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 in the figure) 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.
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.
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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 is president of Portelligent, a TechInsights company. The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.
