Accelerometer Sensitivity Selection for Vibration and Shock Tests: Balancing Range, Resolution, and Reliability
By Tom Resh, Reliability Test Consultant and Training Instructor of Resh Tech LLC - February 2/13/2025
Introduction
Selecting the right accelerometer sensitivity is paramount to obtaining accurate, reliable data in both vibration and shock tests. In industries such as aerospace, automotive, electronics, and defense, vibration and shock environments can greatly influence the performance and longevity of products and components. As vibration and shock tests become more sophisticated, the need to carefully match an accelerometer’s sensitivity to the application’s dynamic range and frequency spectrum grows increasingly critical.
This article reviews the fundamentals of accelerometer sensitivity, highlights key selection criteria, and provides case studies illustrating how organizations approach the challenge of accelerometer specification for different test scenarios.
1. Understanding Accelerometer Sensitivity
Sensitivity is a measure of how much electrical output an accelerometer produces for a given input acceleration. Typically expressed in mV/g (millivolts per g) or pC/g (picocoulombs per g) for charge-mode accelerometers, sensitivity influences several measurement factors:
Dynamic Range: Lower-sensitivity accelerometers can measure higher accelerations before saturating, but at the cost of reduced resolution at low amplitudes.
Resolution: Higher-sensitivity accelerometers provide more detailed resolution at low acceleration levels but may saturate during high-intensity shocks.
Signal-to-Noise Ratio (SNR): Sensitivity affects the overall noise floor and thus the clarity and accuracy of the measured signal.
Frequency Response: Sensitivity is also tied to the internal design (mass, piezoelectric type, or MEMS design), which will affect the usable frequency range.
Balancing sensitivity is key: using the highest possible sensitivity that still covers the peak acceleration without saturating the accelerometer ensures the best possible resolution for your measurements.
2. Factors Influencing Sensitivity Selection
a. Expected Acceleration Level
Vibration tests generally exhibit lower acceleration ranges (from fractions of a g up to a few hundred g).
Shock tests can produce extremely high accelerations (thousands of g or more).
Consequently, a single accelerometer rarely suffices for all tests; specialized sensors with different sensitivities are chosen.
b. Dynamic Range of the Test
Continuous (sine or random) vibration can have moderate peak acceleration but can last for hours.
Shock pulses, such as those in drop tests or explosive events, can reach extremely high peak accelerations, albeit very briefly.
c. Frequency Range
Sensitivity often correlates with the mass and geometry of the accelerometer’s sensing element. A large, high-sensitivity element may have a lower resonant frequency, whereas a smaller element can exhibit a higher resonant frequency but lower sensitivity.
For high-frequency vibration tests (e.g., up to 10 kHz or beyond), smaller accelerometers with lower sensitivity are often used to minimize mass loading and extend the frequency range.
d. Mounting and Test Constraints
If space is limited or mass loading must be minimized (e.g., on small electronic boards or fragile structures), a smaller and typically less sensitive accelerometer might be chosen.
Environmental considerations (temperature, humidity, vacuum) can narrow the choice to certain sensor technologies.
e. Data Acquisition and Electronics
The amplifier and data acquisition system must accommodate the accelerometer’s voltage or charge output. A sensor with very high sensitivity could saturate the amplifier unless adequate gain or range settings are applied. Similarly, if sensitivity is too low, the data acquisition system may struggle to capture low-level signals above the noise floor.
3. Detailed Case Studies
Case Study A: Electronics Vibration Qualification Test
Scenario: An electronics manufacturer must perform qualification testing on a printed circuit board (PCB) assembly designed for automotive applications. The primary concern is random vibration from the engine environment over a range of 10–2,000 Hz, with acceleration levels up to 10 g RMS.
Testing Standard: Based on ISO 16750 and typical automotive OEM specifications for vibration.
Acceleration Range: 0.1 g to ~20 g peak (allowing margin above 10 g RMS).
Frequency Range: 10–2,000 Hz.
a. Selection Process
Sensitivity: A 100 mV/g piezoelectric accelerometer is chosen because it allows good resolution at the sub-g level and can handle up to ~50 g peak before saturation.
Mounting: The accelerometer’s mass (typically a few grams) is low enough not to significantly affect the PCB dynamics.
Data Acquisition: A ±5 V input range from the DAQ system ensures the 100 mV/g sensor can measure up to 50 g without clipping, providing enough overhead for unexpected spikes.
b. Outcome
The high sensitivity provided excellent resolution for detecting subtle resonances in the PCB.
The accelerometer did not saturate during testing, capturing both moderate continuous vibrations and occasional shock transients up to 20 g.
Case Study B: Aerospace Turbine Blade Vibration Test
Scenario: A turbine engine manufacturer needs to measure high-frequency, moderate-amplitude vibrations on turbine blades during engine operation. Frequency content can reach up to 10 kHz with acceleration levels around 100 g peak.
Testing Standard: Some aspects reference MIL-STD-810 for high-frequency vibration, but specialized in-house procedures dominate.
Acceleration Range: 0.5 g to ~150 g peak.
