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Load Cell Basics Technical Q&A Part Two

Interface hosted a series of ForceLeaders webinars this past year covering topics that included multi-axis sensors, instrumentation, torque transducers, custom solutions and more.  We plan to continue this popular series into the new year, based on the feedback we have received about topics of interest and support requirements.

One of the features that we like to include in our events is answering a list of top questions, as well as questions we get from the participants.  In our event, Load Cell Basics, we covered a lot of interesting inquiries as well as the top questions we get about our precision load cell products.

We highlighted the first group of questions in our recent post, Load Cell Basics Technical Q&A Part One. And there is more, below highlights additional questions we addressed during the live event.   A recap of the webinar is also online, reviewing key topics we discussed during the one-hour learning event with force measurement experts Keith Skidmore and Brian Peters.

Is There a Better Way to Tare a Load Cell?

Tare is defined as a deduction from the gross weight of a substance and its container made in allowance for the weight of the container, which is sometimes also called the vessel.  For purposes of this reference, we are noting the container is the actual body of the load cell.

Whether you tare a load cell by electronics or mechanical, preload taring of the system is typically done through instrumentation.  You want to ensure that whatever mechanical tare load you have on the sensor isn’t going to impede on your usable capacity range. As far as the load cell is concerned, there really is no need to accommodate any sort of mechanical tare. These are linear devices if you stay within the range.  On the mechanical preload side, it is important how you preload some of attachments especially for the LowProfile load cells. We recommend to preload if you can. It is what we do for best results so that the load path doesn’t change through the threads, which can affect very small amounts of linearity hysteresis

Does Frequency Measurement Limit Impact How Fast the Load Cell Responds?

This is a function of the mechanical flexure design for that sensor. It is important to note that each sensor is a mechanical spring. Depending on the stiffness or the deflection characteristics of a load cell, you will have different natural frequency characteristics. This information is typically documented on our product data sheets by model and configuration.  Generally, the stiffer the sensor the better it’s going to perform in cyclic applications.

Adding to this, Keith notes that load cells are great at hundreds of hertz. If you are asking about tens of thousands of hertz, they are not. There is a spot where very high frequencies of these standard load cells aren’t as effective. Most of our typical testing customers who are applying cyclical loads to a test specimen will confirm that low profile or strain gage-based load cells are great for their frequency requirements.  Work with our application engineers if you have a frequency requirement that is extremely high.

Tare and Working Up an Error Budget

This question comes in from an engineer that is working up a calculated error budget that has a mechanical tare load on that system already. Before you start your test, if you tare it how does that factor into your initial preload in the system? Will it impact your error budget? The answer is that yes, it can have some impact. You can zero out your temperature effect on zero; however, if you have a lot of preloads, you can’t. You must go back to zero to zero it out. This can impact your error budget. Basically, if you zero out, it eliminates the temperature effect. You cannot make the same assumption in your budget if you have considerable preloads.

Does Interface Recommend Particular Amplifier Instrumentation Products?

We do have a wide variety of amplifier solutions. They range from small embeddable amplifiers up to inline options that are DIN rail mounted.  We also have various digital output instrumentation productions available for ethernet, ether cat, Modbus, and USB.

It is important in your consideration to understand that there are errors that can impact instrumentation performance. Selecting the right instrumentation should be a function of the application and what are the changes in testing environment. Resolution is critical in your decision if you are doing a digital type conversion. There’s a lot of parameters to consider. Our recommendation would be to look at what is available from Interface and contact our application engineers to review your specific requirements.

What is the Most Frequent Problem When Installing a Load Cell?

One of the issues are asked about is usually related to the mechanical installation. It’s not as simple as just a nut, bolt, and threaded connection. There is a lot more to it and it’s often and discounted how important it is to ensure things are aligned properly so that you don’t have any parallel load paths. All the load must go through the load cell to make a proper measurement. For example, if you have a mounting plate and the plates are riding on the heads of the screws of the load cell, it is going to shunt the load. Mechanical installation is often missed in the test set-up design. It is often not given enough weight in importance of your test.

We also often see that users have either inadequate torque in an assembly or perhaps too much torque for the lower capacity load cells.  It’s advised to be careful of the installation torque. We recommend that if you aren’t familiar with mechanical installation, ask us for our detailed installation instructions. We provide guidance in terms of recommended torque values and proper methods of where to attach the sensor. It’s easy to over torque a load cell, as they are designed to see axial loads. We have seen users people crank on things and transmitting too much torsional load through creating performance issues that might not show up right away.

Do Cables and Amplifiers Affect Results of Load Cell Calibration?

The answer is absolutely it can impact your results. It doesn’t mean it will, but it can affect performance and measurement accuracy. For example, if you change the cable length or have an instrument that is a different temperature with cable running out of an oven into cooler temperature room, these things can influence the measurement. Even if the load cell is at a constant temperature but the cable is changing temperature, it will cause a signal to change. That’s not something you want in your measurement.

