Collection of some patents concerning AI erection mechanisms that prove that such systems exist since the first AI's and are essential for AI's to work on the globe.
All patents have in common, that the induced precession rate to erect the gyro must be greater than any drift rate, but smaller than the rate during maneuvers. Typical erection rates are some degrees per minute. Some of the patents sense whether the aircraft flies a maneuver and turn off or switch to another erection rate during the maneuvers to prevent maneuver acceleration induced false erection (maneuver drifts).
The erection mechanism is as important as the gyro in AI‘s. It is the mechanism responsible for the AI to always find level.
A gyro on earth needs support by gimbals and in a gravitational field is subjected to imbalances and friction, so it always precesses. The gyro only takes over during maneuvers, while the erection mechanism is fooled by maneuver accelerations. The gyro itself would be unreliable and therefore useless to show level over more than some minutes. The erection mechanism tells the gyro constantly how to correct the drift and curvature because it knows where the gravity vector points to. But this correction is slow enough that the gyro is not affected too much during maneuvers, in which the erection mechanism gives a false down signal due to maneuver accelerations.
So an AI works by a delicate interplay of the properties of an erection mechanism and a gyro. Both are equally important. One without the other would not work.
By using an erection mechanism the gyro has not to be as precise as in inertial measurement units. If you disable the erection mechanism, the gyro will precess at a faster rate than the earth’s rotation.
Patent of erection mechanism using pendulous vanes in air driven AI's:
Patent US2409659 A, 22. Okt. 1946
Patent of air driven erection mechanism using an arc shaped gravity pendulum in a case that closes some outlet nozzles when out of alignment with the gyro. This produces some unbalance in the forces which erect the gyro. To prevent erection during maneuvers, inertial sensors sense accelerations and turns and activate electromagnets that lock the pendulum in its null position.
Patent US1934774 A, 14. Nov. 1933
Patent describing an erection system, which is responsive to the flow of air for erecting a gyroscope. The erection system includes a plurality of channels having balls disposed therein. The balls are responsive to the gyroscope being tilted from an erect attitude by a first relatively small predetermined angle (0.5°) for rolling in the channels to create an unbalanced flow of air through the erection system, which provides a force to erect the gyro, and are responsive to the gyroscope being tilted by a second relatively large predetermined angle (10°) for rolling in the channels to negate the unbalanced air flow which disables the erection system.
Patent US4527439 A, 9. Juli 1985
Patent using mercury switches to sense direction of gravity and using torque motors to erect the gyro. The patent describes an additional system to reduce the erection rate when a flight maneuver is detected.
US2972893 A, 28. Febr. 1961
Patent using mercury switches to sense direction of gravity and using torque motors to erect the gyro in normal mode. The patent uses an additional mercury switch to detect horizontal acceleration and a spring restrained gyroscope to detect flight maneuvers to switch to pitch/roll erection mode during this maneuvers.
US3077787 A, 19. Febr. 1963
Patent using mercury switches to sense direction of gravity and rotation about vertical axis, using solenoid to displace masses that induce precession to erect the gyro. The rotation sensing mercury switches are used to change the precession rate on maneuvers.
US2842968 A, 15. Juli 1958
Patent of an erection mechanism using free rolling masses in a slowly corotating special cage, deflected by gravity so they produce a torque at the right gimbal to erect the electrical driven gyro in the AI's:
US2603095 A, 15. Juli 1952
US3498146 A, 3. März 1970
Patent of an electrical erection system using a gravity sensing pendulum mechanism and torque motors. Erection signals are cut during maneuvers to prevent gyro errors, by sensing translational movements with accelerometers:
Patent US2969683 A, 31. Jan. 1961
Patent of an electrical erection system using a system of a steel ball, moved by gravity, and an arrangement of 4 coils affected by the position of the ball. The position of the ball changes the inductivity of the coils. This change is converted into an electrical signal that drives two torque motors to erect the gyro.
US2553268 A, 15. Mai 1951
Patent using pneumatic amplification to create a counter torque to erect a gyro. The sensing device consists of a steel ball, that is positioned by gravity, to open and close small air inlets which when closed produce an amplificated air stream which creates a torque.
US3498145 A, 3. März 1970
Note: some of the the linked patents are rather old an scanned documents. The scanners did not work properly on some docs, so they are sometimes difficult to read.