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Load Cells Built for Stress Testing

Stress testing with load cells is an integral part of research, design, and manufacturing processes for various products and components. It helps to ensure that material, equipment, and final products can withstand the stresses they will be subjected to in regular use.

Stress testing with load cells involves applying a known load to a test specimen and measuring the resulting strain. The strain is then used to calculate the stress, which measures the force per unit area.

For destructive stress testing, the test specimen is loaded to failure. The failure load is then used to calculate the ultimate tensile strength (UTS) of the material. In non-destructive testing, the test specimen is loaded to a predetermined stress level and then unloaded. The stress-strain curve is then plotted to determine Young’s modulus and the yield strength of the material.

Selecting the right load cell for any stress testing protocol is important. A detailed review of the sensor’s performance specifications is where to start. Consider the quality of the load cell, along with the materials used to build the testing device and the strain gages.

In designing and building load cells, material composition and build quality play a critical role in the quality, accuracy, and overall lifetime of a load cell. This is especially true when testing involves long, stress-test cycle testing. Interface load cells are designed for optimum fatigue life.

Built for Stress

When looking for a load cell that needs to go the distance over long periods, it’s essential to understand the difference between sensors built for stress and those not. In materials science, the S-N curve is a well-known tool. It is a graphical representation of the number of load cycles required to break a specimen at the range of peak cyclic stress levels.  S-N curves for the high-quality materials used in Interface load cells determine the stress level.

Commonly selected load cells used for high-stress level testing are known as fatigue-rated. Fatigue-rated load cells are designed explicitly for component durability and fatigue test machines where highly cyclical loading is present. These quality load cells resist extraneous bending and side-loading forces.

The table below outlines a load cell strain and safety factor comparison chart, which shows how Interface load cells, including our  1000 Fatigue-Rated Universal LowProfile® Load Cell and 1000 High Capacity Fatigue-Rated Universal LowProfile® Load Cell stack up against generic competitive load cells.

This table compares actual strain levels in Interface LowProfile Load Cells versus generic load cells. The safety factors are a means of visualizing the merit of the various designs. The value of fatigue-rated load cells for fatigue applications is evident from the safety factor data. It is also apparent that Interface load cells with 4 mV/V output have lower stress levels and, therefore, more fatigue resistance than other cells, even though their output is only 3 mV/V or less.

Lower Stress by User Limits

Note that the tests in the safety factor comparison are based on fully reversed load cycles. This type of loading cycle is considerably more stringent than unidirectional loading, which is the more common application of load cells. Suppose a fatigue load cell is repeatedly loaded in only one direction. In that case, the Goodman Law predicts that it can be loaded to about 133% of the bidirectional fatigue-rated capacity with no degradation of its fatigue rating. Conversely, unidirectional loading to a fatigue cell’s rated capacity is much less stressful on the cell than bidirectional. It can be expected to yield a fatigue life well beyond the number of cycles that could be reasonably and economically applied in a verification test program. For additional information on this topic, please refer to Interface’s Load Cell Field Guide under Fatigue Theory.

ADDITIONAL RESOURCES

Fatigue Testing with Interface Load Cells

Beam Stress Test

Force Measurement is Fundamental in Material Testing

Test and Measurement Solutions

LowProfile Load Cells 101

Stainless Steel Load Cells 101

Excitation Voltage 101

Excitation is an electrical signal. The excitation voltage is represented by the volts direct current (VDC). The direct current flows in one direction only. Alternating current (AC) changes direction at times.

Load cell excitation provides a voltage to generate an output signal, sometimes referred to as ‘powering’ the load cell. An output signal from a load cell is typically minimal, so an excitation voltage is needed to power the load cell and ensure the output signal is accurate. The magnitude of the output signal is proportional to the amount of force applied to the load cell. The greater the force, the greater the output signal.

