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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 Releases New ConvexBT White Paper

To meet the demand for the ever-evolving technological landscape, Interface is constantly gathers input from our customers across all industries and global network of test and measurement professionals to understand trends and sensor requirements for today and into the future. These valuable insights drive our new product introduction strategy and evaluations into how we can best solve your challenges.

In this new white paper, Ted Larson, VP Product & Project Management, and James Richardson, Mechanical Engineering Manager, highlight our recent introduction of a revolutionary load button load cell. We have captured the journey of our design story, along with detailing the innovative features, capacities, and benefits of our new ConvexBT Load Button Load Cell. Access the entire paper.

ConvexBT was introduced due to the growing trend of electronics miniaturization going on throughout nearly every hardware industry in the world. Original equipment manufacturers (OEMs) are packing more capabilities into smaller and smaller packages, and as product size shrinks testing sensors and equipment must downsize to match. ConvexBT is engineered to fit in tight spaces to test compression force with ultimate precision. It’s well-suited for industries like medical and industrial, where product miniaturization is prevalent throughout.

You can see some of the other recent ConvexBT highlights and use cases here:

[White Paper] ConvexBT The Most Innovative Load Button Load Cell

Advancing Load Button Load Cell Capabilities with ConvexBT 

Robotic Arm Application Note

Sensor Tips Magazine Highlight of ConvexBT

ConvexBT also includes some incredibly novel design choices that helps with rejection of misaligned loads, as well as temperature compensation. This makes it not only the most accurate load button load cell on the market, but also the most flexible. To learn more about ConvexBT and the unique design, capacity ranges, technical specification and more, download the white paper here.

Robotics in Play with New Animated Application Using ConvexBT

Numerous factors are driving the industry 4.0 revolution. From big data to IoT technology, industrial facilities and manufacturing plants are looking at new ways to automate their process and create a more efficient and cost-effective environment. One of the most important technology advancements in this mix is robotics.

Robotic equipment is a common industry 4.0 innovation used to create an autonomous or semi-autonomous machine capable of carrying out a variety of repetitive tasks that used to take up the time of skilled labor. Some of the tasks or processes that robotics enhance include stock management and logistics, manufacturing automation, janitorial duties and, there are even robotic applications called co-bots that assist human workers when ultra-high precision is needed.

To facilitate the demand for robotics, a variety of sensor and measurement components are necessary to ensure the highest quality and reliability of these application. Many tasks carried out by robotic applications are ultra-precise and require more accuracy than what a human hand or eye can handle.

Sensor technologies embedded in the actual robotics instrument must also be used to constantly calibrate or monitor the robotics. If robotics is used on an automated manufacturing line, any issues with the robotics can disrupt and compromise the entire process. Therefore, robotics manufacturers utilize Interface solutions when they need quality sensors that can monitor the precision of the robotics and ensure that their accuracy and reliability is maintained.

Interface develops high-quality test and measurement solutions designed for hardware testing of all kind. For robotics, our products are frequently used as a component within an OEM device. We understand the premium accuracy and reliability necessary to help develop robotics solutions and have provided both off-the-shelf and custom force measurement solutions designed to meet a variety of applications. We recently created an animated application note on an industrial automation robotic arm using our new light weight, light touch load button load cell, the ConvexBT.

The ConvexBT is designed for testing and also for full integration into the robotic element to measure the force pressure during use.  ConvexBT is available in multiple capacities, including our latest release of the 500lb and 1Klb models.

NEW! Interface Robotic Arm Application Note

A customer came to Interface with a robotic arm product that would be used to lift and move delicate objects, such as a glass bottle, in an automated environment. The goal in using Interface was to find a force measurement product that could ensure the robotic arm did not damage the products it was moving by applying too much force. The main component that Interface products would be applied to is the robotic arms’ clamp. The objective was monitoring the grabbing pressure of the clamp and ensure that the device would stop applying pressure when the necessary force was used to pick up the object without doing damage.

Using its new line of Load Button Load Cells, ConvexBT, and a DMA2 DIN Rail Mount Signal Conditioner, Interface provided a solution that would produce an electric signal on the clamping process that tells a controller to have the device stop applying pressure. Two ConvexBT products were connected underneath the rubber pads on both sides of the robotic arm clamping device. When the clamps made contact and applied pressure, the DMA2 Signal Conditioner converted the signal from the ConvexBT from MV/V to volts to a PLC controller. This signal tells the controller when to have the robotic arm stop applying clamping force.

Ultimately, the two ConvexBT Load Button Load Cells were able to accurately measure the amount of pressure applied to the object the robotic arm was lifting and moving without causing any harm or damage to the object.

This is just one of many examples of force measurement products being used in the robotics and automation industry. As the demand for robotics grows and a wider variety of applications are introduced, Interface will continue to engineer the best solutions to help customers reach the age of Industry 4.0.

To learn more about Interface solutions for the robotics and automation industry, please visit /solutions/. You can also check out our case study on the for industrial automation and robotics use here.

Faces of Interface Featuring James Richardson

Born in Arizona but growing up in the rural area of Cotton City, New Mexico, James Richardson was only exposed to the opportunity of a career in engineering after moving back to Arizona. After graduating as his high school class Salutatorian in 1995, he started college in Eastern Arizona.

