Configure Scaling for Shared Variables

Scaling Numeric Shared Variables

You can use linear and square root scaling for numeric shared variables.

Often an application needs the LabVIEW Datalogging and Supervisory Control (DSC) Module to manipulate the raw data used in the device server to put the data in a form, called engineering units, suitable for the users of the application. You can define the raw range and engineering range for a shared variable to perform simple conversions between the two ranges on the Scaling page of the Shared Variable Properties dialog box. The raw range refers to the values used by the device server. The engineering range refers to the values used by the Shared Variable Engine and the Human Machine Interface (HMI) application.

Linear and Square Root Scaling

Linear scaling is defined by the following equation:

m * raw + b

where

• b is the Engineering Zero Scale
• m is the (Engineering Full Scale – Engineering Zero Scale)/(Raw Full Scale – Raw Zero Scale).

Square root scaling is defined by the following equation:

b + m * sqrt(raw – o)

where

• b is the Engineering Zero Scale
• m is the (Engineering Full Scale – Engineering Zero Scale)/sqrt(Raw Full Scale – Raw Zero Scale)
• o is the Raw Zero Scale

The following table shows shared variable values that range from 0 to 100 scaled to engineering units ranging from 0 to 10:

The following table shows shared variable values that range from 0 to 100 scaled to engineering units ranging from 15 to 30:

Offset scaling delivers more complicated results.

Linear Scaling Example

In this example, a device server returns a voltage from 0 to 5 V. The voltage is related to a position sensor, and the real-world position is measured in centimeters, with 0 V mapped to 50 cm and 5 V mapped to 100 cm.

The following table shows the values of the shared variables after scaling:

Square Root Scaling Example

In this example, a flow meter measures the flow rate of a liquid using a differential pressure reading. The device server provides 4 to 20 mA readings. The actual flow is measured in Gallons Per Minute (GPM), where 4 mA corresponds to 0 GPM and 20 mA corresponds to 100 GPM.

The following table shows the values of the shared variables after scaling:

Assigning Units to a Double Shared Variable

Use the Scaling.Units SharedVariableIO property to assign units to a shared variable. In the previous example, you could specify gallons per minute as a constant to the Scaling.Units property.

Scaling Boolean Shared Variables

You can use inverted scaling for Boolean shared variables. When you enable inverted scaling, the DSC Module inverts the Boolean value when communicating with the server.

Scaling U32 Bit Field Shared Variables

You can use invert mask and select mask scaling for U32 bit field shared variables.

Mask scaling applies only to U32 bit fields. LabVIEW stores a bit field as a number, translating the number back into a 32-bit field when needed. The bit field holds a collection of 32 values, each value limited to 1 or 0. With mask scaling, you can decide which of these array members to pay attention to. Any bit filtered out by the mask appears as a zero. Bits not filtered out appear as zero or one, reflecting the actual value. A mask of 00000000000000001111111111111111 (0x0000FFFF) where 0 marks a position to filter out and 1 marks a place to report, reads the bit array: 10110011100011001100111000110010

as the following: 00000000000000001100111000110010

The following table shows examples of shared variables configured for U32 bit field scaling: