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Extending the Calibration Range of a Transducer

Interface has added a new technical white paper to our library, Extending Transducer Calibration Range by Extrapolation. This detailed engineering report delves into the concept of extrapolating the partial capacity calibration to full capacity, possibly thereby providing an increase in confidence in the extended range. The following is a brief introduction to the white paper and explanation of how extrapolation can increase confidence in your data.

Introduction

Force and torque transducers must be calibrated in a laboratory in order to be useful in their intended application. Applications of the transducers range from relatively basic process measurements to relatively critical calibration of other transducers or equipment. The laboratory calibration consists of loading the transducer with known masses and lever arms or using a comparison method where load is generated by hydraulic or pneumatic means and the transducer under test is compared to a reference transducer. In either method, the cost of calibration equipment rises rapidly with increasing capacity.

Many calibration laboratories have means to calibrate force up to about 10,000 lbf and torque up to about 20,000 lb-in. But capability for higher ranges is scarce. In fact, there are a very limited number of laboratories in the United States that have capability for force over 200,000 lbf and torque over 100,000 lb-in.

There has been some practice in the past by some manufacturers of transducers to calibrate a high capacity transducer at partial capacity and leave the owner to go on hoping and guessing for the sensitivity of the upper end of the capacity. This gives rise to the concept of extrapolating the partial capacity calibration to full capacity, possibly thereby providing an increase in confidence in the extended range.

When Full Capacity Calibration is Not an Option

Strain gage transducers are basically linear. That is, the output follows the input at a near constant ratio. The nonlinearity is routinely measured and typically is in range of ± 0.10%FS or less. This provides for the ability to interpolate values between calibration points with near zero error. But the same is not true for extrapolation which is really estimating values that are beyond the observable range. Conventional wisdom has it, and logically so, that extrapolation is not a valid method of calibration.

Extrapolating is similar to forecasting and that idea helps one realize the liability of it. But the various methods of extrapolation are not all equal. The purpose of this paper is to explore a method that has reasonable validity when economic considerations do not permit a full capacity calibration.

Extrapolation Methods

There are multiple methods of extrapolation. In the white paper, we outline three methods: Linear (0 and last point), Linear (last 2 points) and Poly (calibration points). We also expand upon the best methods for extrapolation by comparing these three methods, as well as demonstrating how to conduct the various methods. The goal of the white paper is to explain how to use extrapolation for best results.

The white paper goes into in-depth details on extrapolation, providing our customers and partners with a blueprint for extending transducer calibration range. If you’re interested in seeing the results and learning more, download the whitepaper here: Extending Transducer Calibration Range by Extrapolation.

For technical questions about Interface transducers and calibration, contact our applications engineers.

You can find additional technical white papers here.

Understanding Load Cell Temperature Compensation

The performance and accuracy of a load cell is affected by many different factors. When considering what load cell will work best for your force measurement requirements, it is important to understand how the impact of the environment, in particular the temperature impact on output.

An important consideration when selecting a load cell is to understand the potential temperature effect on output. This is defined as the change in output due to a change in ambient temperature. Output is defined as the algebraic difference between the load cell signal at applied load and the load cell signal at no load. You can find more detailed information in our Technical Library.

Temperature affects both zero balance and signal output. Errors can be either positive or negative. To compensate for this, we use certain materials that are better suited for hot or cold environments. For instance, aluminum is a very popular load cell material for higher temperatures because it has the highest thermal conductivity.

In addition to selecting the right material, Interface also develops its own proprietary strain gages, which allows us to cancel out signal output errors created by high or low temperatures.

In strain gage-based load cells, the effect is primarily due to the temperature coefficient of modules of elasticity of the force bearing metal. It is common in the industry to compensate for this effect by adding temperature sensitive resistors external to the strain gage bridge which drop the excitation voltage reaching the bridge. This has the disadvantages of adding thermal time constants to the transducer characteristic and of decreasing the output by 10%.

Our load cells are temperature compensated for zero balance. By compensating for zero balance, we can flatten the curve in the relationship between temperature and zero balance. An uncompensated load cell has a much more severe curve, which ultimately affects accuracy and performance.

