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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.

Interface Congratulates Four New Certified Six Sigma Greenbelts

In the engineering and manufacturing world, quality and efficiency are key factors to long-term success. Interface is committed to maintaining the highest standards in our work and products through our training, leadership development and certification initiatives. One of the programs we use to drive continuous improvements and efficiencies is by certifying team members as Lean Six Sigma Greenbelts.

The IASSC Certified Lean Six Sigma Greenbelt™ (ICGB™) is a professional who is well versed in the core advanced elements of Lean Six Sigma Methodology. They lead improvement projects and serve as team members working with their peers on complex improvement projects. These individuals possess a thorough understanding of all aspects of the Lean Six Sigma Method including competence in subject matters contained within the phases of Define, Measure, Analyze, Improve and Control (DMAIC).

We are pleased to announce four newly certified Interface Greenbelts: Ashlesa Mohapatra, Tim Matteson, Jesse Deffenbaugh, and Justin Smith.

Providing this training at Interface is our Director of Quality, Rocky Lee, recently featured in our Faces of Interface series. Rocky is also a certified Greenbelt who is well on his way to being Blackbelt certified, an honor he should achieve this year.

“Greenbelt training serves as the perfect vehicle to provide necessary data collection and analytical skills necessary to complete meaningful studies. It also provides a standardized skillset for future studies.” Rocky Lee, Director of Quality, Interface

We asked the four new Greenbelts about their experience in achieving this valuable new certification. Here are their responses:

“I was interested in joining the certification program because I knew it would equip me with the tools needed to solve problems on the floor in an organized and effective manner. My Greenbelt Certification has helped me improve processes by eliminating defects, while also streamlining production and eliminating waste time, money, and resources. As for the process, it was simple and engaging. We had quizzes at the end of every module, a final exam, and a project. Rocky Lee did a fantastic job in simplifying the concepts by giving us practical examples of problems we face at Interface daily.” Ashlesa Mohapatra, Production Engineer, Interface

“I was interested in getting this certification so I could improve myself by learning new data analysis skills and techniques to help improve efficiency of our processes and to reduce the number of defects and waste in production. The techniques I learned are already helping me in analyzing the impact to performance when changing the gage clamps used for gaging low capacity SSMs. I’ve also learned better ways of analyzing historical creep data and then comparing sample data to the greater trends. I think there is opportunity for improvement when it comes to efficiency and defect reduction, and I think having more employees speaking the same Six Sigma language and using the same methodologies will help in effectively diagnosing issues and then implementing the changes needed.” Jesse Deffenbaugh, Production Engineer, Interface

“I was interested in the Six Sigma course because it adds to my skills as an engineer and compliments my intended career path. I think this course helps to frame issues in terms of quality, the degree of quality that is expected by our internal and external customers, and the cost associated with not meeting those expectations. Rocky was an excellent teacher. I think that having people at Interface with this multi-disciplinary training will help to maintain the overall quality of products as well as the process of making those quality products.” Justin Smith, Engineer, Interface

“I took on this program to further my education and to provide more value to the company. A certification in lean six sigma Greenbelt training will benefit the company and myself, as I will be able to immediately apply this certification in my daily activities. Overall, I believe this process will help with data analysis, building a lean culture, and to be more effective and efficient.” Tim Matteson, Mini OEM Product Quality and Improvement Engineer, Interface

Rocky shared his insights on the significance of the Greenbelt training and how it impacts our work at Interface.

Why is this certification important to Interface?

First, we had a strong interest from our team members. The actual initiative started through requests made to Mark Weathers, VP of Advanced Manufacturing and OEM Products, about the possibility of getting Six Sigma Greenbelt trained and certified at Interface. Mark asked Rocky to lead this effort. The interest was strong enough that Rocky and Mark thought it was a good idea to start it right away.  The decision to move forward with the training coincided with multiple studies that had been started by our engineers.

What does it mean to have a Six Sigma Greenbelt training certification in one’s job?

The DMAIC process provides the framework for attacking problems. The technical knowledge is in the measure and analyze phases of Six Sigma methodology. A great deal of training time is spent in these areas in preparing someone for certification. The impact of this can be seen when a person is leading projects and participating in solving challenges. The framework is valuable for those that apply this technique in their work.

How does this initiative and certification help Interface improve our quality and reduce cost of quality?

Certified Greenbelts approach problems using proven problem-solving tools. This includes using studies and analyses, that when performed by certified leaders is a systematic way to obtain results, have more meaning. This important training drives to actions to identify quantifiable measures that are used to focus on qualitative improvements and results.

How were people chosen to participate?

After the first few people showed interest, Rocky asked other engineers in the Quality and Engineering departments. Ten people signed up and they were divided into two groups. This was to make the classes more interactive during the weekly training and review sessions. The people chosen for the first group already had high-profile projects underway and it would benefit them immediately on their projects.

How long does it take to complete certification?

The course has 34 sections and we were able to complete the course in less than eight weeks.

What is Interface’s plan for additional Six Sigma certifications?

Our plan is to add another Greenbelt class this year. Also, less intensive training classes will be available to employees to increase skills in basic problem-solving, analysis, and statistics. We also plan to have another Blackbelt Certified trainer next year, in addition to Rocky Lee. This will enable Interface to provide continual company-wide basic training that explains the concepts and benefits of Six Sigma. The standard knowledge gained from Six Sigma will help everyone to work smarter and not just harder.

Interface is dedicated to superior quality. From our products to our manufacturing and planning processes, everything we do is with the utmost care and respect to ensure we can exceed our customer’s expectations. Six Sigma training and Greenbelt certifications are important investments into our team members and our customers. We are proud and honored for Rocky’s leadership in this important initiative.

We appreciate our newly certified team members for sharing their experiences and the value of this certification program. Congratulations to our new Interface Greenbelts!