interpolated bounding values – OPC Unified Architecture

3.1.8 interpolated bounding values

bounding values determined by a calculation using the nearest Good value

Note 1

Note 1 to entry: Interpolated Bounding Values using SlopedInterpolation are calculated as follows:

if a non-Bad Raw value exists at the timestamp then it is the bounding value;
find the first non-Bad Raw value before the timestamp;
find the first non-Bad Raw value after the timestamp;
draw a line between before value and after value;
use point where the line crosses the timestamp as an estimate of the bounding value.

The calculation can be expressed with the following formula:

Vbound = (Tbound – Tbefore)x( Vafter – Vbefore)/( Tafter – Tbefore) + Vbefore

where Vx is a value at ‘x’ and Tx is the timestamp associated with Vx.

If no non-Bad values exist before the timestamp the StatusCode is Bad_NoData. The StatusCode is Uncertain_DataSubNormal if any Bad values exist between the before value and after value. If either the before value or the after value are Uncertain the StatusCode is Uncertain_DataSubNormal. If the after value does not exist the before value shall be extrapolated using SlopedExtrapolation or SteppedExtrapolation.

The period of time that is searched to discover the Good values before and after the timestamp is Server dependent, but if a Good value is not found within some reasonable time range then the Server will assume it does not exist. The Server as a minimum should search a time range which is at least the size of the ProcessingInterval.

Interpolated Bounding Values using SlopedExtrapolation are calculated as follows:

find the first non-Bad Raw value before timestamp;
find the second non-Bad Raw value before timestamp;
draw a line between these two values;
extend the line to where it crosses the timestamp;
use the point where the line crosses the timestamp as an estimate of the bounding value.

The formula is the same as the one used for SlopedInterpolation.

The StatusCode is always Uncertain_DataSubNormal. If only one non-Bad raw value can be found before the timestamp then SteppedExtrapolation is used to estimate the bounding value.

Interpolated Bounding Values using SteppedInterpolation are calculated as follows:

if a non-Bad Raw value exists at the timestamp then it is the bounding value;
find the first non-Bad Raw value before timestamp;
use the value as an estimate of the bounding value.

The StatusCode is Uncertain_DataSubNormal if any Bad values exist between the before value and the timestamp. If no non-Bad Raw data exists before the timestamp then the StatusCode is Bad_NoData. If the value before the timestamp is Uncertain the StatusCode is Uncertain_DataSubNormal. The value after the timestamp is not needed when using SteppedInterpolation; however, if the timestamp is after the end of the data then the bounding value is treated as extrapolated and the StatusCode is Uncertain_DataSubNormal.

SteppedExtrapolation is a term that describes SteppedInterpolation when a timestamp is after the last value in the history collection.