Control stations perform the function of a controller and provide additional controls and indicators to allow an operator to manually adjust the controller output to the final control element. Figure 32 shows the front panel of a typical control station. It contains several indicators and controls. Each will be discussed
The setpoint indicator, located in the center of the upper half of the controller, indicates the setpoint (desired value) selected for the controller. The scale may be marked 0% to 100% or correspond directly to the controlled variable (e.g., 0 – 1000 psig or -20°F to +180°F). The setpoint adjustment, located right of the setpoint indicator, is a thumbwheel type adjustment dial that allows the operator to select the setpoint value. By rotating the thumbwheel, the scale moves under the setpoint index line.
The deviation indicator, located left of the setpoint indicator, displays any error (+10% to -10%) between setpoint value and actual controlled variable value. With no error, the deviation pointer stays at mid-scale, in line with the setpoint index mark. If the controlled variable is lower than setpoint, the deviation indicator deflects downward. I higher, the indicator deflects upward. An example of this is shown in Figure 33.
Snap-in tabs, above each end of the output meter, indicate the direction the final control element moves for a change in the output signal. Tabs normally read “open-close” for control valves and “slow-fast” for variable-speed motors, or other appropriate designations. The manual-automatic (M-A) transfer switch, immediately below the output meter, selects operating mode of the controller. A manual output adjust knob, in the center of the M-A transfer switch, varies the controller output signal in manual mode of operation. The knob is rotated clockwise to increase the signal and counterclockwise to decrease the signal. The M-A transfer switch has five positions that alter the mode of operation. The indication is provided by the deviation meter.
AUTO. This is the normal position of the M-A transfer switch. It places the controller in the automatic mode of operation. Also, the deviation meter indicates any deviation between controlled variable and setpoint.
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 value.
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 isindicated 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) control.