Accelerometers

An accelerometer is a sensor that measures acceleration forces, typically in three axes (X, Y, and Z). It has a various use cases including:

• Wearable Devices - Fitness trackers use this to track steps.

  1. X-axis (side-to-side movement), Y-axis (forward-backward movement), Z-axis (up-down movement).

  2. Each step produces a peak in the acceleration data. Compute the magnitude of the acceleration vector where A = sqrt(x² + y² + z²).

  3. Identify peaks that exceed a threshold and sampling under every one seconds.

  4. Count these peaks as steps

     (NOTE: Z-axis and Y-axis are mainly used for step detection.)

• Airbag Deployment - Detects sudden deceleration and triggers airbags during crashes.

  1. This is a safety-critical application and it requires a high-sensitivity accelerometer and sampling more frequently to increase the response time. Also, system needs redundant sensors for accuracy.

  2. A crash event can be detected by exceeding the threshold magnitude of the acceleration vector. Compute the magnitude of the acerbation vector where A = sqrt(x² + y² + z²).

  3. (NOTE: combine this with gyroscope can increase accuracy. Apply high pass filter to remove noise.)

• Vibration Monitoring - Detects vibrations in building and bridges to assess structural integrity.

  1. Compute the magnitude of the acerbation vector where A = sqrt(x² + y² + z²). Apply Fast Fourier Transform to analyze frequency components.

  2. Compare with standard limits such as ISO 2631 for human comfort.

  3. (NOTE: earthquake detection logic is similar to airbag deployment)

• Drones - Helps stabilize a flight.

  1. Compute the tile angle. X-axis (increasing with tilting forward), Y-axis (increasing with tilting sideways), Z-axis (gravity).

  2. Tilt angle roll = arctan(y / sqrt(x² + z²)).

  3. Tilt angle pitch = arctan(x / sqrt(y² + z²)).

  4. Compare with the desired level position and adjust motor speed to stabilize a flight.

  5. (NOTE: Practically, sensor fusion with gyroscope is a must).

• Motion Sensing Controllers - Nintendo Wii.

  1. There are multiple sensors in Wii controller. Accelerometer detects tilt, swings and sudden movements.

  2. Application using basic swings - Tennis, Bowling, Boxing.

  3. Application using tilt - Mario Kart.

The accelerometer is a common sensor that is embedded many different application in our life. Five examples are introduced above. To improve raw data, extra filter and algorithm are required. In this article, without involving too much physics and math, many readers can get familiar with how the accelerometer applies to the world.