TN - Scale Analog Inputs or Outputs

Purpose

Analog signals are often represented by electrical current or voltage in the form of a specific range, such as 4-20 mA. Scaling these values allows users to convert these raw analog inputs and outputs into useful engineering units or command values for better system integration and control.

Summary/Issue Description

Scaling analog signals is essential for converting the raw values, which often correspond to a limited resolution (e.g., 4000 or 8000 point resolution), into a practical, usable range of engineering units (e.g., temperature, pressure, position). Analog inputs and outputs typically have specific lower and upper ranges that need to be translated into corresponding engineering values for precise system operation.

Root Cause

The need for scaling arises from the inherent limitations of analog devices that produce raw signals within a defined range (such as 4-20 mA, or a resolution of 4000 points). These signals must be converted to more meaningful values that can be interpreted by the system's control or monitoring mechanisms. Without proper scaling, the system would not be able to correctly interpret the sensor's data or send the appropriate command values to actuators.

Solution

Example 1 - Scaling an Input

For instance, a temperature sensor is attached to an analog 4-20mA input with a resolution of 4000 points. The temperature sensor has a sensing range of -50 to 250 degrees Celcius which corresponds to the 4-20mA range. The analog input unit should then be reading 4mA at -50 degrees C and 20mA at 250 degrees C. The value the analog input unit converts the input range to its range of resolution, ie 0 to 4000.

In the below graph, the values would be 

X0 = +0.0 (the lower end of the input range)                             Y0 =-50.0 (deg C) (ie the lower converted value)

X1 = +4000.0 (the upper value of the input range)                    Y1 = +250.0 (deg C) (ie the upper converted value)

Note that it is often convenient to use the upper and lower range of the analog input as the data scaling points, but any two known points of input vs output are sufficient to define this linear relationship. 

Example 2 - Scaling an Output

Scaling from a command value of say 0-100% can be done in a similar manner. Lets assume that an actuator is fully closed when it receives a 4mA loop value and fully open when it receives a 20mA loop value.  The analog output used for this device is an 8000 point resolution. Note now the output of the scaling function will be the analog output units command values. 

The values used for scaling in this example would be as follows;

X0 = +0.0 (%) (the lower end of the input range)           Y0 = 0 (analog unit value) (ie the lower converted value)

X1 = +100.0 (%) (the upper value of the input range)    Y1 = 8000 (analog unit value ) (ie the upper converted value)

Date/Revision History

First review 12/23/2024 V1.0

2nd Review 01/13/2025

Author:

Andrea Balderrama, Technical Support Engineer