Full-Featured Pedometer Design Realized with 3-Axis Digital Accelerometer
Neil Zhao · 2010
This article, based on a study of the characteristics of each step a person takes, describes a reference design using the 3-axis ADXL345 accelerometer in a full-featured pedometer that can recognize and count steps, as well as measure distance, speed, and—to an extent—calories burned. The ADXL345’s proprietary (patent pending), on-chip, 32-level first-in, first-out (FIFO) buffer can store data and operate on it for pedometer applications to minimize host processor intervention, thus saving system power—a big concern for portable devices. Its 13-bit resolution (4 mg/LSB) allows pedometers to even measure low-speed walking (where each step represents about 55 mg of acceleration change) with reasonable accuracy. Understanding the Model From the characteristics that can be used to analyze running or walking, we choose acceleration as the relevant parameter. The three components of motion for an individual (and their related axes) are forward (roll), vertical ( yaw), and side (pitch), as shown in Figure 1. The ADXL345 senses acceleration along its three axes: x, y, and z. The pedometer will be in an unknown orientation,