Hi to all !
BH: Thanks for your answers. It doesn’t seem to me that I was trying to
define anything, I just used the definition I found in B:CP about
feedforward and try some examples (experiences) from everyday life (or
with free-livingorganisms) to find out, whether feedforward correspond to evolution
necessity of development in histroy of organisms or not.
BP: It’s important to define feedforward, because it’s easy to forget
that there is no feedback from the result when feedforward is the only
method of control. If the feedforward action is wrong, it will continue
to be wrong every time. If there is anything wrong with the computation
used to calculate the action that is needed, it will never be corrected.
If there is a disturbance that makes the result different from the
calculated result, that difference will never be corrected.
BH: I offered 4 cases and
neither was answered directly. Only some abstractdefinitions.
BP: I thought I would point out some of the details about feedfoward so
you could discover for yourself any problems in your examples, rather
than just telling you about them. I find that to be a useful way to find
out when I have made a mistake, and maybe less embarrassing than having
someone else point it out.
BH: I still think that
however we define feedforward (anticipation,prediction, reflex) it has it’s function in maintaining the
essentialvariables in their limits.
BP: Yes, that’s the result we want, but unfortunately in feedforward
control the “limits” have no effect, because by definition the
feedforward system never learns the results of its actions and has no way
of correcting errors. We can hope that the feedforward system
“maintains essential variables in their limits,” but if it does
not, it will not change its behavior. Only feedback systems sense the
results of their behavior and change their actions if the results are not
right.
BH: So I still think that
development of feedforward was somehow necessary from the perspective of
evolution. I’ll still remain with definition of feedforward given in B:CP
(p.48, 2005)
BP: So will I. But you have to know more about the details to arrive at
correct conclusions. Feedforward means acting without feedback. If the
system somehow is adjusted by some external agency so it acts correctly,
all will be well – until something changes in the environment or inside
the system itself. Then the actions will generate the wrong result, and
will continue to do so.
BH: 1. CASE NATURE AND FREE
LIVING ORGANISMS : It was about how free-livingorganisms in wild nature respond to some signs of predator with
feed-forward. The problem was how could we describe what’s happening
insideanimals, for example antilope, who senses the predator (probably
possibleactual disturbance to essencial variables) and starts suddenly running
awayand there is no attack of predator (for example lion) or the
predatorattacks anyway, after she starts running.
BP: Here the problem is, how does the feedforward system know which
direction is ‘away’? If the antelope just starts running in some random
direction, it is as likely to be running toward the danger as away from
it. This would save the antelopes who happened to be running mostly away
from the danger, but not the others. That’s better, evolution-wise, than
having all the antelopes killed, but it doesn’t give the next generation
any advantage because survival was by pure chance, not because of any
mutation in the animal’s behavior.
It would be much better for antelopes if they could learn to perceive a
relationship between their own positions and the positions of predators,
and control this relationship so as to be sure of increasing the distance
between them and the predator every time, no matter where the predator
is, and no matter whether there is more than one predator (it doesn’t
help much to run away from one predator if that means running toward
another).
BH: I still think that
feed-forward saves life to antilope. We can see that practically everyday
on NG WILD.
BP: No. What you see is antelopes running away from predators. You
imagine that this is being done blindly, without regard to the
relationship between the direction of running and the direction of the
predator. In wildlife programs I have seen, the prey can be seen altering
its path as predators approach and try to cut off its escape, which
suggests that the actions are being varied to counteract a perceived
decrease in distance from the predators.
BH: If I try to follow
definition in B:CP (2005) can we say that antilope issensing the cause of the disturbance (lion) and she anticipates it’s
effects(deadly attack of the lion) thus improving control.
BP: This does not improve control of the relationship to the lion; it
improves the decision about when to start controlling it, a higher-level
process. Once the antelope start to evade the lion, feedback control is
essential; all the animals that simply rushed around without looking to
see the results have been eaten by now, so none are left to
reproduce.
BH: What happens in
antilope? Can we simply say that antilope is anticipating (predicting)
future dangerous situation on the bases of smell and counteract to
something what will acctually occur or never occurs.
BP: Reacting on the basis of smell is very chancy, since smell carries no
directional information (except perhaps that the source is somewhere in
the direction from which the wind is blowing, if there is one). The best
feedforward action that might occur is to start looking around more
alertly, trying to locate where the lion is, so that the action can be
adjusted appropriately to the error.
BH: Whatever happens on
the basis of feedforward, it’s more efficiently than pure control. It’s
maintaining antilope’s essential variables stable, what would not be the
case with only classic control.
