Moving Standard Deviation (MSTD)
This information applies to the CompactLogix 5370, ControlLogix 5570, CompactGuardLogix 5370, GuardLogix 5570, CompactGuardLogix 5380, CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, and GuardLogix 5580 controllers.
The Moving Standard Deviation (MSTD) instruction calculates a moving standard deviation and average for the In signal.
Available Languages
Ladder Diagram
This instruction is not available in ladder diagram logic.
Function Block
Structured Text
MSTD(MSTD_tag, StorageArray);
Operands
Function Block
Operand | Type | Format | Description |
block tag | MOVING_STD_DEV | structure | MSTD structure |
StorageArray | REAL | array | Holds the In samples. This array must be at least as large as NumberOfSamples. |
MOVING_STD_DEV Structure
Input Parameter | Data Type | Description |
EnableIn | BOOL | Enable input. If cleared, the instruction does not execute and outputs are not updated.
Default is set. |
In | REAL | The analog signal input to the instruction.
Valid = any float
Default = 0.0 |
InFault | BOOL | Bad health indicator for the input. If In is read from an analog input, then InFault is normally controlled by fault status on the analog input. When set, InFault indicates the input signal has an error, the instruction sets the appropriate bit in Status, and the instruction holds Out and Average at their current values. When InFault transitions from set to cleared, the instruction initializes the averaging algorithm and continues executing.
Default is cleared. |
Initialize | BOOL | Initialize input to the instruction. When set, the instruction sets Out = 0.0 and Average = In, except when InFault is set, in which case, the instruction holds both Out and Average at their current values. When Initialize transitions from set to cleared, the instruction initializes the standard deviation algorithm and continues executing.
Default is cleared. |
SampleEnable | BOOL | Enable for taking a sample of In. When set, the instruction enters the value of In into the storage array and calculates a new Out and Average value. When SampleEnable is cleared and Initialize is cleared, the instruction holds Out and Average at their current values.
Default is cleared. |
NumberOfSamples | DINT | The number of samples to be used in the calculation. If this value is invalid, the instruction sets the appropriate bit in Status and the instruction holds Out and Average at their current values. When NumberOfSamples becomes valid again, the instruction initializes the standard deviation algorithm and continues executing.
Valid = 1 to size of the storage array
Default = 1 |
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. |
Average | REAL | The calculated average of the algorithm. |
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. |
InFaulted (Status.1) | BOOL | In health is bad. InFault is set. |
NumberOfSampInv (Status.2) | BOOL | NumberOfSamples invalid or not compatible with array size. |
Structured Text
Operand | Type | Format | Description |
block tag | MOVING_STD_DEV | structure | MSTD structure |
StorageArray | REAL | array | Holds the In samples. This array must be at least as large as NumberOfSamples. |
See Structured Text Syntax for more information on the syntax of expressions within structured text.
Description
The MSTD instruction supports any input queue length. Each scan, if SampleEnable is set, the instruction enters the value of In into a storage array. When the storage array is full, each new value of In causes the oldest entry to be deleted.
The MSTD instructions uses these equations for the outputs:
Condition | Action |
|---|---|
Average |  |
Out |  |
The instruction will not place an invalid In value (NAN or 
INF) into the storage array. When In is invalid, the instruction sets Out = In, sets Average = In, and logs an overflow minor fault, if this reporting is enabled. When In becomes valid, the instruction initializes the standard deviation algorithm and continues executing.
INF) into the storage array. When In is invalid, the instruction sets Out = In, sets Average = In, and logs an overflow minor fault, if this reporting is enabled. When In becomes valid, the instruction initializes the standard deviation algorithm and continues executing.You can make runtime changes to the NumberOfSamples parameter. If you increase the number, the instruction incrementally processes new data from the current sample size to the new sample size. If you decrease the number, the instruction re-calculates the standard deviation from the beginning of the sample array to the new NumberOfSamples value.
Initializing the Standard Deviation Algorithm
Certain conditions, such as instruction first scan and instruction first run, require the instruction to initialize the standard deviation algorithm. When this occurs, the instruction considers the StorageArray empty and incrementally processes samples from 1 to the NumberOfSamples value. For example:
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 | Initialize the previous Output and Average. |
Instruction first scan | Initialize Out to zero. Initialize Average to Input Initialize the instruction algorithm. |
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
Each scan that SampleEnable is set, the instruction places the value of In into array storage, calculates the standard deviation of the values in array storage, and places the result in Out. Out becomes an input parameter for function_block_C.
Function Block
Structured Text
MSTD_01.In := input_value;
MSTD_01.SampleEnable := enable_sample;
MSTD(MSTD_01,storage);
deviation := MSTD_01.Out;
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