Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
1998-03-10
2001-11-27
Lewis, Aaron J. (Department: 3735)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204230
Reexamination Certificate
active
06321748
ABSTRACT:
RELATED APPLICATIONS
Not applicable.
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
MICROFICHE APPENDIX
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention deals with the field of piston ventilators for patients in need of respiratory assistance. More particularly, the invention is concerned with a control circuit for a piston ventilator that corrects for errors in the gas pressure delivered to the patient compared to a set point pressure by taking into account the error magnitude, the integration of the error over time and gas leakage in the system. The control circuit also determines the set point pressure as one of a plurality of sequential set point pressures on a pressure trajectory during patient inspiration.
2. Description of the Prior Art
The prior art discloses control devices for piston ventilators in which analog signals representative of motor current and desired current are compared to generate an analog error signal. The error signal is used to drive the piston motor in order to minimize the error. In the prior art, a brushed DC motor was used. The relationship between current and torque, i.e., the torque constant for a DC motor, was used to generate specific pressures. Specifically, torque equals force times distance, therefore, for a given torque on a piston, a particular pressure could be generated.
These prior art control schemes present a number of problems. For example, the torque constant varies with temperature. As a result, the error is never eliminated in a controlled fashion. The prior art uses current control which is a function of how frictionless the piston is. Moreover, the signals delivered to the piston motor do not take into account gas leaks in the system. Also, the prior art analog control schemes do not provide the desired level of precision and flexibility in operation needed for medical applications.
SUMMARY OF THE INVENTION
The present invention solves the prior art problems discussed above and provides a distinct advance in the state of the art. More particularly, the piston ventilator of the present invention uses digital processing to implement a control scheme for eliminating error between actual pressure in the patient circuit and the set point pressure.
The preferred piston ventilator includes a pressure sensor, a motor controller and a digital processor under software control. The processor receives sensor signals from the pressure sensor representative of the actual gas pressure in the patient circuit of the ventilator. In response, the processor determines the gas flow amount from the piston necessary so that the actual gas pressure substantially conforms to a predetermined set point pressure, and delivers motor control signals representative of the gas flow amount to the motor controller.
In preferred forms, the set point pressure is one of a plurality of sequential set point pressures on a pressure trajectory. This trajectory is determined as a function of a predetermined rate of rise as set by the user between a beginning pressure and a target pressure. The beginning pressure is the pressure in the patient circuit at the beginning of patient inspiration and is typically the PEEP pressure (positive expiratory exhalation pressure).
In other preferred aspects of the invention, the motor control signals are a function of the error between the set point pressure and actual gas pressure. More particularly, the motor control signals are a function of the sum of a proportional constant times the error, an integration constant times the error over time, and a leak constant times the actual pressure.
REFERENCES:
patent: 4587967 (1986-05-01), Chu et al.
patent: 4617637 (1986-10-01), Chu et al.
patent: 4726366 (1988-02-01), Apple et al.
patent: 4957107 (1990-09-01), Sipin
patent: 5107830 (1992-04-01), Younes
patent: 5540222 (1996-07-01), Younes
patent: 6076523 (2000-06-01), Jones et al.
patent: WO 9624402 (1996-08-01), None
Hovey Williams Timmons & Collins
Lewis Aaron J.
Nellcor Puritan Bennett
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