Cheese, Catastrophes, & Process Control: Crash Course Engineering #25
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Date: 2022-04-04
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Comments and reviews: 10
HorzaPanda
I did a module on Control Theory as part of my nuclear physics degree, and as Lewis Foster says, PID is important. I thought it might interest people to have a practical example from that:
In order to keep a nuclear reaction steady, you want a set neutron flux in the reactor, one way to control that is with the control rods. These are materials that absorb neutrons (In a PWR you can also add boron to water, in the form of boric acid)
Whatever you do as an intervention, there is always going to be some lag between action and effect
Now, let's say you get a reading that's too high, so you lower the control rods/add more boric acid, you go for a linear approach, so your intervention is directly proportional to your intervention. With this approach, you'll typically overshoot and end up with too low a neutron flux, so you then raise your control rods/take some boric acid out
Well done, you now have set off an oscillation in your neutron flux, and with a proportional/linear response that can quite easily be one that grows with each iteration. For obvious reasons, that is not a good thing to happen in a nuclear reactor.
So you use an integral response to that initial deviation. I find it easier to think of this next bit like it's a mechanical system, with a pendulum, with that initial deviation from your norm being analogous to someone pushing your pendulum. You want to dampen that oscillation so it goes back to being steady, which is where an integral response comes in. You want a response that perfectly dampens that oscillation in the minimum possible time, not one like the linear response which is almost like your pushing it as it comes to the end of its swing
(Note: It's been a while since I've last done this, I might be a little off in places, and I certainly can't remember exactly how to explain how the integral response works in detail)
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I did a module on Control Theory as part of my nuclear physics degree, and as Lewis Foster says, PID is important. I thought it might interest people to have a practical example from that:
In order to keep a nuclear reaction steady, you want a set neutron flux in the reactor, one way to control that is with the control rods. These are materials that absorb neutrons (In a PWR you can also add boron to water, in the form of boric acid)
Whatever you do as an intervention, there is always going to be some lag between action and effect
Now, let's say you get a reading that's too high, so you lower the control rods/add more boric acid, you go for a linear approach, so your intervention is directly proportional to your intervention. With this approach, you'll typically overshoot and end up with too low a neutron flux, so you then raise your control rods/take some boric acid out
Well done, you now have set off an oscillation in your neutron flux, and with a proportional/linear response that can quite easily be one that grows with each iteration. For obvious reasons, that is not a good thing to happen in a nuclear reactor.
So you use an integral response to that initial deviation. I find it easier to think of this next bit like it's a mechanical system, with a pendulum, with that initial deviation from your norm being analogous to someone pushing your pendulum. You want to dampen that oscillation so it goes back to being steady, which is where an integral response comes in. You want a response that perfectly dampens that oscillation in the minimum possible time, not one like the linear response which is almost like your pushing it as it comes to the end of its swing
(Note: It's been a while since I've last done this, I might be a little off in places, and I certainly can't remember exactly how to explain how the integral response works in detail)
reply
Marco
Always assume imperfections in your sensors. I work with drone software. One of the research groups we work with has drones that use barometers to estimate their altitude. They figured it was a great idea to figure their drones landed when the barometer says the height is 0, and completely shut off the motors. Unfortunately, barometers aren't super accurate, and also can't measure the absolute height above ground. The drones ended up falling down a meter or so, and breaking some parts.
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Always assume imperfections in your sensors. I work with drone software. One of the research groups we work with has drones that use barometers to estimate their altitude. They figured it was a great idea to figure their drones landed when the barometer says the height is 0, and completely shut off the motors. Unfortunately, barometers aren't super accurate, and also can't measure the absolute height above ground. The drones ended up falling down a meter or so, and breaking some parts.
reply
angela
Hey Hank and John,
My name is Angela and I'm a huge fan of everything you do. I've read all John's books (waiting for xmas to read hanks) listen to the pods, and of course LOVE vlogbrothers. I'm a musician who just recorded my first EP, but am of course still learning. I was wondering if there was a possibility you could do lessons on music theory. It's really interesting but no one has been very good at explaining it, therefore it's still very confusing to me. Thank you so much!
