MEMS Accelerometer Selection Guide for Motion Sensing Applications
Introduction
MEMS accelerometers enable motion sensing in diverse applications from vehicle safety systems to consumer electronics. This guide provides selection criteria and recommendations for choosing the right accelerometer.
Key Accelerometer Parameters
Measurement Range
The measurement range (full-scale range) determines the maximum acceleration the sensor can measure:
- ±2g to ±4g: Tilt sensing, static orientation, low-g motion
- ±8g to ±16g: General motion sensing, handheld devices, wearables
- ±50g to ±100g: Vehicle dynamics, safety systems, vibration monitoring
- ±200g: Crash detection, high-g shock, pyrotechnic deployment
Select a range that encompasses your maximum expected acceleration with 20-30% margin. Higher range reduces sensitivity (resolution).
Sensitivity
Sensitivity indicates output change per unit acceleration:
- Analog: mV/g (millivolts per g)
- Digital: LSB/g (least significant bits per g)
Higher sensitivity provides better resolution but lower maximum range. For tilt sensing, high sensitivity (1000+ LSB/g) is preferred. For crash detection, lower sensitivity is acceptable.
Noise Density
Noise density specifies self-noise per sqrt(Hz), typically in μg/√Hz. Lower values enable detection of smaller accelerations. Total noise = Noise Density × √(Bandwidth).
- <100 μg/√Hz: High-resolution tilt sensing
- 100-300 μg/√Hz: General motion sensing
- >300 μg/√Hz: High-g applications where noise is less critical
Bandwidth
Bandwidth determines maximum measurable vibration frequency:
- 10-50Hz: Tilt sensing, slow motion
- 100-400Hz: Human motion, vehicle dynamics
- 1-10kHz: Vibration monitoring, machine condition
- >1kHz: Shock detection, impact sensing
Higher bandwidth increases noise (proportional to √BW). Select minimum bandwidth that captures your signal of interest.
Application Guidelines
Vehicle Stability Control
Recommended: MSA-50G (±50g, 40 LSB/g, AEC-Q100)
Key considerations:
- ±50g range covers vehicle dynamics
- AEC-Q100 qualification mandatory
- Good sensitivity for precise measurement
- Wide temperature range operation
Airbag Crash Detection
Recommended: MSA-100G (±100g, 20 LSB/g, AEC-Q100)
Key considerations:
- ±100g range for crash detection
- Fast response for timely deployment
- Built-in self-test for safety validation
- Automotive qualification and traceability
Industrial Vibration Monitoring
Recommended: MSA-50G or MSA-100G depending on expected vibration levels
Key considerations:
- Bandwidth matched to machine rotation frequencies
- Good noise performance for small vibration detection
- Robust package for industrial environment
Tilt Sensing and Inclinometers
Recommended: Low-g accelerometer (±2g to ±4g range)
Key considerations:
- High sensitivity for small angle resolution
- Low noise for stable readings
- Temperature compensation for accuracy
Selection Checklist
- [ ] Measurement range covers expected acceleration + margin
- [ ] Sensitivity adequate for resolution requirements
- [ ] Noise density acceptable for smallest signal
- [ ] Bandwidth covers highest frequency of interest
- [ ] Power consumption acceptable
- [ ] Interface compatible (SPI/I2C/analog)
- [ ] Temperature range covers application
- [ ] Qualification level appropriate
- [ ] Package size fits mechanical constraints
Conclusion
Accelerometer selection requires balancing measurement range, sensitivity, and bandwidth for your specific application. Memsensing offers accelerometers optimized for automotive safety, industrial monitoring, and consumer motion sensing. Contact our FAE for application-specific guidance.
💡 FAE Insights
⚠️ Common Pitfalls
- ✗ Selecting range too close to expected maximum causing saturation
- ✗ Using consumer-grade sensors in automotive applications
- ✗ Bandwidth set too high increasing noise unnecessarily
- ✗ Flexible mounting causing resonance and measurement errors
- ✗ Ignoring cross-axis sensitivity in precision applications
📋 Customer Cases
Automotive Safety Systems Supplier
Automotive
Challenge
The customer was experiencing false airbag deployments in their testing, caused by accelerometer saturation during severe crash tests.
Solution
We recommended upgrading to the MSA-100G with ±100g range and provided guidance on algorithm tuning for the higher-range sensor.
Results
False deployment incidents were eliminated. The system successfully passed all crash test requirements and achieved production qualification with zero safety-related field issues.
Frequently Asked Questions
1. How do I calculate tilt angle from accelerometer data?
Tilt angle can be calculated from accelerometer data using trigonometry. For a single-axis measurement: Angle = arcsin(Acceleration / 1g). For dual-axis (more accurate): Angle = atan2(Ax, Ay) where Ax and Ay are accelerations on perpendicular axes. Important considerations: 1) This only works when stationary or moving at constant velocity (no external acceleration); 2) Use atan2 for full 360-degree measurement; 3) Calibrate for zero-g offset; 4) Consider cross-axis sensitivity. For dynamic applications with motion, combine accelerometer with gyroscope data using sensor fusion algorithms (Kalman filter or complementary filter).
2. What is cross-axis sensitivity and why does it matter?
Cross-axis sensitivity is the accelerometer's response to acceleration perpendicular to its sensitive axis. Ideally 0%, typical values are 1-3%. This means if you have 10g acceleration on the Y-axis, a sensor on the X-axis might show 0.1-0.3g output. Cross-axis sensitivity matters for: 1) Precision tilt sensing where small errors accumulate; 2) High-g applications where cross-axis signals can be significant; 3) Applications requiring precise axis alignment. For most general applications, 2-3% cross-axis sensitivity is acceptable. For precision applications, select sensors with lower cross-axis sensitivity and implement compensation in software if needed.
3. How do I implement shock detection using accelerometers?
Shock detection implementation: 1) Select appropriate range - typically ±50g to ±200g depending on expected shock levels; 2) Set bandwidth high enough to capture shock pulse (typically >1kHz); 3) Implement threshold detection - trigger when acceleration exceeds threshold; 4) Add duration check - confirm shock persists for minimum time to avoid false triggers; 5) Consider direction analysis if shock direction matters. For product damage monitoring, you may want to log shock magnitude and duration. For safety applications like airbags, response time is critical - the detection and decision must complete within milliseconds.
4. Can I use accelerometers for vibration analysis?
Yes, accelerometers are commonly used for vibration analysis and condition monitoring. Key considerations: 1) Bandwidth must cover machine's vibration frequencies (typically 10x running speed in Hz); 2) Noise density determines smallest detectable vibration; 3) Mounting must be rigid to avoid resonance; 4) Sample rate must satisfy Nyquist criterion (2x max frequency). Analysis techniques include: Time domain - RMS vibration level; Frequency domain - FFT to identify specific frequencies; Trending - monitor vibration changes over time. For bearing analysis, look for characteristic frequencies related to bearing geometry. For imbalance, look at rotation frequency. Memsensing accelerometers are suitable for most industrial vibration monitoring applications.
5. What is the difference between DC and AC response accelerometers?
DC-response accelerometers can measure static acceleration (like gravity for tilt sensing) and dynamic acceleration. They have response down to 0Hz. AC-response accelerometers only measure dynamic acceleration above a cutoff frequency (typically 1-10Hz). They cannot measure static tilt. Most MEMS accelerometers are DC-response, making them versatile for both static and dynamic measurements. AC-response accelerometers are less common and used in specialized applications where DC offset rejection is important. For general motion sensing and tilt applications, DC-response is required. For pure vibration analysis where static position doesn't matter, either type can work.