Interface offers thousands of load cell designs, standard use and for hazardous environments. For instance, rocket engine tests subject our load cells to extremely high temperatures. For use in various maritime industry projects, they can be used in very cold coastlines and even submerged in cold water. No matter where you are, environment influences the load cells performance.

If you are concerned about temperature, Interface provides specifications for every load cell we manufacture. The Interface specification datasheet, as see referenced here, is available for download by product. It always includes all the necessary data required to understand the load cell’s ability to perform at the highest-level including compensation range, operating range, effect on zero balance and effect on span.

One thing that is also unique about our products is that while most competitors only compensate for hot temperatures (60 to 160 degrees Fahrenheit), Interface covers both hot and cold thermal compensation from 15 to 115 degrees Fahrenheit, including adjust and verify cycles.

Be sure to tune into Load Cell Basics, where Keith Skidmore discusses temperature compensation.  He notes during this informative presentation that if the temperature is changing during a test that can affect the zero and the output of the load cell. How much effect depends how much temperature is changing and how well the load cell is compensated against the errors, which can be either positive or negative. Good news is they are repeatable from test to test so if you have large temperature swings you can characterize the system and then subtract out the shift if you know the temperature effect on zero.

Interface Application Engineers are available to answer questions regarding the effect of temperature on force measurement data, or the different ways we can help design a solution to compensate for your environment.

The Anatomy of a Load Cell

Have you ever stopped to think about what makes the things we use everyday work? At Interface, our engineers think about what makes up an Interface load cell on the production floor and in our design lab every day.

Whether we are manufacturing a new load cell or speaking to a customer about how it can help solve their test and measurement challenges, we are always thinking about what a load cell can do and how to perfect the process of building one that exceeds all customer expectations in performance, reliability and accuracy.

One thing that people ask us about all the time is, what does it look like inside the pioneering Interface blue load cell? In the photo below, you have a cross-section of a basic load cell identifying each of the components and how it all comes together to provide industries around the globe world-class force measurement solutions.

The first component to understand is the strain gage. This mechanism is embedded in the gage cavity and is a sensor that varies its resistance as it is stretched or compressed. When tension or compression is applied, the strain gage converts force, pressure, and weight into a change that can then be measured in the electrical resistance. You can read more in our recent strain gage 101 blog. Here at Interface, we manufacture our own strain gages in-house to ensure premium quality and accuracy.

The main features of a strain gage are illustrated in the following image:

  1. Grid Lines – strain sensitive pattern
  2. End Loops – provide creep compensation
  3. Solder Pads – used to solder interconnecting wire to the gage
  4. Fiducials – assist with the gage alignment
  5. Backing – insulates and supports foil and bonds the strain gage to the flexure

There are also multiple gage configurations depending on the type of load cell. These include:

  • Linear – measures the strain under bending (used in mini beam load cells)
  • Shear – measures strain under shear force (used in low-profile load cells)
  • Poisson – measures strain under normal stress (used in the Interface 2100 Series Column Load Cells)
  • Chevron – measures strain under torsion (used in the Interface 5400 Series Flange Load Cells)

The next component to understand is the load bearing component of the load cell. It is made up of the hub, diaphragm, outer ring, inner ring and base. This component deflects under load to allow the strain gages to send a signal through the connector to the data acquisition device. Customization can include changing the metal materials used to meet environmental or strength concerns and designing the beam height and thickness to meet certain size and stress considerations.

The mounting ring and connector are also incredibly important to the proper use of a load cell and accurate data collection. The mounting ring is the area in which the load cell is mounted to the test rig to measure force and collect data. It is important to pay attention to mounting instructions because an improperly mounted load cell can cause inaccurate results, as well as damage to the load cell. There are also mounting adapters available to fit a wide variety of test rigs.

The connector is the component that allows the load cell to connect to a data acquisition device. The connector is attached via a wire to the data acquisition device and force data is sent through this device to the user through ethernet or Bluetooth® depending on the load cell and data acquisition device configuration. Interface also sells a wide variety of data acquisition devices.

