Gyroscope Types

For VTOL aircraft to work properly, they must remain stable during hover. To do so, the onboard computers must precisely calculate the attitude of the vehicle (roll, pitch, and yaw) at any given moment. Moreover, to ensure there are no oscillations, the flight computer must also sense the angular-rate (measured in degrees per second) along the 3-axis of the aircraft. Driving the angular rate of an aircraft to zero helps to greatly reduce the oscillations (much like a dampener in a spring-mass system). To calculate angular rate, a computer implements a gyroscope. Gyroscopes are generally divided into three primary categories: Mechanical, MEMS (Micro-Electro-Mechanical Systems), and Light-based (Laser-Ring and Fiber-Optic).

1- Mechanical Gyroscope: This is the traditional type of gyroscope that was first used in the German V2 Rocket and later by the Saturn V Rocket. It is comprised of a large spinning rotor. Prior to launch, the rotor is spun-up and due to the gyroscopic effect, the rotor's axis of rotation remains pointing in the same direction during flight. As the vehicle pitches and rolls, magnetic sensors pick up the angles relative to the rotor axis and a computer can thereby calculate the attitude of the vehicle directly (roll, pitch, and yaw). To calculate the angular rate, the computer performs a time-derivative calculation of the attitude angles.

2- MEMS Gyroscope: This is the most common gyroscope in use today. It can be found almost everywhere, from drones, to smart phones, to flying taxis. This type of gyroscope is incredibly small and can be integrated directly into a chip soldered to a circuit-board. The MEMS gyroscope is based on the Coreolis effect. It consists of several tiny vibrating membranes. As the vehicle rotates along a specific axis a torque is applied to the vibrating membrane which causes acceleration in a perpendicular direction. A sensor can then be used to measure this acceleration from which an angular rate can be directly derived.

3- Laser-Ring Gyroscope: The is the next-generation technology: complex, expensive but very precise. Today, these gyroscopes may be found on certain high-end aircraft. The Laser-Ring Gyroscope is based on the Sagnac Effect. A laser is used to shine two rays of light onto a set of rotating mirrors. One ray of light travels in one direction, and another in the opposite. As the vehicle rolls or pitches, the rays of light travel slightly different distances. As a result, the interference pattern between the rays of light changes. A special device, known as an interferometer, measures the differences in the interference pattern which can be directly correlated to the angular rate. There is a cheaper version of the light-based gyroscope called a fiber-optic gyroscope. It is also based on the Sagnac Effect but does not provide the same level of precision.

Gyroscope technology has evolved significantly over the past 100 years. It will be quite interesting to see what new gyroscope technologies come out in the future. It is worth to mention that modern flight computers can combine data from multiple gyroscopes simultaneously (using an Extended Kalman Filter for example) to obtain a more precise solution for angular rate. In the end of the day, a VTOL aircraft benefits from high-precision sensors but powerful computers and fancy algorithms also play a critical role in establishing stable flight characteristics.