Interface load cells contain proprietary strain gages applied to a Wheatstone bridge, essentially an electrical circuit that changes resistance when subjected to strain. The Wheatstone bridge is comprised of strain gages that are arranged in a specific configuration. When a load is applied to the load cell, the strain gages deform, and their resistance changes. This change in resistance causes the output voltage of the Wheatstone bridge to change.

Interface provides electrical performance data on all specifications represented as VDC MAX, when applicable.  The data for excitation voltage is listed under the electrical section of a transducer model’s specification datasheet, along with other factors, including rated output, bridge resistance, and zero balance.

Sensor Power and Excitation Tips

Load cell excitation is necessary to ensure the accuracy and reliability of load cell measurements.  Here are a few tips to consider regarding excitation and power signals when designing a force measurement system:

  • The output signal from a load cell is expressed in millivolt output per Volt (mv/V) of excitation at capacity.
  • The excitation voltage also affects the magnitude of the output signal. A higher excitation voltage will produce a higher output signal.
  • The output signal is directly affected by the input voltage. It’s essential to maintain a stable excitation voltage.
  • Interface load cells all contain a full bridge circuit. Each leg has a typical bridge resistance of 350 ohms, except for models like our 1500, which have 700 ohm legs.
  • The preferred excitation voltage is 10 VDC, which guarantees the closest match to the original calibration performed at Interface before it is shipped from our factory.
  • A DAQ system won’t always provide stable excitation voltage. Consider using a signal conditioner or DAQ with specific bridge inputs.

Why Load Cell Excitation Matters

Excitation matters in force measurement applications because it provides the power needed to operate the load cell and ensure an accurate output signal. The load cell cannot generate an output signal without excitation, and the force measurement will be inaccurate. In addition, it does influence accuracy, noise, and range.

Accuracy: The excitation voltage powers the load cell and ensures an accurate output signal.

Noise Reduction: The excitation voltage can help to reduce noise in the output signal.

Range: The excitation voltage can help extend the load cell’s measurement limit.

The excitation voltage should be applied to the load cell in a balanced manner. This means the excitation voltage should be applied to both sides of the load cell. The excitation voltage should be stable. This means that the voltage should not fluctuate or drift over time. The excitation voltage should be filtered. This means that any noise in the excitation voltage should be removed.

Excitation 101 in Force Measurement

The excitation voltage determines the sensitivity of the load cell. A higher excitation voltage will result in a more sensitive load cell, which means it can measure smaller forces.

The excitation voltage influences the frequency response of the load cell. A higher excitation voltage will result in a broader frequency response, meaning the load cell can track changes in force more accurately.

Linearity measures how accurately the load cell converts force into an electrical signal. A higher excitation voltage will result in a more linear load cell, meaning the output signal will be more proportional to the applied force.

The excitation voltage is well-regulated to reduce measurement errors. Variations in excitation voltage can cause a slight shift in zero balance and creep. This effect is most noticeable when the excitation voltage is first initiated. The solution is to allow the load cell to stabilize by operating it with a 10 VDC excitation for the time required for the gage temperatures to reach equilibrium. The effects of excitation voltage variation are typically not seen by users except when the voltage is first applied to the cell.

For tips like this, please consult Interface’s Load Cell Field Guide. We also detail remote sensing of excitation and temperature. Download your copy for free here.

It is essential to carefully select the excitation voltage for a load cell application to ensure that it can provide accurate and reliable measurements.

Bending Beam Load Cell Basics

Bending beam load cells are a versatile and cost-effective solution for many weighing and force measurement applications. These types of miniature load cells are small in dimension, which makes them ideal solutions for compact testing environments and for embedding into machines or products for continuous performance measurement.

The use of bending beam load cells expands across industries and applications, for weighing scales, medical devices, industrial process controls, robotic designs, packaging machinery and civil engineering projects.

How Bending Beam Load Cells Work

A bending beam load cell converts a force applied to it into an electrical signal by measuring the flexure of the beam. This is done by attaching strain gages to the beam. When the beam bends, the strain gages change their resistance, which is then converted into an electrical signal by a Wheatstone bridge circuit. The output signal is proportional to the applied load.