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Hey Hank and John,
My name is Angela and I'm a huge fan of everything you do. I've read all John's books (waiting for xmas to read hanks) listen to the pods, and of course LOVE vlogbrothers. I'm a musician who just recorded my first EP, but am of course still learning. I was wondering if there was a possibility you could do lessons on music theory. It's really interesting but no one has been very good at explaining it, therefore it's still very confusing to me. Thank you so much!
reply
Taras
As obvious it might be to -decrease steam flow if something is too hot- it's not always the case. In reality steam is generated somewhere using the power of either engine that is cooled by this very same oil or a battery, that - guess what - is charged by the same engine.
I think it also should be mentioned that the whole process operates under assumption that a) desired behavior is at least marginally stable b) either globally stable or at least has a decent stability region
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As obvious it might be to -decrease steam flow if something is too hot- it's not always the case. In reality steam is generated somewhere using the power of either engine that is cooled by this very same oil or a battery, that - guess what - is charged by the same engine.
I think it also should be mentioned that the whole process operates under assumption that a) desired behavior is at least marginally stable b) either globally stable or at least has a decent stability region
reply
pieter
Murphy's Law: if something can go wrong, it will.
Murphy's Second Law: if more than one thing can go wrong, the thing that causes the greatest damage will go wrong.
Murphy's Third Law (also known as Sod's Law in Britain): Murphy was an optimist.
By limiting yourself to the first law, you kept it at a nice and basic level. I expect a separate episode on the other laws!
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Murphy's Law: if something can go wrong, it will.
Murphy's Second Law: if more than one thing can go wrong, the thing that causes the greatest damage will go wrong.
Murphy's Third Law (also known as Sod's Law in Britain): Murphy was an optimist.
By limiting yourself to the first law, you kept it at a nice and basic level. I expect a separate episode on the other laws!
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Shaine
Nice! I'm taking a program at College called -Instrumentation & Control Engineering-. We learn all about a process (Water tank, DP Transmitter, Rotameter, etc) and have to calibrate it in various situations and in many different ranges (mA or psi. Most people have NO idea what it is I'm studying when I try to explain it. This video is helpful.
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Nice! I'm taking a program at College called -Instrumentation & Control Engineering-. We learn all about a process (Water tank, DP Transmitter, Rotameter, etc) and have to calibrate it in various situations and in many different ranges (mA or psi. Most people have NO idea what it is I'm studying when I try to explain it. This video is helpful.
reply
Dan
You can work and research Process Control following the path of Electrical, Computer, Chemical or Mechanical Engineer, each with their own specificities. It's a tough field, where different engineers need to talk to each other to make it work (especially in the chemical industry)
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You can work and research Process Control following the path of Electrical, Computer, Chemical or Mechanical Engineer, each with their own specificities. It's a tough field, where different engineers need to talk to each other to make it work (especially in the chemical industry)
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baltakatei
06: 00 Wouldn't you want to put the steam valve at the outlet of the heat exchanger? Taking a pressure drop at the inlet would decrease the steam density and and temperature due to adiabatic expansion, wouldn't it?
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06: 00 Wouldn't you want to put the steam valve at the outlet of the heat exchanger? Taking a pressure drop at the inlet would decrease the steam density and and temperature due to adiabatic expansion, wouldn't it?
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freaking
The example about the calculater is flawed because the law states that anything that can go wrong and a calculator is not made up of materials supporting spontaneous combustion
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The example about the calculater is flawed because the law states that anything that can go wrong and a calculator is not made up of materials supporting spontaneous combustion
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Thinker
Another episode about a subject from the field of chemical engineering. Most episodes so far are about chemical engineering.
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Another episode about a subject from the field of chemical engineering. Most episodes so far are about chemical engineering.
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