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Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

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Re: Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

#11

Post by sobellian »

amatic wrote: post #8

>> modelling might in fact be a necessary part of regulation. That seems false. PID is used all over, no models. Very simple, very effective.

The fact that PID loops work for many systems is a consequence of the dynamics of those systems, for example that the integral of velocity is position and the derivative of velocity is acceleration. A vast array of phenomena can be modeled as 2nd order linear ODEs.

Re: Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

#13

Post by theoh »

amatic wrote: post #8

>> modelling might in fact be a necessary part of regulation. That seems false. PID is used all over, no models. Very simple, very effective.

I think this Wiki page discusses it: https://en.wikipedia.org/wiki/Internal_model_(motor_control)

Re: Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

#14

Post by richard_todd »

I don’t see how the last sentence (the brain must model the environment) follows from the admission on the previous page that a regulator can skip the model by taking on unnecessary complexity. It seems there is a built-in assumption that the brain has no unnecessary complexity, if I am following correctly. I wouldn’t be so sure about that! (Although, I should add, the idea that the brain models its environment sounds intuitively true beyond question... I’m just trying to follow the arguments put forth in the paper itself.)

Re: Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

#16

Post by nabla9 »

amatic wrote: post #8

>> modelling might in fact be a necessary part of regulation. That seems false. PID is used all over, no models. Very simple, very effective.

Yes. That sentence is generally untrue if the context is not specified.

Either the authors made rudimentary error or they specified the context. If you read forward, they define the problem and discuss error and cause controlled regulation.

Re: Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

#18

Post by ahartmetz »

amatic wrote: post #8

>> modelling might in fact be a necessary part of regulation. That seems false. PID is used all over, no models. Very simple, very effective.

As a bad physicist ("physics is the art of approximation", some prof said during my studies): Indeed, you only have to model the relevant aspect(s) of the system, which may be completely unrecognizable as the system.

I guess it's somewhat more true in programming, because if you have only ten lines, can you even call it modeling?

That said, I like to say that the best kind of code teaches you something new abou the problem it's solving. Some invariant, widely usable simplification, etc. Still not the same as modeling the system as such, though.

Re: Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

#19

Post by aristus »

jessriedel wrote: post #5
aristus wrote: post #3

It feels like people keep rediscovering the implications of Turing-completness in different domains.

I don't think anyone doubts that being able to regulate a Turing machine would require a system that is Turing complete. But this seems very far from saying that regulating any system requires a complete (isomorphic) model it. Or did you mean something else?

A surprisingly large number of things that we normally don't think of as computers are Turing-complete. It's a very low bar of complexity. And regulating requires modeling requires simulation. That's what Turing's result says: to know whether a program halts you must run it.

Re: Every Good Regulator of a System Must Be a Model of That System (1970) [pdf]

#20

Post by euyyn »

maxerickson wrote: post #10
amatic wrote: post #8

>> modelling might in fact be a necessary part of regulation. That seems false. PID is used all over, no models. Very simple, very effective.

One way to look at a PID controller is that it uses a simple model that is correct in a lot of situations.

Yeah tuning the PID controller is in fact modeling the dynamics of the system. You can't really expect an untuned PID controller to just work.
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