Process High or Low Selector (PHLS)

The Process High or Low Selector (PHLS) instruction selects the highest or the lowest of up to six incoming controlled variables (CVs). The instruction sends the selected CV as output and flags the unselected CVs to track the selected CV.

To avoid problems with ever-decreasing or ever-increasing output, offset the tracking value by an amount equal to the upstream PID/PIDE gain time’s error value.

Use the PHLS instruction to implement an Override Select control strategy. An Override Select strategy provides control of a primary process variable while allowing other process variables to override the output on the final control element, which avoids exceeding constraints.

Available Languages

Ladder Diagram

Function Block Diagram

Structured Text

PHLS(PHLSTag);

Configuration Operands

PHLS Input Structure

Alarms

The PHLS instruction does not have any alarms.

Virtualization

The PHLS instruction does not have any virtualization capability.

Configuration of Strings for HMI

Configure strings for HMI faceplates (FT View) and for the

Logix Designer

configuration dialog box. The strings are set to extended properties of tag items. Configure the strings in

Logix Designer

only.

Monitor the PHLS Instruction

Use the operator faceplate from the PlantPAx library of Process objects for monitoring.

Affects Math Status Flags

No.

Major/Minor Faults

None specific to this instruction. See Index Through Arrays for array-indexing faults.

Execution

Ladder Diagram

Function Block Diagram

Structured Text

In Structured Text, EnableIn is always true during normal scan. The instruction executes when it is in the control path activated by the logic.

Example

The example uses the PHLS instruction to implements part of the pressure control strategy. In this case, two PIDE instructions are used as inputs. The PIDE instructions are for Suction Pressure Override Control and Discharge Pressure Control. The PIDE output values CV (CV to final control element) and E (Loop Error) are used as inputs to the PHLS instruction.

This example also shows PHLS inputs for Initial Value (Inp_InitializeVal) and initialization request (Inp_InitializeReq). In this case, the Initial Value is taken from the speed reference to the pump motor drive. The Initialization

In this example the instruction initialization request flag is set based on the motor’s running and availability status.

Ladder Diagram

Function Block Diagram

Structured text

PIDE(PIDE_01);

PIDE(PIDE_02);

PHLS.Inp_1 := PIDE_01.CV;

PHLS.Inp_E1 := PIDE_01.E;

PHLS.Inp_2 := PIDE_02.CV;

PHLS.Inp_E2 := PIDE_02.E;

PHLS.Inp_InitializeVal := P50_Motor.Val_SpeedRef;

PHLS.Inp_InitializeReq := NOT(P50_Motor.Sts_RunningFwd AND P50_Motor.Sts_Available);

PHLS(PHLS);