BP: I don’t understand why you say that. Feedback control is always more
efficient than feedfoward, when it is possible. If it’s not possible (the
antelope can smell the lion but can’t see it), feedforward may offer a
better chance of survival than doing nothing would offer, but it’s still
not a very good chance. You just assume that feedforward would maintain
the antelope’s essential variables stable, but that’s not enough: you
have to give some reason for saying that this desireable result would
actually be achieved. Under some conditions feedforward can reduce the
variations in essential variables that would otherwise occur, at least
with some probability. But feedback control, when possible, will always
do that better.
BH: Whatever is happening, it’s
saving her life.
BP: You assume it saves her life, but there is a quite large chance that
it won’t.
BH: If she stays and the attack
of predator begins (IN CONTROL MODE OF ANTILOPE) she has less chances to
escape and so to survive or there is even no chances.
BP: But if she sees the lion and is able to run in a direction that is
actually away from the lion, she then has an even better chance of
surviving.
BH: 2. CASE : was about the
drivers and pedestrians. If the driver iscontrolling only speed of the car and course of the car and turn right
inthe crossing, he could run over the pedestrian. If he is controlling
thespeed of the car and the speed of the pedestrian, they both can come
tocrossing at the same time and driver would be forced to stop the car
immediately so risking that some driver behind him bump into his car.
If the driver is anticipating (predicting) the course and speed of his
carand course and speed of the pedestrian, he improves control and make
savedeccision whether to stop or accelerate and safely turn in
crossing.
BP: But now you’re assuming a control system. Feedforward systems don’t
sense the results they produce and change their behavior
accordingly.
BH: So if the driver is sensing
the cause of distrubance (pedestrian) andanticipating it’s effects (possibility of running over the pedestrian),
heis improving his control.
BP: The problem with this mode of “proof” is that you assume
the result before you have shown that this result would actually occur.
You’re imagining that the driver is blind to the pedestrian until it’s
too late to do anything (which does happen occasionally and accounts for
quite a few pedestrian deaths). What about controlling the perceived
position of the car in relation to the pedestrian? Are you declaring that
this is impossible? I realise that the appearance of collision avoidance
is that the controller is predicting the path of the other object, but
this is a very poor way to avoid collisions, though it might work if you
could perceive and compute very rapidly and accurately. A much better way
is just to make sure that the direction from you to the other object,
relative to your direction of travel, is changing at a reasonable rate.
If the direction is not changing, there will be a collision. So alter
your direction until the bearing of the other object starts to change
(either way), and you will most probably miss it.
BH: 3. CASE : In every sport I
mentioned, anticipating (predicting) whatopponent will do is huge advantage and ussually means winning.
Betteranticipation of players means more chances for winning.
BP: You assume that it’s the anticipation of the opponent’s move that
means winning, but you doing this in order to show that anticipation is
what allows winning. That is a logical error, with various names, one
being “begging the question.”
http://en.wikipedia.org/wiki/Circular_logic
Opponents also use your anticipation to get past you, don’t they? In
football it’s called “faking the defender out of his shoes.”
You display the start of a move to the left, and the instant the opponent
anticipates your move, you go the other way.
BH: Some actions of players I think it can be done only with presence of
feed-forward (throw, handing the ball over).
BP: This is something else: you control the way you throw in relation to
the target, so you feel the right accelerations and velocities. That’s
feedback control. But you have no control of any kind after you let the
ball go. You do see the result, so you can make changes in the way you
throw, but not until the next time. That is still feedback
control.
BH: The goal of the game is more
efficiently achieved if the moving of players are anticipated (predicted,
planned). It’s so called tactics.
BP: True, and I would call this feedforward. If it worked as well as
feedback, the game would be over at the start because the best strategy
would always win. But it doesn’t.
BH: So sensing the cause of
disturbance (opponent) and anticipation of effects (opponent’s action)
players are improving control.
BP: Yes. But the whole question rests on just how much improvement you
can get from feedforward, especially when feedback control is
possible.
BH: So feed-forward is somehow
neccessary because everybody wants to win, andmost players and teams make tactics and improve other elements of
training.
BP: No, it’s necessary in order to win only when feedback control is
impossible.
BH: Trainings contains also
the improvement of feedforward actions, so toachieve their goals : to WIN, thus have more money and higher quality
oflife (survival) or in if we say it in Ashby’s language to maintain
essentialvariables in the limits.
BP: You can’t make feedforward control any more effective by pointing out
how important it is to achieve a goal, have more money, or have a higher
quality of life. Those are not goals of feedforward because feedforward
systems have no goals. They only react to inputs with a precalculated
action. It makes no difference to them if the action takes them
closer to some goal assumed by an observer or away from it.
BH: 4. CASE : putting all kind
of vegetables in water is by my oppinionfeed-forward act (anticipating the taste of the soup), and testing it
whilecooking is control act.