Frequency Range: 1Hz–10 kHz (the sensor must reliably capture up to 10 kHz).
a. Selection Process
Sensitivity: A 10 mV/g, shear-mode piezoelectric accelerometer is chosen for its extended frequency range (resonant frequency above 40 kHz) and capacity to measure higher g loads.
Mounting: Special high-temperature epoxy or stud mounting is used to handle engine temperatures and ensure good coupling with minimal mass loading.
Data Acquisition: A ±10 V input module with an anti-aliasing filter is used to reduce noise at high frequencies.
b. Outcome
Despite having a lower sensitivity, the accelerometer delivered accurate data up to 10 kHz without saturating.
Post-analysis correlation with blade natural frequencies confirmed the selection was appropriate, and the low mass of the accelerometer minimized impact on blade dynamics.
Case Study C: Defense High-G Shock Test
Scenario: A defense contractor needs to qualify a missile guidance system for extreme shock events, reaching up to 10,000 g in a fraction of a millisecond.
Testing Standard: MIL-STD-810 method for shock, specifically high-intensity, short-duration shocks.
Acceleration Range: 1,000–10,000 g.
Pulse Duration: Typically 0.5–1.0 ms.
a. Selection Process
Sensitivity: A low-sensitivity (1 mV/g or even 0.5 mV/g) piezoelectric or specialized piezoresistive accelerometer is necessary to avoid saturation.
Frequency Response: The sensor must have a resonant frequency well above the high shock content (often in the tens or hundreds of kHz range).
Mechanical Robustness: The accelerometer must survive repeated high g events without damage.
Data Acquisition: A high-speed DAQ system (sampling at >100 kHz) ensures capturing the rapid shock pulse.
b. Outcome
The chosen accelerometer was able to measure the intense acceleration levels accurately.
Despite the extreme environment, the low sensitivity and rugged design prevented sensor damage and electrical saturation, resulting in reliable shock measurements for system qualification.
Case Study D: Drop Test of Consumer Electronics
Scenario: A smartphone manufacturer must confirm product durability by dropping phones from various heights to evaluate internal damage (acceleration levels can reach hundreds to a thousand g during impact, depending on drop height and surface).
Testing Standard: Industry-specific in-house drop test procedures, referencing IEC 60068-2-31 for drop and topple tests.
Acceleration Range: 10 g to ~2,000 g.
Pulse Duration: Typically 2–10 ms, depending on packaging and phone design.
a. Selection Process
Sensitivity: A mid-range accelerometer of around 10 mV/g often suffices—high enough to detect moderate drops but low enough to avoid saturation on the upper end of shock. In practice, multiple sensor ranges might be used to cover the wide amplitude possibilities. Many modern electronics incorporate a MEMS accelerometer into their products for warranty and reliability purposes. If properly calibrated, this sensor could be used allowing for no additional instrumentation or wiring running out of the phone that could affect the results of the drop test. Smartphones are so compact as well that even the most miniscule size accelerometer may not fit into the tight packaging of a smartphone. However, in this case study we will assume there was appropriate spacing inside of the phone's housing to mount an accelerometer. Additionally, the accelerometer used has a built in wireless transmitter, so that no external wiring is required.
Mounting: Internal fixture or adhesive mounting to the phone’s interior chassis.
Data Acquisition: A multi-channel, high-speed DAQ captures drop events at 20–50 kHz sampling rate.
b. Outcome
Engineers identified the maximum shock the phone experienced on each axis, correlating damage patterns with peak g-levels.
The chosen accelerometer captured the entire shock pulse without clipping, aiding in robust product design improvements.
4. Best Practices and Lessons Learned
a. Always Define the Test Environment First
Before specifying an accelerometer, fully characterize the expected acceleration levels and frequency range, including any potential outliers or worst-case scenarios.
b. Match Sensor Resonance to Frequency Content
Avoid placing critical test frequencies too close to the sensor’s resonance. If high frequencies are vital, smaller or specially designed accelerometers with extended bandwidth may be necessary.
c. Beware of Over-Range Conditions
Even if a standard test suggests a peak of 100 g, there may be unexpected transient spikes. Ensure you have at least 20–30% measurement headroom.
Calibrate and Verify
Regular calibration is crucial. Verify sensors before and after especially demanding shock tests to confirm they have not been damaged or drifted.
Consider the Entire Measurement Chain
Sensitivity selection is only half the picture. Ensure that cables, charge amplifiers (for charge-mode sensors), inline conditioners, and DAQ settings are all aligned to support the sensor’s output levels without causing clipping or excessive noise.
5. Conclusion
Accelerometer sensitivity selection is a delicate balance between resolution and dynamic range. It directly impacts data integrity and the insights gained from vibration and shock tests. By carefully considering acceleration levels, frequency content, mounting constraints, and measurement systems, engineers can effectively choose and deploy accelerometers that deliver precise, actionable data. Each case study demonstrates that success begins with an informed decision on the sensor’s sensitivity—one that aligns with the product’s actual operating environment and the goals of the test program.
Whether characterizing mild PCB resonances under random vibration or capturing extreme shocks in defense applications, the right accelerometer sensitivity ensures the integrity of the results and the safety and reliability of the end product.