Load cells have many configurations and capacities. In fact, we have made tens of thousands of them over the years to meet standard, modified and engineered to order specifications. The load cell diagram above represents a popular low profile “pancake” load cell.  There are many other styles including miniature load cells, bending and dual bending beams, column-style, S-beam and load button load cells. However, even as the shapes and uses change, the anatomy remains relatively similar, with these main components acting as the workhorse of the load cell and providing accurate force data to the user.

For more information on Interface and our wide range of load cells, torque transducers and data acquisition devices check out our product categories on our site or download our product literature here.

Interface Differentiator is Proprietary Strain Gage Manufacturing

Interface products have been heralded for their accuracy, reliability, and quality for more than 50 years. We credit our vertically integrated approach to manufacturing as the most significant factor in our development of industry-leading force measurement products, meaning we control every part of the design, manufacturing and testing of our products before they are shipped to our customers.

The process for how we differentiate ourselves begins with Interface strain gages. By manufacturing our own proprietary strain gages here at our headquarters in Scottsdale, Arizona, we can optimize our load cells to a quality level very few providers can match.

Think of strain gages as the heart and soul of a load cell. These components power every aspect of the device and their quality dictates a significant portion of the load cells’ overall quality. In addition, customization of the strain gages is a critical factor in ensuring the load cell is meeting the specific requirements of a customer’s project.

The last point is critically important because Interface does not just provide one size fits all products. Yes, we have a large standard product line ready to ship. There are many times when we collaborate directly with our customers to understand their application and the challenges that may be present during a force measurement testing program or OEM design. This allows us to offer modified and custom products that are engineered to order.  Whether that comes in the form of an off-the-shelf product within our catalog of more than tens of thousands of options, or a new model using our strain gage technology to meet the needs of a unique application.

An example of our commitment to meeting customer needs is the way we develop our strain gages to compensate for temperature, an environmental factor that can drastically affect the accuracy of force data. Our strain gages are designed and manufactured to counteract the temperature characteristics of the modulus of the load cell structural material.

The benefit to this is that our load cells are temperature-insensitive and do not require modulus compensation resistors, ultimately producing a simpler and more reliable circuit with higher output signal. It also means no dynamic thermal mismatch errors from modulus compensation resistors which cannot be thermally connected with the load cell’s surface at the strain gage location.

In addition, our proprietary strain gages provide several key benefits. Included below are a few of the differentiators available with Interface strain gages:

  • A higher output of 4mV/V, while competitors provide 3mV/V or less, which provides superior performance, flexibility, and accuracy.
  • The ability to perform hot and cold thermal compensation, from 15˚ – 115˚F, while competitors typically only provide heat compensation (60˚ – 160˚F).
  • Eight strain gages per load cell compared to our competitors four gages, which provides superior compensation of eccentric loads to further minimize resulting errors.
  • Our strain gages also offer:
    • Higher signal-to-noise ratio
    • Higher resolution in precision measurement applications
    • Superior fatigue life

Another factor in the development of our strain gages is our expertise and knowledge of the manufacturing process. We have always developed our own strain gages going all the way back to 1968. Therefore, we have learned everything there is to know about it and can guarantee the quality of our load cells in any environment based on this tenured expertise and having manufactured and calibrated millions of force measurement devices.

To learn more about our vertically integrated manufacturing process and the various forms of product and system customization we offer, contact our specialized application engineers.

 

Advancing Load Button Load Cell Capabilities with ConvexBT

Demands for high precision testing utilized for compact designs and in confined spaces is growing. The requirements for quality, accuracy and most importantly reliability are what has driven the experienced engineers at Interface to create the newly released ConvexBT™ Load Button Load Cell product line.

The revolutionary design of the ConvexBT is a first of its kind load button load cell, providing better temperature resistance and more enhanced eccentric load rejection. Miniature load cells categorized as load buttons have been sensitive to off-axis, eccentric or misaligned loads. This means if the load is not exactly perpendicular to the surface it is resting on, the data could become skewed or inaccurate.

