What is Non Linearity?
Almost any load cell specification sheet has a term called static Non-Linearity.
This particular article hopes to explain what the term Non Linearity is.
Load Cell Accuracy: Simple Definition of Non Linearity
In simplistic terms Non Lineratiry tells us how much a load cell's readings might not follow a straight line when we apply force. A lower number means the load cell responds more evenly, which is better for accurate measurements. Depending on how we draw this straight line, the reported non linearity can change for the same load cell.
Load Cell Accuracy: Real Definition of Non Linearity
Non Linearity, % R.O.(Rated Output): This specifies the deviation from a straight line when force is applied. A lower value indicates that the load cell response is more linear, which is ideal for precise measurements.
Reported Non-Linearity can vary for the same cell depending on how this straight line is defined.
Typical definitions are endpoint, best fit straight line, and best fit straight line through zero.
The post explaining SEB uses the best fit straight line through zero to calculate the linearity.
An ideal measurement device has a perfectly linear response to force applied ratio.
However, this is rarely true; most devices have a non-linear ratio.
The purpose of the non linearity calculation is to show how the recorded responses deviate from the ideal ratio.
Non-linearity is typically expressed in the percent of full-scale (% FS).
A line between the initial zero and full-scale points should be drawn to calculate Non Linearity.
This line represents the ideal response ratio that is compared against each of the ascending points.
Calculate the slope using the equations below to draw a line between the two points.
With the slope, the intercept can be calculated using either of the two points used to calculate the slope with the equation below.
With the line properties calculated, use each of the recorded responses in the final calculation below to calculate the % FS of non linearity.
Note: Some manufacturers use higher-order equations to improve their Non-Linearity specification.
Therefore, it is vital to ask them how they calculate non linearity.
At Morehouse, we use the more conservative straight-line approach method.
Calculate Slope
Slope = (0start(force) – FullScale(force)) / (0start(response) – FullScale(response)
Calculate Intercept
Intercept = FullScale(force) – Slope x FullScale(response)
Calculate Non Linearity per Response
Non Linearity = (Point(force) – (Slope x Point(response) + Intercept)) / FullScale(force)
Non Linearity Calculatation Example:
A Morehouse wireless load cell reads 0 at 0 lbf, 1.20003 at 600 lbf, and 2.00010 mV/V at 1000 lbf.
To calculate the slope, the formula would be (0-1000)/(0-2.00010) = 499.975001249937
To calculate the Intercept 1000 – (499.975001249937 * 2.00010) = 0.0
Non-Linearity = (600 – (499.975001249937*1.20003+0))/1000 = 0.0000150
This value is 0.0015 % using the 600 lbf (60 %) point.
Load Cell Accuracy: What is Non-Linearity? - Conclusion
In conclusion, non linearity in load cells refers to the deviation of a load cell's response from an ideal linear relationship between force applied and output signal.
The smaller the non-linearity value, typically expressed as a percentage of full scale (% FS), the more precise and accurate the load cell is in measuring force. Various methods are used to define the reference straight line, including endpoint, best fit, and best fit through zero.
The choice of method can affect the reported non-linearity value, so it is important to understand the calculation method used by manufacturers. At Morehouse, for example, the conservative straight-line method is employed.
By calculating the slope and intercept, non-linearity can be determined for each recorded response, offering insight into the accuracy of a load cell across its operational range.
More Information about Morehouse
We believe in changing how people think about force and torque calibration in everything we do.
This includes setting expectations and challenging the "just calibrate it" mentality by educating our customers on what matters and what may cause significant errors, such as not understanding what Non-Linearity is.
We focus on reducing these errors and making our products simple and user-friendly.
This means your instruments will pass calibration more often and produce more precise measurements, giving you the confidence to focus on your business.
Companies around the globe rely on Morehouse for accuracy and speed.
Our measurement uncertainties are 10-50 times lower than the competition, providing you more accuracy and precision in force measurement.
We turn around your equipment in 7-10 business days so you can return to work quickly and save money.
When you choose Morehouse, you're not just paying for a calibration service or a load cell.
You're investing in peace of mind, knowing your equipment is calibrated accurately and on time.
Through Great People, Great Leaders, and Great Equipment, we empower organizations to make Better Measurements that enhance quality, reduce risk, and drive innovation.
With over a century of experience, we’re committed to raising industry standards, fostering collaboration, and delivering exceptional calibration solutions that build a safer, more accurate future.
Contact Morehouse at info@mhforce.com to learn more about our calibration services and load cell products.
Email us if you ever want to chat or have questions about a blog.
We love talking about this stuff. We have many more topics other than expressing SI units!
Our YouTube channel has videos on various force and torque calibration topics here.
