Welcome to Wikipedia. Although everyone is welcome to make constructive contributions to Wikipedia, at least one of your recent edits, such as the one you made to Feedback, did not appear to be constructive and has been automatically reverted by ClueBot ... <snip> ClueBot ( talk) 20:32, 19 January 2009 (UTC)
Problem with Firefox during editing: characters were getting reversed. Browser wars? I was editing in Notepad and pasting into the browser to check the changes. Somehow, the lower part of the article got deleted. My bad! Trevithj ( talk) 20:29, 27 January 2009 (UTC)
I'd like to apologise for the comment "That was a controversial edit added by a disruptive editor who has been blocked several times." I wasn't referring to you but to Circuit dreamer, who was a principle player in a long edit conflict about introducing the term "negative resistance" into the harmonic oscillator section. I screwed up and confused the term "negative resistance" with "positive/negative feedback", the subject of your edit. In addition, although I meant "disruptive editor" to apply to CD, it looks like I was talking about you. I'd change it if I could, but I don't think there's any way of editing an edit summary. Anyway, I'm sorry for inadvertently dragging your name through the mud. ;) -- Chetvorno TALK 23:33, 29 February 2012 (UTC)
There appear to be three fairly distinct usages for "feedback" (FB) in general - and for "positive feedback" (+FB) by extension.
Three hypothetical cases are described below: one (hopefully) uncontroversial, and the others introducing some areas of ambiguity.
And the same would hold true for a Bear market: reduced prices, reduced investors etc.
Some care needs to be taken with how the 'signal' is defined. Should it include the effect of the outflow in this case?
To summarise, it is possible for one or both influences to be negative/inverting. This may lead to confusion as to how the FB should be defined. Consideration of the entire loop's effect doesn't seem to have this difficulty, but covers a lot of very different cases where signal/control descriptions are important and varied.
You have a nice collection of feedback stuff on your user page – even Chuck Wilts, who was my rock-climbing instructor at Caltech. Dicklyon ( talk) 04:29, 18 June 2014 (UTC)
Systems which utilize feedback for control purposes have become essential elements in modern technology. They range all the way from simple toys to our most complex automatic factories and production equipment. Feedback control is the guidance, more popularly known as "intelligence," that makes modern automation possible, and as such is in a large measure responsible for the ever-increasing productivity and rising standard of living of man. However, the science of feedback control stems not just from the importance of control systems but from the fact that the presence of feedback introduces problems that are peculiar to this class of systems. Examination of typical feedback control systems shows that they constitute a very broad group, with members that show remarkably different form and purpose. However, in spite of their great dissimilarities, they are related by one overriding feature, the use of feedback.
1-1 Definition of feedback.
Although the term feedback appears to have a very simple meaning, a completely general definition is surprisingly difficult. The existence of feedback, particularly when unwanted or of incidental occurrence, is often obscure and difficult to demonstrate. Nevertheless, when feedback is deliberately used for control, its existence and nature are easily ascertained. In a simple feedback system a specific physical quantity is being controlled, and control is brought about by making an actual comparison of this quantity with its desired value and utilizing the difference to reduce the error observed. Such a system is self-correcting in the sense that any deviations from the desired performance are used to produce corrective action. Whether this action is sufficient to eliminate the error is a complicated question that will be the subject matter of a later chapter, but in any case the corrective action must be dependent upon the existence of a difference which results from a comparison process. A system of the type described above can be represented by the diagram shown in Fig. 1-1. In later chapters a systematic symbolism and nomenclature will be introduced for such diagrams, but for present
Trevithj: Items 1 & 3 are clear enough. Item 2, balancing, is the rest of the universe I take it. It is perhaps the nature of any 'miscellaneous' category that it is not so clear, beyond what it doesn't include. My reading of this page and your user page quotes about feedback seem to suggest this category is empty. I'd say it includes the negative feedback amplifier, because this application does not use deviation from an established set-point. So it's not in category 1, and it seems obvious it is not in category 3. One might try to argue that the feedback loop designed to produce a gain of 1/β is a built-in set-point, but that still leaves out the other aspect of error-controlled regulation: the use of a 'gap' to govern the feedback. The departure of the gain from 1/β simply cannot happen in this circuit; the 'gap' is inherently (inevitably) zero (or as close to zero as you wish, by making the open-loop gain large enough). So regulation of a hypothetical 'gap' ("error" correction) is not an issue. Brews ohare ( talk) 13:10, 14 August 2014 (UTC)
"In the conventional negative feedback amplifier, it is difficult to easily correct the phase error and the amplitude error." [1]
"A feedback control system ... includes the necessary elements: the signal path, a means of sampling the output, processing of the feedback signal, and a means of reintroducing the error signal at the input." [2]
"Essential constituents of negative feedback amplifier ... input signal and feedback signal are mixed or processed to get difference or error signal to be applied to the internal or the basic amplifier." [3]
"For the feedback amplifier, an analog subtraction is achieved at the input and, to use feedback control-system notation, the output of the subtractor is an 'error signal'..." [4]
The term "negative feedback amplifier" that we are discussing here is the particular circuit of Black, and is not some complex circuit like Figure 9 in 1, called by these authors a "conventional negative feedback amplifier" involving an amazingly intricate input and feedback mechanism. The reference to "error signal" by Rao is used as a synonym for the difference between the input signal and the feedback signal, and is not an "error" signal in the sense of the departure of the monitored value of some essential variable from its set point. The circuit used in Breed is aimed at a feedback control system and Figure 1 includes a sampling comparator at its output. The negative feedback amplifier is an amplifier not a control device.
