# What's a disturbance?

[From Bruce Abbott (960712.0950 EST)]

Hans Blom, 960711 --

The disturbance in the above diagram makes sense to me only if it is
variable. I truly do not understand a lapse like this from both Rick
and you! And if I don't understand you now, I have never understood
you. Imagine that: talking past each other for, what is it, four, five
years?

Hans, you should think of a disturbance as something whose effect on the
controlled variable requires a compensating output from the control system,
rather than something that purturbs the steady state of the system. A
strong side-wind tends to push the car off the road, and you compensate for
it by steering just a bit into the wind. The tire-angle generates just
enough counter-force to offset the effect of the wind, and if the wind is
steady, you maintain that additional angle -- constant disturbance, constant
output. If there is a small hole in my pool, I can maintain a constant
level of water in the pool by adding water at the same rate at which it
leaks out. The constant rate of leakage constitutes a constant disturbance,
which I offset by continuouly adding water.

When the weather is cool, a home heating system must add heat at a rate
exactly equal to the rate at which the heat leaks out if it is to maintain a
constant inside temperature; the heat leakage constitutes a disturbance to
the system; if the outside temperature is constant then the disturbance is
constant, so long as the thermostat's setting remains unchanged.

You are doubtless well aware of this rather simple fact about control-system
behavior. But it seems that you have been defining "disturbance" a bit
differently from the rest of us and that this has been a source of
confusion. If you define "disturbance" only as some influence on the
controlled variable that continually purturbs its value, then for you,
constant influences like steady side-winds, leaks in the pool, and heat-loss
to the outside air do not constitute a disturbance, whereas in PCT they
would. I hope this dissolves the confusion.

Regards,

Bruce

[Hans Blom, 960712]

(Bruce Abbott (960712.0950 EST))

Hans, you should think of a disturbance as something whose effect on the
controlled variable requires a compensating output from the control system,
rather than something that purturbs the steady state of the system.

Done that, been there. Of course.

A strong side-wind tends to push the car off the road, and you
compensate for it by steering just a bit into the wind. ...

If there is a small hole in my pool, I can maintain a constant level
of water ...

When the weather is cool, a home heating system must add heat ...

Yes, these examples are clear. Now please come up with a plausible
example that starts with: "If gravity is ..., then ...".

Greetings,

Hans

[From Bruce Abbott (960712.1235 EST)]

Hans Blom, 960712 --

Yes, these examples are clear. Now please come up with a plausible
example that starts with: "If gravity is ..., then ...".

I'm not quite sure what you are looking for. How about this:

If gravity is exerting a downward force against my arm (disturbance) while I
attempt to hold it in a horizontal position (reference), then I must exert a
constant upward counterforce (output) to hold it there.

Remove that disturbance (put me into orbit or into a tank of water in which
my arm has neutral bouyancy) and I can relax (zero output). (I am
pretending that the natural tensions produced by relaxed muscle, which
actually tend to bend the arm, are not there; these actually constitute
another source of disturbance to my arm position.)

How'd I do?

Regards,

Bruce