The bending beam load cell is bolted to a support through the two mounting holes. Under the covers, you can see the large hole bored through the beam. This forms thin sections at the top and bottom surface, which concentrate the forces into the area where Interface’s proprietary strain gages are mounted on the top and bottom faces of the beam. The gages may be mounted on the outside surface, as shown, or inside the large hole.

The compression load is applied at the end opposite from the two mounting holes, usually onto a load button that the user inserts in the loading hole.

MB Miniature Beam Load Cell

MB MINI BEAM LOAD CELL

The Interface Model MB is a miniature beam load cell used in test machines and a variety of low capacity applications.

  • Standard Capacities are 5 to 250 lbf (22.2 N to 1.11 kN)
  • Proprietary Interface temperature compensated strain gages
  • Performance to 0.03%
  • Low height – 0.99 in (25.1 mm)
  • Eccentric load compensated
  • ±0.0008% /˚F – max temperature effect on output
  • Low deflection

MBI Overload Protected Miniature Beam Load Cell

Interface’s Model MBI Overload Protected Miniature Beam Load Cell has better resistance to off-axis loads then other similar load cells and is fatigue rated.

  • Standard capacities from 2 to 10 lbf (10 to 50 N)
  • Proprietary Interface temperature compensated strain gages
  • Performance to 0.03%
  • Low height – 1in max
  • ±0.0008% /˚F – max temperature effect on output
  • 10x overload protection

MBP Overload Protected Miniature Beam Load Cell

Our Model MBP series Mini load cells provide a similar performance to Model MB series with the added safeguard of internal overload protection. This patented overload protection is accomplished via hard stops that are EDM machined into the load cell flexure. This provides a greater overload protection (2.5-10lbf ±1000% of full scale capacity, 100 N ±500% of full scale capacity), giving the user added protection in more severe applications.

  • Standard capacities from 2 to 10 lbf (10 to 50 N)
  • Proprietary Interface temperature compensated strain gages
  • 10x overload protection
  • Low height – 0.99 in (25.1 mm)
  • ±0.0008% /˚F temp. effect on output
  • 5′ Integral Cable (custom lengths available upon request)
  • NIST Traceable Calibration Certificate

MBS Parallelogram Load Cell

The Interface MBS Parallelogram load cell is made of lightweight aluminum construction and highly suitable for medical and robotics applications.

  • Capacities from 2.2 to 10 lbf (9.8 to 44.5 N)
  • Lightweight
  • Nonlinearity error 0.02% FS
  • Ideal for OEM applications

Double Bending Beam Cells

A very useful variation on the bending beam design is achieved by forming two bending beams into one cell. This allows the loading fixtures to be attached at the threaded holes on the center line, between the beams, which makes the sensitive axis pass through the cell on a single line of action.

Bending Beam Load Cell Applications

Material testing is a common application for bending beam load cells. This type of miniature load cell measures the forces applied to materials with a high degree of accuracy to determine stiffness, strength and durability of the specimen.

It is quite common to find bending beam load cells in industrial automation machines and robots to precisely measure the forces required for control, safety and efficiency. In robotics specifically, bending beam load cells will measure the force applied to the robot’s arms and grippers. The data is used to control the robot’s movements and to ensure that it is not damaging the objects it is handling.

Aerospace engineering have long used bending beam load cells in design, testing and manufacturing of aircraft and spacecraft. Automotive engineering use bending beam load cells to design and test vehicles for safety and reliability.

Due to Interface’s ability to custom design bending beam solutions that meet strict size, capacity and accuracy requirements, our products are commonly used in medical and healthcare applications.

Bending Beam Application for Medical Device Testing

In this application, the medical device product lab needs to apply known forces to stent and catheters to ensure they pass all necessary strength and flexibility testing. MBP Overload Protected Beam Miniature Load Cell is placed behind the guide wire for the stent or catheter. The motor will spin the linear drive, push the load cell, and guide the wire through the testing maze. The bending beam load cell connects to the DIG-USB PC Interface Module to record and store testing data for analysis. Read more.

