Allen-Bradley LOGIX 5000 Reference Manual page 87

Process Control Instructions Chapter 1
primary loop's process. In this example, if steam flow drops, the steam flow must
be able to increase as a result of the secondary controller's action before the liquid
temperature is affected.
To set up a pair of cascaded PIDE instructions, set the AllowCasRat input
parameter in the secondary loop. This allows the secondary loop to be placed into
Cascade/Ratio mode. Next, wire the CVEU from the primary loop into the
SPCascade parameter on the secondary loop. The SPCascade value is used as the
SP on the secondary loop when the secondary loop is placed into Cascade/Ratio
mode. The engineering unit range of the CVEU on the primary loop should
match the engineering unit range of the PV on the secondary loop. This lets the
primary loop scale its 0-100% value of CV into the matching engineering units
used for the setpoint on the secondary loop.
The PIDE instruction supports several other features to more effectively support
cascade control. Wire the InitPrimary output on the secondary loop into the
CVInitReq input on the primary loop and wire the SP output of the secondary
into the CVInitValue input on the primary. This sets the CVEU value of the
primary loop equal to the SP of the secondary loop when the secondary loop leaves
Cascade/Ratio mode. This allows a bumpless transfer when you place the
secondary loop back into Cascade/Ratio mode. Also, wire the WindupHOut and
WindupLOut outputs on the secondary loop into the WindupHIn and
WindupLIn inputs on the primary loop. This causes the primary loop to stop
increasing or decreasing, as appropriate, its value of CVEU if the secondary loop
hits a SP limit or CV limit and eliminates any windup on the primary loop if these
conditions occur.
Example 3
Ratio control is typically used to add a fluid in a set proportion to another fluid.
For example, if you want to add two reactants (say A and B) to a tank in a constant
ratio, and the flow rate of reactant A may change over time because of some
upstream process upsets, you can use a ratio controller to automatically adjust the
rate of reactant B addition. In this example, reactant A is often called the
"uncontrolled" flow since it is not controlled by the PIDE instruction. Reactant B
is then called the "controlled" flow.
To perform ratio control with a PIDE instruction, set the AllowCasRat and
UseRatio input parameters. Wire the uncontrolled flow into the SPCascade input
parameter. When in Cascade/Ratio mode, the uncontrolled flow is multiplied by
either the RatioOper (when in Operator control) or the RatioProg (when in
Program control) and the resulting value is used by the PIDE instruction as the
setpoint.
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018 87