The "telling example", the source by Pederson, puts the term 'error signal' in quotes to suggest that the terminology is used in a special sense, and this difference is not always interpretable as an 'error'. Here this reference is treating the case where β≡1, which is the special case of a unity gain buffer where the feedback signal is the 'output', not a fraction of the output. In this application, the goal is to produce an exact copy of the input, so the difference between the input signal and the output signal is indeed an error. That is not the case when β is not 1. So I'd agree that the unity gain buffer can be seen as an example of error-controlled regulation, but that does not apply for the general case. Also, this example is unrelated to control of the gain itself, which is forced by the circuit to be 1/β≡1 regardless of feedback.
These sources do not address the basic issue that feedback does not govern gain control. That is "One might try to argue that the feedback loop designed to produce a gain of 1/β is a built-in set-point, but that still leaves out the other aspect of error-controlled regulation: the use of a 'gap' to govern the feedback. The departure of the gain from 1/β simply cannot happen in this circuit; the 'gap' is inherently (inevitably) zero (or as close to zero as you wish, by making the open-loop gain large enough). This fact of circuit topology has nothing to do with feedback or regulation. So regulation of a hypothetical 'gap' ("error" correction) is not an issue."
Can you address this point?
Thanks for your interest Trevith. Brews ohare ( talk) 14:50, 15 August 2014 (UTC)
1. If we accept that feedback opposes/reduces change, then we have to accept that "change" is a basic part of the definition. Otherwise we will have to come up with another definition that doesn't have a synonym for "change" in it.
2. If something changes, its new value and its old value are different. If we don't know the old value, we can't say for sure that there has been a change. So any change is by definition the difference between the old value and the new value.
3. If the feedback opposes this change, it is because the old value is somehow "more correct" than the new value. If that wasn't the case, the feedback wouldn't oppose the change.
4 If the feedback doesn't oppose this change, it is because the new value is somehow "more correct" than the old value. If that wasn't the case, the feedback would oppose the change. In all cases, if there is change, then there is a difference between two values. The feedback treats one of those values as "better" than the other, and responds accordingly. If that were not the case, the feedback wouldn't know which direction to adjust a value, or even if it should be adjusted. A gap is a synonym for a difference. Trevithj ( talk) 09:10, 16 August 2014 (UTC)
A useful discussion, Brews ohare ( talk) 12:31, 16 August 2014 (UTC)
BTW, your analogy "To say regulation of a gap is not an issue because there is no gap is a bit like saying that a fire-prevention program isn't required because no houses have burned down." is wide of the mark. Inasmuch as the gain is 1/β no matter what, and there is no possibility of "fire", the analogy is to say "a fire prevention program isn't required because there is no combustible material." Brews iohare ( talk) 15:04, 15 August 2014 (UTC)
It seems we are in full agreement. How then is it that you disagree on the "correct alternative" of the analogy? Trevithj ( talk) 06:38, 17 August 2014 (UTC)
You appear to ignore the requirement βA >> 1; try A = 106. It also assumes a mode of operation not actually in use, namely, the feedback is switched on and off. I suppose this is a thought experiment, but the rationale hasn't been outlined. More importantly, this arithmetic doesn't address the issue that the operation of this circuit does not involve reduction of a measured 'gap': defined as "actual gain -1/β". Brews ohare ( talk) 13:23, 18 August 2014 (UTC)
Although the negative feedback amplifier does result in a gain of 1/β, I imagine you will agree that the achievement of this goal by this circuit does not in itself imply that it achieves this result by the mechanism of error-controlled regulation? 1, Ashby: Chapter 12: The error-controlled regulator, pp. 219 ff Brews ohare ( talk) 14:09, 18 August 2014 (UTC)
As pointed out by your source, David Mindell, Black's patents and Bell Labs documents do not mention any error-controlled regulatory devices despite the fact that these systems were well-known at the time, going back before Minorsky's automatic pilot for ships used in 1923. 2 Prior to Minorsky's work "some acute observers...noted that the best human operators ... used both anticipation, backing off the power as the controlled variable approached the set-point, and small, slow adjustments when the error persisted. Sperry tried to incorporate these ideas into his devices... In 1922, Nicholas Minorsky presented a clear analysis of the control involved in position control systems and formulated a control law that we now refer to as three-term or PID control." 3
A cynic might suggest that the Bell Labs avoidance of mention of the PID controller was a deliberate act to enforce their patent claims and avoid any challenges that it was a variant of the established art. However, my opinion is that there is no connection. Brews ohare ( talk) 15:58, 18 August 2014 (UTC)
A negative or self-correcting feedback loop describes system behavior that opposes change
Balancing or negative feedback counteracts and opposes change
Negative feedback occurs when a change in input or action of the system is opposed by the output fed back
If the feedback signal reduces the input signal, i.e. it is out of phase with the input [signal], it is called negative feedback.
