MEMS gyroscope sensors and accelerometer are two key inertial sensors in inertial navigation system (INS). They are used to detect changes in the attitude of moving objects. Accelerometers are used to detect changes in the position of moving objects.
Gyroscope Inertial Sensor
Gyroscope is a device for sensing rotation. It can provide a reference coordinate system for the measurement of accelerometer to distinguish the gravity acceleration from the carrier acceleration. Gyroscope also can provide the angular displacement or angular rate of the carrier for the inertial system, fire control system and flight control system.
More than 100 kinds of physical phenomena can be used to sense the rotation of the carrier relative to the inertial space. From the perspective of working mechanism, gyroscopes can be divided into two categories. One is gyroscopes based on classical mechanics, usually called mechanical gyroscopes. The other is gyroscopes based on non-classical mechanics such as vibration gyroscopes, optical gyroscopes and silicon micro-gyroscopes, etc.
Accelerometer Inertial Sensor
MEMS Accelerometers are based on Newton’s law of inertia. An accelerometer is installed on the moving body to measure the specific force of this moving body along a certain direction. That is the difference between the inertial force of the moving body and the gravity. Then the velocity and the distance traveled by the moving body can be calculated through the first and second integration.
There are many methods to measure acceleration, including mechanical, electromagnetic, optical and radioactive. According to the different principle of action and structure, accelerometers used in inertial systems can be divided into two categories. They are mechanical accelerometers and solid-state accelerometers.
Inertial navigation and guidance systems require high precision of gyroscopes and accelerometers. The accelerometer resolution is usually 0.0001g~0.00001g. The gyro random drift rate is 0.01/h or lower. A large measurement range is required, such as the speed range required by military aircraft should reach 10° (0.01°/h ~ 400°/s). Therefore, gyroscopes and accelerometers belong to precision instruments.
Classification of Gyroscope
Classical Mechanics Gyroscope Inertial Sensor
Classical mechanical gyroscopes mainly include rate gyro, liquid floating rate integral gyro, biaxial liquid floating gyro, spherical free-rotor gyro, flexible gyro, dynamically tuned gyro and electrostatic gyro.
1. liquid floating rate integral gyroscope
This is the world’s first inertial level gyroscope developed by the United States in 1955 (that is, the random drift of gyroscope reaches 0.01°/h). It is constructed on the basis of a rate gyro by using a torque motor to make it work. Because it eliminates the friction of the frame through the float assembly, the gyro precision is improved by 1 to 2 orders of magnitude. It is widely used in the navigation and guidance system of aircraft.
2. dynamically tuned gyroscope
The rotor of this gyro is supported by flexible joint. This flexible joint is a frictionless elastic support. It can provide the rotational freedom to the gyro rotor through its own deformation, so that the hinge support mode is no longer needed, and dry friction is completely avoided. As for the elastic recovery torque of the flexible support itself, the dynamic torque of the balance ring is used to overcome it. This compensation method is called dynamic tuning.
3. electrostatic gyroscope
The electrostatic gyroscope uses the electrostatic suction of the electrode on the spherical rotor, and the method of automatically adjusting the electrode voltage, so that the spherical rotor is supported at the center of the electrode. It eliminates the interference torque caused by the mechanical connection of the frame gyro and the flexible gyro. It also avoids the interference torque caused by the liquid floating gyro due to the liquid disturbance, so it is a high-precision gyro. But because of its complex manufacturing process, the cost is high.
Non-Classical Mechanical Gyroscope Inertial Sensor
This kind of gyro mainly includes vibration gyro (including quartz rate gyro, hemispherical resonant gyro, etc.), optical gyro (mainly ring laser gyro, Interferometric fiber optic gyroscope (IFOG), ring resonant fiber optical gyro, ring resonant gyro, etc.) and silicon MEMS gyro.
1. ring laser gyro
Laser gyroscope is a kind of device for angular velocity sensing. It is based on the Sagnac effect in modern physics. Instead of using a mechanical rotor, it uses the resonant frequency difference between the forward and inverse laser beams running along a closed optical path to determine the rotation angle and angular velocity in relative inertial space. Laser gyros are also called solid-state gyros because they have no moving parts that rotate at high speed. Laser gyro is an ideal component for strapdown inertial system because of its incomparable advantages to mechanical gyro. Since the mid-1980s, laser gyro has been dominant in strapdown inertial systems covering the majority of military and civil aircraft.
2. Fiber optic gyroscope / FOG
Fiber optic gyroscope (FOG) is a new type of optical gyroscope based on Sagnac effect. When the gyro rotates relative to the inertial space, the gyro whose output is provided by the phase measurement circuit is called interferometric fiber optic gyroscope (IFOG). It is composed of light emitting diode, beam splitter, fiber and phase detector. FOG does not have the latching problem that laser gyro has. Like the laser gyro, FOG also has no moving parts. It has a very wide dynamic range and low manufacturing cost.
3. MEMS gyroscope
The main body of a MEMS gyroscope is a mechanical component, such as tuning fork, ring or beam which performs high-frequency vibration. To obtain the angular rate, this mechanical component is driven by electrostatic methods. It will get the displacement which is then detected and converted into an electrical signal by capacitance measurements. MEMS gyroscopes are usually made of silicon because of its good mechanical and electrical properties. The most important features of MEMS gyros are their small size (thousands of gyros can be made on a 1mm2 chip) and low cost.
Classification of accelerometers
Mechanical accelerometer Inertial Sensor
Mechanical accelerometers include force-feedback pendulous accelerometers, biaxial force-feedback accelerometers and pendulous integral gyro accelerometers.
Quartz flexible accelerometer is a kind of force feedback pendulum accelerometer. It is a new generation developed on the basis of liquid floating pendulum accelerometer. It consists of flexible rod, pendulum assembly, torque unit and signal unit. For flexible accelerometers, the pendulum component of the sensitive acceleration is not suspended in liquid, but supported by a flexible rod with a thin neck, and it is fabricated by connecting the thin neck and the pendulum together with a whole piece of quartz glass.
Compared with the metal flexible rod, the advantage of using fused quartz flexible rod is that the thermal expansion coefficient is low, and the pendulum assembly changes little with temperature, and thus the scaling factor error is small. Compared with the first generation of liquid floating pendulum accelerometer, quartz flexible accelerometer has the advantages of no support friction torque, lower temperature control, simple structure and lower manufacturing cost.
Solid state accelerometer Inertial Sensor
Solid state accelerometer includes vibration accelerometers, surface acoustic wave accelerometers, electrostatic accelerometers, fiber optic accelerometers, and micromechanical silicon accelerometers.
1. Quartz vibrating beam accelerometer
Quartz vibrating beam accelerometers use the piezoelectric properties of the quartz crystal itself to excite a rigid beam as a resonant element of the meter.
The quartz vibrating beam accelerometer becomes a solid-state, direct-digital output instrument with a crystal oscillator and oscillating electronic circuit, because it replaces the torquer, signaler and zeroing circuit in the force-balanced accelerometer.
It is composed of quartz crystal force-frequency converter (harmonic oscillator), temperature sensor, crystal control oscillation circuit. Because of its high stability of quartz single crystal material, it has excellent scale factor stability, simple structure, small consumption, high resolution, direct digital output and high cost performance.
2. Micromechanical silicon accelerometer
There are many structural forms of micromechanical silicon accelerometers. Most of them are electrostatic balance type. The structure is composed of a pendulum and two plate superposition, that is, the detection mass (pendulum) is made on the silicon wafer, and the frame structure is connected by two silicon flexible beams. In other words, the detection mass, the flexible beam and the frame are all made of the same monocrystal silicon wafer by anisotropic etching. Great progress has been made in the development of force-balanced micromechanical silicon accelerometers.