BP: I would say that putting vegetables in water is a control process. Do
you think you could do that blindfolded and without being allowed to feel
where the vegetables and the water are? Doing this is part of a
higher-level process that controls a sequence called “preparing
dinner.” Each step in the sequence is brought about in the correct
order at the correct time to achieve the goal. If there are disturbances
of the sequence, they are corrected; parts of the sequence are repeated
in the correct order, or the correction is made before the next step
takes place.
I think what you’re missing here is the main point: Feedforward is simply
a fancy name for a stimulus causing a response. If the result of the
response is sensed and the next forward act is modified according to the
result of the previous response, we have a feedback system, not a
feedforward system. The only time you have a real feedforward system is
when the response to the stimulus remains the same every time no matter
what the result of the response is. If the stimulus of dinnertime calls
for putting the celery in a bowl of water, and you find the celery
already in a bowl of water, the feedforward system would put it in
another bowl of water. Timed lawn sprinkers are feedforward systems which
react to a timer by turning on the water even in a rainstorm. A
feedforward system puts the meat in a pan and turns the gas knob even if
the meat has been cooked already, and even if the gas has been turned off
at the main entry to the house. If the gas has been turned off, the
feedforward system still waits 15 minutes, then takes the meat out of the
pan and serves it (if the plate is exactly where it is supposed to be and
not a foot to the left or right).
BH : So we agree that building
houses, skyscrapers, bridges with plans(feed-forward) very much improve control.
BP: No, we do not agree; YOU agree. The fact that the outcome of a
control process takes time to appear, and appears in the future, does not
make it a feedforward process. Building a house would be a feedforward
process if the pieces of wood were put in place and the hammers
were swung at the nails and the walls were pushed upright all without
ever perceiving them or comparing the results with the
blueprint.
BH: Plans in constructing
anything (probably also when engineers construct machines) are
necessary.
BP: Yes. They are specifications for what is to be perceived, not for the
actions that are to be performed. The actions used in building two
identical houses are very different from one house to another. The
carpenters have a saying: “measure twice, cut once.” Building
anything is a process of varying actions to achieve a precise result, and
the precise result is achieved by comparing perceptions continually to
reference perceptions. Feedforward systems have no reference
perceptions.
BH: So the LAW
says. Despite this, some smart guys build houses without plans (only
control).
BP: That is not control, it’s feedforward.
BH: The consequences are
ussually seen. Speccially on muddy groundlike in our country. The houses simply sink into the ground. There are
manythings to predict when building houses.
BP: If a house was built only with feedforward, there would be no way to
predict the result. It would probably be a random pile of lumber with
bent nails lying all over the ground and no structure higher than a
carpenter’s waist.
BP earlier: The safest thing to
do is trust the feedforward, but verify it with feedback control to
handle the many situations in which feedforward fails to help or makes
matters worse.BH : So I think we solve the problem. Can we agree that feed-forward
isimproving control and it’s mostly necessary with activities living
organismsdo, along with feed-back control ?
BP: I said “the many situations in which feedforward fails to help
or makes matter worse.” Is that agreement? I think feedforward does
occur, but that it is too crude to be relied on. If you want predictable
results and precise results and fast results, you need feedback control
systems. We use feedforward only when there isn’t any way to monitor the
results and make corrections as we proceed. If necessary, we’ll inflate a
flat tire without a pressure gauge, but as soon as we can we’ll stop and
measure the pressure before going on. Only a brave man would fill his
bathtub with water and jump into it without feeling the temperature while
filling it.
BH: And can we agree that some
combination of feed-back and feed-forward is ussually present as Arthur
proposed : “Is the combination of feedback and feedforward not
preferable in most situations ?”
BP: I have built many control systems, fixed many others, and observed
still more of them, and have never found that feedforward was a good way
to improve control by more than a very small amount. The most common
result of adding some feedforward to get faster responses was to make the
system unstable and cause oscillations, as well as making the system
respond needlessly to random noise.
I really can’t see why feedforward is getting so much support. Do people
just like the word? Is it to achieve some kind of symmetry with feedback?
Would people be sad or disappointed if we just stopped using the word? Or
is it, as I rather suspect, a throwback to the days when ALL behavior was
thought to be feed-forward, cause-effect, antecedent-consequent,
stimulus-response?
I think that if we just keep in mind that feedforward is an action taken
in response to some stimulus, and taken blindly with no modification on
the basis of its results, we can put feedforward in its proper place. It
really doesn’t work any better in conjunction with control theory than it
did when it was the only theory of behavior.
Best,
Bill P.
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At 03:13 AM 12/5/2009 -0600, you wrote:
