Many people may be unfamiliar with angular velocity sensors , but if we mention its other name—gyroscope—many people will know it.
The principle of a gyroscope
A gyroscope is a device used to sense and maintain direction, designed based on the theory of conservation of angular momentum. Once a gyroscope starts rotating, it tends to resist changes in direction due to the angular momentum of the wheel.
In simple terms, the direction of the axis of rotation of a rotating object will not change when no external force is applied. If you've ever played with a spinning top, you'll know that when a spinning top encounters an external force, the direction of its axis does not change with the direction of that force. Riding a bicycle also utilizes this principle. The faster the wheels spin, the less likely it is to tip over because the axle exerts a force that keeps it level.
Based on this principle, people use it to maintain direction, and the device they create is called a gyroscope. Then, they use various methods to read the direction indicated by the axis and automatically transmit the data signal to the control system.
The basic components of a gyroscope include
1. Gyroscope rotor (often driven by synchronous motors, hysteresis motors, three-phase AC motors, etc., to make the gyroscope rotor rotate at high speed around its own axis, and its speed is approximately constant).
2. Inner and outer frames (or inner and outer rings, which are the structures that enable the gyroscope to obtain the required angular rotational degrees of freedom from its rotation axis).
3. Accessories (referring to torque motors, signal sensors, etc.).
Two important characteristics of gyroscopes
Gyroscopes have two very important fundamental characteristics: one is fixed-axis property and the other is precession. Both of these characteristics are based on the principle of conservation of angular momentum.
Fixed axis
When the gyroscope rotor rotates at high speed, without any external torque acting on the gyroscope, the gyroscope's rotation axis remains stable in inertial space, pointing in a fixed direction; at the same time, it resists any force that changes the rotor's axis. This physical phenomenon is called the gyroscope's axis-fixed property or stability.
Its stability changes with the following physical quantities:
1. The greater the moment of inertia of the rotor, the better the stability;
2. The greater the rotor angular velocity, the better the stability.
The so-called "moment of inertia" is a physical quantity that describes the magnitude of the inertia of a rigid body during rotation. When the same torque is applied to two different rigid bodies rotating about a fixed axis, the angular velocities they obtain are generally different. The rigid body with a larger moment of inertia obtains a smaller angular velocity, that is, it has a greater inertia to maintain its original rotational state; conversely, the rigid body with a smaller moment of inertia obtains a larger angular velocity, that is, it has a smaller inertia to maintain its original rotational state.
precession
When the rotor rotates at high speed, if an external torque is applied to the outer ring axis, the gyroscope will rotate around the inner ring axis; if an external torque is applied to the inner ring axis, the gyroscope will rotate around the outer ring axis. The direction of its rotational angular velocity is perpendicular to the direction of the external torque. This characteristic is called the precession of the gyroscope.
The magnitude of precession is influenced by three factors:
1. The greater the external force, the greater the precession angular velocity;
2. The greater the moment of inertia of the rotor, the smaller the precession angular velocity;
3. The greater the angular velocity of the rotor, the smaller the precession angular velocity.
The Past and Present of Gyroscopes
The gyroscope was invented in 1850 by French physicist Léon Foucault, who was inspired by his study of the Earth's rotation. It is similar to placing a high-speed spinning top on a gimbal and calculating angular velocity based on the direction of the top. It is very different from the compact chip design of today.
After its invention, the gyroscope was first used in navigation (before airplanes were invented), and later in aviation. Because airplanes cannot determine their direction with the naked eye like they can on the ground, and the inability to see clearly during flight is extremely dangerous, the gyroscope was quickly adopted and became the core of flight instruments.
During World War II, countries were frantically developing new weapons. The Germans developed missiles to bomb Britain, which were the prototypes of today's missiles. How could a missile travel such a long distance from Germany to Britain and land on its target? The Germans developed an inertial guidance system. This system uses gyroscopes to determine direction and angular velocity, accelerometers to measure acceleration, and then mathematical calculations to determine the missile's distance and trajectory. It then controls the missile's attitude to try and land where it wanted. However, at that time, neither computers nor instruments were very accurate, so the German missiles had significant deviations. Intended to bomb London, they ended up bombing everywhere, causing considerable panic in Britain.
