Investigation on dynamics of the flow control unit in ventilator systems and its fundamental performance limitations

In the design of a flow servo for critical care ventilator systems, control engineers are often troubled by unwanted oscillations or instability problems since these devices are typically operated under a wide variation conditions. Without knowing the dynamic behavior and its fundamental limitations of the flow control unit in ventilators, even ad-hoc extensions to standard PI control, as usually done in practice, is not able to solve all the problems and satisfy the overall performance required. In this paper, through analytical modeling and experimental identification, the root of the control problem is investigated by analyzing the dynamics of the flow control unit. Here the so called flow control unit includes a type of solenoid operated flow control valve and its exit manifold. A controller is designed by using model linearization methods, and the fundamental control limitation of the flow control unit is examined.