Continuing this issue publicly, which I think still relates to matters
of general interest...
Again, if it gets tedious, we can take it off-list. I will use LaTeX math
notation. The book I use is "Mathematics for Dynamic Modeling", Edward
Beltrami, Academic Press, 1987.
First, "stability" or "instability" is a property not of systems, or
orbits, but of equilibrium points. Thus the ball in the bottom of the
bowl at rest is at a stable equilibrium, the pencil balanced on a point
at an unstable equilibrium. In the thermostat example,
I agree that it is a property of points, or more precisely of regions.
>> far from stability (the stable temperature is perhaps 40 C when the
>> furnace is on and -10 C when it is off), but the temperature oscillates
>> around 20 C. So one can have control without stability.where T is temperature, the point T = 20 C, dT/dt = 0 C/sec is
actually an UNSTABLE equilibrium. In this case, however, there is a
limit cycle, such that points near the unstable eq. spiral outward to
the cycle you described, and points within a neighborhood OUTSIDE the
cycle spiral INWARD to it.
Here Cliff hits exactly my point. At 20C the derivative is NOT zero!
When the furnace is on it is positive, when it is off it is negative.
What I think is important to recognize here is the difference between
continuous feedback (such as exerted by a spring, or by the
gravitational force on a ball in a bowl) and discrete feedback generated
by a thermostat. Discrete feedback is delayed feedback (it takes a
finite amount of time for the temperature to reach the switching point
of the thermostat and for the furnace to fire up or go off), which can
generate `unstable' behaviour if the actual derivatives dT/dt become too
great in magnitude.
Bill Silvert [931230 08:30]