A complete inertial navigation system (INS) mainly consists of an inertial measurement unit (IMU), navigation computer, display terminal and auxiliary modules. As the core sensing front-end of the entire system, the IMU is composed of two essential inertial devices: gyroscopes and accelerometers. The three-axis gyroscope measures the angular velocity and angular variation of the carrier, while the three-axis accelerometer captures carrier linear acceleration and specific force information. The navigation computer resolves real-time velocity, position and attitude data through sensor data fusion and algorithm solving. High-performance inertial devices are the fundamental prerequisite for building high-precision inertial navigation systems.
1. Gyroscope: Core Attitude Sensing Device
A traditional gyroscope refers to a high-speed rotating rigid body installed inside a gimbal framework, featuring two core physical characteristics: stability and precession. Based on these properties, rate gyroscopes for angular velocity measurement and position gyroscopes for angular deviation monitoring are developed. With the integration of optical technology, MEMS micromachining and other emerging technologies, all devices capable of implementing gyroscopic inertial sensing functions are collectively defined as modern gyroscopes.
1.1 Classification of Gyroscopes
Gyroscopes can be classified in multiple dimensions:
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By degree of freedom: Two-degree-of-freedom gyroscopes and single-degree-of-freedom gyroscopes.
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By support mode: Ball-bearing supported gyroscopes, liquid-floated, air-floated, magnetic-floated gyroscopes, flexure gyroscopes (dynamic tuned gyroscopes), and electrostatic gyroscopes.
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By working principle: Rotor gyroscopes relying on high-speed rigid body rotation, and new optical gyroscopes including hemispherical resonant gyroscopes, MEMS gyroscopes, ring laser gyroscopes and fiber optic gyroscopes.
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By precision grade: Ultra-high precision, medium-high precision and low precision gyroscopes.
Ultra-high precision gyroscopes feature a bias stability ranging from 10⁻⁶ °/h to 5×10⁻⁴ °/h, mainly including liquid-floated gyroscopes and electrostatic gyroscopes. The electrostatic gyroscope currently represents the highest precision level of inertial devices.
Medium-high precision gyroscopes cover a precision range of 5×10⁻⁴ °/h to 10⁻¹ °/h. Optical gyroscopes are the most promising products in this category. Ring laser gyroscopes (RLG) are the first-generation optical gyroscopes, while fiber optic gyroscopes (FOG) belong to the second generation. Benefiting from outstanding advantages in precision, volume and environmental adaptability, fiber optic gyroscopes have become the mainstream equipment favored by military applications worldwide.
Low-precision gyroscopes have a bias stability worse than 10⁻¹ °/h. MEMS gyroscopes are the most representative product. Although their single-point precision is limited, their low cost, miniaturization and low power consumption enable extensive civil and tactical-grade application prospects.
1.2 Mechanical-Electrical Gyroscopes (First and Second Generation)
Traditional mechanical-electrical gyroscopes include the first-generation liquid-floated, air-floated and magnetic-suspended gyroscopes, as well as the second-generation flexure gyroscopes and electrostatic gyroscopes. All mechanical-electrical gyroscopes adopt a rigid rotor high-speed rotation structure, with differences mainly reflected in supporting mechanisms.
The first-generation floating gyroscopes achieve extremely high precision, but suffer from complex structures, difficult processing and ultra-high manufacturing costs, limiting their large-scale popularization. To meet the demand for medium-precision, low-cost and high-reliability inertial navigation for aircraft and tactical weapons, second-generation mechanical gyroscopes were developed and optimized.
1.2.1 Flexure Gyroscope
Emerging in the early 1960s, flexure gyroscopes adopt flexible joint support structures, completely eliminating friction interference from mechanical bearings. With the advantages of small size, light weight, high reliability and low cost, they are widely applied in platform and strapdown inertial navigation systems, covering aircraft, missiles, ships and aerospace vehicles.
1.2.2 Electrostatic Gyroscope
As a typical second-generation high-end gyroscope, the electrostatic gyroscope uses an electrostatic field in an ultra-vacuum cavity to suspend a spherical rotor, realizing friction-free ultra-stable rotation. It represents the top-level precision of current inertial technology and is only mastered and applied by a few countries including China, the United States, Russia and France. Due to its huge volume, high price and complex maintenance, it is only equipped on core strategic equipment such as ballistic missile nuclear submarines and aircraft carriers.
At present, second-generation flexure and electrostatic gyroscopes have basically replaced the first-generation floating gyroscopes in mainstream high-precision application scenarios.
1.3 Optical Gyroscopes (Third-Generation Inertial Devices)
Optical gyroscopes are based on the classic Sagnac effect proposed in 1913. The first practical ring laser gyroscope was successfully developed in 1963. Breaking the rigid rotor rotation mechanism of traditional gyroscopes, optical gyroscopes feature no mechanical friction, high stability and fast response, gradually replacing high-cost mechanical-electrical platform inertial navigation systems and becoming the core devices of modern strapdown inertial navigation systems.
