Cascade, Override, Constraint, Ratio, Split Range Refer to DeltaV books on-line as well as your text

Cascade Control p. 310 to 322

Two Controllers, two process transmitters, one valve working toward a master control objective. Experiment 1 demonstrates the need, cascade controllers are used in Experiment 2, but not tested there.

A primary, (a.k.a. outer, master loop) controller output is used to set the set point of a secondary, (a.k.a. inner, slave loop) controller

Frequently used to prevent utility disturbances from affecting the process

Rule of 5, (see McMillan's rule of five) inner loop should be 5 times faster than the outer loop

Note example on p.319, 320

Override or Constraint p. 340 – 346

Override - Two controllers, two transmitters, one valve, working to control to two different objectives.

In Experiment 1, our objective is to control the level at some valve as well as prevent the flow from exceeding a controlled rate. We configure an override control using the control selector block, CTLSL.

This method allows one controller to "take control" over the other.

Constraint Control – In Experiment 3 we will control the valve position of the maximum % open valve of both flow loops at 90% by controlling the pressure controller's set point. Method used for energy reduction, if we had a variable speed pump as a supply, subject of future lecture.

Constraint Control Examples

TemperatureControl

UtilityReturn

Cooling Utility

SGSL

FC

IVP

Variable Speed Control

Fed Batch Constraint Control

Ratio Control p. 333 Permits a second controller to follow a ratio of a master flow controller..

Example: pH control, pH controller output setting a ratio of the reagent flow to the effluent flow. Effluent flow disturbances can adjust the reagent flow rather than have the pH change and adjust the reagent flow.

Split Range - The Splitter (SPLTR) function block This block is used to define the action of two valves from a single controller.

Example: A vessel that needs heating and cooling, half of the output action is devoted to cooling while half is devoted to heating. Also called "Split Range" Can be set by valve position control settings.

TemperatureControl

JacketUtility

Supply

UtilityReturn

Steam

Cooling Water

SPLTR