T&M
• Purpose: validates long-term reliability under operational vibration • Failure mechanism: often leads to stiction or particle contamination due to the wear of protective mechanisms Why the distinction matters Shock and vibration stress the sensor in fundamentally different ways. A sensor rated at thousands of g’s shock survivability may fail under hundreds of g’s continuous vibration. This distinction is crucial for ensuring both sensor survival and performance. Shock survivability refers to non-repetitive extremely high magnitude impacts that can result in system-level failure, whereas vibration tolerance refers to long-term reliability. The MEMS sensor design plays a key role in defining the tolerances for both metrics, shock, and vibration. For example, mechanical stoppers and anti-stiction coating materials are some of the measures used in the design to protect the MEMS structure integrity. The anti-stiction coat creates a low surface energy and/ or electrical insulation, whereas the mechanical stoppers prevent the proof mass from making full contact with the fixed fingers set. Figure 1 shows a simplified representation of a MEMS accelerometer. The mechanical stoppers usually have 4 to 5µm wide crenulations (small bumps) that reduce the contact area under high shock events, which helps avoid stiction. Consider heavy machinery, like a dozer, where accelerometers are used as tilt sensors for proper operation on uneven terrain or for terrain levelling. In this application, the accelerometers may experience continuous random vibration in the tens of g’s (or even over 100g) peak amplitude and require high tilt precision, high temperature stability, and repeatability. An accelerometer like the ADXL357B is the perfect candidate in terms of performance. Even though its full-scale range is limited to
±40g, it can withstand larger vibrations. The vibration safe zone is highly dependent on the sensor’s mechanical design, including its resonant frequency, damping, and the acceleration input required to hit the mechanical stoppers (called mechanical headroom). To illustrate the vibration safe zone, we can examine the mechanical headroom vs. frequency, as shown in Figure 2. This helps engineers understand how much margin exists before the proof mass hits the stoppers, and how the sensor resonant frequency and quality factor play a role in it. As the input vibration gets mechanically amplified by the quality factor, the closer the vibration frequency is to the sensor resonance, effectively reducing the mechanical headroom. Electrical bandwidth and mechanical limits Accelerometers usually have analogue and digital filters on their built-in signal chain, and newer sensors, like the ADXL380, even have a digital equaliser filter, effectively extending its bandwidth flatness up to 4kHz. This is useful for applications like road noise cancellation (RNC), where accurate detection of broadband vibrations is critical for generating effective anti-noise signals. However, it’s important to note that electrical filtering or equalisation does not eliminate the physical excitation of the MEMS structure. The sensor still experiences mechanical stress, and operating beyond its mechanical headroom can result in stiction, fatigue, or structural degradation. Designers must ensure that vibration amplitudes remain within safe mechanical limits, even if the electrical output appears linear across the extended bandwidth. Shock vs. sensor full-scale range It is worth noting that the ADXL357B (±40g range) and the ADXL380 (up to ±16g range) have the same survivability rating
37 ELECTRONICSPECIFIER.COM
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