He later moved to Mesa in 1999 where he took a job working for his uncle at Dewitt Equipment fixing restaurant and cooking equipment like ovens, fryers and microwaves, and along with refrigeration equipment including air conditioning units, freezers, and ice machines. It was also during this time he learned to braze, solder and TIG weld.

At Dewitt, his on the job training for fixing equipment built up his foundation for engineering. The spark that really kicked it off came on a sweltering Arizona summer day when James was repairing an A/C unit on a restaurant’s loose gravel rooftop. The temperature was so high that the gravel began to sink, melting the soles of his shoes. At this point, James realized he enjoyed working with his hands and on advanced equipment; however, it was time to finish his formal education in engineering and pursue a job that included more time inside where there was ample air conditioning.

By this time James had already completed an Associate Degree at Maricopa Community College and he was about 18 months from completing a Bachelor of Science in Mechanical Engineering degree at Arizona State University. Completing this degree, he later earned a Master’s in Engineering Management from Ohio University. Towards the end of his bachelor’s degree, he got an internship at Honeywell Aerospace. His first job after earning his degree was with Enertron Inc., a leading provider of thermal management solutions for the aerospace, military, medical, telecommunications, and IC fab equipment industries. In this role, he designed heat sinks for circuit boards used for lasers, lighting and computers.

After three years with Enertron, he moved to Cleveland Electric Laboratories where he served as an applications engineer working on turbine engine instrumentation. This is where James got his first hands on experience with force measurement equipment. His job was to design instrumentation for strain, temperature, and pressure measurements. At one point he even designed a load pin for a customer.

In his role, he was also introduced to Interface. The company he was working for owned several Interface products and he became familiar with their high-quality and premium accuracy. Then in 2015, a headhunter called him out of the blue to offer him a chance to work for Interface. James was excited about the prospect of working for a company that put quality first. In fact, the thing that hooked him about Interface was the declared focus of “Quality is Our Driving Force,” and the fact that each of the four interviewers reiterated the importance of this statement in their interview.

James joined Interface as a production engineer. He remained in this role for about four years before being promoted to Senior Engineer, and then to his current role as Mechanical Engineering Manager where he leads a team of five other engineers. In this leadership position, James is responsible for overseeing development efforts for some of Interface’s most important product lines including the specialized 1923 and 1925 wireless custom solutions and our downhole products for the energy markets. James was instrumental in the latest new product release, the new ConvexBT Load Button Load Cell.

In addition to this critical role, James also loves to learn about the many ways that Interface products directly affect him and people close to him. This includes how measuring systems ensure the proper weight of food in nutritional planning and packaging, measurement of things like blood donations, and safety test systems for airplanes. The work done at Interface is incredibly important to everyday life and many people don’t even realize it.

In his free time, James can be found spending time with his wife of 21 years and their four children, two sons and two daughters. The family enjoys the outdoors together, partaking in activities like bike rides and hikes. He also brings some of his passion for engineering home. He’s intrigued by the possibilities of 3D printing and owns a printer himself. He’s designed and printed things like bowties, wallets, wall-mounts for various gadgets, and even toys for the kids. In case you missed it, the photo of James is his own 3-D printed bowtie. It was a big hit at the Interface holiday party.

Another interesting fact about James is that throughout his career he’s tried to connect with co-workers from different countries by learning their language. Throughout his life he’s learned a little bit of Polish and German, and is fluent in Spanish, which he learned while spending two years as a missionary in South America.

We asked James to describe his thoughts on his career in engineering in another language. He responded, “Un dicho o una frase que a mí me gusta pensar, cuando algo no sale buenisimo, es: “Siempre hay una manera mejor.” This translates to a saying or phrase that I like to think of when something doesn’t turn out great, “There is always a better way.”

To learn more about the ConvexBT, check out the datasheet here:

ConvexBT

ConvexBT Load Button Load Cell Featured Online at Sensor Tips

Interface’s newest product release, ConvexBT, is featured in the Sensor Tips, the respected online resource publication for electronics engineering challenges of today and tomorrow.

CLICK HERE TO READ THE COMPLETE SENSOR TIPS ARTICLE

As sensor requirements for force measurement are being utilized as miniature-sized components, the load button load cell ConvexBT is designed to give precision level performance in force measurement.  The new release is designed for accuracy and flexibility.

As shared in Industry Today, the ConvexBT capabilities far exceed what is available in these grow dimension requirements due to specifications to make devices and products more compact and convenient. Industries such as medical, industrial automation and products reliant on advanced communications technology need to validate these products with force-sensing solutions that can fit in confined spaces and provide extremely accurate data.

ConvexBT product comes in two different sizes: 3/8-inch, and 1/2-inch, which are all manufactured using 17-4 PH heat treated stainless steel. These options provide a wide measurement range from 10 to 250 lbf, a compensated temperature range of 60° to 160°F, and an operating temperature range of -40° to 175°F.

Additional specifications for ConvexBT include:

  • 2.00 ± 20% mV/V rated output
  • ± 0.25 nonlinearity as a percentage of full scale
  • ± 0.25 hysteresis as a percentage of full scale
  • ± 0.50 static error band as a percentage of full scale

Download the complete ConvexBT specifications datasheet and STP / CAD files here.

Check out introduction video for ConvexBT, the next generation in force measurement device.