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Dynamically Tuned Gyroscope(DTG) for Inertial Navigation System

Inertial navigation system(INS) provides information on position and attitude of various vehicles such as airplanes, ships and missiles. Inertial measurement unit(IMU) that is heart of INS consists of a set of gyroscopes and accelerometers which measure respectively angular rate and linear acceleration in three dimensional inertial space. Aiding apparatus including global positioning system(GPS) and star tracker can be added to improve the INS performance. Navigation computer and data processing electronics integrate the output from gyroscopes and accelerometers to calculate the angular and linear displacements of the vehicle.

Historically the gyroscope was first devised to find an inertial reference in study of Earth's rotation. Because of its large angular momentum, the spinning wheel of the gyroscope would stay fixed in the inertial space. After this invention, many kinds of electro-mechanical gyroscopes were developed including single degree-of freedom floated rate integrating gyroscope(FRIG) and 2 degree-of-freedom dynamically tuned gyroscope(DTG). Since then innovative gyroscopes has been brought into the real world by virtue of the progress of science and technology in various fields and have broad spectrum of civil and military applications. For instances, optical gyroscopes such as fiber-optic and ring laser gyroscopes employ the phenomenon so called the Sagnac effect and, vibrating and micro-electro-mechanical gyroscopes utilize essentially the Coriolis force.

DTG is one of the most successful electro-mechanical gyroscopes ever made. With 0.01∼100[deg/hr] accuracy, it is smaller and cheaper, and has less structural complexity when compared to its predecessor FRIG. When the wheel is spinning, the universial joint(or suspension) produces negative spring effect in the direction perpendicular to its spin axis. The wheel becomes a free rotor at a particular speed, so called the “tuning frequency”when the magnitude of the negative spring constant equals that of the static one. The spinning wheel maintains its original attitude in the inertial space regardless of case rotation at this spin speed.

Even though DTG has a simple structure, it is a very precise instrument. Most of DTGs can measure 0.1 degree drift for an hour and at the same time, 400[deg/sec] which is more than one revolution for a second. Hence it is not so surprising that to fabricate DTG, advanced technology in material processing, machining and electronics is required. The assembling, tuning and testing DTGs are also formidable tasks.

In early 1990s, Agency for Defense Development(ADD) initiated the development of DTG in cooperation with university and industry. It took nearly 7 years for ADD to complete the design, material processing, fabrication, tuning and testing to have its own model. The acquired gyro, named KDG117, has 0.1[deg/hr] accuracy and can be used in various stabilizers and medium grade INSs. KDG117 is now inserted into a Korean mid-range tactical missile which is at the final stage of development. The flight test was conducted twice and the mission was successfully accomplished. KDG117 shows very competitive performance and environmental endurance to other gyroscopes of its class. Two most distinguished features of KDG117 are very low temperature dependency and negligibly slow turn-on transient drift, which are vital for high-precision rapid strike weapon systems.