We just had a big snowfall yesterday. Eighteen inches of snow compressed to ten inches of pure white concrete while I was watching it. By the time I got out, it was too late. Four hours later, the snow blowing was done and so was I.
But, you can make lemonade out of anything.
So, today I'm going to talk about open loop guidance and closed loop guidance.
I work on the Ares I launch vehicle. We're responsible for the software that determines how the Ares vehicle gets the payload (Orion) where it's going. We have a target, defined to be a direction and a velocity. Ares does it with two stages. Orion takes that and either rendezvous with the ISS or the hardware that will get it to the moon.
But let's talk about a hypothetical rocket with only two stages, the First Stage (FS), we'll say is a brute force rocket capable of leaping tall buildings in a single bound and catching bullets in its teeth. The Upper Stage (US) is cut from finer clother. It's the Energizer Bunny. Not as strong as the FS, it just goes on and on and on. The FS uses open loop guidance and the US uses closed loop guidance.
Now, let's imagine you're snow blowing up and down the driveway and for some anally retentive reason you want to snowblow a straight line. (Those of you down South that don't quite understand that water can freeze outside and fall to the ground naturally, think of cutting your grass.) So, you're walking up the hill and trying to go straight. You could sight on a distant object that lines up where you want to go and walk towards it. You're not taking into account where you are on the driveway, how straight you already are or how fast you're going. You just keep sight of that object and walk towards it. Every time you glance at it, you aim towards it. This is open loop guidance, a form of open loop control. Washing machines, heaters and the First Stage use open loop control. It's a form of control that does not take into account your current state but only the state where you want to be. It's ideal for the Big Brute where little mistakes aren't meaningful and just waste energy.
Let's change the scenario. Let's say you're walking up that hill, the distant point is still fixed in your gaze, but now you're watching the snow you're cutting, seeing how straight your path is, how much it deviates from the previous path. You take into account you're going up a hill-- the snow blower is going to slow down--the fact that it's a right wheel driver that gives it a kick in that direction, the tilt to the road, how much you wish this were over and how much you want a beer, etc. You are, in point of fact, taking into account your current state and using it to figure out how to best reach that target. That's closed loop guidance and it's a form of closed loop control. It uses feedback. It takes into account the current state of the vehicle and determines the control output. Most modern cruise control systems, power steering and the Upper Stage use closed loop control. Long and slow, it gets you there precisely where you want to go.
Strange things occur to you while blowing snow.
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