<Bob Clark (940427.1530 EDT)>
Bill Powers (940422.0600 MDT) Subject: predictions; DME
This was the first topic in your post. To simplify the discussion, I
am experimenting with trying to keep each post to a single topic.
Thermostat. Bob Clark (940421.2155) Subject:RE: CONFLICTS - RKC.
In response to my brief remark about the discontinuous operation of
the usual residential heating system, you pointed out that the
operation is smoothed by thermal inertia, resulting in the familiar
cycling of the system. As you point out:
If you stretch out the time scale accordingly, the thermostat
contacts are always rapidly opening and closing, the continuous
variable being the "smoothly" changing duty cycle.
Yes, of course. However, human comfort is primarily concerned with
the average temperature. The cycling is usually ignored -- except by
the engineer who wants to improve the accuracy of temperature control.
Later in your same post, you refer to the thermostat's "anticipation"
adjustment.
What this adjustment does is make the contacts move closer together
if the furnace is off or move farther apart (that is, reduce the
contact pressure) if it is on. The effect is to make the furnace
think the temperature has changed farther than it has, so the
furnace turns off before the set-point is reached, and on sooner
than it otherwise would have as the temperature falls.
This "adjustment" consists of a small variable resistor that adds a
small amount of heat to the temperature sensitive element. This
heat, added to the thermostat assembly, reduces the time for the
thermostat to reach its set-point. This does not provide any form of
"rate feedback."
The existence, in spinal reflexes, of a signal "proportional to the
rate change of length" is interesting. But has no relation to the
thermostat system.
Your anthropomorphic terminology, "make the furnace think," seems
unnecessary.
My concern in the post noted above, was the illustration of an
idealized concept with a familiar example. In this case, the
thermostat, the example is not very satisfactory. It only controls
the household temperature indirectly. As you point out, the duty
cycle, not the room temperature, is the controlled variable! Control
of temperature is a "side-effect!" Yet the result is adequate for
human needs. [Incidentally, the system tends to be less satisfactory
when diurnal temperature variations are large, as in spring and fall.]
One can extend the physical and engineering details as far as
desired, but the thermostatic system is really not a very good example
of an elementary, isolated control system.
Regards, Bob Clark