Control system design for a continuous positive airway pressure ventilator
© Chen et al; licensee BioMed Central Ltd. 2012
Received: 31 August 2011
Accepted: 1 February 2012
Published: 1 February 2012
Continuous Positive Airway Pressure (CPAP) ventilation remains a mainstay treatment for obstructive sleep apnea syndrome (OSAS). Good pressure stability and pressure reduction during exhalation are of major importance to ensure clinical efficacy and comfort of CPAP therapy. In this study an experimental CPAP ventilator was constructed using an application-specific CPAP blower/motor assembly and a microprocessor. To minimize pressure variations caused by spontaneous breathing as well as the uncomfortable feeling of exhaling against positive pressure, we developed a composite control approach including the feed forward compensator and feedback proportional-integral-derivative (PID) compensator to regulate the pressure delivered to OSAS patients. The Ziegler and Nichols method was used to tune PID controller parameters. And then we used a gas flow analyzer (VT PLUS HF) to test pressure curves, flow curves and pressure-volume loops for the proposed CPAP ventilator. The results showed that it met technical criteria for sleep apnea breathing therapy equipment. Finally, the study made a quantitative comparison of pressure stability between the experimental CPAP ventilator and commercially available CPAP devices.
Nasal continuous positive airway pressure is a prevalent and effective treatment for patients with obstructive sleep apnea syndrome (OSAS) characterized by repetitive episodes of complete or partial upper airway obstruction that occurs during sleep [1–3]. CPAP devices delivered a positive trans-mural pressure during the throughout respiratory cycle to prevent the collapse of the upper airway. Actually, constant CPAP levels affect the impedance of airway circuit and gas leak, especially the tidal volume and breathing frequency. The key problem to be resolved in designing CPAP devices with good compliance is how to synchronize them with patient's spontaneous breathing, that is, CPAP devices should automatically increase pressure levels at the beginning of inspiration to maintain therapeutic pressure and decrease pressure at the beginning of expiration to facilitate patient's expiration. The present work aims to develop the CPAP ventilator with better pressure stability. The study also tries to come up with some parameters to objectively evaluate the performance of different CPAP devices in an attempt to make CPAP therapy more comfortable and acceptable.
Pressure range: 4-20 hPa (4-20 cmH2O)
Sound levels: ≤30 dB (at 10 hPa)
In order to meet the above design requirements, it is very important to select a sensitive pressure sensor and a motor and blower with good performances such as fast response time and low noise level. In this design, we use an integrated silicon pressure transducer (model MPXV5004GC7U by Freescale Semiconductor, Inc., USA) with pressure range from 0 to 40 hPa and an application-specific CPAP blower (ebm-papst, Germany) that is able to change its speed rapidly to respond to the requirement of the patient. The blower's maximum air flow is 530 ± 10% l/min, maximum back pressure 48 ± 20% cmH2O, life expectancy at nominal speed of 30000 r/min more than 20000 h, while the mass is only 0.262 kg. The low noise design offers CPAP patients peaceful and quiet sleep.
A. System Structure
B. Controller Design
where u 2 is motor input voltage (unit: volt), and p is blower output pressure (unit: cmH2O). It can be seen that the error between the model (in red) and the experimental data (in blue) is very small. Therefore we use the piecewise linear function u 2 (P) to design the feed forward controller.
where k is the sample interval, K P , K 1 and K D , are the proportional, integral and differential gains controller, respectively. Ultimately, the command input voltage u(S) of the motor driver is equivalent to u 1 (S) plus u 2 (S).
PI controller parameters at different pressure setting
It is also observed that in Figure 6 the airway pressure fluctuation is ± 1.5 cmH2O while the output port pressure fluctuation is only ± 0.5 cmH2O. First, we don't expect the motor speed makes an excessive large or small change to avoid pressure unable to go back to the setpoint before the beginning of the next breathing cycle. Hence, a pressure threshold of 3 cmH2O is set in the computer. When the offset between the setpoint and the actual pressure is greater than 3 cmH2O the PI controller doesn't work any more. In normal CPAP therapy, the patient is directly connected to the output port. However, in Figure 5 additional breathing tube and the analyzer are included in the patient circuits. In this scenario, resistance of additional breathing tube and the flow sensor (between inlet and outlet of the analyzer and with dynamic resistance < 2 cmH2O at 60 lpm) is a non-negligible factor that impedes gas flow and undermines the automatic pressure compensation function played by the CPAP ventilator. Besides, gas leak in the exhalation port also partly weakens the contribution from the PI controller. Taken together, these reasons result in the observed airway pressure fluctuation.
The aim of this part is to make a comparison of pressure stability between the experimental CPAP ventilator and commercially available CPAP devices. The test setup is completely similar to that in Figure 5. Flow, volume and pressure are continuously recorded and stored using the VT for Windows PC software.
Another specification that needs to be taken into account is device's noise level. With the output pressure set at 10 cmH2O as specified by ISO 17510 we acoustically compare their A-weighted sound power levels and find no significant difference. Further, taking measurements using a sound level meter (TES 1350A, Taiwan), the A-weighted sound power level caused by the experimental CPAP is 44.5 dB and that is 5.8 dB above the A-weighted background level of extraneous noise, 38.7 dB. This device additional noise level is slightly smaller than 6 dB specified by ISO 17510.
CPAP Clinical Performance Results for 6 Subjects
Using a fast-response blower, pressure transducer and microprocessor, an experimental CPAP ventilator has been built for the treatment for OSAS. A compensation that comprises feed forward control and feedback control integrating PI compensator is proposed to maintain the therapeutic pressure and minimize pressure variations during spontaneous breathing. Comparison of pressure-time curves and P-V loops indicates that a CPAP ventilator with PI pressure feedback control outperforms the ventilator without pressure feedback control.
Taking a close look at Figure 7, one finds when the CPAP pressure is set at 8 cmH2O even if the experimental CPAP ventilator operates in the open-loop mode, the output port pressure variations are still less than ± 2 cmH2O. Hence, the present study shows that a fast-response motor/blower is the dominant factor determining performance of a CPAP ventilator.
A future study is needed to further eliminate the uncomfortable feeling of exhaling against positive pressure. A feasible solution is to let the controller actively reduce the CPAP pressure during the expiration and then return the setting CPAP pressure before the onset of inspiration. In fact, several commercial CPAP ventilator companies such as Respironics and Resmed have introduced this new technology to their own products [14–16]. To implement this, besides using a pressure sensor, an additional flow sensor is needed to determine the transitional points between exhalation and inhalation. The flow sensor typically has fast response time of 1 to 3 ms, ranges from ± 200 to ± 1000 sccm and features bi-directional sensing capability. It senses the patient's breathing effort by monitoring airflow amount and direction. Then this flow signal is used to control a sleeve valve whose one of the ports is pneumatically connected to the blower pressure and the other is an exhaust outlet. Since the airflow direction is reversed from exhalation to inhalation it is technically easy to identify the breathing phase and to adjust the pressure correspondingly. Another advantage that the flow transducer offers is it makes it possible to compensate flow loss caused by air leaks, which in turn is helpful in maintaining constant CPAP [17, 18].
The author would like to thank Dr. Khosrow Behbehani for valuable support and guidance, Prof. Hai-Ming Xie for providing the gas flow analyzer, and Dr. Hong-guang Xi contributed with helpful discussions and suggestions.
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