Interface designed the ConvexBT™ Load Button Load Cell to confine misaligned loads to the primary axis of the cell providing superior performance in comparison to similar products on the market in repeatability, better data and reproducible results.

As technology advances, there is a growing demand to make devices and products more compact and convenient. This trend is happening across industries and is especially prevalent in medical, industrial automation and products reliant on advanced communications technology. To design and validate these products, our customers need force-sensing solutions that can fit in confined spaces and provide extremely accurate data. This is the driving force behind the development of ConvexBT, the next generation in force measurement device.” – Ted Larson, VP product management and marketing, Interface.

CONVEXBT FEATURES AND SPECIFICATIONS

The newly released 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 non-linearity 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

Other load cell load buttons designs have also been extremely sensitive to temperature conditions. Interface has redesigned its ConvexBT ultra-precision product line of load buttons to ensure that this is no longer something the user has to account for by taking the sensing technology disrupted by temperature out of the cable, and designing it directly into the load button.

The new available ConvexBT models include the following capacities:

  1. ConvexBT Model LBSU-10 lbs 3/8″
  2. ConvexBT Model LBSU-25 lbs 3/8″
  3. ConvexBT Model LBSU-50 lbs 3/8″
  4. ConvexBT Model LBSU-100 lbs 1/2″
  5. ConvexBT Model LBSU-250 lbs 1/2″

Additional model capacities will be available this year.  You can view the complete product specifications as well as technical guide by visiting the product page here.

ConvexBT was developed through a combination of intense research into growing technology trends in force measurement and actively collaborating with our customers to understand their unique challenges, By introducing the industry’s most advanced and versatile ultra-precision load button load cells, we are solving the test and measurement challenges associated with miniaturization of existing and new technologies.” – Greg Adams, CEO at Interface

The revolutionary ultra-precision line of ConvexBT™ Load Button Load Cells uniquely uses multi-point calibration for testing force on miniaturized products and within confined spaces where accuracy is paramount to success and safety. The requirements are critical to common buyers of miniature load cells, especially for use in medical devices, robotics and in industrial automation applications

In addition to its ability to solve test and measurement challenges with compact devices, another key benefit of ConvexBT is its versatility in that it can be used as a traditional test and measurement solution. It can also be installed into OEM components and devices as an advanced miniature sensing solution to collect accurate real-time force data on the product as it is in use.

ConvexBT is available now under the product family of Interface Mini® Load Cells. The product is part of a growing line of Interface Load Button Load Cells. The new ConvexBT model LBSU specifications are available here: /product-category/load-button-load-cells/.

Read more about Off-Axis Loads and Temperature Sensitive Applications here.

Instrumentation Options in Test and Measurement

Force and torque measurement technologies such as load cells and torque transducers are a single part of an overall system often used for test and measurement projects and programs. Instrumentation is also a key component of force and torque measurement systems. Instrumentation tools are functional for visualizing and logging the sensor data.

When considering all the options for your project, product designers and engineers need to evaluate the type of instrumentation required to read and gather the sensor output and display the results.

Common questions to ask in preparing your test and measurement project, building a system or setting up a lab:

  • Where are you going to connect your sensor technology and how?
  • Do you need to store your data?
  • Do you prefer an analog or digital output device?
  • Are you going to plug-in your instrumentation or use hand-held, wireless or Bluetooth connectivity?
  • How will your data output be displayed?
  • How many channels do you need for your project or program?

These are all questions related to instrumentation devices and how they interact with and connect to your test and measurement products. Because of the wide variety of instrumentation options, from transmitters and indicators to data logging, it is critical to carefully review the features, specifications, capacities for each. Engineers and testers should review capabilities for data collection of a device, connectors and adapter requirements, and how the device works with specific types of load cells, torque transducers, multi-axis sensors, and other testing equipment.

A valuable tip is to spend time reviewing the specifications of any instrumentation device you are considering, as well as speak with an experienced application engineer. The critical model and design details are provided in the product datasheet to help in your selection.