Bending Beam Application for Vertical Farming

Vertical farming is the production of produce in a vertical manner using smart technology systems, while indoors using an irrigation system. A wireless force measurement solution is needed to monitor the amount of water being used, to ensure the produce is being watered just the right amount. Interface suggests installing four MBI Overload Protected Miniature Beam Load Cells under each corner of the trays of the produce to accurate measure the weight during watering. A JB104SS 4-Channel Stainless Steel Junction Box connects to each bending beam cell and to a WTS-AM-1E acquisition module. The device wirelessly transmits the sum weight to the WTS-BS-1-HA Wireless Handheld Display for multiple transmitters, and the WTS-BS-6 Wireless Telemetry Dongle Base Station. Interface’s Wireless Telemetry System monitored and weighed the amount of water being used on the produce in this vertical farming system to increase yield and conversation. Read more here.

Additional Resources

How Do Load Cells Work?

The Basics Of Shear And Bending Beams

Interface Mini™ Load Cell Selection Guide

Introducing Interface Load Cell Selection Guides

The Anatomy Of A Load Cell

Mini Load Cells 101

Load Cell 101 And What You Need To Know

Rod End Load Cells 101

Rod end load cells are designed to measure the tension and compression forces applied to a rod or similar structure. This type of load cell consists of a rod with a threaded connector. The load cell’s strain gage measures the deformation caused by the applied force. As the force is applied to the rod, it deforms slightly, causing a change in the electrical resistance of the strain gage.

To provide a complete analysis of the forces being applied to the structure, rod end load cells are often used in pairs, with one load cell measuring tension and the other measuring compression.

Interface Rod End Load Cells utilize proprietary temperature compensated strain gages for high-accuracy measurement. For example, our popular REC Rod End Load Cell is fatigue rated and designed to go up to 100,000,000+ fully reversed cycles which makes them ideal for test article failure tests. Our REC models are resistant to off-axis and impact loading with performance to .04%. They are environmentally sealed and made of stainless steel. They are easy to integrate in actuators.

Rod end load cells are commonly found in test labs and used in industrial environments, mechanical systems, and testing machines. They are adaptable and reliable force measurement devices utilized for applications in various industries.

Common Rod End Load Cell Use Cases

  • Material Testing: Rod end load cells are widely used in material testing applications, such as tensile testing, compression testing, and fatigue testing. They help measure the applied forces accurately and provide data for analyzing material properties, structural integrity, and performance.
  • Machine Force Monitoring: Rod end load cells are used in industrial machinery to monitor and control forces applied to rods, shafts, or other structural components. They help ensure that the machinery operates within safe load limits, preventing overloading and potential failures.
  • Hydraulic and Pneumatic Systems: Rod end load cells are employed in hydraulic and pneumatic systems to measure the tension or compression forces experienced by cylinders, pistons, or actuators. This enables accurate force control and monitoring for proper system use.
  • Robotics and Industrial Automation: Rod end load cells are integrated into robotic systems to measure the forces exerted by robotic arms or grippers. This allows for precise force control, feedback, and safety features in tasks such as assembly, material handling, and force-sensitive operations.
  • Calibration and Test Lab Equipment: Rod end load cells are utilized in calibration and testing equipment, such as force testers and dynamometers. They provide reliable and traceable force measurements, ensuring the accuracy and calibration of the testing instruments.
  • Aerospace and Automotive Industries: Rod end load cells find application in both aerospace and automotive industries for various purposes, including component testing, structural analysis, quality control, and safety testing. There are many use cases to rod end load cells for multiple industries, as found in our application solutions.
  • Research and Development: Rod end load cells are used in research and development activities across different fields, enabling precise force measurements for studying material behavior, product development, and prototype testing.

