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Systems: Control theory Start‑class High‑importance

This article is within the scope of WikiProject Systems, which collaborates on articles related to systems and systems science.SystemsWikipedia:WikiProject SystemsTemplate:WikiProject SystemsSystems Start This article has been rated as Start-class on Misplaced Pages's content assessment scale. High This article has been rated as High-importance on the project's importance scale. This article is within the field of Control theory.

Discussion 2004

The present form of the article describes the term "feed-forward" in a confusing way. At least as far as uses in aspects of "control theory" are concerned "feed-forward" and "open loop" have more in common than the corresponding current Misplaced Pages articles teach. It would be great if some experts voiced their thoughts and could share them with the public - thanks.

My first thought on seeing this page was that it would be best moved to Feed-forward neural network or some such, but looking at what links to it, it seems you are intending more of a page about the term "feed-forward" in general. My worry in that case is that you'll find yourself with not enough to write, and will be informed that Misplaced Pages is not a dictionary. I guess that depends what other fields use the term: perhaps it could end up as more of a disambiguation page - IMSoP 01:44, 4 Apr 2004 (UTC) (I'm not following you around, honest, I just fired up recentchanges and saw something I recognised)

Eh, physiology? Is this some new kind of "living perceptron"? Oh well, I guess I'll stop interrupting you and assume you know what you're doing. The reason I put "refers to" rather than "is", by the way, is because "feed-forward" isn't a noun - or I've never seen it used as one, anyway; so if you wanted to use is, you'd have to have a noun for the adjective feed-forward to modify: "...a feed-forward network is one which..." - IMSoP 02:05, 4 Apr 2004 (UTC)

Firstly, thanks. I like it when people review what I enter.

Secondly, I think you may be on the right track (I will move this page to "Feed-foreward regulatory network". Bensaccount 02:08, 4 Apr 2004 (UTC)

I guess I was trying to merge OneLook dictionary and textbook information in an attempt to figure out what "feed-forward" was. Bensaccount 02:19, 4 Apr 2004 (UTC)

Well, I can only comment on what I know, but in Artificial Intelligence, a feed-forward network would be an artificial neural network - such as a simple multi-layer perceptron - which had multiple layers connected in series, such that no neuron received input from any neuron to which its own output contributed. In other words, one in which there is no feedback from later processing to influence earlier layers. Such architectures allow more complex processing than single-layer networks, but are more easily analysable than more complex architectures such as fully-connected networks.

I won't copy that onto the page just yet, because I'm not entirely sure whether that was or wasn't the sense of the term you were originally aiming to explain, but I think you have become confused if you are associating perceptrons with physiology. - IMSoP 19:18, 4 Apr 2004 (UTC)

Feed-forward seems to be used in two contexts: All I know is that in the physiology context, feedforward is a type of neural regulatory system of the nervous system. I dont know the computing meaning. I seem to have mixed the two meaning up because I didn't realize they were different things. If you can fix it you should, I will be back later.Bensaccount 20:04, 4 Apr 2004 (UTC)

Quoting from the entry:

When a hill is encountered the car slows down below the set speed. This speed error causes the engine throttle to be opened further, bringing the car back almost to its original speed. Almost its original speed because a feedback system needs some residual error that can be multiplied by loop gain to provide the necessary correction factor for the duration of the hill.

This is incorrect, but I'm not sure how to succinctly explain what is wrong. I'll go on about it here, instead.  :-)

What the article says is partly true: for a feedback control system which has only a proportional term, there will always be a residual error which the proportional term is unable to resolve.

In the early days of closed-loop control (in factories, say of a chemical process), people first discovered you could regulate the output rate of a process by measuring the deviation from a desired rate (set-point) and then altering the input flows by an amount proportional to that deviation.

