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 set the ProxyExt Conversion property to Linear. You must then enter your calculated scale and offset values in the Linear fields. Also, you must set the “Units” in its Linear Calibration Ext to match an input point’s facets—otherwise a fault status occurs.
This also applies to any linearly-responding ResistiveInputPoint using point facets other than ohms.
Use the following formulas to calculate the Linear scale and offset values:
where: x1 and x2 are the Ndio values (e.g voltage), and y1 and y2 are the corresponding desired units.
-
Scale
Scale = (y2 - y1) / (x2 - x1)
-
Offset
Offset = y1 - (Scale * x1)
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 Shunt” (if available), 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 12 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.