What is non-linearity?

In its broadest sense, non-linearity refers simply to a departure from something that is linear.

In the world of transducers, non-linearity is the maximum deviation in output between a transducer’s sensitivity curve and a linear representation of its true sensitivity curve drawn between nominal zero and full scale. Non-linearity is measured on increasing input only, and is expressed as a percent of full-scale output.

An example of non-linearity for a transducer is ±0.15% F.S. Determining non-linearity for a transducer raises a question of how to create the linear representation of a transducer’s true sensitivity. Often, a best fit straight line, which is based on the least squares method, is employed.

Best Fit Straight Line Compared to Ideal Sensitivity

When a best fit straight line is used, transducer non-linearity is simply the greatest deviation between the transducer’s sensitivity curve and the best fit straight line obtained mathematically using the least squares fit method.

Non-Linearity based on Best Fit Straight Line

In other cases, a terminal point straight line is used to determine transducer non-linearity. The terminal point straight line is drawn between nominal zero and output at full scale.

Terminal Point Straight Line

Terminal point straight line is often a more practical best straight line, as it is easy to understand and implement. The user simply takes the output at zero and the output at full scale and assumes a straight line relationship.

Using a terminal point straight line results in a greater (worse) value for non-linearity than using a best fit straight line obtained mathematically.

Non-Linearity based on Terminal Point Straight Line

Note: Honeywell uses the terms linearity and non-linearity interchangeably. Honeywell uses the terminal point, straight line method and least squares, fit best straight line to determine its transducer’s non-linearity. The datasheets indicate the method used when quoting specifications.