Gas flow control method in a blower based ventilation system

• US 7,487,773 B2
• Filed: 09/24/2004
• Issued: 02/10/2009
• Est. Priority Date: 09/24/2004
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett LLC (Medtronic Plc)
• Inventors: Li, Kun
• Primary Examiner(s): Douglas; Steven O
• Application Number:
  US10/949,853
• Publication Number:
  US 20060065270A1
• Time in Patent Office: 1,600 Days
• Field of Search: 128/204.21, 128/204.23, 128/204.18, 128/200.24, 128/207.22, 128/204.22
• US Class Current: 128/204.18
• CPC Class Codes: A61M 16/0057 Pumps therefor
• CPC Class Codes: A61M 16/0069 the speed thereof being controlled by respiratory parameters, e.g. by inhalation
• CPC Class Codes: A61M 16/026 specially adapted for predicting, e.g. for determining an information representative of a flow limitation during a ventilation cycle by using a root square technique or a regression analysis
• CPC Class Codes: A61M 2016/0039 in the inspiratory circuit
• CPC Class Codes: A61M 2205/3355 Controlling downstream pump pressure
• CPC Class Codes: A61M 2205/3365 Rotational speed

Abstract

The invention is directed to a system and method for controlling the flow of gas from a medical ventilator into a patient'"'"'s lungs. The control system provides for a non-linear feedforward controller to correct for disturbances caused by back pressure at the outlet of the blower of the medical ventilator. For this purpose, a pressure transducer is provided to measure the back pressure. Additionally, the invention allows for a feedback controller to correct for the differences between the rate of the actual gas flow and the targeted gas flow rate. For this purpose a flow rate transducer is provided. The control system may account for each of the gas flow rate error and the back pressure disturbance to provide for a quick and accurate adjustment to achieve the targeted gas flow rate.

Claims

1. A method of controlling the rate of gas flow induced by a variable speed fan in a breathing assistance system, comprising the steps of:

receiving a user setting for a target flow rate;

measuring a back pressure experienced by the fan;

calculating a target fan speed for producing the target flow rate set by the user based at least on the measured back pressure; and

controlling the variable speed fan based at least on the calculated target fan speed.

2. The method of claim 1, further comprising the steps of:

setting a time span for achieving the target flow rate; and

repeating the entire process when the time span has concluded.

3. The method of claim 1, further comprising the steps of:

measuring an actual flow rate generated by the fan;

calculating the difference between the actual flow rate and the target flow rate; and

controlling the variable speed fan based at least on (a) the measured back pressure and (b) the calculated difference between the actual flow rate and the target flow rate.

4. The method of claim 1, wherein:

the fan includes a variable speed electric motor and a fan mechanism; and

the setting the variable speed fan step includes sending a pulse width modulated electrical signal to the electric motor.

5. The method of claim 1, further comprising the steps of:

measuring an actual fan speed;

calculating a difference between the actual fan speed and the target fan speed calculated based on the measured back pressure; and

controlling the variable speed fan based at least on the difference between the actual fan speed and the target fan speed calculated based on the measured back pressure.

6. The method of claim 1, comprising calculating the fan speed for producing the target flow rate using a non-linear function between fan speed, target flow rate, and current back pressure, wherein the non-linear function is preprogrammed into the breathing assistance system.

7. The method of claim 1, wherein the back pressure measured is that produced by a patient'"'"'s lungs while the breathing assistance system is connected to the patient.

8. The method of claim 1, wherein each step is continued until the target flow rate is achieved.

22. The method of claim 1, wherein:

calculating the target fan speed for producing the target flow rate comprises calculating a target motor speed for a motor associated with the variable speed fan; and

controlling the variable speed fan based at least on the calculated target fan speed comprises controlling the motor based at least on the calculated target motor speed.

9. A method of controlling a gas flow rate from a breathing assistance system, including the steps of:

providing a breathing assistance system having a variable speed fan, an air hose, an electronic control system, an air flow transducer, and a pressure transducer;

connecting the air hose of the breathing assistance system to a patient'"'"'s lungs;

repeatedly blowing air from the variable speed fan through the air hose and into the patient'"'"'s lungs;

monitoring the air flow through the air hose using the air flow transducer;

monitoring the pressure within the air hose using the pressure transducer;

using a feedforward controller of the electronic control system to calculate a first desired fan speed based at least on signals from the pressure transducer, including resolving a non-linear equation relating fan speed with gas flow rates and pressure;

calculating a target fan speed using the electronic control system based at least on (a) signals from the air flow transducer and (b) the first desired fan speed calculated based at least on signals from the pressure transducer; and

adjusting the variable speed fan based at least on the target fan speed.

10. The method of claim 9, further comprising the steps of:

providing a speed transducer configured to measure the speed of the fan;

monitoring an actual fan speed of the variable speed fan; and

adjusting the variable speed fan to correct for differences between the actual fan speed and the target fan speed.

11. The method of claim 9, wherein the non-linear equation includes variables preprogrammed into the feedforward controller.

12. The method of claim 9, wherein the electronic control system further includes a feedback controller which calculates a second desired fan speed based at least on the air flow in the air hose, the second desired fan speed being used as input for calculating the target fan speed.

