Micromod Micro-DCI: 53SL5100B Single Loop Controller - Manual (Page 59)

53SL5100B Single-Loop Controller
INSTRUCTION MANUAL
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7 RATIO (PID) CONTROLLER
7.1 Ratio (PID) Controller Operation Overview
The Ratio (PID) Controller is used where one variable, called the controlled variable, must be automatically
maintained in definite proportion to another variable, called the wild variable.
Transmitting meters (e.g., flow meters) must be installed in each variable line. Signals from the controlled and
wild variable meters (Analog Input 0 and Analog Input 1, respectively) are received by the Ratio Controller,
which compares them and calculates the required correction that is applied as an output signal (Analog
Output 0) to a final control element (e.g, valve) in the controlled variable line. The final element in the
controlled variable line is controlled to alter line throughput so that the predetermined ratio between the two
lines is maintained.
The predetermined ratio is set at the controller front panel with the Ratio/Local (R/L) pushbutton in R. If the
proportion between the wild variable and the controlled variable can not be maintained, the controller
activates a signal (CCO0) to an alarm. These events are illustrated in Figure 7-1.
Figure 7-1. Ratio (PID) Controller Process Loop
While in Ratio (R) control, the setpoint pushbuttons modify the desired ratio value (controlled variable to wild
variable) by altering the location of the ratio tick bar, which is immediately covered by the setpoint arrow
indicator when the push button is released. Because the setpoint indicator moves to cover the ratio tick bar,
altering the ratio setting indirectly affects the setpoint value. When in Local (L) control, the pushbuttons
modify only the setpoint value. In Local control, the controlled variable line functions as a Standard Feedback
Control Loop (see Section 5).
 NOTE: To minimize sudden process changes, adjust the local setpoint value
to cause the setpoint pointer to cover the ratio tick bar before switching to ratio
control (Ratio).