C.28.CM:5.2 - Construct the measurement and device mechanisms separately
An idealized regulated supply has command C=12 volts and a fixed 6-ohm resistive load. Its display reads D=14 volts. The practical question is whether correcting the display can reduce the load current.
Construct separate variables for delivered voltage V, display D and current I. Use the load relation I=V/6. Two accounts fit the displayed value:
| Account | Proposed mechanisms | Current implied by the account |
|---|---|---|
| Display bias | V=C; D=V+2. | V=12 and I=2 amperes. |
| Supply offset | V=C+2; D=V. | V=14 and I=7/3 amperes. |
Setting the displayed number to 12 replaces the display mechanism in either model. It leaves V and I unchanged. Changing C to 6 instead gives V=6, I=1 in the first model and V=8, I=4/3 in the second, if the stated offsets and load relation remain valid. The observation D=14 alone does not choose between the accounts.
A separately qualified voltage or current observation can discriminate these idealized models. The need is a measurement of the physical quantity with a suitable independent basis, not another copy of the same display.
Change the situation: the displayed value is fed into an automatic controller for the next command. The earlier absence of a display-to-device path no longer applies over that horizon. Add D(t) → C(t+1) → V(t+1) and obtain the controller law before predicting the later current. The current instant and later controlled behavior are different questions. C.28.MR then performs the chosen replacement in the model actually constructed.