Drone Fireworks Rod End Load Cell Application

Drone fireworks have become increasingly popular in recent years. During drone firework and light shows, drones are equipped with LED lights, flying in synchronized patterns to create displays in the night sky.  Four rod end styled Interface WMC Sealed Stainless Steel Miniature Load Cells  are installed to the necessary propeller motors measure the attached LED lights. Each are connected to a WTS-AM-1E Wireless Strain Bridge Transmitter Modules. The WMC’s measure the weight of the LED lights to monitor weight shifting or any uneven weight distributions. Data results are wirelessly transmitted through the WTS-BS-4 Wireless Base Station.

Interface offers a wide range of rod end load cells. The following highlights our standard models for this type of load cell. Customization is an option if a rod end load cell is needed to be designed into machines or used as a component within a specific product.

Interface Standard Rod End Load Cells

REC Rod End Load Cell stainless steel mini with capacities from 1K to 50K lbf (5 kN to 220 kN).

WMC Rod End Load Cell industrial grade rod end type has capacities 15K to 200K lbf (65 kN to 900 kN).

WMC Sealed Stainless Steel Miniature Load Cell environmentally sealed in capacities from 5 to 500 lbf (22 to 2200 N). Submersible versions also available.

WMC Sealed High Capacity Stainless Steel Miniature Load Cell ranging from 1K lbf to 10K lbf (5 kN to 45 kN). Submersible versions also available.

WMCP Overload Protected Stainless Steel Miniature Load Cell With Male Threads is an excellent safeguard in rugged applications with capacities 1.1 to 2.2 lbf (500 to 1000 gmf).

WMCFP Overload Protected Sealed Stainless Steel Miniature Load Cell With Female Threads in 1.1 to 2.2 lbf (500 to 1000 gmf) capacities are stainless steel, sealed, and environmentally protected.

WMCF Miniature Sealed Stainless Steel Load Cell is an industrial design with capacities from 5 to 10 lbf with female threads.

MTFS Miniature Tension Force Load Cell is a small sized tension load cell available in capacity ranges from 1 kN to 100 kN (224.8 to 22.5K lbf).

NEW! ITCA Tension And Compression Load Cell is ideal for measuring both tensile and compressive forces 2.2K lbf to 330.6K lbf (1 MT to 150 MT). The standard metric threads at each end of the load cell are designed to accept standard spherical seating rod-end bearings. Customization is available.

Benefits of Interface Rod End Load Cells

#1 High accuracy: Our rod end load cells provide high accuracy in measuring the tension or compression force applied to a rod or similar structure. They are capable of measuring very small changes in force, making them suitable for precise measurement requirements and applications.

#2 Range of measurement and dimensions: Interface rod end load cells are available in a wide range of measurement capacities and compact size making them suitable for use in a variety of industrial and mechanical applications.

#3 Durability: Rod end load cells are designed to withstand harsh environments, high loads, and repetitive use. They are made from ruggedized materials, stainless steel or aluminum, so that can withstand exposure to moisture, dust, and other environmental factors.

#4 Versatility: Rod end load cells can be used in a variety of applications including material testing and manufacturing processes. They can be easily integrated into existing actuation systems and are compatible with a variety of instrumentation and control systems.

Rod end load cells, as highlighted in our recent Testing Lab Essentials Webinar, are used in applications that involve the measurement of tension or compression force on a rod or similar structure, such as in material testing or in the calibration of testing machines.

Load Cells Versus Piezoelectric Sensors

Load cells and piezoelectric sensors are used in all types of measurement applications. While both types of sensors are used to measure similar physical quantities, they work on different principles and have distinctive characteristics.

By simple definition, load cells measure the amount of force or weight being applied to them. The amount of force a load cell is engineered to measure is numerated by the capacity of the model specification and design, such as 50lbf (pounds-force) or 5kN (kilonewton). When a force is applied to the load cell, the metal body deforms slightly, which changes the resistance of the strain gages. This change in resistance is then measured and used to calculate the amount of force being applied to the load cell.

Piezoelectric sensors work on the principle of piezoelectricity. They are made of materials that generate an electric charge in response to mechanical stress, such as pressure or vibration. Piezoelectricity is a property of certain materials that allows them to generate an electric charge in response to applied mechanical stress, such as pressure or vibration. The word “piezo” comes from the Greek word for “squeeze” or “press,” which refers to the fact that these materials generate an electric charge when they are squeezed or pressed. When a force is applied to a piezoelectric sensor, it generates a voltage proportional to the amount of force being applied. This voltage can then be measured and used to calculate the force or weight being measured. Piezoelectric sensors are most often used in vibration and pressure tests.

Load cells are more suitable for applications where high accuracy is required, as they are more sensitive than piezoelectric sensors in detecting smaller changes in force. Load cells are characteristically more robust and can withstand higher loads without being damaged. Piezoelectric sensors, on the other hand, can be more fragile and may require more careful handling to avoid damage.

Load Cell Advantages

  • Higher accuracy: Load cells are more accurate than piezoelectric sensors, especially when measuring low loads. Load cells can provide precise and reliable measurements with minimal error, making them ideal for applications that require high accuracy. Read: Specifying Accuracy Requirements When Selecting Load Cells
  • Lower sensitivity to temperature changes: Load cells are less sensitive to temperature changes than piezoelectric sensors. This means that load cells can maintain their accuracy even when the temperature changes, while piezoelectric sensors may need to be calibrated frequently to maintain accuracy. Read: Understanding Load Cell Temperature Compensation
  • Better linearity: Load cells have a more linear response than piezoelectric sensors, which means that their output is more predictable and easier to calibrate. This is particularly important in applications where accurate and repeatable measurements are critical.
  • Higher durability: Load cells are more robust and can withstand higher loads without being damaged. This makes them suitable for applications where high loads are present, such as in heavy machinery or construction.
  • Lower cost: Load cells are often less expensive than piezoelectric sensors, making them a more cost-effective choice, especially for OEM use cases.

Piezoelectric sensors are used in a wide range of applications that require the measurement of vibration or acceleration. For example, piezoelectric sensors can be used in machinery and equipment to monitor vibrations and detect potential problems, such as imbalances or misalignments. They are the sensors used in cars to measure pressure, such as in tire pressure monitoring systems or fuel injection systems. Piezoelectric sensors are found in ultrasound imaging to generate and detect sound waves and in musical instruments, such as electric guitars or electronic drum kits, to convert vibrations into electrical signals for amplification.

In selecting the right load cell for any project, check out our new Load Cell Selection Guide. It is a useful resource to determine the capacity, capability and design features that are best suited for your applications. You can also check out How to Choose the Right Load Cell.

Load cells and piezoelectric sensors have distinctive characteristics and advantages, thus specific application requirements will determine the choice of sensor. For questions about selecting the right sensor for your application, contact our solutions engineers.

Additional Resources

How Do Load Cells Work?

LowProfile Load Cells 101

Get an Inside Look at Interface’s Famously Blue Load Cells

Load Cell Basics Sensor Specifications

Interface Load Cell Field Guide

 

 

What is Moment Compensation?

Moment compensation refers to a process of adjusting or counterbalancing the effects of an external force or torque, known as a moment, on a system or object. This is often done in engineering or physics contexts where precise control and stability are required, such as the design of force measurement applications.

Moment compensation is often used to prevent unwanted movements or deformations in systems, to ensure precision and accuracy in measurements, or to maintain stability and control during operation. Moment compensated load cells improve accuracy by compensating for the impact of external forces and moments on the measurement, allowing for more precise and reliable measurements.

