An Inertial Measurement Unit (IMU) is an electronic device that uses accelerometers and gyroscopes to measure acceleration and rotation, which can be used to provide position data.
IMUs are essential components in unmanned aerial systems (UAVs, UAS and drones) – common applications include control and stabilization, guidance and correction, measurement and testing, and mobile mapping.
The raw measurements output by an IMU (angular rates, linear accelerations and magnetic field strengths) or AHRS (roll, pitch and yaw) can be fed into devices such as Inertial Navigation Systems (INS), which calculate relative position, orientation and velocity to aid navigation and control of UAVs.
There are many types of IMU, some of which incorporate magnetometers to measure magnetic field strength, but the four main technological categories for UAV applications are: Silicon MEMS (Micro-Electro-Mechanical Systems), Quartz MEMS, FOG (Fiber Optic Gyro), and RLG (Ring Laser Gyro).
Silicon MEMS IMUs are based around miniaturized sensors that measure either the deflection of a mass due to movement, or the force required to hold a mass in place. They typically perform with higher noise, vibration sensitivity and instability parameters than FOG IMUs, but MEMS-based IMUs are becoming more precise as the technology continues to be developed.
MEMS IMUs are ideal for smaller UAV platforms and high-volume production units, as they can generally be manufactured with much smaller size and weight, and at lower cost.
FOG IMUs use a solid-state technology based on beams of light propagating through a coiled optical fiber. They are less sensitive to shock and vibration, and offer excellent thermal stability, but are susceptible to magnetic interference. They also provide high performance in important parameters such as angle random walk, bias offset error, and bias instability, making them ideal for mission-critical UAV applications such as extremely precise navigation.
Higher bandwidth also makes FOG IMUs suitable for high-speed platform stabilization. Typically larger and more costly than MEMS-based IMUs, they are often used in larger UAV platforms.
RLG IMUs utilise a similar technological principle to FOG IMUs but with a sealed ring cavity in place of an optical fiber. They are generally considered to be the most accurate option, but are also the most expensive of the IMU technologies and typically much larger than the alternative technologies.
Quartz MEMS IMUs use a one-piece inertial sensing element, micro-machined from quartz, that is driven by an oscillator to vibrate at a precise amplitude. 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 rate. These factors make it ideal for inertial systems designed for the space- and power-constrained environments of UAVs.
More information:
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