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Among other industries, inertial navigation systems (INS) are used to help navigate many types of vehicle (land, sea and air). They are different to navigation systems used by the general public, i.e. GPS, as they track your position relative to where you were last, and not through a satellite coordinate system.
The term ‘inertial navigation system’ encompasses a wide range of technologies that use the same principles. In this article, we look at what they are, how they work, and how they are employed in the automotive industry.
What is an Inertial Navigation System?
Inertial navigation systems (INS) are a navigation system that can be used to track your position. They are similar to global positioning systems (GPS) – which gives an absolute position using a longitudinal and latitudinal co-ordinate system. Inertial navigation systems are different. Instead, they track the user's current position relative to their last known position.
Inertial navigation sensors come in many different forms. However, they all use a combination of motion sensors (accelerometers), rotation sensors (gyroscopes) and magnetic sensors (sometimes) to continuously calculate the location, velocity and orientation of a vehicle.
Inertial navigation systems use a process called “dead reckoning”. This is where you take information from a source and turn it into a movement, which is then added to the last known position. This allows the user to see where they are now located.
Many may question why inertial systems are used when GPS can accurately locate your current position. The answer is this – an inertial navigation system works out its current position in relation to where it started and can track the movement from its starting position. Because of this feature, inertial navigation systems are widely used in aerospace, submarine, missile defense and automotive industries.
How Do They Work?
Inertial navigation systems create an estimated value for both the linear acceleration and angular velocity of the vehicle (that the system is attached to). The systems then integrate these values, allowing for the velocity vector and body attitude to be deduced. The position of the vehicle is then calculated by integrating the velocity vector.
Inertial navigation systems are made up of two distinct parts – the inertial measurement unit (IMU) and the navigation computer. The IMU is the collective name for the accelerometers and the gyroscopes that measure linear acceleration and angular velocity; and IMUs commonly contain three orthogonal rate-gyroscopes and three orthogonal accelerometers to determine these values in 3-dimensions.
Inertial navigation systems generally use one of two different navigation computers. This are either stable platform systems, or strapdown navigation systems.
In stable platform systems, the inertial sensors are placed on a platform away from any external rotational motion. Gimbals are used to mount the platform, and this allows freedom across all three axes. The platform is stabilized using mechanical gyroscopes that rotate independently to the inertial navigation system.
This means that the platform does not rotate when the navigation system is rotating. The global frame is also kept aligned by rotating the gimbal frames. This then allows information on the orientation of the vehicle, which can be combined with the data collected from the accelerometers to locate the current position.
On the other hand, strapdown navigation systems do not move independently to the inertial navigation system. Instead, they are “strapped down”, i.e. they are firmly mounted onto the system. This means that the values are measured in the body frame rather than the global frame.
The gyroscopes used in strapdown systems are different to those in the stable platform and are usually based around microelctromechanical systems (MEMS).
The strapped down systems also use a different tracking approach. These systems transform the signals from the three accelerometer into global coordinates (x,y,z) and combine it with the orientation values generated by the MEMS gyroscopes.
For strapdown systems to create a localized position in a 3D space, the axes of the sensors must be placed perpendicular (90°) to each other. By taking measurements in all three directions, the navigation computer can understand how the vehicle is moving and rotating.
Inertial Navigation Systems in the Automotive Industry
Internal navigation systems are used on automobiles for mobile mapping applications. This is a method of collecting geospatial information and dynamically mapping the environment in which the vehicle moves through. Inertial navigation systems are preferable over GPS because they can efficiently map the contours, slopes, surrounding vegetation and even the condition of the road.
The sensors and cameras that collect this data needs to be highly accurate and precise. Inertial navigation systems offer a much better mapping quality than any other technique, because they can locate and position features of interest whilst the vehicle is moving. Additionally, GPS is not suitable for this application as many features can block the satellite signal, and the satellites above Earth do not always provide a clear line of sight. Therefore, GPS would not be a reliable mapping instrument.
Sources:
OXTS: https://www.oxts.com/
http://www.oxts.com/technical-notes/why-is-it-important-to-use-an-inertial-navigation-system-ins-on-a-mobile-mapping-vehicle/
Skybrary: https://www.skybrary.aero/index.php/Inertial_Navigation_System_(INS)
Cambridge University: https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-696.pdf
Universidade de Coimbra: http://home.deec.uc.pt/~jlobo/publications/jlobo_isie1995.pdf
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