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.




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 -