Most load cells are sensitive to orientation and loading concentricity. When external forces or moments are introduced, measurement errors are more common and reduce the accuracy of the readings. These external forces or moments can come from various sources. Examples of external forces or moments that can affect the accuracy of load cells and require moment compensation:

  • Off-axis loading: When the load is applied off-center to the load cell, it creates a moment that can introduce errors in the measurement.
  • Temperature changes: Changes in temperature can cause thermal expansion or contraction of the load cell, which can introduce measurement errors.
  • Vibration: Vibrations from nearby equipment or processes can cause the load cell to vibrate, creating measurement errors.
  • Changes in orientation or position: Changes in the orientation or position of the load cell can cause gravitational forces or other external forces to act on the load cell, affecting the measurement.
  • Torque: When a load cell is subject to torque, such as twisting or bending forces, it can introduce measurement errors.
  • Wind or air currents: Air currents or wind can create external forces on the load cell that can affect the measurement

A load cell that is moment compensated can minimize or eliminate these errors, resulting in higher accuracy. Load cells with moment compensation can be more sensitive to slight changes in the load, as it can compensate for any external forces or moments that might affect the measurement.

Moment Compensation is an Interface Differentiator

Interface’s moment compensation process reduces force measurement errors due to eccentric loads by deliberately loading cell eccentrically, rotating load, monitoring and recording output signal, and then making internal adjustments to minimize errors. Every product we ship must pass moment compensation specifications and performance requirements. Every Interface LowProfile™ load cell is moment compensated to minimize sensitivity to extraneous loads, a differentiator from other load cell manufacturers.

When load cells are moment compensated, they can be used in a wider range of applications, including those with complex or dynamic loads, which might be difficult or impossible to measure accurately using a load cell without moment compensation. Interface’s LowProfile Load Cell models have the intrinsic capability of canceling moment loads because of its radial design. The radial flexure beams are precision machined to balance the on-axis loading.

Moment compensated load cells are designed to counteract the external forces or moments by using a configuration of strain gages and electronics that can detect and compensate for these forces. The strain gages are arranged in a way that allows the load cell to measure the force applied to it in multiple directions, and the electronics can then use this information to calculate the impact of external forces and moments on the measurement.

Interface uses eight gages, as opposed to the four used by many manufacturers, which helps to further minimize error from the loads not being perfectly aligned. Slight discrepancies between gage outputs are carefully measured and each load cell is adjusted to further reduce extraneous load sensitivity to meet exact specifications.

Moment compensation improves the stability of a load cell, particularly in situations where the load is off-center or subject to torque. This can prevent the load cell from shifting or becoming damaged, leading to more consistent and reliable measurements. It also improves the durability of a load cell, as it can help protect it from the impact of external forces or moments that might cause damage or wear over time.

ADDITIONAL RESOURCES

Addressing Off-Axis Loads and Temperature Sensitive Applications

Contributing Factors To Load Cell Accuracy

Off-Axis Loading 101

How Do Load Cells Work?

Load Cell 101 and What You Need to Know

Get an Inside Look at Interface’s Famously Blue Load Cells

Strain Gages 101

 

How Do Load Cells Work?

What is the most frequently searched question searched related to Interface and the products we manufacture? It may seem overly simple to test engineers and frequent buyers of Interface force measurement solutions, but to many it is an important question. What do inquisitive users of the internet want to know? They want to how load cells work.

Diving into this question, we learned that many understand the purpose of a load cell. A load cell converts an applied mechanical force, whether that is tension, compression, or torsion, into a measurable electrical signal. Any change in force, increases or decreases the signal output change in proportion.

There are fewer people that understand how a force transducer works. After 55 years making load cells, we thought we should help provide an answer to an incredibly good question. Here is a quick technical brief on how a load cell works.

Interface Tech Talk Answers How Do Load Cells Work

A load cell has two basic components. It has a spring element that is often known as a flexure that mechanically supports the load to be measured and a deflection measurement element that responds to flexure movement resulting from the application of force.

In simpler terms, there is a bending beam under the load and when weight or force is applied, the change in bend (deflection) results in change in output.

A load cell’s basic function is to take applied force and convert it into an output signal that provides the user with a measurement. This process of converting a force into data is typically completed through a Wheatstone bridge that is comprised of strain gages.

