The Complete Handbook of Accelerometers

Chapter 1: Understanding Accelerometers

An accelerometer is a device that measures proper acceleration, which is the acceleration of a body relative to freefall. In practice, this means it measures the force exerted on a proof mass inside the sensor. This is not the same as the coordinate acceleration you might calculate from a change in position over time. For instance, an accelerometer sitting on a table reads roughly 9.8 m/s² upward, not zero, because the table is pushing up against the sensor against gravity. A GPS receiver would show the same table as having zero velocity, but the accelerometer tells a different, and often more useful, story.

This distinction matters in nearly every application. In an inertial navigation system for an aircraft or a submarine, the accelerometer measures the specific force acting on the vehicle. The navigation computer then integrates this signal over time to track velocity and position. In a car, a single-axis accelerometer is the core of the airbag deployment system. When the vehicle decelerates suddenly, the sensor detects the force of the impact and triggers the airbag within milliseconds. In a smartphone, a three-axis accelerometer detects orientation by measuring the constant pull of gravity, allowing the screen to rotate when you turn the phone. The same sensor can also detect shake or freefall, which is how your phone disables the battery contacts before it hits the pavement.

This chapter is about understanding what these devices actually are, how they perform their measurements, and why they are indispensable