This discussion was initiated by the hypothesis that the negative feedback amplifier does not use error-controlled regulation. The null hypothesis is of course that it does use error-controlled regulation. While I can't say we can clearly reject the null hypothesis (yet), the search has been most informative and serves to reaffirm something I have suspected for a while: the negative feedback amplifier is a more complicated example than it seems! Even its inventor had trouble explaining the essential concepts, and I am beginning to see why.
I'm going to try and outline what I see as some of those complications here. In particular, there are two terms that I have found problematic. Some discussions (hopefully not this one) tend to lump them together, or be ambiguous in their usage.
The feedback keeps the overall gain close to the desired gain by adjusting the actual gain - not that straight-forward, especially considering that Aol isn't altered by any of this.
The other variables in the system can be derived from three variables: I, Aol and β. β is fixed, so the feedback responds to changes in I and Aol. Changes in I don't affect the overall gain, because the feedback changes I2 (and so actual gain) in direct proportion to the changes. Changes in Aol have some affect on overall gain, but relatively little. These two responses are difficult to untangle.
From an error-control POV, it seems at first glance that β is the reference value, because it is fixed. But then the essential variable is O. Does that make -βO the error signal? If so, what use does the summation component serve? On second glance, error-control would expect the reference and the measured values to be compared (in the summation component). But that would make -βO the measured value, I the reference value, and I2 the error value. While perhaps feasible, it is confusing to regard I as a reference.
So even if this could be made to work, the error-control approach is evidently an uncomfortable fit. The suggestion to go with a higher-level concept common to both makes a lot of sense. We may perhaps split hairs as to whether "opposes change" or "reduces change" is the more generic term, but I feel that the proposed wording has it right. Trevithj ( talk) 09:20, 23 August 2014 (UTC)
Mainly strategic. The distinction between error-control and desensitization is not easy to communicate. It is best left to a sub-section or the dedicated page, which can do it justice. If the amplifier features in the lede, it will continue to attract debate that clouds the main issue of generic definition.
Also, while "error" may be too narrow a term here, it is difficult to discuss "change" without some sort of reference point. So best to stick to examples with clear and agreed-upon reference points. Thermostats or steam governors would do a better job, IMO. Trevithj ( talk) 19:43, 27 August 2014 (UTC)
Apparently you are correct in thinking understanding the amplifier is too much for many. Of course, if they stuck to sources, the problem of dissuading them from erroneous opinions would be avoided. As things stand, this article will have a wrong statement of what is negative feedback, and will give the impression that it is limited to error-controlled regulation.
This unfortunate situation would not happen if WP policy on sources were observed. But that is not going to happen here, and WP is stuck with inaccuracy. Brews ohare ( talk) 01:36, 28 August 2014 (UTC)
Trevithj: If I is the input signal and O is the output signal, the gain is O/I , and the algebra from the block diagram then insists the gain is :
for any size of BA and there are no loopholes to say this is 'mainly' the gain. So I don't know what 'mainly' means in your remark. Perhaps you mean that the gain is not determined by the feedback value B? I think Kal means the gain is determined by the feedback circuit in the general case, and of course is 1/B if A is large enough. As you say, this is the overall gain, the closed-loop gain, but it is also the actual gain. I don't know what you mean by suggesting the actual and the closed-loop gain are different things.
