Interpretation of Force Measurement System Errors
Understanding the potential sources of error is crucial to accurate force measurement. Interface provides technical support resources online to assist load cell users. As detailed in the Interface Load Cell Field Guide, system errors are important to differentiate from load cell performance questions.
Often, terms like resolution, repeatability, and reproducibility are used interchangeably, leading to confusion. It’s important to remember that these are system parameters, not just load cell characteristics. They are influenced by a variety of factors, including:
- Proper load cell application ensures the load cell is correctly installed and aligned within the system.
- Force measurement systems and mechanical fixtures are of high quality, and the mechanisms used to apply force are stable.
- The accuracy and stability of the electrical instruments used to measure the load cell output.
Defining Resolution
Theoretically, load cell resolution is virtually limitless. In a perfectly controlled environment with stable temperature, minimal external forces, and high-gain electronics, a load cell can detect incredibly small changes in force. However, real-world conditions can present challenges. Factors like temperature fluctuations, building vibrations, and air movement can all introduce noise into the system, making it difficult to achieve the theoretical resolution. Even the stability of hydraulic forcing systems can impact maintaining a consistent force.
Resolution Tip: Excessive resolution can be a hindrance. If your system is overly sensitive, the display constantly fluctuates due to minor instabilities, making it difficult to obtain a reliable reading. Review your set-up and instrumentation selection.
Defining Repeatability
Repeatability refers to the ability of a system to produce the same output when the same force is applied multiple times. Often, poor repeatability is mistakenly attributed to the load cell itself. However, a variety of factors can contribute to inconsistent readings, including:
- Mechanical connections: Loose connections in fixtures can introduce movement and variability.
- Rigidity of the load frame: Flexing or instability in the test or load frame can alter force application.
- Repeatability of the force measurement system: Inconsistent force application from hydraulics or other system components.
- Load application speed: Applying a load too quickly can result in overload and impact force accuracy.
- Reading time consistency: Variations in reading times can introduce creep effects.
- Electronics stability: Temperature drift, cable and power fluctuations, and noise in the electronics can all affect readings.
Repeatability Tip: To improve repeatability, ensure all mechanical connections are secure, the load frame is rigid, and the force system is stable. Apply loads smoothly and consistently and maintain consistent reading times.
Defining Reproducibility
Reproducibility involves obtaining the same measurement results when the same force is applied in different setups. This becomes important when comparing measurements taken in different locations or with different equipment. Even seemingly minor changes, such as swapping out a fixture, can affect reproducibility.
Reproducibility Tip: Interface Gold Standard Load Cells are designed to minimize the impact of setup variations. Their low moment sensitivity makes them less susceptible to misalignment issues, and their permanently installed loading stud ensures consistent loading. Combined with high output and low creep, these features make them an excellent choice for applications where reproducibility is critical.
By understanding these concepts and implementing best practices, you can improve the accuracy and reliability of your force measurement system.
You can find this information in the essential Interface Load Cell Field Guide. This resource, available for download, is an engineering and force measurement resource that defines technical terms, applications, procedures, and characteristics.
ADDITIONAL RESOURCES
How Do You Define A Load Cell Error Budget?
What is Static Error Band Output?
Mechanical Installation Load Cell Troubleshooting 101
