Rezaf Diary

I'll write about my daily life, as a PhD Student at the University of Illinois. I'll put some photos from my town, Urbana, and some music.

Sunday, February 04, 2007

Open loop vs closed loop control

Do you ever wonder what the hell is a closed loop control system? How doews it differ from an open loop control system? If so, read on!

Theoretically, an open loop control system should be able to do whatever their closed loop counter parts do. All you need is a 100% accurate mathematical model of your system, and every other environmental factor which could affect the systems operation.

Ok, we want to control real stuff. In reality you can never model a system accurately 100% (it is just too hard!) , and it's even harder to account for environmental factors (or noise if you like). Thus you see a good amount of deviation from what you modeled in your real implementation.

How does a feedback control system work? It doesn't care about the internals of the system, just looks at it as a black box. It will look at the outputs of the system and will say: "hm... I'd like the output to be 10, but it is 8, so let's increase the input a bit. ok, now it's better, we're getting to 10... oh oh , we passed 10 and are now at 10.5, oh boy, let's decrease the input a bit. Ok, that's better, output is at 10 now, I'm done! But I'll keep an eye on the output in case an unknown noise comes in, in which case I can compensate for it by changing the input."

Now the problem with the feedback control is that since our systems are real (the tech word is causal) and our controller cannot foresee the future (DUH, we're talking about real world!) and moreover it doesn't know that much about the system internals and noise, We
will usually have overshoot (like the scenario I told you, i.e. we want 10, but for a short period we get 10.5) and oscillation (like 8...9...10...10.5...9.75...10.... unless you set your feedback controller to work really slow.

So to sum it up, open loop should work fine and beautifully, but only in your dreams! put an open loop controller before a real system, and you'll see deviations, which the controller cannot even correct. In real world we use feedback control systems all the time.

As an interesting side note, our brain controls our limbs in a feedback control system, which is set to overshoot and oscillate a bit for fast responses. Check it out: stretch your right arm, and then try to quickly bring your palm in front of your face, and stop it there. You'll surely see the overshoot and oscillation!

Note: Click on the photos to enlarge them.

If you feel interested in any of them and want to get the original file, please don't hesitate to email me!


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