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).
The mechanical gyroscope 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. Interestingly, in those days, and particularly in the V2 days, the computer was entirely analogue. All calculations were performed by analogue components with zero software programming. For the V2 Rocket, to ensure that the vehicle remains stable, a standard PID controller was implemented (Proportional, Integral and Derivative). The angular rate from the gyroscope (in the form of a voltage level) was fed directly into a simple electric circuit where a combination of capacitors, inductors and resistors performed the calculation before sending a separate voltage signal to the rocket thrust vectoring control gimbals. In those days, in order to tune the rocket, an engineer would have to swap out electrical components in order to change the individual PID values.
The MEMS gyroscope 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 Coriolis 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. An accelerometer can then be used to measure this acceleration and output a small voltage. By the Coriolis effect, this voltage is directly proportional to the torque input on the membrane and thereby the rotational rate. An ADC component (Analogue to Digital converter) built on the chip can then convert this voltage into digital data and provide the angular rate (typically measured in radians per second). From here, the pre-programmed firmware takes over and often uses an Extended Kalman Filter to combine data from several gyroscopes and accelerometers to obtain a fairly accurate estimate of the vehicle attitude (roll, pitch, yaw) and their respective rotational rates. However, because the rotational rate has to ultimately be integrated in order to calculate the exact angle, a small error can build up over time. This causes an issue known as “attitude drift” – indeed this is a typical problem for MEMS based sensors as over time, hours leading into days, the attitude data may drift so much and no longer be useful. Most times, aircraft manufacturers must compensate this drift through the use of additional sensors, such as downward facing cameras which tracks the ground to determine if the vehicle has truly rotated or is suffering from an artifact of the integration error.
The Laser-Ring Gyroscope (LRG) 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 interference pattern which can then 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. The reason why these devices are complex and expensive is obvious – they require some pretty fancy scientific equipment to work well including an interferometer, rotating mirror and a laser. However, thanks to their incredible accuracy, even though we still require an integral to determine attitude, the LRG is not as susceptible to attitude drift as the MEMS gyroscope.
All in all, 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 no matter how fancy the gyroscope is, modern flight computers must combine a variety of sensors to obtain a more precise solution for angular rate and vehicle attitude, there is just no way around that. In addition, modern VTOL aircraft can take advantage of powerful computers running sophisticated algorithms to further help reduce errors in sensor measurements and ensure a smooth and stable flight profile.