I agree that the formula for the input to the A block, I-BO is not controversial, being simple algebra. Its value I/(1+BA) is determined by I, B, and A, and determines O as AI/(1+AB). I interpret GliderMaven's difficulties with all this stem from his confusion with error-controlled regulation. He thinks that the analysis of Kal is erroneous because it omits mention of adjusting deviation from a setpoint and so, in his view, misses the entire point of feedback. What GliderMaven misses is that his conception of feedback is only one form, and because all published analyses disagree with him that this form applies here, that should give him pause. Although one could take it that the feedback network implied a setpoint value for B there is no measurement of a 'gap' (by some invisible 'thermostat'), and no minimization or closing of a 'gap' by some (invisible) regulator. As Kal shows, desensitization to fluctuations can be achieved differently. Brews ohare ( talk) 05:23, 30 August 2014 (UTC)
Ashby's "essential variable" is one parameter (perhaps of a set) that determines the state of the system. The output O is not such a variable. For example, in some systems, there is no output, such as homeostatic systems that serve only to control the state of the system by setting the essential variables to their set point. Examples might be blood pressure, or body temperature where the only 'output' is a reaction that reduces the departure of the system from its proper condition. For process control, on the other hand, the process may convert some 'input' to some product 'output' and the output is a result of the system state. For example, a bottle washer accepts input as dirty bottles, output as clean bottles, and the essential variables might be water pressure, water temperature, and amount of soap sprayed on the bottles. The setpoints are established empirically or using some analytical model, departures are measured and trigger correction, constituting feedback, That kind of system is close to the amplifier, where the input is a signal (dirty bottle) and the output is a larger signal (clean bottle), and the result depends upon β, arguably an essential variable (like pressure, temperature, etc.). However, β is not controllable, so there is no error control on β and we just hope the feedback network doesn't wander off base. If βA is small, however, the output signal does depend upon A, and A is not predictable. Unfortunately perhaps, the negative feedback amplifier has no mechanism to monitor A, nor to affect its essential variables. So there is no error control involving A.
One can imagine a more elaborate bottle washer in which the output did affect the system: that could happen if the setpoints were adjusted depending upon how clean the bottles are. So some optical assessment of 'clean' could be measured at the output and sent to an algorithm relating 'clean' to temperature, pressure, and quantity of soap, and the setpoints adjusted. Perhaps that would automate the system to deal with different species of 'dirty', maybe just a classification like 'tough to clean - use a power wash cycle' and 'easy to clean - use a rinse cycle'. This use of the output to control system operation seems to introduce something different from the notion of an 'essential variable' because a property of the output (how 'clean' it is) is a product characteristic, not itself a system parameter. However, the amplifier does not fit this situation because no setpoint adjustments occur. In particular, the output signal doesn't affect β or A, and so is not used to change the internal state of the amplifier.
A third possibility has occurred to me in which feedback from the output affects the input. An example would be a bottle washer where the monitoring of 'clean' at the output is used to adjust the orientation of bottles entering the washer, or perhaps to adjust the conveyer speed (neither of which is under jurisdiction of the essential variables pressure, temperature, or amount of soap). The amplifier might be thought of this way, as the output seems to change the input as suggested by I-βO. However, with the amplifier there is no monitoring of the output signal to see whether it meets some criterion (like 'clean' or 'not clean'). In particular, the output signal amplitude is not monitored to see if it is 1/β times the input signal. The setting of the gain by the feedback network is determined by circuit topology as the response to the (so-called) 'error' I/(1+βA). That is to say, not by adaptation to the output signal, but set by the input signal I and the internal fixed values of β and A. One could imagine a more sophisticated amplifier, but it wouldn't be the simple negative feedback amplifier under discussion.
The upshot is this: none of these forms of error control apply. These points may be clear to you already. I apologize for any belaboring. Brews ohare ( talk) 22:57, 23 August 2014 (UTC)
Thanks for your prompt provision of full details for one of the reviews at Cybernetics:_Or_Control_and_Communication_in_the_Animal_and_the_Machine, plus the addition of a couple more excellent references. Good work also on the (bizarrely troublesome) discussions at Negative feedback. DaveApter ( talk) 10:22, 3 September 2014 (UTC)
Hi Trevith: I am taken aback by your support of the suppression of this discussion with Dicklyon. Of course, I understand your position is that the error signal Se=Si./(1+βA), although determined by the variables Si., A and β, is not determined separately from the feedback circuit because you believe these parameters are themselves not determined by the signal source, the open-loop amplifier and the feedback network operated in isolation from the feedback circuit. This contention does not square with Rashid's customary textbook formulation, his Eqs. 10.1-10.3, but there it is.
