We have a winner!

[Paul George 940715 10:00]

[From Rick Marken (940714.1445)]

Me:

By convention and practice the system is seen as controlling D in order to
provide control of F

Oops :frowning: The only variable in the loop that is controlled (kept at a
specified level against disturbance) is H.

Ack. I just meant that that is the way process engineers usually talk about it.
And while PCT usually just works within the context of the control loop, In
process control, the Instruments and the equipment performing the process are
viewed as part of the 'System'. The system boundary is in a slightly different
place. Also from a terminology standpoint (backward from PCT) the thing acted
upon is seen as being controlled, the sensors just tell you what happened (you
hope :slight_smile: {sound familiar?}.

However, the control system writ small (essentially F1) only cares
about H & B.

Well, it really only cares about (that is, controls) H; the system cares
what value H is -- it "wants" H to equal A and it will do what it can to make
that happen. The system doesn't care what value B is; B just varies around as
necessary (depending on disturbances -- E -- and changes in functional
relationships in the loop) to keep H matching A.

I understand that is the way it is usually modeled. But is it necessarily
impossible that B is also 'perceived'? To anthromorphisize a bit, If I push on
a door lightly and don't perceive it moving, I then push harder. To increase
the force I must have some idea how hard I was pushing originally. Now, in this
particular case I have sensors that tell me how hard my hand is pressing, or
how hard my muscles are contracting. But the concept of direct feedback when I
can't detect (indirectly) C or D doesn't seem invalid on the face. The signal
exists, and the control loop could 'wire itself' that way.