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Strain Gage Design Under Eccentric Load WRSGC Presentation

By Ashlesa Mohapatra, product design engineer, Interface

In the global marketplace, Interface is well known as providing the force measurement industry’s most reliable and accurate products. One of the key reasons that Interface consistently earns this recognition is because we manufacture our own strain gages. Products engineered and manufactured at Interface use our proprietary strain gages, and each designed for the specific transducer model based on the application type and environment for use.

As an example of our dedication to quality and excellence in performance as it pertains to strain gages, I recently shared a technical presentation on the negative effects of eccentric load and how strain gage design can reduce these challenges.

Below is a brief recap of this presentation made to the attendees of the Western Regional Strain Gage Committee meeting that took place in Tempe, Arizona in October 2022. The summary explains why strain gage design can make all the difference in quality versus poor performance with load cells.

Interface redesigned the strain gages on one of our mini load cells, the LBSU Miniature Load Cell Load Button, also known as our ConvexBT – The Most Innovative Load Button Load Cell. Our goal in the redesign was to create more controlled and repeatable loading, in turn creating a more predictable output. Our research focused on strain gage designs for load cells where mechanical moment compensation is not feasible.

The main challenge with this initiative was overcoming the errors associated with eccentric loading by making the installation process smoother through a redesign.  This is difficult because strain gages are very small in size and therefore more difficult to work with, in addition they are extremely sensitive to the environment with factors like temperature, humidity, cleanliness and electric interference all potentially effecting performance.

Before diving into the redesign, I would like to touch on eccentric loading and the errors it will cause, as well as the varied factors in strain gage manufacturing that can lead to errors causing eccentric load. There are two types of eccentricity: loading and mounting. Eccentric load results from improper loading or mounting of the strain gage, which leads to off-axis loads and bending. This causes several problems including distorted measurement results, decreased load cell accuracy, and diminishing life of the load cell.

When a strain gage is mounted on the load cell incorrectly or gages are badly bonded, it will almost always be an error source and contribute to mounting errors. Also, when strain gages are not bonded to the load cell at appropriate temperature and humidity, it leads to bubbles under the gage. Chemical composition of the strain gage is critical, such as the adhesive between the foil and backing, based on the application in which load cell will be used in a lab, machine, or testing program.

With these factors in mind, we set out on a redesign continuous improvement project. The previous design of this products strain gages was rectangular in shape. So, when the load cell was loaded, eccentrically or not, the strain field would not pass through because of shape. Therefore, we began to look at other shapes for our strain gage design, ultimately landing on a circular “diaphragm” style strain gage that allow strain fields to pass through.

One of the features of this newly designed strain gage is the proprietary adhesive foil we used to adhere the foil to the backing. This adhesive provided a great deal of benefit including a lower modulus of elasticity making it resilient to adhesive failure, and the elasticity also allows for better flow.

Another feature is the full bridge gage pattern we used that provides three key advantages. This includes fewer solder joints and reduced risk for electrical shorts due to simplified wiring, reduced symmetry error, and consistent thermal performance.

One process improvement we wanted to point out was that in our calibration process we only used 5V excitation voltage. Most manufacturers use 10V to calibrate their load cells. Due to lack of thermal mass in the thin diaphragm design of our strain gage, the zero will shift due to high voltage and low poor heat dissipation with 10V. We use a 5V excitation voltage to calibrate these miniature load cells instead of the alternative to prevent overheating of the cell.

To further improve the design, we enhanced the inspection process. Our diaphragm gages are quality inspected for accurate mounting with visual and electrical testing. Visual testing includes checking for air bubbles under the gage, badly bonded edges, unreliable solder connections and flux residues. Electrical tests include checking for electrical continuity and insulation resistance.

We then moved our attention to the circuit board. Some manufacturers use a circuit board in the cable due to the limited space within the cell to improve zero balance zero balance and to better compensate for temperature. However, bending or moving this cable would put pressure on the board and shift the zero. Therefore, we elected to install an abradable compensation resistors inside the flexure instead of the cable. This keeps the compensation resistor close to the gages and is intimately bonded to the body of the sensor to improve the reaction time of the cell to temperature.

To evaluate and confirm that our design was superior, we assessed three different strain gage styles: the rectangular gages (discreet gages), patch gages, and our diaphragm gage. Each of the gage styles were placed on three different load cells and loaded at one degree centricity. This test was run at 45 degree increments eight times. The results showed diaphragm style provided more reproducible result under eccentric load compared to other gages.

This was an interesting undertaking that taught the project team a lot about strain gage design and eccentric load. What I took away from this experience, other than a superior design for our ConvexBT Load Button Load Cells, is that any commercially successful product has a strong process behind it. You also need to have a clearly defined process that includes a continuous improvement plan. Interface Minis are a popular product line that has been around for many years. As soon as a product like this hits a point of stagnation, it will lose its hold on the market. I am proud of our team’s ability to avoid stagnation by taking critical steps to improving the Mini product line, maintaining our reputation for having the best quality, accurate and reliable products no matter the capacity available for precision force measurement.

Western Regional Strain Gage Committee (WRSGC), a technical division of the national Society for Experimental Mechanics (SEM), was established to promote a free interchange of information about strain measurement techniques using strain gages.