1.3.1 Ring Laser Gyroscope (RLG)
The ring laser gyroscope calculates carrier angular velocity by detecting the optical path difference and phase difference of two counter-propagating laser beams in a closed optical loop. After decades of iterative optimization, RLG technology has become fully mature in miniaturization and engineering.
Since the 1980s, RLG-based strapdown inertial navigation systems have been widely promoted and applied in military fields in the United States, Russia, France and other countries. They are equipped on various tactical and strategic equipment including fighter jets, launch vehicles, cruise missiles, satellites, torpedoes and military combat vehicles. In the 1990s, laser inertial navigation systems were popularized in mainstream military aircraft, aerospace vehicles, warships and deep-space detectors, becoming the mainstream third-generation inertial navigation technology.
1.3.2 Fiber Optic Gyroscope (FOG)
The fiber optic gyroscope is an upgraded second-generation optical gyroscope based on the Sagnac principle. It uses coiled optical fiber to build a long-distance optical loop, greatly improving detection sensitivity and resolution, and effectively solving the laser lock-up problem of ring laser gyroscopes.
Modern high-end fiber optic gyroscopes achieve a bias stability of 0.0002 °/h, and mass-produced medium-precision products reach 0.1 °/h. International mainstream manufacturers including Honeywell and Northrop Grumman have developed instrument-level FOG products with ultra-low noise and ultra-high stability. With core advantages such as long service life, instant start-up, strong shock and vibration resistance, insensitivity to gravity and wide dynamic range, FOGs are gradually replacing electrostatic gyroscopes in high-precision strategic navigation scenarios.
The United States takes the lead in FOG theoretical research, optical device development and engineering application. Japanese and European manufacturers have realized large-scale production of medium and low-precision fiber optic gyroscopes for automotive and tactical navigation, while China, Russia, South Korea and other countries have also achieved breakthroughs in high-end FOG technology.
2. Accelerometer: Core Specific Force Measuring Device
The accelerometer is another core component of the inertial measurement unit. It senses the specific force of the carrier in three-dimensional space, measures linear acceleration and gravitational acceleration, and provides original data for the navigation computer to resolve real-time velocity, displacement and position information.
2.1 Classification of Accelerometers
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Input-output characteristics: Ordinary type, integral type and quadratic integral type accelerometers.
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Working principle: Pendulum accelerometers (liquid-floated, flexure pendulum, integral pendulum) and non-pendulum accelerometers (vibrating beam, electrostatic accelerometers).
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Measurement dimension: Single-axis, dual-axis and three-axis accelerometers.
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Precision grade: High precision (better than 10⁻⁴ m/s²), medium precision (10⁻² ~ 10⁻³ m/s²) and low precision (lower than 0.1 m/s²).
3. MEMS Inertial Devices: Miniaturized Low-Cost Inertial Technology
Modern battlefield and civilian intelligent equipment have put forward urgent demands for miniaturization, low power consumption and low-cost inertial navigation systems. MEMS (Micro-Electro-Mechanical System) technology, a multidisciplinary cutting-edge technology developed in the late 20th century, perfectly meets this development trend.
MEMS inertial sensors are chip-level devices fabricated by semiconductor micro-nano processing technology with silicon or quartz as the core material. Quartz-based inertial devices feature high stability but high processing difficulty and cost; silicon-based devices have simple processing, low cost and easy mass production, and their performance has been greatly improved with the maturity of processing technology.
3.1 Technical Advantages of MEMS IMU
Compared with traditional mechanical and optical inertial devices, MEMS inertial measurement units have outstanding advantages of ultra-small size, ultra-light weight and ultra-low cost. MEMS accelerometers have achieved mature commercialization, with individual product accuracy reaching the strategic navigation level. MEMS gyroscopes are gradually iterated from low-precision civilian grade to medium-precision tactical grade, fully meeting the application requirements of small UAVs, tactical missiles and intelligent unmanned equipment.
3.2 Global R&D and Typical Products
The United States is the global leader in MEMS inertial device research and industrialization. Draper Laboratory, UC Berkeley, Honeywell, Litton and other institutions and enterprises have mastered core mass production technologies. European, Japanese, Korean and Russian enterprises also have mature R&D and manufacturing capabilities in the MEMS field.
Typical high-performance products include the SENSONOR STIM202 high-precision three-axis MEMS gyroscope, with a full-temperature compensated bias stability of 0.5 °/h and a weight of only 55g, representing the top level of industrial-grade MEMS gyroscopes. Litton’s SiAC silicon accelerometer features a wide measuring range and ultra-low bias error, widely used in tactical weapon guidance and small unmanned vehicle navigation control.
4. Development Trend of Inertial Devices
Inertial navigation devices have completed three generations of iteration from mechanical-electrical gyroscopes to optical gyroscopes and MEMS micro-inertial devices. High-end applications continue to pursue ultra-high precision and ultra-high stability, with optical gyroscopes gradually replacing traditional mechanical strategic-grade devices. Medium and low-end tactical and civilian scenarios are rapidly popularizing high-performance, low-cost MEMS inertial devices. The integration, miniaturization, high precision and low power consumption of inertial measurement units have become the core development direction of modern inertial navigation technology.
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