You may mitigate temperature by shielding the cable or you could have a fatter cable which could have less of an effect. Some of these things can be calibrated out. If you add a long cable and then apply loads to your load cell, it considers that there is a longer cable, and you can completely calibrate out the effect of the cable length on the signal dropping due to the resistance. Some instruments use sense to measure the voltage at the load cell. In that case, it is insensitive to any cable effect. You can change cable lengths and it doesn’t make any difference.

Is a 10kn (2,250) lb. Force Application Too Much for a 2,000 lb. Load Cell?

Yes, it’s above the calibrated range and thus you can’t guarantee performance. Load cells will typically stay linear over capacity but there’s no guarantee on the accuracy. This is especially true if you overload with frequency. You’re starting to exercise the load cell in areas where we’d recommend you stay away from to keep the results accurate. Oftentimes, you’re better off going with a larger sensor. Pay attention to the actual output. That’s one advantage of most of our products, especially in the LowProfile range.

Is There Maximum Sampling Frequency for Strain Gage Load Cells?

It is going to depend somewhat on the model of the load cell. Hundreds of hundreds of hertz are certainly adequate. If you start getting close to the natural frequency posted on our product data sheet, we recommend you review the application with our application engineers.

We know engineers who use load cells for safety impact testing. For example, you might need to sample 10 kilohertz. The load cell doesn’t make a difference for how fast you sample the signal, but those cells can only respond in typically a millisecond response time. There are no moving parts or active circuitry; however, typically testing is going to test something until it fails. It is suggested to sample it at a couple thousand hertz to make sure you capture that exact point where the system fails or spikes. If you do have some concern over creep error that’s introduced, record what that non-zero value is as soon as you release that load. That is an accurate measurement of how much creep error was introduced being symmetric.

We exercise our load cells before calibration. It’s relatively quick and typically done in hydraulic frames which are automated to test where they cycle the load in short cycle intervals, like 30 seconds. It’s really the magnitude of the exercising, not how long it lasts. If load is being tested at 120% for an hour, that’s no better than 10 seconds, other than it introduced a lot of creep for an hour. Exercising means you must wait longer for the creep to return, so if the load is a fast cycle, you can check the zero quickly.

If you have additional questions for our experts, we recommend that you visit our technical support resources. We have commonly asked questions, installation guides and manuals and more.  We are also here to support you. Contact our technical support team if you need assistance.

Additional Resources

 

Load Cell Field Guide

Load Cell Basics Sensor Specifications

Load Cell Basics Technical Q&A Part One

Interface 1200 Precision LowProfile Load Cell Series Product Highlight

How to Choose the Right Load Cell

Load cells are used to test and confirm the design of hardware, components, and fixtures used across industries and by consumers. From the structural integrity of an airplane to the sensitivity of a smartphone touchscreen, there’s a load cell available to measure force. In fact, here at Interface we have over tens of thousands of products used in force measurement, for all types of different applications.

How do engineers and product designers go about choosing the right load cell for a specific application or testing project?

Have no fear, Interface has put together a short guide on choosing the load cell that is right for you. This blog will cover the basic questions to answer when selecting a product, as well the most important factors affecting load cell choice.  Be sure to watch the online video, Load Cell Basics, that highlights key factors of consideration when choosing the right load cell for additional insights.

The basic questions you need to consider when selecting a load cell include:

  • What are the expected loads? What is the minimum and maximum load you’ll be measuring?
  • Is there any potential for higher peak loads than what you intend to measure? What are these expected peak forces?
  • Is it tension, compression, or both?
  • Will there be any off-axis loads? If so, what is their geometry? Do you want to measure them too?
  • Will it be a static, dynamic or fatigue measurement?
  • What is the environment in which you’ll be conducting your test? Will the load cell need to be sealed?
  • How accurate do your measurements need to be? Do they need to be at the highest accuracy of ±0.02-0.05% or within ±0.5-1%?
  • What additional features, accessories and instrumentation does your application require to complete a test?
  • Do you need standard electrical connectors or customized options? What about additional bridges or amplifiers?
  • How are you planning to collect and analyze the data output from the load cell?

Next, these are the most important factors affecting accuracy, which will have a heavy influence over the load cell you choose. It’s important to understand how your application and the load cell will be affected by each of the factors, which include:

  • Mechanical – Dimensions and Mounting
  • Electrical – Output and Excitation
  • Environmental – Temperature and Moisture

One of the most important factors in choosing the right load cell is understanding how it will be mounted for testing or as a component within a design. There are a wide variety of mounting types including threaded connections, inline, through hole or even adhesive. Understanding the mounting type that suits your application is critical to getting the correct data because a poorly mounted load cell will distort the results and can damage the load cell.

The mounting process also requires you to understand which direction the load is coming from, in addition to any extraneous loads that may be present. The load cell mating surface is also an important factor. For example, when using our LowProfile® load cells without a pre-installed base, the best practice is to ensure that the mating surface is clean and flat to within a 0.0002-inch total indicator reading and is of suitable material, thickness, and hardness (Rc 30 or higher). Also make sure that bolts are torqued to the recommended level.

