An inertial measurement unit (IMU) is an electronic device that uses accelerometers and gyroscopes to measure acceleration and rotation and can be used to provide position data.
IMUs are an important component of unmanned aerial systems (UAVs, UAS, and drones) and common applications include control and stabilization, guidance and correction, measurement and testing, and mobile mapping.
Raw measurements output from an IMU (angular rate, linear acceleration, and magnetic field strength) or AHRS (roll, pitch, and yaw) can be fed into devices such as an inertial navigation system (INS) to calculate relative position, direction, and speed to help UAV navigation and control.
There are many types of IMUs, some of which incorporate magnetometers to measure magnetic field strength, but the four main technology categories for drone applications are: silicon MEMS (microelectromechanical systems), quartz MEMS, FOG (fiber optic gyroscopes), and RLG (Ring Laser Gyroscope).
Silicon MEMS IMUs are based on tiny sensors that measure the deflection of a mass due to motion, or the force required to hold the mass in place. They typically have higher noise, vibration sensitivity, and instability parameters than FOG IMUs, but as technology continues to advance, MEMS-based IMUs are becoming more accurate.
MEMS IMUs are well suited for small UAV platforms and high-volume production units because they can often be manufactured at smaller size and weight and at lower cost.
The FOG IMU uses solid-state technology based on a beam of light propagated through a coiled optical fiber. They are less sensitive to shock and vibration and have excellent thermal stability, but are susceptible to magnetic field interference. They also offer high performance in important parameters such as angular random walk, bias offset error and bias instability, making them ideal for mission-critical UAV applications such as extremely precise navigation.
The higher bandwidth also makes the FOG IMU suitable for high-speed platforms and stable. They are larger and more expensive than MEMS-based IMUs and are typically used on large UAV platforms.
The RLG IMU uses a similar technical principle to the FOG IMU, but uses a sealed ring cavity instead of an optical fiber. They are generally considered the most accurate option, but are also the most expensive IMU technology and are often much larger than alternatives.
Quartz MEMS IMUs use a one-piece inertial sensing element micromachined from quartz, driven by an oscillator to vibrate at precise amplitudes. The vibrating quartz can then be used to sense angular rate, producing a signal that can be amplified and converted into a DC signal proportional to the angular rate. These factors make it ideal for inertial systems designed for space- and power-constrained UAV environments.
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