Strain Gage Load Cells: A strain gage is typically constructed of an exceptionally fine wire or metal foil that is arranged in a grid-like pattern. Strain gages are strategically placed on the load cell flexure and bonded securely, such that the force induced deflection of the flexure causes the gages to stretch or compress. Thus, when tension or compression is applied, the electrical resistance of the strain gages changes and the balance of the Wheatstone bridge then shifts positive or negative. Fundamentally, the strain gages convert force, pressure, or weight into a change that can then be measured as an electrical signal.

Why use strain gages in load cells? Strain gage characteristics include thermal tracking, temperature compensation, creep compensation, frequency response, and non-repeatability. The major advantage of the strain gage as the deflection measuring element is the fact that it has infinite resolution. That means that no matter how small the deflection, it can be measured as a change in the resistance of the strain gage.

The strain gage is the critical foundation of a load cell and the most vital component for accurate and reliable measurements. One thing to understand about Interface load cells is that we develop our own strain gages in-house using a proprietary manufacturing process to ensure premium performance.

In addition to strain gage load cells, there are also two different less common load cells that use a diverse types of data collection method. This is defined as pneumatic and hydraulic methods.

Pneumatic: These load cells are typically used for measuring lower weights with high degrees of accuracy. They measure weight in terms of force-balance, meaning that weight is reported as a change in pressure. Key advantages of pneumatic load cells are their resistance to electrical noise and inability to spark, in addition to their low reactivity to temperature changes.

Hydraulic: As the name suggests, these load cells utilize fluid pressure for measurement. Like pneumatic load cells, hydraulic load cells balance force by measuring weight as a change in pressure, and the pressure of the fluid rises because of an increase in force. These load cells have no electric components, allowing them to perform well in hazardous conditions.

How to choose the right load cell?

Load cells seem like an extremely basic piece of equipment used to measure different forces such as weight, compression, tension, torsion, or a combination of these. It can be on a single axis or across multiple axes. However, there are many distinct types of strain gages and load cells that are designed for a variety of environments and force measurement testing requirements.

Specifications of a measurement sensor validate the design capabilities and capacities, including the amount of measurement that can be used for a particular device before you exceed the limits.

The field of force measurement has the same types of constraints as any other discipline. It starts with considerations of weight, size, cost, accuracy, useful life, and rated capacity. This also means considerations for extraneous forces, test profile, error specifications, temperature, altitude, pressure, and environment are particularly important when choosing a load cell.

The major difference in strain gages is the base material used in the manufacturing process. Varied 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. It is why Interface has excelled in building precision load cells for five and half decades and continues to be a trusted supplier to industry market leaders, innovators, engineers, and testing houses around the world. It is what we do best. It is what we know.

Our team of engineers and manufacturing experts use expertise that has built over time, applications, and load cell experience. A load cell starts as a raw piece of steel, aluminum, or other metal. It is machined, gaged, wired, finished, and calibrated by experts in load cell production, machinists, and quality engineers.

If you are just beginning to work with products that require accurate force measurement, we would suggest that you speak with an application engineer who can help you understand the load cell that will fit best for your use case.

When shopping for a load cell it is important to know the type of force that you need to measure, the size of the application, the environment in which you will be measuring the application, the accuracy of data needed, the type of communication output that will work with your current test system and if there are any unique details about your application, like extreme or hazardous conditions.

ADDITIONAL RESOURCES

Interface Load Cell Field Guide

Interface Presents Load Cell Basics

LowProfile Load Cells 101

Load Cell 101 and What You Need to Know

Technical Library

LowProfile Cutaway

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, for standard use and for use in 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 cell’s performance.

If you are concerned about temperature, Interface provides specifications for every load cell we manufacture. The Interface specification datasheet, as 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.

Watch our recorded webinar Load Cell Basics, where Keith Skidmore discusses temperature compensation.  He notes during this informative presentation that if the temperature is changing during a test, it 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.