If you’re conducting a fatigue measurement, it’s also important to address the frequency and magnitude of load cycles with your load cell provider. Factors to address include single mode versus reverse cycles, deflection versus output resolution, and material types. Interface offers a wide variety of fatigue-rated load cells that are perfect for these types of applications.

Another consideration in choosing the right load cell is the electrical signal. Load cells work by converting force into an electrical signal. Therefore, it’s important to understand the electrical output type necessary for your application, which could include millivolt, voltage, current or digital output. You can find the excitation voltage data on our website for each of our load cells. Additional considerations include noise immunity, cable length and proper grounding.

The environment is also a critical factor in ensuring accurate performance of your load cell. Interface provides load cells in a variety of material types including aluminum, steel, and stainless steel. Each material has a variety of properties that make them more suitable for different environments. For a more in-depth perspective on the different strengths and weaknesses of materials, please read our blog titled, Considerations for Steel, Stainless Steel and Aluminum Load Cells. For applications where load cells need to be submerged in liquid or enter an explosive environment, we also have a variety of harsh environment and IP rated load cells, in addition to load cells suitable for high humidity or splash resistance. Learn more about our intrinsically safe load cells here.

Learn more about choosing the right load cell in these online resources:

WATCH: Load Cell Basics with Keith Skidmore

WATCH: How to Choose a Load Cell with Design Engineer Carlos Salamanca

READ: Load Cell Field Guide

VISIT: Interface Technical Library

To learn more about choosing the right load cell for any application, connect with our applications engineers about the force measurement needs for your next project at 480-948-5555.

Test Stand Applications for Force and Torque

In the world of test and measurement, test stands are essential equipment for manufacturers and testing engineers. The test stand provides a host of different testing products in a single “cabinet-like” structure. These systems have been used for a long time to gather data on various functions of products during the product test phase.

Test stands works like a mobile test lab, hosted by a frame and containing one or more force or torque sensor components, software, and data acquisition instrumentation and accessories. Force stands are typically motorized or manual.  Motorized test stands, also known as mechanical or electrical, have the advantages of controlling performance by applying modes such as speed, cycles, and time into the testing procedure. The more advanced testing stands are frequently used for repetitive high-performance testing requirements, validating accuracy and quality. Manual test stands are used for simple testing protocols and frequently used in education programs.

There are a wide variety of testing devices and sensor products that are used as part of the entire test process. As parts roll off the production line, the test stand will sit at the end of the line where the test engineer can immediately load the product into the test rig. Test stands help to streamline the test process by providing all available test functions in a single, mobile application.

Interface is a supplier of choice for precision components of various capacities and dimensions for test stand configurations requiring precision and accuracy in performance. Interface load cells, torque transducers, and instrumentation equipment are commonly used in numerous product test applications by engineers, metrologists, testing professionals and product designers around the world.

Included below are a few examples of specific test applications and the Interface components used in the different style testing stands.

Linear Test Stand

In this example, an Interface customer wanted to add a crush test to their test stand to measure the force it took to deform a piece of material. Interface provided an Model 1210 Load Cell with an internal amplification of 0-10VDC output.

The load cell was installed into the load string of the customer’s load frame, and the scaled analog output from the load cell was connected to the customer’s test stand instrumentation. When the force levels reached the crushing point, the customer’s software was able to read the output of the amplified load cell and record the value.

See the application note for the Linear Test Stand here.

Motor Test Stand

In the quality control lab at a major automotive manufacturing company, a test engineer needed to test, record, and audit the torque produced by a new motor design under start load. Interface supplied the new AxialTQ® Rotary Torque Transducer that connected between the motor and the differential, on the drive shaft, that could measure and record these torque values.

Based on the data collected using the AxialTQ transducer, along with the AxialTQ Output Module, and a laptop, the test engineer was able to make recommendations to optimize the amount of torque created by the new motor design.

See the application note for the Motor Test Stand here.

Verification Test Stand

In this application, a customer needed a test stand application to verify that its load cell was in good, working order. Interface helped to create a solution that used a load cell to verify the customer’s load cell. The solution involved the customer’s supplied verification load frame and an Interface Model 1210 Precision LowProfile® Load Cell connected with a Model SI-USB 2-Channel PC Interface Module.

The customer was able to install their load cell and Model 1210 Precision LowProfile Load cell into the verification load frame. Applied forces were displayed and recorded by Model SI-USB PC Interface Module for review and record keeping on customer’s computer. This allows the customer to have a proven load cell verification test stand at their disposal to ensure its test load cell is always in working order.

See the application note for the Verification Test Stand here.

These are just a few examples of the different types of test stands that Interface can provide off-the-shelf or custom force measurement solution components. If your project involves a mechanical test stand and you are interested in learning more about adding force sensors, please contact our application engineers.