Gyroscopes

Definition

A gyroscope is a sensor that measures the rotation rate with respect to an inertial frame.

It will thus measure all the subsequent changes in orientation when it's rotated.

Since the Earth coordinate frame is rotating, a gyroscope lying on the ground will also measure the Earth rotation rate (approximately 15°/h).

Gyroscope output is typically expressed in °/s, or in rad/s.

How do we use gyroscopes

Rotation rates can be integrated at high frequency to measure precisely the sensor orientation. This principle is used at the base of all inertial sensors (Vertical Gyroscopes, AHRS, MRU, INS).

They are also used in navigation (Position and Velocity) determination in Inertial navigation Systems (INS).

Highest performance gyroscopes (typically closed loop FOG based) are so accurate that they can measure the Earth rotation without any bias estimation, and determine a heading without any external heading reference. These sensors are called Gyrocompasses. There are many other techniques that allow to measure heading without doing pure gyro-compassing, and this allows to release the constraints on gyro technology.

Principle

SBG Systems gyroscopes use 2 principles to measure rotation:

Coriolis effect

Coriolis effect is used by MEMS gyroscopes. When a proof mass is vibrating in the X direction, a rotation on the perpendicular axis Z, will generate a force causing a corresponding motion on the perpendicular axis Y.

There are several MEMS architectures, typically based on vibrating beams, or vibrating rings.

The actual gyroscope performance may vary depending on MEMS element size and quality, sensor packaging, mechanical and signal processing.

Sagnac Effect

This effect is used by optical gyroscopes such as Fiber Optic Gyroscopes (FOG).

Two light sources are looped traveling in opposite direction: one clockwise, the other one counterclockwise. When there is no rotation, the light from both sources arrive at the same time, however if there is a rotation one of the path will be longer and the other-one shorter. This will create interference that can be observed to deduct the angular speed.