Advanced Navigation‘s high-accuracy navigation solutions for unmanned systems are built on fiber optic gyroscope (FOG) technology.
The exact origin of the first gyroscope remains unknown, but has transcended generations to become an essential tool in navigation.
However, observing how a spinning top maintains balance—even on an inclined surface—may have inspired the concept that evolved into the gyroscope.
The Basics of Gyroscope Sensor Technology
Using a flat disc with a pin through its center as a spinning top allowed the balancing effect to be studied more closely. Increasing the disc’s mass, especially around its edges, and spinning it faster extended its ability to remain balanced. Greater spin speed or moment of inertia increases angular momentum.
To study this phenomenon further, researchers mounted the spinning disc within a mechanical frame. This frame allowed manipulation without physically touching the disc.
Bearings or similar mechanisms minimized friction between the disc’s axle and the frame. Rotating the disc in the frame and adjusting the frame’s angle revealed a phenomenon where the spinning disc resisted repositioning. This effect, unknown to early observers, is called gyroscopic precession.
Understanding Gyroscopic Precession
Gyroscopic precession describes how a spinning object reacts to tilting its axis of rotation. The object resists the tilt with a torque perpendicular to both the external force and its axis of rotation, following the right-hand rule.
A practical example is an unbalanced spinning top. Instead of falling over or righting itself, actions that would violate angular momentum conservation, the top moves in a circular pattern. The torque from gyroscopic precession acts 90° to the gravitational force, causing circular motion.
Rather than directly counteracting an applied torque, the spinning object’s resistive force occurs perpendicular to the applied force.
Utilizing the Gyroscope with a Gimbal
Attaching the spinning disc to a gimbal enabled more controlled study of its properties. A gimbal is a frame assembly that allows separate components to move independently. In this setup, the inner gimbal holding the disc axle connects to an outer gimbal at a 90° angle. Pivot points minimize friction, preserving gyroscopic effects.
The outer gimbal is attached to a fixed frame, creating the classic mechanical gyroscope capable of rotating along the X, Y, and Z axes.
A classical mechanical gyroscope design. The spinning disc (A) is mounted to the inner gimbal (B), which is mounted at 90 ° to the outer gimbal (C). The outer gimbal is able to rotate with respect to the mounting frame. This arrangement provides independent rotation of the gimbals, allowing the spinning disc to retain its original axis of rotation regardless of the orientation of the gyroscope
Gyroscopic Motion & Angular Momentum Conservation
Gyroscopic motion reflects a spinning object’s tendency to maintain its rotational alignment. This occurs due to angular momentum, which resists changes to the axis of rotation.
The gimbal setup demonstrates that a spinning disc’s rotational axis remains stable even when the gyroscope tilts or rotates in 3D space. This stability stems from angular momentum conservation. Any orientation changes in the gyroscope adjust the gimbal positions, ensuring the disc retains its original axis of rotation. Faster spinning discs exhibit higher angular momentum, enhancing resistance to reorientation.
The Gyrocompass: a Practical Navigation Tool
The electric motor enabled continuous, high-speed spinning of gyroscopes, making them viable for extended use. Navigators recognized the potential of using gyroscopes aligned with Earth’s axis of rotation to determine true North, bypassing the inaccuracies of magnetic compasses.
In the early 20th century, Hermann Anschütz-Kaempfe developed the gyrocompass, a functional navigation tool for steel-hulled vessels. Unlike magnetic compasses affected by ferrous metals, gyrocompasses provide accurate headings relative to true North. Methods for determining heading include graduated rings that indicate gimbal rotation or torque measurements reflecting directional changes.
Inertial Navigation Systems (INS)
The rise of autonomous systems necessitated highly reliable navigation and control technologies. Inertial navigation systems (INS) emerged as a crucial component, providing roll, pitch, and heading data for vehicles.
INS devices use sensors to detect linear motion and gyroscopes for rotational changes. Magnetometers or fiber-optic gyroscopes can supply heading information, while GNSS often provides absolute position data. When GNSS signals are unavailable, INS uses dead reckoning to estimate position changes based on motion data.