13. The method of claim 12, wherein the feedback controller and the feedforward controller work in unison to calculate the target fan speed based at least on both the air flow and pressure within the air hose.

14. The method of claim 12, wherein the feedback controller includes a proportional gain circuit, an integral gain circuit, and a differential gain circuit which together calculate the second desired fan speed.

15. The method of claim 14, wherein the proportional gain circuit, the integral gain circuit, and the differential gain circuit rely upon preprogrammed gain variables in their calculations.

23. The method of claim 9, wherein:

calculating the target fan speed using the electronic control system comprises calculating a target motor speed for a motor associated with the variable speed fan; and

adjusting the variable speed fan based at least on the target fan speed comprises adjusting the motor based at least on the calculated target motor speed.

16. A method of controlling the rate of gas flow induced by a fan including a fan outlet in a breathing assistance system, including the steps of:

setting a target gas flow rate;

setting a time span for the gas flow rate;

calculating a first fan speed to induce the target flow rate within the time span;

adjusting the fan based at least on the first fan speed;

measuring a first gas flow rate generated by the fan at the conclusion of the time span;

measuring a first back pressure experienced at the fan outlet at the conclusion of the time span;

calculating a second fan speed based at least on the first gas flow rate and the first back pressure to correct for the difference between the first gas flow rate and the target gas flow rate; and

adjusting the fan based at least on the second fan speed.

17. The method of claim 16, wherein the setting a time span step includes setting a new time span each time a time span concludes, whereby the measuring, calculating and adjusting steps are repeated until inspiration is complete.

18. The method of claim 16, wherein calculating the second fan speed includes resolving a non-linear equation relating fan speed, back pressure, and the target gas flow rate.

19. The method of claim 18, wherein calculating the second fan speed further includes resolving a proportional-integral-derivative equation to correct for differences between the first gas flow rate and an actual gas flow rate to determine the second fan speed.

24. The method of claim 16, wherein:

calculating the first fan speed comprises calculating a first motor speed for a motor associated with the fan;

adjusting the fan based at least on the first fan speed comprises adjusting the motor based at least on the calculated first motor speed;

calculating the first fan speed comprises calculating a second motor speed for the motor; and

adjusting the fan based at least on the second fan speed comprises adjusting the motor based at least on the calculated second motor speed.

20. A system for controlling the gas flow induced by a breathing assistance system, comprising:

a variable speed motor connected to a fan having a fan outlet;

a first transducer configured to measure a pressure experienced at the fan outlet and to generate an electrical signal indicative of the pressure;

a second transducer configured to measure a rate of gas flow generated by the fan and to generate an electrical signal indicative of the gas flow rate;

an electronic feedforward control circuit configured to receive the electrical signal from the first transducer and to generate a first signal indicative of an adjustment to the speed of the variable speed motor;

an electronic feedback control circuit configured to receive the electrical signal from the second transducer and to generate a second signal indicative of an adjustment to the speed of the variable speed motor;

an electronic motor controller configured to vary the speed of the variable speed motor based at least on the first and second signals generated by the feedforward controller and the feedback controller;

a third transducer configured to measure the speed of the fan and to generate an electrical signal indicative of the fan speed; and

the electronic motor controller is further configured to receive the electrical signal generated by the third transducer.

21. The system of claim 20, further comprising:

a user interface configured to receive manual input of a target gas flow rate and to generate a signal indicative of the target gas flow rate; and

the electronic motor controller is further configured to receive the signal generated by the user interface.

25. A method of controlling a breathing assistance system including a variable speed fan, an air conduit, an air flow sensor, a pressure sensor, and a fan speed sensor, the method comprising:

monitoring air flow through the air conduit using the air flow sensor;

monitoring pressure within the air conduit using the pressure sensor;

calculating a target fan speed based at least on signals from the air flow sensor and the pressure sensor;

adjusting the variable speed fan based at least on the target fan speed;

monitoring an actual fan speed of the variable speed fan; and

adjusting the variable speed fan to correct for differences between the actual fan speed and the target fan speed.

26. A method of controlling the rate of gas flow induced by a variable speed fan in a breathing assistance system, comprising the steps of:

setting a target flow rate;

measuring an actual flow rate generated by the fan;

calculating the difference between the actual flow rate and the target flow rate;

measuring a back pressure experienced by the fan;

calculating a target fan speed for producing the target flow rate set based at least on the measured back pressure and the difference between the actual flow rate and the target flow rate; and

controlling the variable speed fan based at least on the target fan speed.

27. A method of controlling the rate of gas flow induced by a variable speed fan in a breathing assistance system, comprising the steps of:

setting a target flow rate;

measuring a back pressure experienced by the fan;

calculating a target fan speed for producing the target flow rate set based at least on the measured back pressure;

controlling the variable speed fan based at least on the calculated target fan speed;

measuring an actual fan speed;

calculating a difference between the actual fan speed and the target fan speed calculated based on the measured back pressure; and

controlling the variable speed fan based at least on the difference between the actual fan speed and the target fan speed calculated based on the measured back pressure.