PID Controller Design for Ventilator

Ricketty C · 2020-08-16T01:44:03+00:00

I understand the basics of PID design, but if you can't describe the thing being controlled, how can you design the controller other than trial and er ror? The "plant" is a motor on a tall reducing gear (~300:1) turning an arm that presses on a bag producing an air flow with the loop controlled by a press ure measurement. One issue I'm seeing discussed is a tradeoff on the PWM resolution vs. freq uency. Presently they are using 3.6 kHz with 8 bit PWM control. I kinda w onder if a sigma-delta might be better, but that might require some externa l logic. They seem to be shy of pushing the CPU too much even after changi ng from an Arduino CPU at 20 MHz to an ARM CM4F at 80 MHz. The big concern is the overshoot when ramping up the pressure between exhal e and inhale. In general, would it be better to simply jump the pressure s et point at once and let the PID controller do its thing, optimizing the re sponse time as best as possible controlling overshoot -or- would it be bett er to run up the pressure set point over a period of time which would seem to place less demand on the PID controller? The model of the lung seems to include a spring constant (I think of this a s a capacitor) in parallel with a dissipative element (a dashpot or resisto r in electronics). The motor is highly geared to a relatively lightweight arm pushing on a bag with air passing through a tube of relatively low rest riction. So initially the dominate opposition to flow will be the dissipat ion/resistor, i.e. proportional to the rate of airflow. This in turn is pr oportional to the arm speed (although not constant through the stroke due t o the bag geometry). The arm speed is what is controlled by the PWM (appro ximately). The lung model shows the dashpot and spring in parallel, but I'm not sure t hat's appropriate. The response to air entering the lung will be the sum o f the airway resistance (dashpot) and the lung compliance (spring) which wo uld be a series combination to...

Aug 16, 2020 Last reply: 3 years ago 80 Replies

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Ricketty C 6 years ago

Show me wrong. What do you know about the project? If I am delusional, there has to be lots of evidence of it. Or are you the one who is delusional? I'm just sayin'...

Rick C. --+- Get 1,000 miles of free Supercharging --+- Tesla referral code - https://ts.la/richard11209

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PID Controller Design for Ventilator

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