The constant of proportionality was determined empirically. Immediately, folks noticed that the output of the system never quite reached the set point, but always ended up a bit below it. The operators quickly realized that they could measure this steady-state error and nudge the input controls up a bit by hand, bringing the system up to the exact desired set point value. As the system approaches the set point, the error drops to zero and so this so-called "integral term" in the feedback equation ceases to affect the systems behavior.

The operators were doing something akin to integrating the residual error over time, and adding that (times a constant of proportionality) back to the input rates. Soon, people realized that this too could be done automatically.

And the car's cruise control, being just such a system, does the same thing. It doesn't need any residual error to maintain the velocity; it simply compares the velocity to the set point and alters the throttle setting according to the control law of the system. See also "PID loop control".

Agreed, the page is incorrect, I was quite shocked to read something so wrong on the Wiki, I'll try to fudge it, but I don't really get what the point the person writing it was trying to make.

Hello chaps. This is a tricky one. As a cybernetician a strict distinction between feedforward and feedback to me has always seemed elusive. Just because an envirionmemtal parameter is monitored doesn't necessarily imply feedforward. If the controlling model is seen as forecasting it still need not imply feedforward. Anyway this graphic may help. It's redrawn after Fig 1-1 in Mrosovsky's "Rheostasis" OUP 1990. It's defintely worth discussing!

Incidentally someone ought to discuss homeostasis or homeodynamics as Yates calls it, in one of these Wiki entries on Feedback. Stasis seems incorrect for an essentially dynamic process even though convergence to a fixed point is implied. Mrosovsky's fixed point moves hence his title but whither the much loved Rheostat of the electrical laboratory? This probably a bit off topic and should be raised in the Homeostasis entry but maybe it illuminates some of the difficulties.--Nick Green 17:08, 2 September 2006 (UTC)

Discussion 2009

The whole subject of feedback and open loop control seems to be badly understood everywhere, not just here on Wiki. I tried to get some help in improving these pages by reading Encyclopedia Brittanica but that was equally bad.

I will try to make it all a bit clearer, but it is going to be difficult since I am not an expert. OTOH, all the experts these days are so specialised it probably needs someone like me to provide a more general overview. A B McDonald (talk) 12:59, 21 April 2009 (UTC)

The new diagram is partly based on that by Nick Green on this page. I'll wait to see if there are any reactions to my changes before going on to correct the feedback articles. A B McDonald (talk) 15:30, 21 April 2009 (UTC)

Hi Folks

How annoying (but tantalising), to come across such a relatively common but poorly defined term. I have encountered it often in neuroscience. For its meaning here, can I suggest:

"In neuroscience, feedforward describes a neural system where the signal travels from input, through various synapses, to output, and where the input signal does not vary according to the effect of the environment or later elements of the system. Contrasted with feedback where the input signal is modified by the response of later parts of the system or the system's environment"?

This seems to be the sense in which it is used in neuroscience but I've never encountered a definition, only derived this from context - though it does seem to harmonise with Feedforward neural network... Anthony (talk) 07:08, 8 July 2009 (UTC)

-- Though I haven't witnessed or been involved in much of the past of this article, I suggest, for starters, that a split be considered. I'm an electrical engineer and having biological/psychological elements mixed in with control theory is a bit bothersome. It also slightly confuses me when I try to look for places to make additions. (A textbook I own gave me some good ideas about examples to use regarding control theory, such as how feed-forward techniques improve the tracking of a sinusoidal or parabolic signal.)

Just a thought, my two cents.

98.238.110.152 (talk) 16:21, 30 December 2009 (UTC) R. Smith

IMHO you should add new sections as needed under Applications. That is where these items appear in Feedback entry. I notice that on the Feedback page the applications have been ordered alphabetically, so if you are adding any it might be worthwhile reordering the ones here. If I were you I would go ahead and make the changes. I don't think that you will receive any criticism, unlike me when I try to insert my diagram into the Feedback pages :-( A B McDonald (talk) 20:56, 16 January 2010 (UTC)

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