Dicklyon has a more specific view of the failure of the textbook formulation, that although the 'input-only' form of the error signal is in fact decided by variables whose values can be found externally, a different point is that the form Se=Si./(1+βA) rests upon the basic assumption that the gain A of the open-loop amplifier is defined by So=A Si, and this definition is inapplicable within a more general view of the feedback amplifier (more general than Rashid's and other textbook formulations), especially where noise is considered that can result in an output even with no input. This viewpoint appears possible, although so far completely unsourced, so I don't understand why you do not support its discussion. Brews ohare ( talk) 17:04, 1 November 2014 (UTC)
My point of concern is this: it appears that there is something you are working very hard at not seeing. Trevithj ( talk) 07:16, 4 November 2014 (UTC)
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Welcome to Wikipedia. Although everyone is welcome to make constructive contributions to Wikipedia, at least one of your recent edits, such as the one you made to Feedback, did not appear to be constructive and has been automatically reverted by ClueBot ... <snip> ClueBot ( talk) 20:32, 19 January 2009 (UTC)
Problem with Firefox during editing: characters were getting reversed. Browser wars? I was editing in Notepad and pasting into the browser to check the changes. Somehow, the lower part of the article got deleted. My bad! Trevithj ( talk) 20:29, 27 January 2009 (UTC)
I'd like to apologise for the comment "That was a controversial edit added by a disruptive editor who has been blocked several times." I wasn't referring to you but to Circuit dreamer, who was a principle player in a long edit conflict about introducing the term "negative resistance" into the harmonic oscillator section. I screwed up and confused the term "negative resistance" with "positive/negative feedback", the subject of your edit. In addition, although I meant "disruptive editor" to apply to CD, it looks like I was talking about you. I'd change it if I could, but I don't think there's any way of editing an edit summary. Anyway, I'm sorry for inadvertently dragging your name through the mud. ;) -- Chetvorno TALK 23:33, 29 February 2012 (UTC)
There appear to be three fairly distinct usages for "feedback" (FB) in general - and for "positive feedback" (+FB) by extension.
Three hypothetical cases are described below: one (hopefully) uncontroversial, and the others introducing some areas of ambiguity.
And the same would hold true for a Bear market: reduced prices, reduced investors etc.
Some care needs to be taken with how the 'signal' is defined. Should it include the effect of the outflow in this case?
To summarise, it is possible for one or both influences to be negative/inverting. This may lead to confusion as to how the FB should be defined. Consideration of the entire loop's effect doesn't seem to have this difficulty, but covers a lot of very different cases where signal/control descriptions are important and varied.
You have a nice collection of feedback stuff on your user page – even Chuck Wilts, who was my rock-climbing instructor at Caltech. Dicklyon ( talk) 04:29, 18 June 2014 (UTC)
Systems which utilize feedback for control purposes have become essential elements in modern technology. They range all the way from simple toys to our most complex automatic factories and production equipment. Feedback control is the guidance, more popularly known as "intelligence," that makes modern automation possible, and as such is in a large measure responsible for the ever-increasing productivity and rising standard of living of man. However, the science of feedback control stems not just from the importance of control systems but from the fact that the presence of feedback introduces problems that are peculiar to this class of systems. Examination of typical feedback control systems shows that they constitute a very broad group, with members that show remarkably different form and purpose. However, in spite of their great dissimilarities, they are related by one overriding feature, the use of feedback.
1-1 Definition of feedback.
Although the term feedback appears to have a very simple meaning, a completely general definition is surprisingly difficult. The existence of feedback, particularly when unwanted or of incidental occurrence, is often obscure and difficult to demonstrate. Nevertheless, when feedback is deliberately used for control, its existence and nature are easily ascertained. In a simple feedback system a specific physical quantity is being controlled, and control is brought about by making an actual comparison of this quantity with its desired value and utilizing the difference to reduce the error observed. Such a system is self-correcting in the sense that any deviations from the desired performance are used to produce corrective action. Whether this action is sufficient to eliminate the error is a complicated question that will be the subject matter of a later chapter, but in any case the corrective action must be dependent upon the existence of a difference which results from a comparison process. A system of the type described above can be represented by the diagram shown in Fig. 1-1. In later chapters a systematic symbolism and nomenclature will be introduced for such diagrams, but for present
Trevithj: Items 1 & 3 are clear enough. Item 2, balancing, is the rest of the universe I take it. It is perhaps the nature of any 'miscellaneous' category that it is not so clear, beyond what it doesn't include. My reading of this page and your user page quotes about feedback seem to suggest this category is empty. I'd say it includes the negative feedback amplifier, because this application does not use deviation from an established set-point. So it's not in category 1, and it seems obvious it is not in category 3. One might try to argue that the feedback loop designed to produce a gain of 1/β is a built-in set-point, but that still leaves out the other aspect of error-controlled regulation: the use of a 'gap' to govern the feedback. The departure of the gain from 1/β simply cannot happen in this circuit; the 'gap' is inherently (inevitably) zero (or as close to zero as you wish, by making the open-loop gain large enough). So regulation of a hypothetical 'gap' ("error" correction) is not an issue. Brews ohare ( talk) 13:10, 14 August 2014 (UTC)
"In the conventional negative feedback amplifier, it is difficult to easily correct the phase error and the amplitude error." [1]
"A feedback control system ... includes the necessary elements: the signal path, a means of sampling the output, processing of the feedback signal, and a means of reintroducing the error signal at the input." [2]
"Essential constituents of negative feedback amplifier ... input signal and feedback signal are mixed or processed to get difference or error signal to be applied to the internal or the basic amplifier." [3]
"For the feedback amplifier, an analog subtraction is achieved at the input and, to use feedback control-system notation, the output of the subtractor is an 'error signal'..." [4]
The term "negative feedback amplifier" that we are discussing here is the particular circuit of Black, and is not some complex circuit like Figure 9 in 1, called by these authors a "conventional negative feedback amplifier" involving an amazingly intricate input and feedback mechanism. The reference to "error signal" by Rao is used as a synonym for the difference between the input signal and the feedback signal, and is not an "error" signal in the sense of the departure of the monitored value of some essential variable from its set point. The circuit used in Breed is aimed at a feedback control system and Figure 1 includes a sampling comparator at its output. The negative feedback amplifier is an amplifier not a control device.
