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A gyroscope senses change in orientation of a device, and when paired with an accelerometer, is an excellent tool for measuring the orientation of an object in 3D space.
In an earlier post, we defined a piece of technology that is helping to shape the future—the IMU sensor. Today, we are going to take a deeper look at one motion sensor in particular, the gyroscope.
A gyroscope senses change in orientation of a device, and when paired with an accelerometer, is an excellent tool for measuring the orientation of an object in 3D space. Gyroscopes determine angular velocity (ω) typically measured in radians/second. The integration of angular velocity provides orientation information (if an initial orientation is provided or a value can be assumed) across three axes: pitch, roll and yaw.
A gyroscope enables tracking of the twists, turns and rolls of an object in motion. Access to more accurate orientation information has wide-ranging practical applications, for instance, helping a land based robot account for obstacles it runs over, translating a person's real world movement into a virtual world, or helping to orient an aircraft in flight.
A traditional, mechanical gyroscope is a simple wheel that’s mounted on 2-3 gimbals (pivoted supports, typically rings, that let the wheel rotate on a single axis.) Gyroscopes were first used in the mid-19th century for early renderings of the earth’s orbit and have since been used in the inertial navigation systems of aircrafts, ships, spacecrafts and satellites. However, despite its ubiquitous use, the traditional mechanical gyroscope is limited to its moving parts.
Let’s explore modern gyroscope types and their applications:
Ring Laser Gyroscopes operate on the Sagnac Effect, which in short says that a split beam of light traveling the same path in opposite directions will undergo phase changes when the whole apparatus experiences angular velocity. In the diagram shown, a laser is split into two along two paths of equal length and are received at a detector. As the RLG rotates clockwise, the beam moving clockwise effectively travels a slightly longer path and slowing down its reception by the detector. The counterclockwise beam is traveling against the rotation, effectively shortening the path, increasing its speed relative to the other laser. The opposite effect occurs under a counterclockwise rotation.
By measuring the phase changes while the device is rotating, angular velocity can be ascertained.
FOGs also use the Sagnac effect, but compounds the effect multiple times by using multiple coils for the light to travel (as opposed to a single ring like with the RLG). Additionally, the rings are not part of the laser. Instead, the light beams are propagated by an external laser. Once again, the light beam propagating in the same direction as the rotation will have a somewhat longer path delay than the beam that’s running against the rotation. This results in a differential phase shift, measured with interferometry.
The differential phase shift is effectively multiplied by each additional coil that the FOG uses. This multiplication allows the FOG to have an increased sensitivity in general over the RLG. However, with a single traversal of light through the ring, a RLG is more accurate as its phase shift is proportional to the rotation itself, and not the derivative, like a FOG would have to use.
MEMS (microelectromechanical systems) use a combination of mechanical oscillation and Coriolis force. The Coriolis force is the inertial force that acts in a direction perpendicular to the rotation axis. Inside the MEMS gyro, imagine a capacitive block that oscillates at a fixed rate in opposite phase with another block. As the device rotates, the blocks’ Coriolis forces move them slightly in opposite directions (both perpendicular to the rotation axis), due to the blocks’ phase differences. This difference in force changes the capacitance of the plate underneath it to measure the overall angular rate of an object. In reality, the blocks are more like meshes, but the same basic principle applies.
MEMS gyroscopes are typically 3-axis, but can be single axis or dual axis. Different levels of quality are also available, depending on application requirements. For instance, consumer grade gyros tend to be cheaper than ones intended for industrial or automotive use, which operate at wider temperature ranges and are designed and tested to have more consistent performance in order to meet stringent safety requirements.
Modern RLG, FOG, and MEMS-based gyroscopes all address limitations of traditional gyroscopes. And with their varied strengths, they are able to support next-gen technologies in all fields.
In the consumer space in particular, MEMS-based gyroscopes are growing in popularity and adding functionality to a lot of consumer devices, due to their ease of integration, size, and cost.