Item History: Inertial navigation systems based on the electrically suspended gyroscope (ESG) have proven to be, over the last thirty three years, the most accurate implementation of inertial navigation. The history of the ESG dates back to the definition of the ESG concept by Professor Arnold Nordsieck of the University of Illinois in 1954. Honeywell Corporation began concept feasibility of the ESG in 1956. The initial motivation for the ESG was the promise of increased accuracy, improved reliability, reduced size and power consumption compared to existing mechanical gas bearing gyroscopes that dominated the inertial navigation market during the 1960’s. The exceptional performance of the ESG is due to the elimination of the relatively large random drift rates induced by imperfect bearings of the conventional gyroscopes.
The ESGs do require considerable more software development and processing, than previously associated with legacy gyroscopes, to accommodate compensation for their complex, but highly deterministic, drift and pickoff errors.
The ESG is a “free-rotor”, two-degree-of-freedom gyro where the spinning rotor ball is supported in a vacuum by an electric field. The Honeywell ESG consists of a hollow beryllium rotor, a ceramic envelope containing six hexahedral support electrodes, a miniature vacuum pump, two optical pickoffs, the suspension electronics, spin coils and damping coils. Optical pickoffs observe readout patters on the rotor surface. In effect these patterns make the spin axis visible so that its position relative to the cavity can be determined. In the gimbaled mode, which is the primary application of ESGs, the pickoff outputs are processed to provide gimbal drive signals to null the pickoff output.
After performing studies on examining the applicability of towards strapdown configurations, it was determined that gimballed space stable configurations offered the most potential. Although several aircraft programs have used and one still uses the ESG, its most demanding applications have been associated with attack and strategic submarine applications. From 1961 to 1975, research and development, of an ESG based navigator were focused towards the strategic submarines and tests were conducted at the Naval Strategic Systems Navigation Facility in Broooklyn, N.y. and aboard the USS Compass Island.
By 1968, Autonetics (subsequently Rockwell International), entered the ESG arena. The Autonetics Micro-ESG is a two degree-of-freedom instrument providing a reference plane for navigation. The gyro consists of a rotor, an envelope, mechanical vacuum housing fixtures, and a motor. The rotor is spherical and is of solid beryllium, spinning in a vacuum, and suspended by a servoed electrostatic field. The suspension charges are generated on each of four electrode pairs. These electrodes are spherical octants plated on a beryllia substrate. The envelope consists of the plates and the substrates and is made as two mating hemispheres. Each electrode pair produces a force on the rotor proportional to the displacement of the rotor from the center of the cavity. The rotor and envelope assembly are mounted in a stainless steel vacuum housing with electrical feedthroughs. The housing is surrounded by three orthogonal sets of motor windings used for rotor spin up, spin down, orienting the rotor spin axis with respect to the gyro envelope, and fixing the spin axis with respect to the rotor (polhode damping).
The pickoff mechanism which determines the relative orientation of the spin axis is sensed by observing a radial mass unbalance intentionally placed in the gyro rotor by adding mass to one side. This unbalance causes a displacement of the rotor center of mass from the center of geometry resulting in a sinusoidal variation of rotor-electrode capacitances at rotor frequency as seen by the case-fixed electrodes. The rotor is spun up along its major moment of inertia to a nominal speed of 2590 revolutions per second (rps). This spin frequency is far above the natural frequency of the suspension servo (approximately 800 Hz). As a consequence, the rotor spins very nearly about the center of mass. The Micro-ESG rotor speed is controlled by the suspension servos and is held within a fraction of a rps of the nominal speed.
The same four electrode pairs used for suspension are also used to provide four capacitive-coupled pickoff signals. These four signals are demodulated in a signal conditioner. The outputs, referred to as Mass Unbalance Modulation (MUM), are the modulation envelopes of the pickoff signals. The required spin axis attitude information is contained in the relative amplitudes and phase relationships of these MUM signals. The four pickoff signals may be used to form a two-axis pickoff with the null at the center of the No. 1 plate. This null pickoff is used when the gyro is to be used in a gimbaled system. Alternatively, the four signals may be used to form a wide angle readout that unambiguously determines the three direction cosines elating the spin axis postion to a set of case fixed axes. This latter form of pickoff would be used in a strapdown mechanization, which was not implemented tactically.
It was in the mid 1970s, when requirements for the longer range TRIDENT I missile were formulated along with needs for longer times between inertial navigator resets, that the ESG was introduced into the Fleet. With the Strategic System’s Project Office’s (SSPO) characteristic combination of stretching the envelope of technical performance while minimizing subsystem risk, the ESG was first introduced in 1979 as a “monitor” for the SINS Mk 2. Furthermore, as an additional risk mitigation, the ESG monitor vendor selected by SSPO as Autonetics, the legacy SINS manufacturer. Pictures of an Autonetics ESGN monitor are also shown below.