A respiratory therapy device comprising: a mechanical ventilator configured to provide mechanical ventilation to a patient in a pressure support ventilation or pressure support ventilation/intermittent mandatory ventilation (PSV/IMV) mode in accordance with a PSV pressure setting; a power of breathing (PoB) or work of breathing (WoB) estimator configured to generate an estimated PoB or WoB signal for the patient based on sensor data; an error calculator configured to compute an error signal E ( s) as a difference between the estimated PoB or WoB signal and a set point PoB or WoB value; a controller having a controller transfer function C ( s) and configured to input the PSV pressure setting equal to C ( s) E ( s) to the mechanical ventilator; and a patient adaptation component (52, 54, 56, 60); wherein the mechanical ventilator, the PoB or WoB estimator, the error calculator, and the controller are operatively interconnected to form a single-input, single-output (SISO) closed loop feedback control system in which the mechanical ventilator and a connected patient and the PoB or WoB estimator define the controlled system whose single input is the PSV pressure setting and whose single controlled output is the estimated PoB or WoB signal; wherein the patient adaptation component comprises an electronic device programmed to fit parameters of a model of the controlled system transfer function to estimated PoB or WoB signal and PSV pressure setting data generated by the operating SISO closed loop feedback control system and to adjust parameters of the controller transfer function C ( s) to maintain stability of the closed loop transfer function of the SISO closed loop feedback control system. a mechanical ventilator configured to provide mechanical ventilation to a patient in a pressure support ventilation or pressure support ventilation/intermittent mandatory ventilation (PSV/IMV) mode in accordance with a PSV pressure setting; a power of breathing (PoB) or work of breathing (WoB) estimator configured to generate an estimated PoB or WoB signal for the patient based on sensor data; an error calculator configured to compute an error signal E ( s) as a difference between the estimated PoB or WoB signal and a set point PoB or WoB value; a controller having a controller transfer function C ( s) and configured to input the PSV pressure setting equal to C ( s) E ( s) to the mechanical ventilator; and a patient adaptation component (52, 54, 56, 60); wherein the mechanical ventilator, the PoB or WoB estimator, the error calculator, and the controller are operatively interconnected to form a single-input, single-output (SISO) closed loop feedback control system in which the mechanical ventilator and a connected patient and the PoB or WoB estimator define the controlled system whose single input is the PSV pressure setting and whose single controlled output is the estimated PoB or WoB signal; wherein the patient adaptation component comprises an electronic device programmed to fit parameters of a model of the controlled system transfer function to estimated PoB or WoB signal and PSV pressure setting data generated by the operating SISO closed loop feedback control system and to adjust parameters of the controller transfer function C ( s) to maintain stability of the closed loop transfer function of the SISO closed loop feedback control system.