[From Bruce Abbott (2018.02.26.1155 EST)]
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GeneralNegativeFeedbackLoops_18.02.26.pdf (250 KB)
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The part you got wrong is “only
control systems oppose disturbances.� The fact that
you can’t just push your hand through a table top
proves that equilibrium systems oppose disturbances,
too. The downward force that you exert on the table
top is resisted by the counterforce that develops as
your downward force compresses the table at the area
of contact with your hand. Think of the table top as
a very stiff spring that is being compressed. Unless
your force is sufficient to break the table, the
counterforce will grow until it equals the downward
force compressing the table top, and there your hand
will be stopped – disturbance resisted.
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As Richard Kennaway, Martin Taylor,
and I have been saying, negative feedback is present
in both control systems and equilibrium systems; the
important difference lies in the source of the
counteracting output.Â