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 Basics Sensor Specifications
Load Cell Basics Technical Q&A Part One
Interface 1200 Precision LowProfile Load Cell Series Product Highlight
