0 - 100 (AUTO side). In this position, the controller is still in automatic mode. However,
the deviation meter now indicates the approximate value of the controlled variable. The
deviation meter deflects full down for zero variable value, and full up for 100% variable
MAN. This position places the controller in the manual mode of operation. Controller
output is now varied by adjusting the manual output adjust knob. This adjustment is
indicated on the output meter. The deviation meter indicates any deviation between
controlled variable and setpoint.
0 - 100 (MAN side). The controller is still in the manual mode of operation, and the
deviation meter indicates controlled variable value (0% to 100%) as it did in the 0-100
(AUTO side) position.
BAL. In many cases, controller output signals of the automatic mode and manual mode
may not be the same. If the controller were directly transferred from automatic to manual
or manual to automatic, the controller output signal could suddenly change from one
value to another. As a result, the final control element would experience a sudden change
in position or "bump." This can cause large swings in the value of the process variable
and possible damage to the final control element.
Bumpless transfer is the smooth transfer of a controller from one operating mode to
another. The balance (BAL) position provides this smooth transfer when transferring the
controller from the automatic to manual mode. In the BAL position, the controller is still
in the automatic mode of operation, but the deviation meter now indicates the difference
between outputs of manual and automatic modes of control. The manual output is
adjusted until the deviation meter shows no deflection. Now, the controller can be
transferred smoothly from automatic to manual.
To ensure a bumpless transfer from manual to automatic, the manual output signal,
indicated by the output meter, is adjusted to match the controlled variable value to
setpoint. This will be indicated by no deflection of the deviation meter. Once matched,
the M-A transfer switch can be switched from manual (MAN) to automatic (AUTO)
Self-Balancing Control Stations
The self-balancing M-A control station shown in Figure 34 has several controls and indicators
that are basically the same as those shown on the M-A control station in Figure 32. However,
there are some which differ. In addition, the controller does not require a balancing procedure
prior to shifting from one mode (manual or automatic) to another.