Notch Filter (NTCH)
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.
Available Languages
Ladder Diagram
This instruction is not available in ladder diagram logic.
Function Block
Structured Text
NTCH(NTCH_tag);
Operands
Function Block
Operand | Type | Format | Description |
|---|---|---|---|
NTCH tag | FILTER_NOTCH | Structure | NTCH structure |
FILTER_NOTCH 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 false. |
WNotch | REAL | The filter center frequency in radians/second. If WNotch < minimum or WNotch > maximum, the instruction sets the appropriate bit in status and limits WNotch.
Valid = see Description section below for valid ranges
Default = maximum positive float |
QFactor | REAL | Controls the width and depth ratio. Set QFactor = 1 / (2*desired damping factor). If QFactor < minimum or QFactor > maximum value, the instruction sets the appropriate bit in Status and limits QFactor.
Valid = 0.5 to 100.0
Default = 0.5 |
Order | REAL | Order of the filter. Order controls the sharpness of the cutoff. If Order is invalid, the instruction sets the appropriate bit in Status and uses Order = 2.
Valid = 2 or 4
Default = 2 |
TimingMode | DINT | Selects timing execution mode. 0 = Periodic mode 1 = Oversample mode 2 = Real time sampling mode2 For more information about timing modes, see Function Block Attributes.
Valid = 0 to 2
Default = 0 |
OversampleDT | REAL | Execution time for oversample mode.
Valid = 0 to 4194.303 seconds
Default = 0 |
RTSTime | DINT | Module update period for real time sampling mode
Valid = 1 to 32,767ms
Default = 1 |
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. |
WNotchInv (Status.1) | BOOL | WNotch < minimum or WNotch > maximum |
QFactorInv (Status.2) | BOOL | QFactor < minimum or QFactor > maximum |
OrderInv (Status.3) | BOOL | Invalid Order 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 to true 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 |
|---|---|---|---|
NTCH tag | FILTER_NOTCH | structure | NTCH structure |
See Structured Text Syntax for more information on the syntax of expressions within structured text.
Description
The NTCH instruction uses the Order parameter to control the sharpness of the cutoff. The QFactor parameter controls the width and the depth ratio of the notch. The NTCH instruction is designed to execute in a task where the scan rate remains constant.
The NTCH instruction uses this equation:
where i is the Order operator with these parameters limits (where DeltaT is in seconds):
Parameter | Limitations |
|---|---|
WNotch second order
LowLimit |  |
WNotch fourth order
LowLimit |  |
HighLimit |  |
QFactor | LowLimit = 0.5
HighLimit = 100.0 |
Whenever the value computed for the output is invalid, NAN, or 
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.
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
No
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 NTCH instruction attenuates a specific resonance frequency. Typically, these resonance frequencies are directly in the range of response being regulated by the closed loop control system. Often, they are generated by loose mechanical linkages that cause backlash and vibration in the system. Although the best solution is to correct the mechanical compliance in the machinery, the notch filter can be used to soften the effects of these signals in the closed loop regulating scheme.
The following diagram shows the ideal gain curve over a frequency range for a specific center frequency and Q factor. As increases, the notch becomes wider and shallower. As decreases; the notch becomes deeper and narrower. The instruction may be set for an order of 2 or an order of 4. Higher orders take more execution time.
This example is the minimal legal programming of the NTCH 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.
Function Block
Structured Text
NTCH_01.In := frequency_input;
NTCH_01.WNotch := center_frequency;
NTCH_01.QFactor := Notch_width_depth;
NTCH(NTCH_01);
Notch_output := NTCH_01.Out;
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