TN Hist211 Wonderware Historian Interpolation Types

Description

This article describes the different Interpolation Types in the Wonderware Historian.

• Author: Joseph Hefner
• Published: 04/02/2015
• Applies to: Historian 9.0 and later

Details

Wonderware Historian has 2 possible types of Interpolation for integer and real data types. The default Interpolation Type is Stairstep for integer and Linear for real. This default type can be changed from within the Historian by going to Configuration Editor \ System Configuration \ Parameters as below:

By default, when you create a new attribute in Wonderware Application Server, it will have an Interpolation type of System Default. This means it will use whatever Interpolation type the Historian has set for that data type.

Why does the Interpolation type matter?

In the trend below, both the red and blue line have data changing from 0 to a high number on the trend scale. Notice how the red line has an interpolated line drawn between the 2 points where the blue line maintains the original value until the value changes and then instantly changes to the new value. This is due to linear Interpolation of the red line.

With Linear Interpolation, the Historian calculates a new value at the cycle boundary by interpolating between the 1st stored value before the boundary and the first stored value after the boundary. If either of these values is NULL, it returns the last stored value before the boundary. Below is a description of how the value is calculated:

Expressed as a formula, Vc is calculated as:

Vc = V1 + ((V2 - V1) * ((Tc - T1) / (T2 - T1)))

With StairStep Interpolation, no interpolation occurs. The value at the cycle boundary is assumed to be the same value as the last stored value before the cycle boundary. The last known point is returned with the given cycle time. If no valid value can be found, a NULL is returned.