Understanding Load Cell Frequency
Load cell “frequency” isn’t just a technical term; it’s a key factor in determining how effectively and accurately your sensor performs.
The natural frequency of a load cell is the frequency at which it will oscillate freely when subjected to a sudden force or disturbance and then allowed to vibrate without any further external input. It’s often referred to as the “ringing frequency.”
A load cell’s frequency response describes its ability to accurately measure and reproduce a dynamic (time-varying) load over a range of frequencies. It indicates how consistently the load cell’s output signal tracks the input force as the frequency of that force changes.
A load cell’s usable frequency response is generally a fraction (e.g., 1/5th to 1/10th) of its natural frequency. Exceeding this range can distort the output signal in both amplitude and phase. This characteristic is crucial for applications involving rapidly changing loads, impacts, or vibrations, such as crash testing, fatigue testing, or material dynamics, as a load cell with an inadequate frequency response for a given application may miss rapid force peaks or introduce significant measurement errors.
From fundamental sampling rates to understanding natural frequencies, optimizing frequency can significantly impact your test and measurement load cell application. Let’s explore some common questions and insights regarding load cell frequency.
Sampling Frequency and Load Cell Response
A common question is whether a maximum sampling frequency exists for strain gage load cells. While the optimal rate varies by load cell model, hundreds of hertz are generally sufficient for most applications.
However, if your sampling frequency approaches the load cell’s natural frequency value, typically listed on our product data sheets, it’s advisable to consult our application engineers. Exceeding the natural frequency can introduce inaccuracies or even cause damage.
Consider impact testing, where engineers may need to sample at 10 kilohertz. While the load cell doesn’t set your sampling speed, its mechanical response time is crucial, typically within a millisecond. Load cells are passive devices with no moving parts or active circuitry, meaning their inherent mechanical properties govern their response. When pushing systems to failure, sampling at a couple of thousand hertz is a good practice to accurately capture spike events or the exact point of failure.
It’s also important to note the relationship between cycling and creep error. When exercising a load cell before calibration, the magnitude of the load, not the duration, really matters. For instance, testing at 120% for 10 seconds is just as effective as testing at 120% for an hour, but helps avoid introducing significant creep. A quick check of the zero value after releasing the load can assist in accurately assessing any introduced creep error for fast cycles.
How Does Frequency Measurement Limit Load Cell Response?
The speed at which a load cell responds is intrinsically linked to its mechanical flexure design. Consider each sensor as a mechanical spring. The stiffer the load cell and the lower its deflection, the higher its natural frequency will be. This characteristic, documented on our product data sheets, directly impacts performance in cyclic applications. Generally, a stiffer sensor handles cyclical loads more effectively.
Load cells typically perform exceptionally well at hundreds of hertz. However, their effectiveness diminishes at tens of thousands of hertz, and standard load cells become less suitable for extremely high-frequency applications. For most testing projects that apply cyclical loads to test specimens, strain gage-based LowProfile load cells will meet their frequency requirements.
Understanding Natural Frequency: Lightly Loaded vs. Heavily Loaded Systems
The concept of a load cell’s natural frequency is fundamental to understanding its dynamic response.
Lightly Loaded Use Case
When a load cell is lightly loaded, such as with a weigh pan or small test fixture, its dynamic response can be observed by tapping its active end and monitoring the output on an oscilloscope. The frequency of this vibration, or the natural frequency of free oscillation (fO
Natural frequency is a typical value that acts as a guide rather than a controlled specification. It’s determined by the load cell’s design, effectively functioning as an equivalent spring-mass system. The “spring” represents the stiffness of the flexure’s thin measurement section, while the “mass” corresponds to the live end of the cell and any attached fixtures. Adding mass to the active end of the load cell will inevitably lower the total system’s natural frequency.
Static calibration is analogous to a DC parameter, while dynamic response reflects an AC parameter. Employing logarithmic scaling for frequency and output allows us to display a broader range of values and observe common characteristics as straight lines on graphs, simplifying analysis. It’s important to remember that in most installations, the resonances of the attaching fixtures, test frame, and driving mechanism will often dominate the load cell’s response.
Heavily Loaded Use Case
In scenarios where the load cell is tightly coupled into a system with component masses significantly heavier than the load cell itself, the load cell behaves more like a simple spring connecting the driving and driven elements. In these cases, the challenge for the system designer lies in analyzing the interaction of the system’s masses with the load cell’s very stiff spring constant.
There is no direct correlation between the load cell’s unloaded natural frequency and the heavily loaded resonances observed in the user’s system.
Load Cell Frequency Tips for Optimal Performance
Understanding load cell frequency is essential for accurate and reliable measurements. Considering these factors, you can ensure your load cell performs optimally for your needs. Here are a few key tips:
- Know Your Natural Frequency: Always check the product data sheet for your load cell’s natural frequency. This is your baseline for understanding its dynamic limits.
- Match Sampling to Application: Hundreds of hertz are usually sufficient for general use. For high-speed events like impact testing, sampling at a few thousand hertz can be critical to capture data accurately.
- Mind the Mass: Any additional mass added to the active end of your load cell will lower the overall system’s natural frequency. Factor this into your design.
- Stiffer is Better for Cycles: If your application involves frequent cyclic loading, opt for a stiffer load cell, as they generally perform better in these scenarios.
Finally, be sure to consult the experts. If you have exceptionally high-frequency requirements or are unsure about your setup, don’t hesitate to consult with our application engineers. They can provide tailored guidance.
Be sure to download a copy of Interface’s Load Cell Field Guide as a reference and review the illustrated examples of frequency. Also, to tune into the frequently asked questions in our Load Cell Basics Q&A, watch the following:
The full video, other educational references, and webinars are available on our Interface Force YouTube channel. If you like to explore technical topics and 101 articles, bookmark our ForceEDU online resource center.
