Introduction
Sensors are essential in wearable devices, and accelerometers can detect position and orientation. This article analyzes the working principle of three-axis accelerometers to help readers better understand their applications.
1. Working Principle of Accelerometers
An accelerometer measures the acceleration experienced by the sensing element. Without detailing the internal fabrication, the sensor typically contains a suspended sensing element inside a cavity; inertial forces cause voltage changes that are converted to digital values by an internal ADC.
For a three-axis accelerometer, it measures acceleration along the X, Y, and Z axes, as shown in the figure below.
When stationary, the sensor experiences gravity along one axis, so one axis reads 1 g (i.e. 9.8 m/s^2). In practice, calculations use 1 g as the standard acceleration unit, or 1/1000 g (mg). Because the output is from an ADC, the concepts of full-scale range and resolution apply. For example, the LIS3DH supports +/-2g, +/-4g, +/-8g, and +/-16g ranges. For step counting, +/-2g is usually sufficient: after accounting for 1 g of gravity, there is still up to 1 g of dynamic acceleration to detect. Resolution depends on register bit depth. The LIS3DH stores each axis reading across two 8-bit registers (16 bits total). Because acceleration can be positive or negative, the 16-bit value is a signed integer, so the usable range is 15 bits. With a +/-2g range, resolution = 2 g / 2^15 = 2000 mg / 32768 = 0.061 mg.
In the static posture shown above, the Z axis measures +1 g while X and Y measure 0. If the device is rotated (for example when a phone screen flips), one axis will read 1 g and the others 0. In real measurements, the zero values may show small offsets rather than exact zeros.
During motion, the X, Y, and Z readings all change. Walking exhibits characteristic acceleration patterns because a step involves lifting and placing the foot. Vertical acceleration increases when the foot leaves the ground due to ground reaction force, reaches a maximum when the foot is highest, then decreases as the foot moves down until contacting the ground where vertical acceleration reaches a minimum. Forward acceleration is produced by friction between the foot and ground, increasing when both feet contact the ground and decreasing when one foot is airborne.
2. Applications of Accelerometers
1. Motion wakeup while stationary
Using an OR logic mode, set a small motion threshold and monitor X and Y axes (ignore the Z axis while it measures 1 g). If either X or Y exceeds the threshold for a certain duration, treat the device as wakeup.
2. Free-fall detection
In free fall, Z acceleration and gravity cancel out, producing approximately 0 g, and X and Y remain unchanged. Using an AND logic mode, set a small threshold. If all three axes are below the threshold for a short period, register a free-fall event.
3. Orientation and posture recognition
Examples such as detecting a phone flip use the static gravity vector to determine device orientation.