The "telling example", the source by Pederson, puts the term 'error signal' in quotes to suggest that the terminology is used in a special sense, and this difference is not always interpretable as an 'error'. Here this reference is treating the case where β≡1, which is the special case of a unity gain buffer where the feedback signal is the 'output', not a fraction of the output. In this application, the goal is to produce an exact copy of the input, so the difference between the input signal and the output signal is indeed an error. That is not the case when β is not 1. So I'd agree that the unity gain buffer can be seen as an example of error-controlled regulation, but that does not apply for the general case. Also, this example is unrelated to control of the gain itself, which is forced by the circuit to be 1/β≡1 regardless of feedback.
These sources do not address the basic issue that feedback does not govern gain control. That is "One might try to argue that the feedback loop designed to produce a gain of 1/β is a built-in set-point, but that still leaves out the other aspect of error-controlled regulation: the use of a 'gap' to govern the feedback. The departure of the gain from 1/β simply cannot happen in this circuit; the 'gap' is inherently (inevitably) zero (or as close to zero as you wish, by making the open-loop gain large enough). This fact of circuit topology has nothing to do with feedback or regulation. So regulation of a hypothetical 'gap' ("error" correction) is not an issue."
Can you address this point?
Thanks for your interest Trevith. Brews ohare ( talk) 14:50, 15 August 2014 (UTC)
1. If we accept that feedback opposes/reduces change, then we have to accept that "change" is a basic part of the definition. Otherwise we will have to come up with another definition that doesn't have a synonym for "change" in it.
2. If something changes, its new value and its old value are different. If we don't know the old value, we can't say for sure that there has been a change. So any change is by definition the difference between the old value and the new value.
3. If the feedback opposes this change, it is because the old value is somehow "more correct" than the new value. If that wasn't the case, the feedback wouldn't oppose the change.
4 If the feedback doesn't oppose this change, it is because the new value is somehow "more correct" than the old value. If that wasn't the case, the feedback would oppose the change. In all cases, if there is change, then there is a difference between two values. The feedback treats one of those values as "better" than the other, and responds accordingly. If that were not the case, the feedback wouldn't know which direction to adjust a value, or even if it should be adjusted. A gap is a synonym for a difference. Trevithj ( talk) 09:10, 16 August 2014 (UTC)
A useful discussion, Brews ohare ( talk) 12:31, 16 August 2014 (UTC)
BTW, your analogy "To say regulation of a gap is not an issue because there is no gap is a bit like saying that a fire-prevention program isn't required because no houses have burned down." is wide of the mark. Inasmuch as the gain is 1/β no matter what, and there is no possibility of "fire", the analogy is to say "a fire prevention program isn't required because there is no combustible material." Brews iohare ( talk) 15:04, 15 August 2014 (UTC)
It seems we are in full agreement. How then is it that you disagree on the "correct alternative" of the analogy? Trevithj ( talk) 06:38, 17 August 2014 (UTC)
You appear to ignore the requirement βA >> 1; try A = 106. It also assumes a mode of operation not actually in use, namely, the feedback is switched on and off. I suppose this is a thought experiment, but the rationale hasn't been outlined. More importantly, this arithmetic doesn't address the issue that the operation of this circuit does not involve reduction of a measured 'gap': defined as "actual gain -1/β". Brews ohare ( talk) 13:23, 18 August 2014 (UTC)
Although the negative feedback amplifier does result in a gain of 1/β, I imagine you will agree that the achievement of this goal by this circuit does not in itself imply that it achieves this result by the mechanism of error-controlled regulation? 1, Ashby: Chapter 12: The error-controlled regulator, pp. 219 ff Brews ohare ( talk) 14:09, 18 August 2014 (UTC)
As pointed out by your source, David Mindell, Black's patents and Bell Labs documents do not mention any error-controlled regulatory devices despite the fact that these systems were well-known at the time, going back before Minorsky's automatic pilot for ships used in 1923. 2 Prior to Minorsky's work "some acute observers...noted that the best human operators ... used both anticipation, backing off the power as the controlled variable approached the set-point, and small, slow adjustments when the error persisted. Sperry tried to incorporate these ideas into his devices... In 1922, Nicholas Minorsky presented a clear analysis of the control involved in position control systems and formulated a control law that we now refer to as three-term or PID control." 3
A cynic might suggest that the Bell Labs avoidance of mention of the PID controller was a deliberate act to enforce their patent claims and avoid any challenges that it was a variant of the established art. However, my opinion is that there is no connection. Brews ohare ( talk) 15:58, 18 August 2014 (UTC)
A negative or self-correcting feedback loop describes system behavior that opposes change
Balancing or negative feedback counteracts and opposes change
Negative feedback occurs when a change in input or action of the system is opposed by the output fed back
If the feedback signal reduces the input signal, i.e. it is out of phase with the input [signal], it is called negative feedback.