Key areas to consider in your review and design of a force and torque measurement systems include:

  • Excitation
  • Outputs
  • Performance standards
  • Environmental performance
  • Power
  • Mechanical definitions
  • Connections
  • Protocols

There are dozens of instrumentation options available through Interface including signal conditionersoutput moduleshigh-speed data loggersportable load cell indicatorsweight indicators, and junction boxes. Here are some of our latest additions and most popular instrumentation products:

Download our Instrumentation Brochure
Download our NEW Digital Instrumentation Brochure

Terms and Definitions

To help get you started on the process of selecting the right instrumentation for your project, we have compiled a list of common terms used for instrumentation and in force measurement and sensor technology product descriptions.

  • Accuracy: The closeness of an indication or reading of a measurement device to the actual value of the quantity being measured. Usually expressed as ± percent of full-scale output or reading.
  • Adapter: A mechanism or device for attaching non-mating parts.
  • Amplifier: A device that draws power from a source other than the input signal and which produces as an output an enlarged reproduction of the essential features of its input.
  • Analog Output: A voltage or current signal that is a continuous function of the measured parameter.
  • Analog-to-Digital Converter (A/D or ADC): A device or circuit that outputs a binary number corresponding to an analog signal level at the input.
  • Bluetooth: A standard for the short-range wireless interconnection of mobile phones, computers, and other electronic devices.
  • Bus Formats: A bus is a common pathway through which information flows from one computer component to another. The common expansion bus types include, Industry Standard Architecture (ISA), Extended Industry Standard Architecture (EISA), Micro Channel Architecture (MCA), Video Electronics Standards Association (VESA), Peripheral Component Interconnect (PCI), PCI Express (PCI-X), Personal Computer Memory Card Industry Association, (PCMIA), Accelerated Graphics Port (AGP), Small Computer Systems Interface (SCSI).
  • Calibration: Process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual value being measured.
  • Communication: Transmission and reception of data among data processing equipment and related peripherals.
  • Controller: Controllers deliver measurement and control functions that may be used in a wide variety of applications. They feature compact form and versatility in systems that require precise measurement of weight or force combined with processing and storage.
  • Digital Output: An output signal which represents the size of an input in the form of a series of discrete quantities.
  • Environmental Conditions: All conditions in which a transducer may be exposed during shipping, storage, handling, and operation.
  • Frequency: The number of cycles over a specified time period over which an event occurs. The reciprocal is called the period.
  • Indicator: Load cell indicators are often needed where the force, load or weight measurement needs to be displayed to a user visually and displaying the results on a PC is not feasible.
  • Intelligent Indicator: Intelligent Indicators ensure sensor equipment is used for the correct amount of time, thereby helping to safeguard against mistakes or purposeful misuse.
  • Output: The electrical signal which is produced by an applied input to the transducer.
  • Protocol: A formal definition that describes how data is to be exchanged.
  • Range: Those values over which a transducer is intended to measure, specified by its upper and lower limits.
  • Signal Conditioner: A circuit module which offsets, attenuates, amplifies, linearizes and/or filters the signal for input to the A/D converter. The typical output signal conditioner is +2 V dc.
  • Strain Gage: A measuring element for converting force, pressure, or tension into an electrical signal.
  • Transducer Electronic Data Sheet (TEDS): Provides a force or torque transducer with electronic identification, allows sensor instrument to be “Plug & Play Ready” meets IEEE 1451.4
  • Wireless: Broadcasting, computer networking, or other communication using radio signals, microwaves, and other signals.

If you still have questions about load cells, torque transducers, and the instrumentation options please give us a call at 480-948-5555 or visit www.interfaceforce.com.

For some of the key terms, we used an online reference you can find here: Source

Strain Gages 101

A strain gage is a sensor that varies its resistance as it’s stretched or compressed. When tension or compression is applied, the strain gage converts force, pressure, and weight into a change that can then be measured in the electrical resistance.

At the heart and soul of every load cell is a strain gage. This is the pinnacle technology that allows engineers to collect and analyze force data. In the industry, it is known as force measurement.

