrated that the respiratory state-i.e., the expiratory state (flow from the simulator to the pressure buffer) and the inspiratory state (flow from the pressure buffer to the simulator)-can be estimated from the differential pressure <b>Δ<i>P</i></b>, the pump voltage <b><i>V</i></b>, and the PWM duty value <b>d</b>, with respect to the disturbance flow generated by the respiratory airflow simulator. It was also confirmed that such respiratory state estimation remains valid even when the duty value <b>d</b> of the pump is being actively modulated to control the internal pressure of the PAPR hood. Furthermore, based on the estimated respiratory states, a theoretical investigation was conducted on constant pressure control inside the PAPR and on the inverse pressure control aimed at supporting respiratory activity-namely, pressure control that assists breathing by depressurizing when expiratory motion is detected and pressurizing when inspiratory motion is detected. This study was conducted as part of a research and development project on public-oriented PAPR systems, which are being explored as alternatives to lockdown measures in response to airborne infectious diseases such as COVID-19. The present work specifically focused on improving the wearing comfort of the PAPR.