Method and apparatus for improving flow and pressure estimation in CPAP systems
A CPAP apparatus in which the swing in pressure at the patient interface is adjusted by regulating the air flow from the flow generator through an air delivery conduit taking into account a pressure drop representative of the inertance of the airflow in the air delivery conduit during the increase of air flow from the flow generator.
1. A method comprising:
sensing a flow through an air delivery conduit of a respiratory therapy device;
sensing a pressure at a flow generator of the respiratory therapy device;
calculating a derivative of the sensed flow;
converting the derivative of the sensed flow to a converted value based on a comparison of the derivative of the sensed flow to a threshold;
estimating a pressure loss in the air delivery conduit based on the sensed flow and the converted value;
estimating a pressure in a patient interface of the respiratory therapy device by subtracting the estimated pressure loss from the sensed pressure; and
controlling the respiratory therapy device based on the estimated pressure in the patient interface.
2. The method of claim 1 , wherein the pressure loss (P drop ) in the air delivery conduit is estimated as:
P drop =K 1 Q 2 K 2 Q+K L dQ/dt,
where Q is the sensed flow through the air delivery conduit, K 1 and K 2 are empirically determined constants, K L is an analytically determined flow inertance, and dQ/dt is the converted value of the derivative of the sensed flow through the air delivery conduit.
3. The method of claim 2 , wherein the flow inertance K L is analytically determined as:
K L =ρI/A,
where ρ is the density of the air in the air delivery conduit, I is the length of the air delivery conduit, and A is the cross sectional area of the air delivery conduit.
4. The method of claim 1 , wherein converting the derivative of the sensed flow comprises clamping the derivative of the sensed flow.
5. The method of claim 4 , wherein clamping the derivative comprises setting the derivative to zero for negative values of the derivative.
6. The method of claim 4 , wherein clamping the derivative comprises cutting off the derivative at a maximum positive value of the derivative.
7. The method of claim 6 , wherein the maximum positive value is 2.5 L/s/s.
8. The method of claim 1 , wherein controlling the respiratory therapy device comprises controlling the flow generator by maintaining the estimated pressure in the patient interface to be substantially equal to a desired pressure.
9. The method of claim 1 , wherein controlling the respiratory therapy device comprises delivering a forced oscillation.
10. A respiratory therapy device comprising:
a flow generator,
a patient interface,
an air delivery conduit for delivering air from the flow generator to the patient interface,
a pressure sensor for sensing a pressure at the flow generator,
a flow sensor for sensing a flow through the air delivery conduit, and
a controller configured to:
calculate a derivative of the sensed flow through the air delivery conduit;
convert the derivative of the sensed flow to a converted value based on a comparison of the derivative of the sensed flow to a threshold;
estimate a pressure loss in the air delivery conduit based on the sensed flow and the converted value;
estimate a pressure in the patient interface by subtracting the estimated pressure loss in the air delivery conduit from the sensed pressure at the flow generator; and
control the flow generator based on the estimated pressure in the patient interface.
11. The respiratory therapy device of claim 10 , wherein controlling the flow generator comprises controlling the flow generator to maintain the estimated pressure in the patient interface to be substantially equal to a desired pressure.
12. The respiratory therapy device of claim 10 , wherein controlling the flow generator comprises delivering a forced oscillation with the flow generator.
13. The respiratory therapy device of claim 10 , wherein converting the derivative of the sensed flow comprises clamping the derivative of the sensed flow.
14. The respiratory therapy device of claim 13 , wherein clamping the derivative comprises setting the derivative to zero for negative values of the derivative.
15. The respiratory therapy device of claim 13 , wherein clamping the derivative comprises cutting off the derivative at a maximum positive value of the derivative.