Nonlinearity 101
When we talk about measuring force or weight, there is an assumption of a straight-line relationship between the applied force and the output signal. For a high-quality load cell, this is the goal. But in the real world, this relationship is rarely a perfect straight line. Nonlinearity is an accuracy specification of a load cell that quantifies this deviation.
Imagine plotting the output of a load cell (voltage, for example) against the force applied (in pounds or kilograms). A perfect straight line connecting the output at no load (the minimum load) to the output at the maximum rated load (the maximum load) is the ideal response. This line represents what we would expect from a perfectly linear sensor. The actual response is the curve you get from the real-world measurements. It’s the actual output of the load cell at various points of applied force.
Nonlinearity is the algebraic difference between a point on the actual response curve and the corresponding point on the ideal straight line. This difference is typically expressed as a percentage of the load cell’s Full Scale (FS) output.
Why Understanding Nonlinearity is Important
Nonlinearity is a crucial specification because it directly impacts the accuracy of your measurements. A load cell with a high nonlinearity percentage will give you inaccurate readings, especially at the intermediate points of its measurement range. For example, a load cell with 1% nonlinearity might be very accurate at its minimum and maximum loads, but its reading at 50% of its capacity could be off by a significant amount.
The measurement of nonlinearity is part of a comprehensive characterization process. While the ideal definition considers all points, it’s common to measure this value at a few specific points, often at 40% to 60% of the full scale. This range is chosen because it typically represents where the maximum deviation from linearity occurs. The value is then reported as a percentage of full scale (%FS). A lower percentage indicates a higher-quality, more linear load cell, which in turn provides more accurate measurements across its entire operating range.
It’s easy to confuse nonlinearity with another important specification: hysteresis. While both relate to measurement accuracy, they describe different phenomena:
- Nonlinearity: This refers to the deviation from a straight line during a single, continuous loading cycle (e.g., from zero to full load).
- Hysteresis: This refers to the difference in output at a specific load point when you are increasing the load versus when you are decreasing the load.
A load cell can have low nonlinearity but high hysteresis, and vice versa. For high-precision applications, you need a load cell that excels in both of these specifications.
Engineering Tips from Load Cell Designers
- There is actually no “Accuracy” value on data sheets.
- Accuracy is detailed by performance defined as Max Error for SEB, Nonlinearity, Hysteresis, Nonrepeatability, Creep, Side Load Sensitivity, and Eccentric Load Sensitivity.
- Apply specifications according to the application to ensure you select the right sensor for your requirements.
- Know that %FS, %RO, %, %/°F are time related specifiations.
By understanding nonlinearity, you can better interpret load cell datasheets and select the right sensor for your application, ensuring the accuracy and reliability of your force and weight measurements.
Additional Resources
What is Static Error Band Output?
Expert Tips for Essential Load Cell Specifications
Demystifying Specifications Webinar
