Notes

Closed loop control system!

What is it? Why is it important? What's the difference from open loop systems? All this in an exciting whatsapp documentary!

Let's start with open loop control system. You have an input, you have a process and you have an output. It's very simple to explain with an example.
You press _volume up_ on your remote control (input), some magic happens (process) and volume changes (output).
You set the clothes drier to (I don't know how yours works) cotton, full dry -> drier works for 2 hours -> clothes dry
You set the air conditioner on cold - cold air blows out - car gets colder.
What they all have in common is that the input is unaware of the output. If the process is affected, for example if batteries in the remote controler are low then the volume won't change. The drier is unaware how dry clothes are. If you put in a big batch they may come out still wet. The air conditioner (admittedly a cheap/old one) will blow cold air as much as it can even when it gets too cold.

Closed loop systems address some of those weaknesses. They're more advanced. They adapt input to the output you wish to get. It usually goes like this: You set an input, a process happens, output happens. Output is _measured_ and input is adapted until the output is what it should be (preferrably automatically). For example
Your drier now measures the moisture in the air blowing out of the drier and when it reaches a certain value it finishes the task.
You set the air conditioner to 22°, it's going to blow cold air until it a sensor reports it has reached that temperature and then stop.

These closed loop systems have what's called a _negative feedback loop_. A whole field of mathematics and engineering is dedicated to studying its nature for complex systems, but we'll keep it simple.

Difference amplifier substracts output (ambient temp in the example) from input (desired temperature). Difference amplifier is usually fed electrical signals which usually need amplifying, that's why its called amplifyer even though it doesn't do that in our example. The main point is that, the bigger the difference between input and output, the stronger the response. If you remember sitting into a car that's been in the sun and turning on (working) air conditioner - it blows really hard at the beginning. And just a little bit when it's maintaining temperature.

I'm sure you had no problem understanding any of this and wonder where complexity comes in. Just to give you a taste of how things can become problematic.

Say you're trying to warm up a really big hallway in winter. Your thermometer is on the opposite side to the air conditioner (which can heat too obviously :p). The difference between what thermometer detects and what you want is pretty big at the beginning so the heater will work 100%. The room begins to warm up, but it's noticeably warmer on the heater side of the room. By the time the heater side is pleasantly warm, the thermometer side is still cool, so the heater will still work 100%. When the warmth catches up on the thermometer side it's already too hot on A/C side. The thermometer reports temperature has reached desired value so the heating stops. But the now hot air from heater side eventually gets to the thermometer which will now trigger cooling to happen. So the A/C will start cooling its side, but by the time that gets to the thermometer it's already too cold for the A/C side.
Basically you get in a neverending oscillation between too hot and too cold because the there's such a time delay. Oscillations are a big problem in control theory and requires good models of situations to prevent them from happening.

Then let's say you're designing a nuclear reactor. In case of the core overheating you can do multiple things. You can insert more nuclear moderator (apparently the thing that slows down nuclear reactions is called that). You can also increase the flow of cooling water. Both will do the job and keep the core at operational values; which one to choose in what circumstances? That depends on a shitload of other factors (do you need to produce more electricity? Is the coolant pressure normal? Has there been a tsunami?), all of which have to be considered.


These are all manmade systems. But they can also be found in nature; in human body, in climate, in population of rabbits, practically everywhere.
Human body (It's a marvel of maintaining equilibrium, couldn't help but list many examples because human body never ceases to amaze me):
Open loop
Touch hot plate -> pain signal travels to spine where it triggers a muscle signal -> jerk away from hot hot hot before brain is even aware
Shine bright light to the eyes -> i don't even know where this signal is processed* -> iris contracts
*I checked wikipedia. The pupillary light reflex is fascinating and has a very technical wiki, mathematical model included oO. Apparently the signal travels to a ganglion right behind the eye and that one triggers an iris contraction. In both eyes! Regardless of which one was shone upon.

Closed loop
Input - normal body temperature. Process - activating/deactivating sweat glands. Controlling blood flow to skin blood vessels. Output - current temperature.

Numerous other systems are regulated (to homeostasis) without our conscious knowledge. Blood sugar levels, blood pressure are quick examples.

Input - hold breath. Process - CO2 accumulates in blood. CO2 reacts with water to form H2CO3, an acid, so higher concentrations of CO2 means higher blood acidity. Body has receptors for blood acidity and will increase desire for breathing until it surpasses your will to hold it. Output - breathe
Hunger, thirst and a bunch of others work similarly.

Input - grab a glass of water. Process - hand moves toward glass. Eyes detect difference between current location of hand and location of glass -> adjust input so the hand moves in the right direction.
Moving more to conscious closed loop systems^^

Input - driving a car from A to B in a city. Process - every time you drive on Sievekingsallee you get a jam, which reinforces your dislike for the road. Output - start looking for alternative routes.
Terrible case of psychological negative reinforcement :p



I promised climate and population of rabbits too huh
The climate is actually self-regulatory to a degree. In various way, but the main one is this: The sun is heating up the earth, but the hotter it becomes, the more heat it radiates back into space. So the earth doesn't just heat up and become a molten rock. The radiating back into space is basically the negative feedback loop. If it got more energy from the sun it would radiate out more, eventually settling at a new equillibrium. I don't want to go into global warming though :p

Rabbits are eaten by wolves (supposedly). More rabbits = more prey for wolves. More prey = easier living for wolves ->
more wolves. More wolves -> more rabbits dead = less rabbits. So the rabbit population has a negative feedback loop involving wolves, and wolf population has a negative feedback loop involving rabbits. Neat huh.

I remember driving home from uni one day when it dawned on me how fundamental the closed-loop systems were. I drove with a cruise control which kept speed on its own. It monitors speed and adds engine power if it's below it. Off the highway I noticed myself trying to keep the same distance between me and the next car - distance too short, I slow down; distance too long, I speed up. I kept the car in the road lane by constantly subconsciously looking how far I was from left and right side. I turned the steering wheel on the turns, the sharper the turn, the more the steering was rotated. All of those are basically negative feedback loops. You make one and you start seeing them everywhere -.-.