Editorial Feature

The Basics of Accelerometers

Jun 20 2014

Image Credits: Igor Podgorny/shutterstock.com

An accelerometer is a sensor used to measure the proper acceleration of an object, by calculating the displacement of mass. Piezoelectric, piezoresistive, and capacitive sensors are the most common sensors found within commercial accelerometers, and they all operate by converting mechanical energy into electrical energy.

Modern accelerometers are micro-electro-mechanical systems with a seismic mass and cantilever beam. They are operated based on the deflection of seismic mass under the effect of external acceleration.

Basic Principle of Accelerometers

The operation principle for most accelerometers is based on Hooke’s law and Newton’s law.

According to Hooke’s law of extension and compression, a force (F) exerted on a spring, with a spring constant (k), that then extends over a distance (Dx), is represented by the equation F = kDx.

Newton’s second law states that a force (F) acting on a mass (m), produces an acceleration (a) and that the equivalence between them presented with the equation F = ma.

Acceleration Measurement

Consider a mass that freely moves on a base, attached using a spring. The spring initially remains unstretched with no force acting on the mass. If the entire setup is then accelerated in one direction, the spring is stretched; thereby, providing the required force of acceleration. This is shown in the following equation:

ma = kDx

The above equation used for acceleration measurement can be reduced as follows:

a = kDx/m

Using this equation, the linear displacement of the spring can be measured. The equation can still be used to measure the compression of the spring, even when the acceleration is provided in the opposite direction.

The mass is sometimes referred to as the ‘seismic mass’ or ‘test mass,’ as it converts the force of acceleration into linear displacement of the spring.

Design variations in accelerometers are often based on the linear displacement of the spring measurement method.

In recent years, thermal or convective accelerometers have been introduced. In these designs, the test mass and spring system found in most MEMS devices are replaced with a quantity of fluid inside a dome and is temperature-controlled via a heater. The thermal bubble is displaced by movement and temperature sensors in the device, allowing its location to be derived. Therefore, the mechanical acceleration corresponding to this displacement can be calculated.