DESIGN
T&M
pulse severity, the impact velocity can be estimated using Equation 1:
where V is velocity in m/s, A is peak acceleration in m/s², and D is pulse duration in milliseconds. For example, both a 200g shock lasting 3ms and a 3000g shock lasting 0.2ms result in an impact velocity of approximately 3.8m/s, as specified in IEC 60068-2-27. This equivalence allows for more accessible test setups while still approximating the mechanical energy of a standard shock event. Conclusion Shock survivability and vibration tolerance are often misunderstood as interchangeable specifications, yet they represent fundamentally different stress profiles and failure mechanisms in MEMS accelerometers. Shock events are rare but intense, requiring robust mechanical design to prevent catastrophic damage, while vibration is persistent and insidious, demanding long-term reliability and resistance to wear. Understanding these distinctions is critical for engineers selecting sensors for harsh environments. By considering factors such as mechanical headroom, damping, and system-level robustness, designers can ensure that the chosen sensor not only survives but performs reliably over time. Ultimately, matching the sensor’s specifications to the application’s mechanical stress profile is essential for achieving both durability and precision. With the right sensor and a clear understanding of its limitations and strengths, engineers can confidently deploy MEMS technology in even the most extreme conditions. Acknowledgement Thank you to Mark Looney and Tyler Dunn for their input.
Figure 2. ADXL357B mechanical headroom vs. frequency. The mechanical headroom decreases near resonance, highlighting the importance of damping in sensor design. The ADXL357B is rated to 70g peak amplitude continuous random vibration with frequency content from 0Hz to 2kHz, based on MIL-STD-883 Method 2007, Test Condition C specifications.
as a ±400g range sensor like the ADXL373, which is 10,000kg peak amplitude, 0.1ms pulse width half-sine wave shock profile. However, the ADXL373 has a much higher vibration mechanical headroom. The shock tolerance rating can be seen as a system-level test. Not only is the MEMS sensor put under test, but also the integrity of wire bonds, die attach, package, and even solder joints need to withstand this test. These MEMS sensors from ADI might be able to survive repeated shocks even higher than 10,000g without structural failure, but it might not be the case for other parts of the system that form part of the device. Shock testing Replicating standardised shock tests in-house is often challenging due to the need for specialised equipment – such as drop towers or shock tables – that can be several meters tall and precisely controlled. As a result, engineers frequently seek practical alternatives. One common approach is to reduce the peak acceleration while increasing the pulse duration, maintaining equivalent shock energy. This is based on the principle that the area under the acceleration-time curve (velocity) is a key indicator of shock, as it correlates with the impact energy. For a half-sine shock
38 ELECTRONICSPECIFIER.COM
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