Strain gages are made through a photo-etch process using a flexible backing and a very thin foil. The way a strain gage works is when the backing and foil stretches or compresses, resistance goes up and down respectively. We know this as force. Think of stretching like a three-lane highway switching to two lanes, and vice versa for compression with two lanes going into three. As the load cell’s internal strain gage experiences force, it sends a signal with a precise measurement of the amount of force it’s experiencing.

There are many different types of strain gages for a variety of environments and force measurement needs. The major difference in strain gages is the base material used in the manufacturing process. Different materials are used when a load cell needs to perform optimally in a variety of temperatures, humidity levels, and elevations. Matching the correct strain gage and a load cell to the customer’s needs is critical to accuracy.

“Here at Interface, we pride ourselves on developing the most accurate force measurement tools, and it starts with our proprietary manufacturing of the strain gage.”  Scott Dunne, Production Engineering Manager

More than 52 years ago, when our founder Richard F. Caris started Interface, he purchased over a mile of foil, which is the base material used in strain gages. Caris understood the only way to ensure Interface customers received quality results from their force measurement products was to control every aspect of engineering design, product development, and production.

The key ingredient to our precision accuracy and reliability is the fact that we have vertically integrated the entire manufacturing process from design to production and have a deep understanding of the materials necessary to suit every client’s need for optimal results

Many load cell makers purchase their strain gages from a third party. This means there’s more variability in their manufacturing process and you often find the variances in their materials clash and diminish the accuracy, or they are not correctly suited for the customer’s project requirements.  Interface makes all their own strain gages.

We have learned everything there is to know about strain gage manufacturing and can guarantee the quality of our load cells in any environment based on this tenured expertise and having manufactured and calibrated hundreds of thousands (ok, millions) of force measurement devices. And here’s a fun fact, although we’ve manufactured hundreds of thousands of load cells and strain gages, we haven’t even used half of the original mile of foil we purchased in 1968. Good product managed well can go a long way!

For more information on Interface’s commitment to accuracy and reliability, we have written The Load Cell Field Guide, the definitive resource on load cells. It is available on Amazon. You can also download our latest technical white paper, Contributing Factors to Load Cell Accuracy, for free by clicking here.

Contributor:  Scott Dunne, Production Engineering Manager, Interface

Interface 2019 Load Cell Field Guide Now Available on Amazon

The Interface Load Cell Field Guide, the most thorough guide in the industry for understanding and utilizing load cells and strain gages, has been updated for 2019 and re-released on Amazon. The popular primer from leaders in force measurement solutions provides in-depth information for understanding and using load cells and strain gages to accurately measure force.

The 2019 Load Cell Field Guide is the ultimate tool for engineers and students who are learning about and using load cells for test and measurement projects and products. The book is available today in paperback on Amazon and can be ordered for $15 here.

“The creation of this instructional guide was driven by a value included in Interface’s mission to always go above and beyond,” said Joel Strom, CEO, Interface, Inc. “We believe this informative reference is a helpful resource for engineers, STEM students, and universities around the world because it comes from a company that’s recognized as the pioneer in load cell design and manufacturing. Sharing our knowledge benefits all industries testing force.”

The “Interface Load Cell Field Guide” was first published in 2014 and has been distributed throughout the world. In the newly released 2019 edition, Interface updated the book to include more than 120 pages of instructive content that covers force measurement topics including types of load cells, general uses of load cells, load cell characteristics and various test application use cases. This type of essential information provides value to all load cell users and force measurement enthusiasts.

Science, technology, engineering, and math (STEM) is a strategic focus of the 51-year-old load cell manufacturer. The introduction of Interface’s University Program earlier this year provides the Load Cell Field Guide to support global education and STEM programs. The guide was authored, edited and published by a team of expert Interface engineers with deep experience in the force measurement industry.

For more information on the “Interface Load Cell Field Guide” or University Program, please call 480-948-5555 or visit /the-load-cell-field-guide/.

Press release: https://www.prnewswire.com/news-releases/new-2019-load-cell-field-guide-from-interface-now-available-on-amazon-300911460.html