Interface is a proud member and sponsor of WRSGC. Our engineers participate in the technical conferences, in both presentation and attendance. Interface’s Product Design Engineer Ashlesa Mohapatra presented at the event held in Arizona, October 17-19, 2022.

Why Product Design Engineers Choose Interface

Load cells and torque transducers of all types play a critical role in the test and measurement landscape and are used widely by engineers to qualify product design, ensure safety, prove reliability and durability. Force measurement solutions are being utilized more frequently in original product designs and components today due to the dependencies on force feedback during design, testing, prototyping and for monitoring force in the finished goods.

Interface has been a long-time partner of product design engineers. They utilize our force sensors throughout the design process.  Interface sensor technologies are preferred by product design engineers because of the precision, accuracy, range of options, and quality. Across industries, our load cells and torque transducers are being used as a feature in all types of equipment, industrial solutions and consumer products, large and small.

In search of a solution to a problem, design engineers use our products in every stage of the process.

Ask + Qualify: Often in consultation with our application engineers, they gather key sensor specifications, design features, capacity and capability requirements. This initial step is critical in ensuring the right type of sensor is selected and that it will fit into the test plan.

Planning + Design Files:  As the plan is created by the engineer, they will turn to Interface to gather the datasheet details, along with our design files. Interface has thousands of products and each has their own unique drawing. These are all available on our site by product. If you need help finding a design file, submit CAD requests here.

Prototyping:  Once the plan is committed to and prototyping is the next step, if any of the specifications require a custom or modified design, our engineers will team with the design and test engineer to ensure exact specs meet the final design requirements.

Test + Measurement:  This is where accuracy and quality matter most. The designs are tested and data is gathered to determine if the problem is actually solved.  Often our sensors are paired with instrumentation and other products that are utilized in the lab to validate all the use cases and design specs.  It also provides valuable insights into improvements and modifications, if needed.

Manufacture + Launch:  The final step that Interface can play a significant role is building sensors at scale that are used as embedded components. We work with many OEMs in building unique parts for their product designs.

Product Solution Types for Design Engineers

Interface force measurement products are the preferred solutions for design engineers because of our extensive catalog of products. We offer four product solution types for design engineers:  standard products available on our site, engineered to order products to match a specific use case test, custom solutions that are built to specifications and requirements of the engineer and OEM products that are designed in as a part of a product.  We’ve worked across thousands of applications as noted in our extensive applications catalog and have developed a product list of over 30,000 SKUs to get started in the design process.

Designing Brake Pedals for Gaming

In the gaming world, load cells are extremely popular for racing game pedals to simulate the actual real-world experience. These load cells capture the force of a human foot pressing down on the pedal more accurately in game, especially when you are using products like our Bluetooth Brake Pedal Load Cell.  This specialized load cell is also available in wireless.  Brake pedal design engineers will create a series of tests to ensure that the pedal is performing as intended, and also to ensure that all the use cases are validated with accurate measurement data.  This helps in the final design tests and initial prototyping, as well as full release. Further details on this consumer product application can be found in the Gaming Simulation Brake Pedal App Note. Read more about gaming solutions here.

Designing Tractor Linkage Draft Controls

Farmers need to measure the forces applied on their tractor’s draft control, between the tractor and any linked-on attachments. Measuring the force helps the farmer sense any strains on the hitch of the tractor and will be needed in order to apply any specific settings to the draft control when the tractor encounters rough terrain. As this problem was being solved, the maker shared their requirements with Interface application experts. During the qualification process with the design engineer, Interface suggested the WTSLP Wireless Stainless Steel Load Pin, which is a wireless load pin that can be installed directly in the hitch and replace the normal shear pin of the tractor. Force results are then transmitted wirelessly to the WTS-BS-4 USB Industrial Base Station, where they can view the results of the testing via a connected computer. We also provided the WTS toolkit to provide the testing software. The engineer was able to view test results on the WTSBS-1-HS Handheld Display for Single Transmitters in real-time. Using this solution, the design engineer was able to determine the specific draft control settings to assemble with the tractors. They also were able to design in features to make real-time adjustments during the tractor’s use.

Designing Fitness Equipment and Machines

A fitness machine manufacturer wanted multiple load measurement systems for their different fitness machines such as the elliptical, leg press, rowing machine, and the cable machines. The goal of designing sensors into the equipment is to ensure the machines are functioning properly to prevent injuries. The sensors can also be used for trainers who want to conduct strength and endurance tests. Interface provided a combination of products including the WMCFP Overload Protected Sealed Stainless Steel Miniature Load Cell, SSB Sealed Beam Load Cells, and AT103 Axial Torsion Force and Torque Transducers. Paired with Interface’s proper instrumentation, the forces can be measured, graphed, and displayed during the testing stage. Interface’s products all effectively measured forces needed for those working out or undergoing athletic training. Not only did it ensure the designed machines were working properly, but it also helped those using them to track their endurance performance and consider future design enhancements based on consumer use.

These are just a few examples on the industrial and consumer side of how sensors can add critical capabilities to products across the board. Our sensors help to validate, measure, test, prototype and launch new products that are safe, reliable and durable. We’d also like design engineers to understand the wide range of monitoring and real-time sensing capabilities available through Interface. To get started, ask. We are here to help in the product design process.

Additional Resources

OEM Solutions- Turning an Active Component into a Sensor

OEM Brochure

Making the Case for Custom Solutions