Understanding Uncertainty in Load Cell Calibration

In force measurement testing, accuracy is the most critical factor in ensuring the data you collect can help to identify challenges, failures and opportunities in the product design and development cycle. Here at Interface, we have mastered the art of load cell accuracy by employing a vertically integrated manufacturing process that allows us to control the development of our products most critical components.

Even the most high-end manufactured load cells and finely tuned components endure accuracy degradation over continued use. Therefore, we have also invested in equipment and talent with deep expertise in load cell recalibration, as well as offering gold and platinum standard calibration systems to customers. Recalibration is recommended on an annual basis, or of course, whenever our customers feel they need to confirm they are getting the right data out of their load cells.

One of the key factors of calibration and recalibration is understanding how to estimate practical uncertainty in load cell calibration. Measurement uncertainty is defined as an estimate of the range of measured values within which the true value lies or, alternatively, the degree of doubt about a measured value. In every application, there will be an uncertainty requirement on the force measurement. The equipment used to make the measurement must be traceable to a realization of the SI unit of force (the newton) within this required uncertainty.

Each application is different in terms of its uncertainty requirement. For instance, an application testing force in the aerospace and defense or medical sector will include a much more stringent uncertainty requirement than something like a commercial scale used to measure someone’s weight or food. It is critical to understand the uncertainty requirement on the application to ensure the force measurement device used is calibrated to handle the project.

How does one go about estimating uncertainty in load cell calibration? The first thing to understand is the GUM, a guide to the expression of uncertainty in measurement. This guide establishes general rules for evaluating and expressing uncertainty in measurement that are intended to be applicable to a broad spectrum of measurements.

Next, we have included a list of different considerations, as we measure uncertainty here at Interface. These factors will help guide you as you determine uncertainty for yourself. This list includes:

  1. Determine what parameter is to be measured and the units of measure.
  2. Identify the components of the calibration process and the accompanying sources of error.
  3. Write an expression for the uncertainty of each source of error.
  4. Determine the probability distribution for each source of error.
  5. Calculate a standard uncertainty for each source of error for the range or value of interest.
  6. Construct an uncertainty budget that lists all the components and their standard uncertainty calculations
  7. Combine the standard uncertainty calculations and apply a coverage factor to obtain the final expanded uncertainty.

It is also important to consider the different methods of load cell calibration. There are three different methods, and each has an approximate feasible expanded uncertainty. The different calibration methods include:

  • Direct dead weight – this method is the best for accuracy at 0.005% uncertainty, but it is slow, and the equipment is space inefficient.
  • Leveraged dead weight – middle of the road for accuracy at 0.01% uncertainty, and slow and space inefficient.
  • Hydraulic force generation comparison – this method has reasonable accuracy at 0.04% uncertainty and is also the fastest and most space-efficient option.

The final point is the sources of error in calibration. Error is defined as the difference between the measured value and the true value. There is a long list of different factors that can cause error and increase uncertainty. These factors may include drift, creep, misalignment, or environmental factors such as temperature. To compensate for this, it is important to understand the various formulas that can be used to find the true value based on the given measurement and the various factors for error.

To learn more about uncertainty and the different ways users can address uncertainty and overcome it, please give us a call at 480-948-5555, or visit our website to contact our Application Engineers.

Contributor:  LaVar Clegg, Interface

Source: NCLSI Measurement Training Summit 2014

Test Stand Applications for Force and Torque

In the world of test and measurement, test stands are essential equipment for manufacturers and testing engineers. The test stand provides a host of different testing products in a single “cabinet-like” structure. These systems have been used for a long time to gather data on various functions of products during the product test phase.

Test stands works like a mobile test lab, hosted by a frame and containing one or more force or torque sensor components, software, and data acquisition instrumentation and accessories. Force stands are typically motorized or manual.  Motorized test stands, also known as mechanical or electrical, have the advantages of controlling performance by applying modes such as speed, cycles, and time into the testing procedure. The more advanced testing stands are frequently used for repetitive high-performance testing requirements, validating accuracy and quality. Manual test stands are used for simple testing protocols and frequently used in education programs.

There are a wide variety of testing devices and sensor products that are used as part of the entire test process. As parts roll off the production line, the test stand will sit at the end of the line where the test engineer can immediately load the product into the test rig. Test stands help to streamline the test process by providing all available test functions in a single, mobile application.

Interface is a supplier of choice for precision components of various capacities and dimensions for test stand configurations requiring precision and accuracy in performance. Interface load cells, torque transducers, and instrumentation equipment are commonly used in numerous product test applications by engineers, metrologists, testing professionals and product designers around the world.

Included below are a few examples of specific test applications and the Interface components used in the different style testing stands.

Linear Test Stand

In this example, an Interface customer wanted to add a crush test to their test stand to measure the force it took to deform a piece of material. Interface provided an Model 1210 Load Cell with an internal amplification of 0-10VDC output.

The load cell was installed into the load string of the customer’s load frame, and the scaled analog output from the load cell was connected to the customer’s test stand instrumentation. When the force levels reached the crushing point, the customer’s software was able to read the output of the amplified load cell and record the value.

See the application note for the Linear Test Stand here.

Motor Test Stand

In the quality control lab at a major automotive manufacturing company, a test engineer needed to test, record, and audit the torque produced by a new motor design under start load. Interface supplied the new AxialTQ® Rotary Torque Transducer that connected between the motor and the differential, on the drive shaft, that could measure and record these torque values.

Based on the data collected using the AxialTQ transducer, along with the AxialTQ Output Module, and a laptop, the test engineer was able to make recommendations to optimize the amount of torque created by the new motor design.

See the application note for the Motor Test Stand here.

Verification Test Stand

In this application, a customer needed a test stand application to verify that its load cell was in good, working order. Interface helped to create a solution that used a load cell to verify the customer’s load cell. The solution involved the customer’s supplied verification load frame and an Interface Model 1210 Precision LowProfile® Load Cell connected with a Model SI-USB 2-Channel PC Interface Module.

The customer was able to install their load cell and Model 1210 Precision LowProfile Load cell into the verification load frame. Applied forces were displayed and recorded by Model SI-USB PC Interface Module for review and record keeping on customer’s computer. This allows the customer to have a proven load cell verification test stand at their disposal to ensure its test load cell is always in working order.

See the application note for the Verification Test Stand here.

These are just a few examples of the different types of test stands that Interface can provide off-the-shelf or custom force measurement solution components. If your project involves a mechanical test stand and you are interested in learning more about adding force sensors, please contact our application engineers.