Exploring the Dynamics of Vibration Testing: A Comprehensive Analysis of Power Spectral Density (PSD) Differences

Monday, January 22 2024

In the relentless pursuit of product excellence, Highly Accelerated Life Test (HALT) and Highly Accelerated Stress Screen (HASS) testing stand as sentinels, ensuring the robustness and reliability of the innovations that shape our world. Among the key metrics, Power Spectral Density (PSD) emerges as a critical element, measuring the  signal's power content versus frequency, which are used to stimulate a product during testing.

Understanding the Importance of PSD in Reliability Testing

Whether in prototyping with HALT or during production with HASS, the primary goal is to apply repetitive shock vibration energy that excites the natural frequencies of the product, thereby accelerating mechanical fatigue failures. The significance of the PSD of the input energy lies in directing energy towards frequencies that induce resonance in the product. Consequently, comparing PSDs becomes a valuable tool for monitoring changes over time or assessing new components. 

The Limitations of Current Frequency Response Comparison Methods

Traditional methods of frequency response comparison often fall short in effectively showcasing the differences between two PSDs. Enter the Normalized PSD Difference method, a novel approach developed at Qualmark to bridge this gap. This method provides a concise and visually intuitive way to highlight energy shifts across the frequency band between two measured response curves. 

Calculating the Normalized PSD Difference

The Normalized PSD Difference method involves a step-by-step calculation process. Here are the core components:  

1. Power Calculation: Determining the power in g²RMS of two PSD curves.

2. Normalization: Scaling both curves to the same power level (1 gRMS).

3. Plotting the Difference: Subtracting values of one series from another and plotting the resulting curve.

4. Area Calculation: Determining the area under the difference curve as a metric for the total amount of difference. 

Interpreting the Results

Once normalized, the difference curve provides insights into where and to what magnitude vibration energy shifts across frequencies. By summing the area under the difference curve and dividing it by the frequency band measured, the Normalized Total Difference delivers a unified metric to answer the question, "How different are these PSDs?" 

Acceptable Differences and Application Considerations

Determining what is acceptable in PSD differences is not a one-size-fits-all equation. It depends on whether the natural frequencies of the products under test are being excited. Peaks and clusters of energy shifting across the frequency band become key indicators. A case-by-case assessment is crucial, as shown in examples with different frequency and energy shifts. 

Industrial Implications: Practical Applications in Various Sectors

1. Automotive Industry: Monitoring and comparing PSD changes in vehicle components during HALT and HASS testing to enhance overall reliability.

2. Electronics Manufacturing: Assessing the PSD of electronic components to ensure consistency and durability in diverse operational environments.

3. Aerospace and Defense: Analyzing PSD differences in critical components to identify potential weaknesses and enhance the structural integrity of aircraft and defense systems. 

Conclusion: Bringing Clarity to PSD Comparisons

With the Normalized PSD Difference method, the landscape of comparing energy shifts between two PSD curves becomes standardized. This method not only facilitates quick comparisons of multiple products in a HASS fixture but also aids in monitoring changes in the PSD of a bare HALT/HASS table over time. The transition from subjective to objective PSD analysis is now more achievable, providing valuable insights for users in the HALT & HASS community. 

As we strive for better quality and higher product reliability, the journey through testing in vibration chambers becomes more nuanced and efficient with the Normalized PSD Difference method.

If you need reliable HALT services, turn to ESPEC North America. At ESPEC Test Centers (previously Qualmark), our highly skilled experts provide exclusive testing services on advanced HALT systems. To discuss your testing needs, reach out on our website.