Design Properties That Affect Controller Gain and Available Bandwidth of Non-Minimum Phase Membrane Humidifiers

The water vapor transfer across a membrane exhibits non-minimum phase behavior. This paper shows that the competing dynamics of heat and mass transfer cause the membrane humidifier to have a non-minimum phase zero. Even though the non-minimum phase zero exists in the disturbance-output loop, it will limit the feedback controller gain because the disturbance-output loop is coupled with the input-output loop. The membrane properties and heat transfer parameters affect the non-minimum phase zero location. The impact on available feedback control gain and system bandwidth is analyzed in relation to changes of the non-minimum phase zero during hardware design.