This box was probably manufactured somewhere around the original rollout of the Boeing 747, circa 1970, a date supported by the 1975 retest tag marking on the gyro. While size and weight are always considerations in flight, the Sperry box is no lightweight, with a cubic case design measuring about 25cm on a side and a weight in the neighborhood of 10kg. While case and internal components make liberal use of cast aluminum, the collective effect is still one of a pretty chunky piece of avionics.
Fundamentally, the Vertical Gyro is used in aircraft to measure both bank angle (roll) and attitude (pitch). The instrument name comes from the fact that at the center of the design is a spinning mass whose spin axis is aligned in the vertical direction. Beyond this, I'll take a crack at explaining how the Sperry box works with apologies in advance for the near-certain errors in references and descriptions of exactly what's what and what each piece is doing. I'm confident that there are some highly skilled folks in the audience to set me straight if and when I wander into the weeds.
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The Sperry box is no lightweight, with a cubic case design measuring about 25cm on a side and a weight in the neighborhood of 10kg. (Click to view full image) |
Internal division
Overall the instrument is internally divided into an upper bay that houses the electro-mechanical apparatus of the gyro proper while the lower half encloses all system electronics, the two separated by the floor of the gyro tub. A single DB-25 connector joins the two halves, and a single external Cannon connector provides interface to the aircraft.
As you might expect, two degrees of freedom equates to two axes of motion and a gimbal (pivoted support) is seen for each of the direction changes to be monitored (pitch and roll). Both of these gimbals reside in a meaty frame that in turn is mounted to the outer case with rubber suspension points, presumably to reduce the impact of small-scale box vibrations on gyro output. Given that the instrument is hard-mounted somewhere in the aircraft, the buzz and vibes from all the other parts of the plane carriage must be kept at bay.
Housed in the inner gimbaled structure is the spinning mass—or flywheel—quite literally the heart of the gyro. An armature is used to spin up the flywheel—and keep it spinning—to begin the gyroscopic process. The flywheel device is a surprisingly heavy but truly gorgeous piece of turned metal, chock full of various-sized drill divots to assist in achieving perfect balance after shaping. I've no idea what the spin speed of the flywheel might be, but it's probably way up there (pilots know of the characteristic whine of aircraft gyros) so even a minor out-of-balance condition is intolerable.
In the most fundamental terms, the larger outer gimbal senses roll, while the inner gimbal—which houses the flywheel assembly—senses pitch. As many may have learned from the "spinning bike wheel" experiment, the flywheel wants to keep its orientation and while the aircraft and hard-mount gyro box go through pitch and roll changes, the flywheel wants to stay right where it is.
How the angular displacement between flywheel and roll/pitch gimbals gets sensed is less obvious. There are clearly visible windings in each of the two gimbals (labeled Gimbal Sense Coils) that may be used to detect displacement by induction. More specifically, these coils are probably part of a synchro or resolver apparatus that drives an inner coil with an AC signal, while an outer coils set detects angular offset based on induced voltages in the differently phased set of outer windings. There was no potentiometer visible so the synchro/resolver approach seems most likely as the means of displacement measurement.
With all of the connections to be made from the outside to the inner aspects of the spinning gimbals, electrical feeds are amazing bits in and of themselves. Flywheel armature drive currents, synchro/resolver signals and probably others are passed using delicate wire-brush contacts made to special posts protruding from each gimbal axis. The posts each have delicate contact bands to pass seven connections in a rotating contact to individual wires corresponding to each band on the post for connection to interior wiring harnesses. A picture is worth a thousand words, so see the related image for more clarity.
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Engineering prowess brings the system together. (Click to view full image) |
Analog art
Electronics underneath must do all of the coil driving and angular displacement detection, and the boards mounted inside are all of a purely analog nature. One board has power transistors suggestive of driving the armature to spin up the flywheel, while another (one of two identical boards) looks to be a multichannel amplifier perhaps used for amplitude/phase detection and displacement output from the synchro/resolver coils on each gimbal axis. Although decidedly old school in construction and component density, the boards are nonetheless noble examples of the analog art.
Engineering prowess brings the system together but flight-line maintenance personnel have to keep the planes flying so external markings on the instrument are made to ensure careful and correct installation. Flight axis alignment marked on top and a polarized set of mounting points ensure that pitch and roll do not get reversed and climb/dive/bank-left/bank-right indications remain correctly oriented. The labeling also speaks to the need for caution in installation, with labeling of "Handle Tenderly," "Delicate Instrument" and "Handle With Care" making clear the fragile nature of the box.
While spinning-mass gyros are still probably used in many aircraft, this box was taken out of service, perhaps to be replaced by a ring-laser gyro or other upgraded variant to improve weight and accuracy. Even MEMS-based gyros are starting to make their way into aviation as replacements, theoretically eliminating traditional mechanical or optical complexities of the incumbent technologies altogether.
Still, despite its age, the Sperry instrument struck me as a beautiful piece of mechanical and electronic craftsmanship, right down to the plentiful set of hand-soldered wire connections and carefully tied cable harnesses. Even if I got all the details wrong on how it works, many of you can probably still enjoy the pictures.
- David Carey
President, Portelligent
Keywords: Sperry Systems Flight aircraft gyro gyro Boeing 747
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