This discussion was initiated by the hypothesis that the negative feedback amplifier does not use error-controlled regulation. The null hypothesis is of course that it does use error-controlled regulation. While I can't say we can clearly reject the null hypothesis (yet), the search has been most informative and serves to reaffirm something I have suspected for a while: the negative feedback amplifier is a more complicated example than it seems! Even its inventor had trouble explaining the essential concepts, and I am beginning to see why.
I'm going to try and outline what I see as some of those complications here. In particular, there are two terms that I have found problematic. Some discussions (hopefully not this one) tend to lump them together, or be ambiguous in their usage.
The feedback keeps the overall gain close to the desired gain by adjusting the actual gain - not that straight-forward, especially considering that Aol isn't altered by any of this.
The other variables in the system can be derived from three variables: I, Aol and β. β is fixed, so the feedback responds to changes in I and Aol. Changes in I don't affect the overall gain, because the feedback changes I2 (and so actual gain) in direct proportion to the changes. Changes in Aol have some affect on overall gain, but relatively little. These two responses are difficult to untangle.
From an error-control POV, it seems at first glance that β is the reference value, because it is fixed. But then the essential variable is O. Does that make -βO the error signal? If so, what use does the summation component serve? On second glance, error-control would expect the reference and the measured values to be compared (in the summation component). But that would make -βO the measured value, I the reference value, and I2 the error value. While perhaps feasible, it is confusing to regard I as a reference.
So even if this could be made to work, the error-control approach is evidently an uncomfortable fit. The suggestion to go with a higher-level concept common to both makes a lot of sense. We may perhaps split hairs as to whether "opposes change" or "reduces change" is the more generic term, but I feel that the proposed wording has it right. Trevithj ( talk) 09:20, 23 August 2014 (UTC)
Mainly strategic. The distinction between error-control and desensitization is not easy to communicate. It is best left to a sub-section or the dedicated page, which can do it justice. If the amplifier features in the lede, it will continue to attract debate that clouds the main issue of generic definition.
Also, while "error" may be too narrow a term here, it is difficult to discuss "change" without some sort of reference point. So best to stick to examples with clear and agreed-upon reference points. Thermostats or steam governors would do a better job, IMO. Trevithj ( talk) 19:43, 27 August 2014 (UTC)
Apparently you are correct in thinking understanding the amplifier is too much for many. Of course, if they stuck to sources, the problem of dissuading them from erroneous opinions would be avoided. As things stand, this article will have a wrong statement of what is negative feedback, and will give the impression that it is limited to error-controlled regulation.
This unfortunate situation would not happen if WP policy on sources were observed. But that is not going to happen here, and WP is stuck with inaccuracy. Brews ohare ( talk) 01:36, 28 August 2014 (UTC)
Trevithj: If I is the input signal and O is the output signal, the gain is O/I , and the algebra from the block diagram then insists the gain is :
for any size of BA and there are no loopholes to say this is 'mainly' the gain. So I don't know what 'mainly' means in your remark. Perhaps you mean that the gain is not determined by the feedback value B? I think Kal means the gain is determined by the feedback circuit in the general case, and of course is 1/B if A is large enough. As you say, this is the overall gain, the closed-loop gain, but it is also the actual gain. I don't know what you mean by suggesting the actual and the closed-loop gain are different things.
