ilator hacked together by amateurs, no matter how well-meaning they are
There are a couple of companies we are talking to about running the design through a certification process and into manufacturing. So that will be co vered. When we hit certain bumps in the road the solution is to not worry about it since the company taking it over will be redoing that anyway. Thi s is not a rational to take technical shortcuts, more procedural things. S o the documentation will be sparse I expect.
That is my single biggest issue with the process. They are not doing a pro per requirements analysis. This means many aspects of the design are not p lanned out properly and various sections have been redesigned several times and may be again in the future as we find bends in the road.
Oh well, keeps me off the streets. I need to design a high side current mo nitor since the one in the motor controller has very poor accuracy. There is a nominal current ratio, but at lower currents the tolerance is worse th
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Rick C.
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Ok I see what you mean, imagining like a tire-pressure gauge tapped off the tube somewhere, as a crude analogy. Whether it's the bag pushing air in, or the patient blows back, or both, the pressure in the tube will rise. So worst-case it would undershoot, not overshoot. I think.
I was just wondering because while a sedated patient's lungs may be like an...RC circuit you say? an awake patient has a diaphragm and they can use it to fight the vent, in some situation perhaps. The physiology of conscious patients on ventilators is unclear to me I'd assume they can and will do whatever at some point. I'd think you'd want a controlled pressure ramp-up to ensure they're not doing that until it levels off. Then the PID kicks in for the constant rate phase and if they fight it at that point the control loop naturally ends the cycle early. and I'd guess in the pro stuff too many short cycles where the desired tidal volume hasn't been delivered triggers an alarm.
Welcome to having clients, even hardware ppl are expected to be "agile" nowatimes
The Bird Mark 8 was a pretty simple, totally hydro-mechanical gizmo.
1940s tech, used in hospitals in the US well into the 1970s and probably longer elsewhere.
"This is not an exact science, probably at best a guess":
Still, probably saved way more old ladies than it killed.
Some device like OP is talking about with an electronically-controlled squeeze-bag seems to intrinsically limit the potential hazardous failure modes. You only have so much air to work with and the mechanical "squeezer" can only physically move so fast.
High-performance ventilators use electric blowers and there's a lot more to go wrong, there. Good news! The schematic of the blower board I looked at had a 555. The lil 555, out there saving lives every day.
In a modern Western hospital run by insurance companies with other better options available? No, probably not.
In a field hospital in a crisis, or in the third world, where there's nothing else available and there aren't bureaucrats up your ass? They'll throw tin cans and twine at a situation if it's thought it would be of help.
The Hippocratic Oath says "First do no harm.." but if the patient is clearly dyin' anyway, y'know.
On a sunny day (Sun, 16 Aug 2020 08:13:04 -0700 (PDT)) it happened Ricketty C wrote in :
Well, OK, but I want the 'operator' out. An operator 24/7 is fatal if he does not pay attention. The amount of air and its oxygen percentage (assuming it can also mix in extra oxygen) is something that must be checked breath by breath, try it ! It is not as simple as a pumping in - out .
That said with over 20,000 fatal medical mistakes made in Germany alone each year
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automating what counts is interesting. Best of luck with it.
A "Christian Nation" that regularly uses capital punishment is an odd concept.
In Singapore they just hang the condemned; it's quick and cheap and they don't worry too much about whether when they violate some Abrahamic commandment, which they don't have, that they should at least be nicer about breaking it, so they may go about their business afterwards secure in the knowledge they are still good Christians in the eyes of God. like the first Commandment actually says "Thou shalt not kill, except if..."
What it actually says is thou shall not commit murder. The commandment against "false witness" is also misinterpreted as a commandment against lying in general.
Interpreting it as "thou shalt not murder" which may be closer to the original Hebrew meaning, 5000 years ago, adds an escape route. what's a murder? "killing of an innocent person." Well what's an innocent person. Someone who isn't guilty. Who convicts people and determines if they're guilty? Humans do. Based on what. On what their particular ideas of what is "moral" in a particular situation.