However, since then, inertial guidance systems based on gyroscopes have been widely used in aerospace. Today's missiles still contain this system, and with the stimulation of demand, gyroscopes are constantly evolving.
Traditional inertial gyroscopes mainly refer to mechanical gyroscopes. Mechanical gyroscopes have very high requirements for manufacturing processes and are complex in structure, and their accuracy is limited by many factors.
Since the 1970s, the development of modern gyroscopes has entered a completely new stage. The basic concept of the modern fiber optic gyroscope was proposed in 1976, and since the 1980s, it has developed very rapidly. At the same time, laser resonant gyroscopes have also made significant progress. Due to its advantages such as compact structure, high sensitivity, and reliable operation, the fiber optic gyroscope has completely replaced the traditional mechanical gyroscope in many fields, becoming a key component in modern navigation instruments.
Meanwhile, there has been a breakthrough in laser gyroscopes, which measure rotational angular velocity through optical path difference. They have similar advantages to fiber optic gyroscopes, but are more expensive.
The gyroscopes used in our smartphones are MEMS (microelectromechanical systems) gyroscopes. Their accuracy is not as high as the fiber optic and laser gyroscopes mentioned earlier. They need to refer to data from other sensors to function. However, they are small, consume little power, are easy to digitize and make intelligent, and are especially low in cost and easy to mass-produce. They are very suitable for devices that require large-scale production, such as mobile phones, automotive traction control systems, and medical equipment.
Classification of gyroscopes
Based on the degrees of freedom of rotor rotation, gyroscopes are divided into two-degree-of-freedom gyroscopes (also known as three-degree-of-freedom gyroscopes) and single-degree-of-freedom gyroscopes (also known as two-degree-of-freedom gyroscopes). The former is used to measure the attitude angles of an aircraft, while the latter is used to measure the attitude angular velocity; therefore, single-degree-of-freedom gyroscopes are often referred to as...
However, they are usually classified according to the support method used in the gyroscope:
Ball bearing free gyroscope
It is a classic gyroscope. Using ball bearings for support is the earliest and most widely used support method. Ball bearings rely on direct contact, resulting in high frictional torque and relatively low gyroscope accuracy, with a drift rate of a few degrees per hour. However, they are reliable and are still used in applications where high precision is not required.
A free rotor gyroscope (two-degree-of-freedom gyroscope) can measure two attitude angles using angle sensing elements on the inner and outer ring axes.
Liquid float gyroscope
Also known as a float gyroscope. The inner frame (inner ring) and rotor form a sealed spherical or cylindrical float assembly. The rotor rotates at high speed within the float assembly, and a buoyancy fluid fills the space between the float assembly and the shell to generate the required buoyancy and damping. A gyroscope where the buoyancy is equal to the weight of the float assembly is called a fully floating gyroscope; a gyroscope where the buoyancy is less than the weight of the float assembly is called a semi-floating gyroscope.
Because buoyancy is used for support, frictional torque is reduced, resulting in higher accuracy for gyroscopes. However, friction still exists because they cannot be positioned precisely. To compensate for this deficiency, magnetic levitation is usually added to liquid buoyancy. In this case, the weight of the float assembly is supported by the buoyant fluid, while the thrust generated by the magnetic field keeps the float assembly suspended in a central position.
Modern high-precision single-degree-of-freedom liquid-floated gyroscopes are often triple-float gyroscopes that combine liquid floatation, magnetic floatation, and dynamic pressure air floatation. These gyroscopes offer higher accuracy than ball bearing gyroscopes , with a drift rate of 0.01 degrees per hour. However, liquid-floated gyroscopes require high machining precision, strict assembly, and precise temperature control, resulting in higher costs.
electrostatic gyroscope
Also known as an electro-floating gyroscope. It consists of a hollow, spherical metal rotor surrounded by uniformly distributed high-voltage electrodes that create an electrostatic field , supporting the high-speed rotating rotor with electrostatic force. This method is a type of spherical support, allowing the rotor to rotate not only around its axis of rotation but also in any direction perpendicular to that axis; therefore, it belongs to the free-rotor gyroscope category.
An electrostatic field only exhibits attraction; the closer the rotor is to the electrode, the stronger the attraction, which makes the rotor unstable. By using a support circuit to change the force on the rotor, it can be kept in a centered position. Electrostatic gyroscopes use non-contact supports, eliminating friction , resulting in high accuracy and a drift rate as low as 10⁻¹⁰ degrees/hour. However, they cannot withstand significant shocks and vibrations. Their disadvantages include complex structure and manufacturing processes, leading to higher costs.