Modern Gyroscope Sensor Technology
Advancements in electronics, computing, photonics, and manufacturing have transformed gyroscope technology over the past century. While the fundamental concept remains unchanged, innovations have improved accuracy, reduced size and weight, and lowered costs. These advancements support applications across commercial, industrial, and defense sectors.
Modern demands for precise data collection have further driven gyroscope development. Navigation systems for space exploration, subsea research, robotics, and unmanned vehicles rely on highly accurate gyroscopes. These systems contribute to reducing costs, increasing efficiency, and minimizing environmental impacts.
MEMS Gyroscopes
Micro-electromechanical systems (MEMS) gyroscopes, developed from integrated circuit technology in the 1960s, combine electrical and mechanical components in a miniaturized chip form. MEMS devices are cost-effective and widely used in inertial navigation systems, from commercial products to tactical applications.
MEMS gyroscopes often use the Coriolis effect to detect rotational motion. In such systems, a proof mass oscillates within a frame, and rotation induces vibrations perpendicular to the motion. These vibrations generate capacitance changes proportional to the sensed rotation.
MEMS gyroscopes measure rotation within the device’s inertial reference frame but cannot provide heading information. Magnetometers or other sensors are required for this purpose.
This graphic depicts the operation of a simple MEMS Coriolis effect gyroscope. The springs (A) hold the proof mass (B) in position within the inner frame (C), creating the drive axis. The inner frame is isolated from the outer frame (D) using springs (E) set at 90 ° to the drive axis, creating the sense axis.
The inner frame has several protruding fingers (i). The fixed electrodes (ii) make up differential capacitors, with a protruding finger from the inner frame between the capacitor electrodes. During rotation, the Coriolis effect causes movement of the proof mass / inner frame against the direction of rotation that results in a change in capacitance that is proportional to the rate of rotation.
Fiber-Optic Gyroscopes
Fiber-optic gyroscopes, introduced in the 1970s, offer high accuracy and resistance to drift, making them ideal for advanced navigation and strategic applications. A FOG consists of optical fiber coils, a laser source, and an optical receiver.
FOGs operate using the Sagnac effect, where light beams traveling in opposite directions through a rotating coil experience phase shifts. This phase shift enables precise rotation measurements. While FOGs are larger and more expensive than MEMS gyroscopes, their ability to determine true North and deliver superior performance makes them indispensable in high-end applications.
In the graphic, the FOG is rotating about the Z-axis. During rotation, the phase shift of the light can be seen as a difference in arrival time of the two light beams at the optical receiver.
Phase shift in the light occurs only during rotation; once the rotation stops, the light will again be in phase. Note that this is for illustrative purposes only and not necessarily a depiction of the actual technology.
Ring Laser Gyroscopes
The ring laser gyroscope (RLG), introduced in the early 1960s, marked a significant advancement in optical gyroscope technology. Leveraging the Sagnac effect and lightwave interferometry, RLGs use controlled, narrow-bandwidth light to measure rotational movement.
The key distinction between RLGs and fiber-optic gyroscopes (FOGs) lies in how light is propagated: RLGs utilize a resonant cavity defined by mirrors, while FOGs employ optical fiber coils.
A typical RLG setup includes two laser beams traveling in opposite directions along a mirrored path, forming the “ring.” The frequency difference between the beams is then measured to determine angular velocity. While FOGs are often considered superior due to their longer light paths and higher resolution, RLGs remain widely used due to their long-standing production history and proven reliability, particularly in commercial aviation.
The basic operation of a ring laser gyroscope. A laser source (A) emits beams of light simultaneously in opposite directions. The light bounces off mirrors (B) that direct it to an optical receiver (C). Any rotation of the device will cause the light beams to reach the optical receiver at different times due to the Sagnac effect, which can be extrapolated into a measurement of rotation
Single-Axis and Hybrid Optical-MEMS Gyroscopes
Certain fiber-optic and ring laser gyroscopes are designed for single-axis rotation measurement, often tailored to applications requiring precise heading (Z-axis) determination, such as in maritime vessels and submarines.
Alternatively, hybrid gyroscopic systems combine optical gyroscopes with micro-electromechanical systems (MEMS) gyros. This approach balances cost, size, and weight by using high-precision optical gyros for heading while deploying smaller, cost-effective MEMS gyros for roll (X-axis) and pitch (Y-axis).
