Second-Order Lead Lag (LDL2)

This information applies to the CompactLogix 5370, ControlLogix 5570, Compact GuardLogix 5370, GuardLogix 5570, Compact GuardLogix 5380, CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, and GuardLogix 5580 controllers.

The LDL2 instruction provides a filter with a pole pair and a zero pair. The frequency and damping of the pole and zero pairs are adjustable. The pole or zero pairs can be either complex (damping less than unity) or real (damping greater than or equal to unity).

Available Languages

Ladder Diagram

This instruction is not available in ladder diagram logic.

Function Block

Structured Text

LDL2(LDL2_tag);

Operands

Function Block

Operand	Type	Format	Description
LDL2 tag	LEAD_LAG_SEC_ORDER	Structure	LDL2 structure

LEAD_LAG_SEC_ORDER Structure

Input Parameter	Data Type	Description
EnableIn	BOOL	Enable input. If false, the instruction does not execute and outputs are not updated. Default is true.
In	REAL	The analog signal input to the instruction. Valid = any float Default = 0.0
Initialize	BOOL	Request to initialize filter control algorithm. When true, the instruction sets Out = In. Default is cleared.
WLead	REAL	The lead corner frequency in radians/second. If WLead < minimum or WLead > maximum, the instruction sets the appropriate bit to true in Status and limits WLead. If the WLag:WLead ratio > maximum ratio, the instruction sets the appropriate bit in Status to true and limits WLag Valid = see Description section below for valid ranges. Default = 0.0
WLag	REAL	The lag corner frequency in radians/second. If WLag < minimum or WLag > maximum, the instruction sets the appropriate bit to true in Status and limits WLag. If the WLag:WLead ratio > maximum ratio, the instruction sets the appropriate bit to true in Status and limits WLag. Valid = see Description section below for valid ranges Default = 0.0
ZetaLead	REAL	Second order lead damping factor. Only used when Order = 2. If ZetaLead < minimum or ZetaLead > maximum, the instruction sets the appropriate bit to true in Status and limits ZetaLead. Valid = 0.0 to 4.0 Default = 0.0
ZetaLag	REAL	Second order lag-damping factor. Only used when Order = 2. If ZetaLag < minimum or ZetaLag > maximum, the instruction sets the appropriate bit to true in Status and limits ZetaLag. Valid = 0.05 to 4.0 Default = 0.05
Order	REAL	Order of the filter. Selects the first or second order filter algorithm. If invalid, the instruction sets the appropriate bit to true in Status and uses Order = 2. Valid = 1 to 2 Default = 2
TimingMode	DINT	Selects timing execution mode. 0 = Periodic mode 1 = Oversample mode 2 = Real time sampling mode Valid = 0 to 2 Default = 0
RTSTimeStamp	DINT	Module time stamp value for real time sampling mode. Valid = 0 to 32,767ms Default = 0

Output Parameter	Data Type	Description
EnableOut	BOOL	Indicates if instruction is enabled. Cleared to false if Out overflows.
Out	REAL	The calculated output of the algorithm.
DeltaT	REAL	Elapsed time between updates. This is the elapsed time in seconds used by the control algorithm to calculate the process output.
Status	DINT	Status of the function block.
InstructFault (Status.0)	BOOL	The instruction detected one of the following execution errors. This is not a minor or major controller error. Check the remaining status bits to determine what occurred.
WLeadInv (Status.1)	BOOL	WLead < minimum value or WLead > maximum value.
WLagInv (Status.2)	BOOL	WLag < minimum value or WLag > maximum value.
ZetaLeadInv (Status.3)	BOOL	Lead damping factor < minimum value or lead damping factor > maximum value.
ZetaLagInv (Status.4)	BOOL	Lag damping factor < minimum value or lag damping factor > maximum value.
OrderInv (Status.5)	BOOL	Invalid Order value.
WLagRatioInv (Status.6)	BOOL	WLag:WLead ratio greater than maximum value.
TimingModeInv (Status.27)	BOOL	Invalid TimingMode value. For more information about timing modes, see Function Block Attributes.
RTSMissed (Status.28)	BOOL	Only used in real time sampling mode. Set when ABS (DeltaT - RTSTime) > 1 millisecond.
RTSTimeInv (Status.29)	BOOL	Invalid RTSTime value.
RTSTimeStampInv (Status.30)	BOOL	Invalid RTSTimeStamp value.
DeltaTInv (Status.31)	BOOL	Invalid DeltaT value.

Structured Text

Operand	Type	Format	Description
LDL2 tag	LEAD_LAG_SEC_ORDER	structure	LDL2 structure

See Structured Text Syntax for more information on the syntax of expressions within structured text.

Description

The LDL2 instruction filter is used in reference forcing and feedback forcing control methodologies. The LDL2 instruction is designed to execute in a task where the scan rate remains constant.

The LDL2 instruction uses these equations:

When:	The instruction uses this Laplace transfer function:
Order = 1
Order = 2

with these parameter limits (where DeltaT is in seconds):

Parameter	Limitations
WLead first order LowLimit
WLead second order LowLimit
HighLimit
WLead:WLag ratio	If WLead > WLag, no limitations If WLag > WLead: No minimum limitation for WLag:WLead First order maximum for WLag:WLead = 40:1 and the instruction limits WLag to enforce this ratio Second order maximum for WLag:WLead = 10:1 and the instruction limits WLag to enforce this ratio
ZetaLead second order only	LowLimit = 0.0 HighLimit = 4.0
ZetaLag second order only	LowLimit = 0.05 HighLimit = 4.0

Whenever the value computed for the output is invalid, NAN, or Plus or Minus sign

INF, the instruction sets Out = the invalid value. When the value computed for the output becomes valid, the instruction initializes the internal parameters and sets Out = In.

Affects Math Status Flags

Major/Minor Faults

None specific to this instruction. See Common Attributes for operand-related faults.

Execution

Function Block

Condition/State	Action Taken
Prescan	EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is false	EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is true	EnableIn and EnableOut bits are set to true. The instruction executes.
Instruction first run	N/A
Instruction first scan	Recalculate coefficients.
Postscan	EnableIn and EnableOut bits are cleared to false.

Structured Text

Condition/State	Action Taken
Prescan	See Prescan in the Function Block table.
Normal Execution	See Tag.EnableIn is true in the Function Block table.
Postscan	See Postscan in the Function Block table.

Example

The LDL2 instruction can attenuate between two frequencies or can amplify between two frequencies, depending on how you configure the instruction. Since the Lead and Lag frequencies can be set to values that are larger or smaller than each other, this instruction may behave as a Lead-Lag block, or, as a Lag-Lead block, depending on which frequency is configured first. Note that higher orders increase the execution time for the filter instruction.

This example is the minimal legal programming of the LDL2 function block and is only used to show the neutral text and generated code for this instruction. This is for internal purposes only and is not a testable case.