Load Cell Performance Starts by Design Webinar Recap

Interface recently hosted an in-depth technical seminar, Load Cell Performance Starts by Design, emphasizing the engineering perspective.

To kick off the detailed discussion, we answer the following question: What is a load cell? A load cell transducer converts a mechanical force into an electrical signal. It is designed to measure various loads, including tension, compression, torsion, shear, and moment. Load cells are characterized by their lack of moving parts, which ensures durability and minimal wear and tear.

Five key factors influence the design of a load cell. Accuracy requirements determine the precision and reliability of measurements. Performance results must be consistent and repeatable. Quality must be of the highest standard, both for manufacturing and materials. Durability is crucial in determining long operational life and resistance to damage. Application suitability will determine the type of load cell selected for the specific project. Find the recording on Interface’s YouTube Channel.

Load Cell Design: How They Work

Most load cells operate on the principle of strain gage technology. A force applied to the load cell causes a deflection in the flexure (the load-bearing component). This deflection leads to strain, which alters the electrical resistance of strain gages attached to the flexure.

Interface load cells use proprietary strain gages configured in a Wheatstone bridge arrangement. When a force is applied, the strain gage resistance changes, unbalancing the bridge and producing a measurable and ratiometric output voltage proportional to the applied force. This output is typically specified in mV/V.

Sensor Specifications

There are many types of load cells, including LowProfile, miniature, load pin, shear beam, bending beam, rod end, canister, and multi-axis.

Selecting the right load cell requires careful consideration of mechanical, electrical, environmental, and performance specifications, including:

• Capacity: The maximum load the cell can measure, safe overload, and point of damaging results.
• Extraneous Loads: Torque, side load, and off-axis loading.
• Mounting: Proper mounting is crucial for accurate measurements, detailing live-end and dead-end considerations.
• Electrical: Signal output, excitation voltage, and cable length.
• Environmental: Temperature and humidity resistance and thermal factors.

Specifications to consider include nonlinearity, the deviation from a linear output, and hysteresis, the difference in output between loading and unloading cycles. Static Error Band (SEB), a general measure of overall accuracy, is an excellent place to start when reviewing specs. Nonrepeatability defines the variation in output for repeated loadings under identical conditions. Eccentric load sensitivity is the effect of off-center loading, and creep is the change in output over time under constant load.

During the webinar’s specification section, load cell engineer Jacob Wahlstrom provides valuable insights into creep specifications and the timing of a creep measurement. Watch the recording here.

Anatomy of Load Cells

The science of force measurement is best understood by looking inside the sensor.  During this portion of the technical event, Brian Peters highlights different models by dissecting the designs to understand better how they factor into building precision measurement devices.

Along with the visual views, the discussion focuses on the main components of load cells, including flexure, strain gages, electrical connections, and customizable features like bridge counts, cables, and connectors.

Strain gages are the heart of the load cell, meticulously measuring the subtle deformations of the flexure under stress. The webinar explored the diverse configurations of these gages, including linear, shear, Poisson, and Chevron types, each designed to capture specific strain patterns. Jacob delved into the construction of foil strain gages, highlighting the importance of components like grid lines, end loops, and backing material in achieving accurate and stable measurements. Find out why Interface LowProfiles use eight strain gages and our 1800s use 16 for ultimate performance accuracy.

Furthermore, the webinar emphasized the critical role of temperature compensation techniques in minimizing the influence of thermal variations on strain gage readings. Different gage foil types, such as Karma and Constantan alloys, were examined for their unique characteristics and suitability for various applications. Learn why Interface prefers one type of alloy for our proprietary strain gages.

The manufacturing process of strain gages involving photolithography was also discussed, along with the crucial aspect of gage placement for optimal sensitivity and accurate capture of principal strains.

Engineering Perspective of Flexure and Model Design

Understanding flexure design’s intricacies emphasizes the impact on load cell performance. Flexures, the load-bearing components that deform under stress, are carefully engineered to ensure accurate and reliable force measurements. The webinar explored various aspects of flexure design, including:

• Material Selection: Different materials, such as aluminum, steel, stainless steel, and titanium, are chosen based on strength, corrosion resistance, and temperature stability. The webinar highlighted the trade-offs in selecting the optimal material for a given application.
• Geometry Optimization: The shape and dimensions of the flexure are crucial for achieving desired performance characteristics. For instance, in LowProfile load cells, the beam height and thickness are carefully balanced to maintain shear stress and avoid bending, which can introduce measurement errors.
• Stress Concentration Management: Load cell design engineers aim to minimize stress concentrations in the flexure, particularly around features like threads and holes. This ensures uniform stress distribution and prevents premature failure.
• Mounting Considerations: The webinar discussed the importance of proper mounting techniques to avoid introducing unwanted stresses or misalignments that can affect accuracy. This includes considerations for bases, threaded connections, adhesives, and load introduction points.
• Environmental Protection: Flexures are often designed with features to protect them from environmental factors like moisture and temperature fluctuations. This can involve sealing techniques, specialized coatings, or corrosion-resistant material choices.

Specific examples of flexure designs were presented for different load cell types, including:

• LowProfile Load Cells: The webinar highlighted design features like loading rings with reduced area to maintain flatness and perpendicularity, ensuring axial force alignment.
• Load Button Load Cells: The challenges of designing flexures for miniature load cells were discussed, emphasizing the importance of precise gage placement and stress distribution.
• Multi-Axis Load Cells: The webinar showcased innovative flexure designs that measure forces and torques in multiple dimensions, highlighting the complexity of capturing these complex strain patterns.
• Load Pin Load Cells: We explored design considerations for load pins, including internal vs. external gaging and stress concentration management.

The webinar provided a comprehensive overview of how flexures are engineered to achieve optimal load cell performance in various applications. It explored these diverse design considerations and showcased real-world examples. Watch the complete event below.

Moment compensation techniques were explained, emphasizing their role in reducing errors caused by eccentric loads. The webinar covered temperature compensation methods, including self-temperature and modulus compensation, to ensure accurate measurements across varying temperatures. Review: What is Moment Compensation? and Understanding Load Cell Temperature Compensation

Summary

This webinar isn’t just a theoretical overview of load cell technology; it’s a practical guide for engineers who rely on accurate force measurements. Understanding the nuances of load cell design, strain gage technology, and model and flexure engineering design criteria is fundamental for ensuring reliable and meaningful data in any application.

Engineers can make informed decisions about sensor selection and the intricacies of how different load cell types operate, from miniature beams to multi-axis sensors. Knowing how factors like temperature, eccentric loading, and creep can influence measurements empowers engineers to minimize errors and optimize testing plans and use cases.

Furthermore, a deeper understanding of strain gage characteristics, including their construction, temperature compensation techniques, and optimal placement, allows engineers to maximize sensitivity and accuracy. Ultimately, this knowledge translates to more robust and reliable force measurements, leading to better product design, improved process control, and more confident data analysis using load cell technologies.

Be sure to download our Load Cell Field Guide for a quick reference. We also have an entire educational series, ForceLeaders 101, highlighting technical topics. If you have questions, please contact our representatives, distributors, and application engineers. We are vested in your success!

ADDITIONAL RESOURCES

Interface Load Cell Field Guide

Demystifying Specifications Webinar Recap

Interface Presents Load Cell Basics