Navigation is estimation the position, orientation and velocity of a vehicle.If inertial sensors are utilized for the navigation,it is called inertial navigation systems.The inertial sensors which are accelerometers and gyros are based on inertial principles , the acceleration and angular velocity are measured with them and they are always relative to inertial space.
The aim and the areas of usage of INS
INS have been developed for a wide range of vehicles.INS estimates the position of vehicles when the information from GPS are not available.INS estimates the position,velocity until GPS is acquired. The working principle of INS is;
*Measurements are evaluated via INS Algorithm
*The information about position and velocity are get
* Position can be found with measurement of acceleration and orientation can be found with
angular velocity.
*Developing an error model of the system
*Reducing error via using kalman filter
INS can be used in techonogies for defence industry, unmanned vehicles, space vehicles and the all situaitions , which include position estimation are impossible to done by people , can be done by INS.(Such as high pressure or temperature situations.)
Types of INS
Gimballed INS
An accelerometer as an instrument that measures acceleration along a single axis. Taking three accelerometers, with their sensing axes orthogonal. Arrange them so that their axes are aligned north- south, east-west, and vertical. To maintain this orientation when the vehicle manouvres, the accelerometers are suspended in a set of three gimbals that are gyro-stabilized to maintain the direction. The gimbals have a bearing at each end. Each has a motor, built around one of the bearings and at the other end a synchro (an electromagnetic angle-measuring device). The synchro on the innermost gimbal thus measures azimuth (or heading), the synchro on the middle gimbal measures pitch, and that on the outer gimbal measures roll. The innermost gimbal can be thought of as a 'stable platform' on which are mounted the gyros and accelerometers. This is called a gimballed platform.
Figure2: Gimballed System
Strapdown INS
Gimballed INS can be very reliable, accurate, and good value for money. However, the gimbal
arrangement is mechanically very complex. It contains delicate sliprings; the motors dissipate
power, thus the instruments see a varying thermal environment as the gimbals move about ;mechanical resonances are unavoidable. The accelerometers and gyros are mounted in body coordinates and are not mechanically moved. Instead, a software solution is used to keep track of the orientation of the IMU (and vehicle) and rotate the measurements from the body frame to the navigational frame. This method overcomes the problems encountered with the gimballed system, and most importantly reduces the size, cost, power consumption, and complexity of the system.
Figure3: Complete strapdown system (it is not our system)
Inertial Measurement Unit
Several inertial sensors are often assembled to form an Inertial Measurement Unit (IMU).Typically the unit has 3 accelerometers and 3 gyros (x, y and z). In a strapdown IMU, all inertial sensors are attached to the unit (no mechanical movement). In a gimballed IMU, the gyros and accelerometers are isolated from vehicle angular movements by means of gimbals.
(We will write about frames and transformations)
