What is an IMU and what is it used for?
An Inertial Measure Unit (IMU) is a device able to measure and report attitude (roll, pitch and yaw), velocity, changes in altitude and gravitational forces acting on an aircraft.
IMUs are a main component of the inertial navigation systems used in aircraft, unmanned aerial vehicles (UAVs) and other unmanned systems, as well as missiles and even satellites.
Inertial navigation depends only on input from sensors directly contained within the platform and that have no reference to external, artificial input (e.g. GNSS) and so is not susceptible to tampering or hacking.
In an inertial navigation system, the data collected and reported by an IMU is processed through a computer in order to calculate current position based on velocity and time. Velocity and time calculations can be used to provide dead reckoning.
The IMU intelligently compensates for the disadvantages of some sensors by fusing input from others less affected, thus obtaining an output with reduced noise and less drift.
An IMU is typically composed of:
Accelerometers: measure the gravitational forces in a fixed coordinate system. For example, an accelerometer at rest on the surface of the Earth will measure '-1g', or -9.8 m/s2. When the platform is in motion the forces of inertia will be added. For this reason, accelerometers are often said to provide a 'noisy' signal.
Gyroscopes: measure angular velocity. A mechanical gyroscope includes a spinning wheel or disc. Thanks to conservation of angular momentum any change in the orientation of the axis of the spinning wheel will be registered by the sensor; the change in orientation of the platform may therefore be calculated. Different technologies and physical principles are used in the construction of gyroscopes. These include the most precise Fiber Optic Gyroscopes (FOG) based on the Sagnac effect and also the less precise Micro Electro-Mechanical (MEMS) units which are based on calculation of the Coriolis force by means of tiny vibratory structures. Gyroscopes are essential for calculation of orientation, but may suffer from drift - even when static. FOGS gyros are generally much more accurate than MEMS units.
Magnetometers: measure magnetism. A simple type of magnetometer is a compass, which measures the direction of the Earth's magnetic field in 2D. In recent years, magnetometers have been miniaturized (e.g. MEMS sensors). The Earth's magnetic field is a 3-dimensional vector that, like gravity, can be used to determine long-term orientation.
One of the biggest challenges facing any cutting-edge Attitude & Heading Reference System / Inertial Navigation System (AHRS/INS) is the ability to perform a mission in a degraded environment. In navigation, dead reckoning is the process of calculating one’s current position by using a previously determined one, and advancing it based upon known or estimated speeds (integrated) over elapsed time and course.
UAV Navigation develops and manufactures high-end autopilots and as such is completely committed to products which are capable of precise and reliable navigation. Navigation in degraded environments (e.g. with no GNSS input) is a major objective.
The equipment used to produce the following results is the VECTOR, UAV Navigation's cutting-edge range of autopilots. VECTOR autopilots features a highly advanced, high-end, MEMS-based AHRS/INS. It has been designed for system integration in avionics packages or other attitude sensing applications, and includes:
- Attitude Heading & Reference System (AHRS)
- Inertial Measurement Unit (IMU)
- Inertial Navigation System (INS)
- Air Data System (ADS)
- GNSS (only for data comparison)
VECTOR family has accumulated thousands of hours of flight time and has proven itself in a variety of highly dynamic environments, giving outstanding results.
Attitude and Heading Reference System (AHRS). Consists of sensors (gyroscopes, accelerometers and magnetometers) that provide attitude information for the platform. The difference between an IMU and an AHRS is the post processing system. The IMU reports data to an additional device that computes attitude and heading. These computers usually use Kalman filters to estimate. The AHRS can typically be found within an Electronic Flight Instrument System (EFIS) as used in many manned aircraft cockpits.
