Understanding load cell frequency

The “frequency” of a load cell is not just a technical term, but a key factor that determines how effectively and accurately your sensor works.

The natural frequency of a load cell is the frequency at which a sound oscillates freely when subjected to a sudden force or disturbance and then allowed to oscillate without further external influence. It is often referred to as the "oscillation frequency."

The frequency response of a load cell describes its ability to accurately measure and reproduce a dynamic (time-varying) load across a range of frequencies. It indicates how consistently the load cell's output signal follows the input force as the frequency of that force changes.

The usable frequency response of a load cell is generally a fraction (e.g., 1/5 to 1/10) of its natural frequency. Exceeding this range can distort the output signal in both amplitude and phase. This characteristic is critical for applications with rapidly changing loads, shocks, or vibrations, such as crash tests, fatigue tests, or material dynamics, as a load cell with a frequency response inadequate for a particular application may miss rapid force peaks or introduce significant measurement errors.

From basic sampling rates to understanding natural frequencies, optimizing frequency can significantly impact your load cell application for testing and measurement. Let's explore some frequently asked questions and insights about load cell frequency.

Sampling frequency and load cell response

A common question is whether there is a maximum sampling frequency for strain gauge load cells. While the optimal rate varies depending on the load cell model, hundreds of Hertz are generally sufficient for most applications.

However, if your sampling frequency approaches the load cell's natural frequency, which is typically specified in our product datasheets, it is advisable to consult our application engineers. Exceeding the natural frequency may result in inaccuracies or even damage.

Think about them Impact test, where engineers require a sampling rate of 10 kilohertz. While the load cell doesn't determine the sampling speed, its mechanical response time is critical and is typically within a millisecond. Load cells are passive devices with no moving parts or active circuitry, meaning their inherent mechanical properties determine their response. When systems are stressed to failure, sampling at a few thousand hertz is good practice to accurately capture peak events or the exact point of failure.

It's also important to consider the relationship between cycles and creep error. When testing a load cell prior to calibration, it's the amount of load applied that matters, not the duration. For example, testing at 120% for 10 seconds is as effective as testing at 120% for an hour, but will help prevent the introduction of significant creep error. A quick check of the zero reading after the load is released can help accurately assess any introduced creep error during rapid cycling.

How Does Frequency Measurement Limit Load Cell Response?

The speed with which a load cell responds is inseparable from its mechanical Bending construction connected. Consider each sensor as a mechanical spring. The stiffer the load cell and the lower its deflection, the higher its natural frequency. This property, documented in our product data sheets, directly impacts performance in cyclic applications. In general, a stiffer sensor can better handle cyclic loading.

Load cells typically perform exceptionally well at hundreds of hertz. However, at tens of thousands of hertz, their effectiveness decreases, and standard load cells are less suitable for extremely high-frequency applications. For most testing projects where cyclic loads are applied to the test specimens, LowProfile load cells based on strain gauges the frequency requirements.

Understanding the Natural Frequency: Lightly Loaded vs. Heavily Loaded Systems

The concept of the natural frequency of a load cell is fundamental to understanding its dynamic behavior.

Use case: Lightly loaded

When a load cell is lightly loaded, such as with a weighing pan or small test fixture, its dynamic response can be observed by tapping its active end and monitoring the output signal on an oscilloscope. The frequency of this oscillation, or the natural frequency of free vibration (fO), can be calculated by measuring the time (T) for one complete cycle. Extreme caution must be exercised when applying shock to avoid damaging the load cell.

The natural frequency is a typical value that serves more as a guideline than a controlled specification. It is determined by the Load cell design which effectively functions as an equivalent spring-mass system. The "spring" represents the stiffness of the thin measuring section of the bend, while the "mass" corresponds to the active end of the cell and the devices attached to it. Adding mass to the active end of the load cell inevitably leads to a reduction in the natural frequency of the overall system.

Static calibration corresponds to a DC parameter, while dynamic response reflects an AC parameter. Using logarithmic scaling for frequency and power allows us to represent a wider range of values ​​and observe common features as straight lines on graphs, simplifying analysis. It is important to remember that in most installations, resonances of the mounting fixtures, test frame, and drive mechanism often dominate the load cell response.

Highly loaded use case

In scenarios where the load cell is tightly integrated 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 is to analyze the interaction between the system masses and the very stiff spring constant of the load cell.

There is no direct relationship between the unloaded natural frequency of the load cell 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 will help ensure your load cell performs optimally for your needs. Here are a few key tips:

• Know your natural frequency: Always check the product datasheet for your load cell's natural frequency. This is your foundation for understanding dynamic limits.
• Adjust sampling to the application: Hundreds of hertz are usually sufficient for general use. For high-speed events, such as impact testing, sampling at a few thousand hertz can be critical for accurate data acquisition.
• Be mindful of mass: Any additional mass added to the active end of your load cell will lower the natural frequency of the entire system. Consider this in your design.
• Stiffer is better for cycling: If your application involves frequent cyclic loading, you should choose a stiffer load cell as they generally perform better in these scenarios.

Finally, be sure to consult the experts. If you have unusual high-frequency requirements or are unsure about your setup, don't hesitate to consult our application engineers. They can provide you with tailored advice.

Be sure to download a copy of Interface's Load Cell Field Guide Download it for reference and see the illustrated frequency examples. Also, check out the frequently asked questions in our Q&A on load cell basics: