Setting Shared Variable Deadbands (DSC Module)

LabVIEW 2017 Datalogging and Supervisory Control Module Help

Edition Date: March 2017

Part Number: 371618K-01

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You can set deadbands only for network-published shared variables with a floating-point data type.

A deadband is a filter that eliminates noise from data. LabVIEW compares any changes in value from a data point to the previous value. The new value replaces the old value only if the difference between the new value and the previous value exceeds the deadband.

Note  Set deadbands in shared variables as a percentage of the value range of a data point.

The Shared Variable Engine (SVE) uses update deadband and logging deadband values to eliminate unnecessary processing on insignificant data value changes. A deadband defines what constitutes a significant change. The SVE ignores an operation if the change in data is not considered significant. By increasing the deadband size, you can reduce the strain on the SVE, though this might compromise data resolution.

Deadbanding Interaction

You can configure the following deadband settings when you configure individual shared variables: update deadband, logging deadband, and alarm deadband.

The update deadband affects how the SVE updates values. Logging deadband and alarm deadband values operate on the values that have passed through the update deadband value. If you set the update deadband value greater than the logging or alarm deadband values, the update deadband might interfere with the logging or alarm deadband settings, resulting in inadequate historical logging or alarm management.

The OPC server applies the deadband you set in the Configure OPC Client I/O Server dialog box before sending the data to the SVE. The SVE applies the logging deadband upon receiving the data. The effects of this deadband setting also can apply to the update deadband and alarm deadband values. Refer to the OPC server documentation before you change the deadband settings for an OPC Client I/O server. Not all OPC servers support deadbands.

Setting Deadbands for OPC Client I/O Servers

The deadband is the percent of change within a range that a value from the OPC server must change before the OPC server communicates the value to the SVE. Settings made in the OPC server determine the range for each data item. Refer to the OPC server documentation for more information about that server. Set the deadband for an OPC Client I/O server in the Configure OPC Client I/O Server dialog box.

Setting Update Deadbands

When you set an update deadband value, the SVE compares any new value acquired to the existing value. The new value replaces the old value only if the percentage difference between the new value and the previous value equals or exceeds the deadband. Set the update deadband value by using the Update Deadband:Enabled and Update Deadband:Source or Update Deadband:User SharedVariableIO properties.

For example, for a data point with a range of values of 0 to 100, set the update deadband to 1%. The existing value in the SVE is 12.3. If the SVE reports a new value of 13, the SVE does not update because the change in value did not equal or exceed the deadband. If the SVE reports a new value of 11, it updates because the difference is greater than the deadband.

Setting Logging Deadbands

When you set a logging deadband value, the SVE compares any new value acquired to the existing value. The SVE logs the new value only if the percentage difference between the new value and the previous value equals or exceeds the deadband. Set the logging deadband value in the Value Deadband field of the Logging page of the Shared Variable Properties dialog box. You also can set the logging deadband value using the Logging:Value Deadband SharedVariableIO property.

The default setting for the logging deadband is 1%.

For example, for a data point with a range of values of 0 to 100, set the logging deadband value to 2%. The last value the SVE logged was 12.3. When the SVE updates to 11, it does not log the updated value because the change in value is less than the 2% deadband. The value in the SVE must be equal to or greater than 14.3 or equal to or less than 10.3 for the SVE to log the data point. If the value in the SVE is 14.5, the SVE logs this new value because the value exceeds the 2% deadband. The next value that the SVE logs then must be equal to or greater than 16.5 or equal to or less than 12.5.

Setting Alarm Deadbands

When you set an alarm deadband value, the LabVIEW Datalogging and Supervisory Control (DSC) Module compares any new value acquired to the existing value. The SVE clears an active alarm if the percentage difference between the new value and the previous value equals or exceeds the deadband. Set the alarm deadband using the Alarming:AlarmName:Deadband SharedVariableIO property, where AlarmName is HI_HI, HI, LO_LO, LO, RoC, U32 Bit Field, Bad Status, or Boolean, or when you configure alarms for a shared variable.

For example, for a data point with a range of values of 0 to 100, set a LO condition alarm at a value of 12 with a deadband of 1.5%. The alarm condition does not trigger until the SVE value drops to 12 or below. The alarm stays active until the SVE value rises to 13.5 or greater.

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