An accelerometer is a sensor that measures the specific force (or the rate of velocity change). The specific force represents all inertial accelerations, including changes of velocity in the local coordinate frame, and reaction to gravity.

Its output is typically expressed in m/s².

How do we use them

Accelerometers can be used in several ways in inertial systems.

In typical AHRS or Vertical Gyroscopes systems, the accelerometers are used to measure the direction of gravity vector, that is used as vertical reference.

In modern Inertial navigation Systems, the accelerometer signal is integrated once to obtain velocity information, and twice to obtain a position information.

Note that accelerometer signal must be used in conjunction with gyroscope information to account for any rotations.

Principle of operation

An accelerometer can be schematized as a proof mass attached to a spring. The displacement is measured to estimate the acceleration.

Accelerometer principle 2

MEMS capacitive technology

Typical MEMS accelerometers employ a capacitive design. A mobile comb is fixed to a fixed comb through a spring.

When no acceleration is present, the mobile comb is centered in such ways that all capacitors have the same value. However, in presence of accelerations in the sensitive axis, the mobile comb moves away which creates a different capacitance on each side of the comb. The difference in capacitance is proportional to the acceleration.

A set of analog components and Analog to Digital Converters are used to convert this differential capacitance into an acceleration in natural units.

Diagram MEMS Accelerometer

Diagram MEMS Accelerometer Acceleration