Scale and offset calculation (linear)

Unless you want a VoltageInputPoint or VoltageOutputWritable to operate with facets (units) of volts, you must set the point’s facets to the desired units, and change the Conversion property in its ProxyExt. Typically, you select a Conversion of Linear, except in the case of VoltageInputPoint used for a 4-20mA sensor, which requires a conversion of 500 Ohm Shunt, with Linear as the secondary conversion.

Linear conversion is also appropriate for a linearly-responding ResistiveInputPoint using facets other than ohms—although this may be less common than a non-linear application.

Linear requires you to enter your calculated scale and offset values in the Linear fields. Also, if an input point, you must set the “Units” in its Linear Calibration Ext to match an input point’s facets—otherwise a fault status occurs.

Use the following formulas to calculate the Linear scale and offset values:

where: x1 and x2 are the Nrio values (e.g voltage), and y1 and y2 are the corresponding desired units.

Scale

Scale = (y2 - y1) / (x2 - x1)
Offset

Offset = y1 - (Scale * x1)

See example 1 and example 2.

Example 1. Scale and offset calculation example 1

You have a 6-to-9Vdc actuator to control with a VoltageOutputWritable, in terms of 0-to-100% input. Configure this point’s facets to have units: “misc,” “percent” (%).

Set the ProxyExt’s Conversion property to Linear, and enter these calculated scale and offset values:

Scale

Scale = (y2 - y1) / (x2 - x1)

Scale = (100% - 0%) / (9V - 6V) = 100/3 = 33.3333333
Offset

Offset = y1 - (Scale * x1)

Offset = 0% - (33.3333333 * 6V) = 0 - 200 = -200

Example 2. Scale and offset calculation example 2

You have a linear 4-to-20mA differential pressure sensor to read with a VoltageInputPoint (using an external 500 ohm resistor wired across the input), reading effectively 2V to 10V. The range of this sensor is from -10 in.wc. to +10 in.wc. Configure this point’s facets to have units: “pressure,” “in/wc”.

Set the ProxyExt’s Conversion property to use “500 Ohm Conversion”, then “Linear” as secondary, and in the Linear fields enter these calculated scale and offset values:

Scale

Scale = (y2 - y1) / (x2 - x1)

Scale = (+10in.wc - -10in.wc) / (10V - 2V) = 20/8 = 2.5
Offset

Offset = y1 - (Scale * x1)

Offset = -10in.wc - (2.5 * 2V) = -10 - 5 = -15

Figure 20 shows this point’s configuration in a Workbench dialog. Note that the Linear Calibration Ext for this point will require its “Units” property to be configured to match the facets for this point—otherwise, the point will have a fault status. See the Linear Calibration Ext section for related details.