Flexible gyroscope
A gyroscope whose rotor is mounted on an elastic support device. Among flexible gyroscopes, the dynamically tuned flexible gyroscope is the most widely used. It consists of an inner flexible rod, an outer flexible rod, a balance ring, a rotor, a drive shaft, and a motor.
It relies on the dynamic reaction torque (gyroscopic torque) generated by the balancing ring's torsional motion to balance the elastic torque generated by the flexible rod support, thus making the rotor an unconstrained free rotor. This balance is called tuning.
Flexible gyroscopes are inertial components that developed rapidly in the 1960s. Due to their simple structure, high accuracy (similar to liquid-floated gyroscopes), and low cost, they have been widely used in aircraft and missiles.
laser gyroscope
Its structural principle is completely different from the gyroscopes mentioned above. The laser gyroscope is actually a ring laser. It does not have a high-speed rotating mechanical rotor, but it uses laser technology to measure the angular velocity of an object relative to inertial space, thus having the function of a rate gyroscope.
The structure and operation of a laser gyroscope are as follows: A triangular cavity is made of a material with an extremely low coefficient of thermal expansion. Three mirrors are installed at each vertex of the cavity to form a closed optical path. The cavity is evacuated, filled with helium-neon gas, and electrodes are installed to form a laser generator.
The laser generator produces two laser beams directed in opposite directions. When the ring laser is stationary, the optical path lengths of the two laser beams are equal, resulting in the same frequency. The frequency difference between the two beams is zero, and the interference fringes are zero.
When a ring laser rotates about an axis perpendicular to the plane of the closed optical path, the optical path length of the beam that is in the same direction as the rotation increases, the wavelength increases, and the frequency decreases; the other beam does the opposite, thus creating a frequency difference and forming interference fringes.
The number of interference fringes per unit time is proportional to the rotational angular velocity. Laser gyroscopes have a drift rate as low as 0.1 to 0.01 degrees per hour, high reliability, and are unaffected by linear acceleration, etc. They have been applied in the inertial navigation of aircraft and are a promising new type of gyroscope.
MEMS gyroscope
This refers to silicon microelectromechanical gyroscopes. The vast majority of MEMS gyroscopes rely on alternating Coriolis forces caused by mutually orthogonal vibrations and rotations. MEMS (Micro-Electro-Mechanical Systems) refers to a complete micro-electromechanical system that integrates mechanical elements, micro-sensors, micro-actuators, signal processing and control circuits, interface circuits, communication, and power supply.
MEMS gyroscopes utilize Coriolis' theorem to convert the angular velocity of a rotating object into a DC voltage signal proportional to the angular velocity. Their core components are mass-produced using doping, photolithography, etching, LIGA, and packaging technologies.
Its main features are:
1. Small size and light weight, with side lengths all less than 1mm, and the core of the device weighs only 1.2mg .
2. Low cost.
3. High reliability, with a working life of over 100,000 hours, and can withstand an impact of 1000g.
4. Large measurement range.
Over a century ago, Léon Foucault invented the gyroscope for scientific research. Today, this little device has revolutionized our lives. Gyroscopes not only serve as indicating instruments, but more importantly, they can function as sensitive components in automatic control systems, acting as signal sensors.
As needed, gyroscopes can provide accurate signals such as azimuth, level, position, speed, and acceleration, so that pilots or autopilots can control aircraft, ships, or space shuttles to fly along a certain route. In the guidance of missiles, satellite launch vehicles, or space exploration rockets, these signals are directly used to complete the attitude control and orbit control of the vehicle.
As stabilizers, gyroscopes enable trains to travel on single rails, reduce the swaying of ships in wind and waves, and stabilize cameras mounted on airplanes or satellites relative to the ground, among other things.
As a precision testing instrument, gyroscopes can provide accurate orientation references for ground facilities, mine tunnels, underground railways, oil drilling, and missile launch silos.
Without it, there would be no airplanes, no rockets, no modern life—something its inventor probably never imagined. This tiny gyroscope makes our world a better place.
In the following sections, we will learn more about the application of gyroscopes in people's daily lives.
