Andrew Sarangan wrote:


Your analogy with driving tells me a little about your line of
thinking. In that case, why does the car to slow down when it hits a
steep hill? It is due to the inability of the engine to respond fast
enough for the sudden demand in power.

No, it is because there is always a lag in a real world feedback control
system or it goes into oscillation.

The lag is due to what is called the margin of stability.

It is possible to design a control system that is virtually instantaneous.

This is called a critically damped system.

The problem with that is that if anything changes, like linkages wear,
the system can easily go underdamped and it goes into oscillation.

You don't want to be in a vehicle at 65 MPH with the cruise control
going into oscillation.

So for safety, cruise control systems are over damped, i.e. have a
large margin of stability.

For a fair explanation of control systems and stability, see
http://en.wikipedia.org/wiki/Control_theory

snip rest

--
Jim Pennino

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