This (otherwise not bad) NR article doesn't really go far enough. _all_ use of the Ten Commandments to justify actions of the state cheapen the Ten Commandments.
g being controlled, how can you design the controller other than trial and error?
at presses on a bag producing an air flow with the loop controlled by a pre ssure measurement.
equency. 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 changin g from an Arduino CPU at 20 MHz to an ARM CM4F at 80 MHz.
ale and inhale. In general, would it be better to simply jump the pressure set point at once and let the PID controller do its thing, optimizing the r esponse time as best as possible controlling overshoot -or- would it be bet ter to run up the pressure set point over a period of time which would seem to place less demand on the PID controller?
as a capacitor) in parallel with a dissipative element (a dashpot or resis tor 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 res triction. 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 pro portional to the arm speed (although not constant through the stroke due to the bag geometry). The arm speed is what is controlled by the PWM (approxi mately).
that's appropriate. The response to air entering the lung will be the sum of the airway resistance (dashpot) and the lung compliance (spring) which w ould be a series combination to obtain the resulting air pressure. Well, ma ybe that is right for the mechanical model, but in the electrical equivalen t if pressure is the same as voltage it would be a series arrangement.
n by a P only controller, is there any way it could ring? I was shown data measured that showed huge ringing from an initial step function in the set point.
low regulated cycles. I expect to see similar results with either method.
Here is a cheap, patented ventilator that doesn't need PID control (volume of air delivered is regulated mechanically by the angular motion of a cam):
"Only God can judge. However God seems unavailable at the moment to take this particular case, so I, as God's lawfully-authorized acting representative, hereby declare your execution as an obvious criminal to be God-sanctioned."
On Sunday, August 16, 2020 at 11:45:57 AM UTC-4, Ricketty C wrote:
en wrote:
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ntilator hacked together by amateurs, no matter how well-meaning they are
n through a certification process and into manufacturing. So that will be c overed. When we hit certain bumps in the road the solution is to not worry about it since the company taking it over will be redoing that anyway. This is not a rational to take technical shortcuts, more procedural things. So the documentation will be sparse I expect.
oper requirements analysis. This means many aspects of the design are not p lanned out properly and various sections have been redesigned several times and may be again in the future as we find bends in the road.
onitor since the one in the motor controller has very poor accuracy. There is a nominal current ratio, but at lower currents the tolerance is worse th
hmmm running it through certification process with 'documentation is rather sparse'? My suggestion is to get the documentation as it will be part of the assuran ce case that will be made to the FDA (or whoever). Ultimately, the FDA cer tifies the as-built system and will need to see proper design docs and *ra tional* that when reasoned about, can make the argument that the system wi ll behave as required and is safe. Understanding PID controllers is one thing. Writing a discrete version of the PID control law is something else. Understanding the system dynamics a nd developing is control loop goes beyond understanding a PID controller. I know nothing about how a ventilator works. I do know a bit about develop ing control systems. From the number of posts and your 'wondering' of what the control variable is, it sounds like one need to make a model of the s ystem in order to better understand the components and linkages and develop a realistic transfer function. I am quite surprised that the system is ch aracterized as a simple first order system. For a voltage controlled DC motor the rotational speed/torque transfer func tion is first order. Closed loop control rotational speed/voltage is always a second order system - in fact, it is the quintessential example in any f irst control theory course. And there is a 'simplifying assumption' - if o ne assumes the time constant of the electrical circuit is much sm aller than the time constant of the load dynamics, the transfer functio n may be reduced to a first order transfer function. Not clear that this simplifying assumption is applicable in this case.
So, don't know the open loop transfer function of the system? run a frequen cy test and measure the response. Create a simulink model. Understand whe re the poles and zeros of the system are and determine the natural frequen cy of the system. Once one understands what they are dealing with then one can think about an effective control approach...there is usually more to it than throwing a P ID controller at the system. Gee, what happens if noise gets coupled into the system? Does your design compensate for that (hardware and digitally) You should also consider doin g stability analysis to see if your controlled introduced an oscillation. With all due respect, this is not a design problem for amateurs (no insinua tion about your skills or experience - just a lesson learned from spending lots of time in the control and certification domain (both FDA and Mil) ).
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