I agree that the formula for the input to the A block, I-BO is not controversial, being simple algebra. Its value I/(1+BA) is determined by I, B, and A, and determines O as AI/(1+AB). I interpret GliderMaven's difficulties with all this stem from his confusion with error-controlled regulation. He thinks that the analysis of Kal is erroneous because it omits mention of adjusting deviation from a setpoint and so, in his view, misses the entire point of feedback. What GliderMaven misses is that his conception of feedback is only one form, and because all published analyses disagree with him that this form applies here, that should give him pause. Although one could take it that the feedback network implied a setpoint value for B there is no measurement of a 'gap' (by some invisible 'thermostat'), and no minimization or closing of a 'gap' by some (invisible) regulator. As Kal shows, desensitization to fluctuations can be achieved differently. Brews ohare ( talk) 05:23, 30 August 2014 (UTC)
Ashby's "essential variable" is one parameter (perhaps of a set) that determines the state of the system. The output O is not such a variable. For example, in some systems, there is no output, such as homeostatic systems that serve only to control the state of the system by setting the essential variables to their set point. Examples might be blood pressure, or body temperature where the only 'output' is a reaction that reduces the departure of the system from its proper condition. For process control, on the other hand, the process may convert some 'input' to some product 'output' and the output is a result of the system state. For example, a bottle washer accepts input as dirty bottles, output as clean bottles, and the essential variables might be water pressure, water temperature, and amount of soap sprayed on the bottles. The setpoints are established empirically or using some analytical model, departures are measured and trigger correction, constituting feedback, That kind of system is close to the amplifier, where the input is a signal (dirty bottle) and the output is a larger signal (clean bottle), and the result depends upon β, arguably an essential variable (like pressure, temperature, etc.). However, β is not controllable, so there is no error control on β and we just hope the feedback network doesn't wander off base. If βA is small, however, the output signal does depend upon A, and A is not predictable. Unfortunately perhaps, the negative feedback amplifier has no mechanism to monitor A, nor to affect its essential variables. So there is no error control involving A.
One can imagine a more elaborate bottle washer in which the output did affect the system: that could happen if the setpoints were adjusted depending upon how clean the bottles are. So some optical assessment of 'clean' could be measured at the output and sent to an algorithm relating 'clean' to temperature, pressure, and quantity of soap, and the setpoints adjusted. Perhaps that would automate the system to deal with different species of 'dirty', maybe just a classification like 'tough to clean - use a power wash cycle' and 'easy to clean - use a rinse cycle'. This use of the output to control system operation seems to introduce something different from the notion of an 'essential variable' because a property of the output (how 'clean' it is) is a product characteristic, not itself a system parameter. However, the amplifier does not fit this situation because no setpoint adjustments occur. In particular, the output signal doesn't affect β or A, and so is not used to change the internal state of the amplifier.
A third possibility has occurred to me in which feedback from the output affects the input. An example would be a bottle washer where the monitoring of 'clean' at the output is used to adjust the orientation of bottles entering the washer, or perhaps to adjust the conveyer speed (neither of which is under jurisdiction of the essential variables pressure, temperature, or amount of soap). The amplifier might be thought of this way, as the output seems to change the input as suggested by I-βO. However, with the amplifier there is no monitoring of the output signal to see whether it meets some criterion (like 'clean' or 'not clean'). In particular, the output signal amplitude is not monitored to see if it is 1/β times the input signal. The setting of the gain by the feedback network is determined by circuit topology as the response to the (so-called) 'error' I/(1+βA). That is to say, not by adaptation to the output signal, but set by the input signal I and the internal fixed values of β and A. One could imagine a more sophisticated amplifier, but it wouldn't be the simple negative feedback amplifier under discussion.
The upshot is this: none of these forms of error control apply. These points may be clear to you already. I apologize for any belaboring. Brews ohare ( talk) 22:57, 23 August 2014 (UTC)
Thanks for your prompt provision of full details for one of the reviews at Cybernetics:_Or_Control_and_Communication_in_the_Animal_and_the_Machine, plus the addition of a couple more excellent references. Good work also on the (bizarrely troublesome) discussions at Negative feedback. DaveApter ( talk) 10:22, 3 September 2014 (UTC)
Hi Trevith: I am taken aback by your support of the suppression of this discussion with Dicklyon. Of course, I understand your position is that the error signal Se=Si./(1+βA), although determined by the variables Si., A and β, is not determined separately from the feedback circuit because you believe these parameters are themselves not determined by the signal source, the open-loop amplifier and the feedback network operated in isolation from the feedback circuit. This contention does not square with Rashid's customary textbook formulation, his Eqs. 10.1-10.3, but there it is.
Dicklyon has a more specific view of the failure of the textbook formulation, that although the 'input-only' form of the error signal is in fact decided by variables whose values can be found externally, a different point is that the form Se=Si./(1+βA) rests upon the basic assumption that the gain A of the open-loop amplifier is defined by So=A Si, and this definition is inapplicable within a more general view of the feedback amplifier (more general than Rashid's and other textbook formulations), especially where noise is considered that can result in an output even with no input. This viewpoint appears possible, although so far completely unsourced, so I don't understand why you do not support its discussion. Brews ohare ( talk) 17:04, 1 November 2014 (UTC)
My point of concern is this: it appears that there is something you are working very hard at not seeing. Trevithj ( talk) 07:16, 4 November 2014 (UTC)
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