An inertial navigation system comprises two-distinct parts; the first is the IMU (inertial measurement unit)—sometimes called the IRU (inertial reference unit). This is the collective name for the accelerometers and gyros that provide acceleration and angular velocity measurements. The second part is the navigation computer. The navigation computer takes measurements from the IMU and uses them to calculate the relative position, orientation and velocity of the INS.
There are essentially two kinds of navigation computers in use; stabilised platforms and strap-down navigators.
Stabilised platforms use real, spinning mechanical gyroscopes to stabilise a platform that rotates independently to the INS. So, as the inertial navigation system rotates, the stabilised platform inside it does not. In this way, the system learns about its orientation and can make use of the measurements from the accelerometers. The downsides of this type of system are gimbal lock (see gyros for a full explanation), the high cost and complexity.
In contrast, the sensors inside a strap-down navigator do not move independently of the INS. They are, if you like, strapped down. This overcomes many of the problems associated with stabilised platforms and is the main reason why inertial navigation systems are now affordable to a lot more people. Unlike the spinning, mechanical gyros inside a stabilised platform, the gyros used inside a strap-down navigator are typically MEMS (microelectromechanical systems), which don’t appear to have any moving parts. In fact, it’s better to think of them as angular rate sensors, rather than gyros, although that’s what they’re typically called.
In order to capture the measurements needed for navigating in 3D space, the axes of the inertial sensors are laid out in a mutually perpendicular way. In other words, each axis is at 90° to the other two (as shown in the image below).
This image shows the three axes (xyz) that the inertial navigation system uses to measure movement and orientation. The position of the axes within the INS is fixed, so they rotate with it. Each axis measures in both directions—the arrowheads in the image show which direction is read as positive. For example:
- If the INS accelerates in the direction of the green arrow, the y-axis will show a positive acceleration.
- If the INS accelerates in the opposite direction, we would see a negative acceleration along the y-axis.
Angular velocity about each axis is also measured.
- If the INS rotates as indicated by the circular, red arrow, we would see a positive reading on the x-axis gyro.
- If the INS rotates in the opposite direction we would see a negative value on the x-axis gyro.
By taking measurements along (and about) the x-, y- and z-axes, the navigation computer can understand how it is moving and rotating. In the IMU frame image, you can clearly see how the measurement xyz-axes are laid out on one of our products. You can also see a circular arrow, showing how the x-axis gyro measures angular velocity.
It’s worth pointing out that although each arrow points in one direction, the gyros and accelerometers still measure in both directions along or about each axis. The arrows simply indicate which direction the sensors see as positive movement. So if the product accelerates down (in the direction of the blue arrow), the z-axis accelerometer would indicate a positive value; if the product accelerated upwards, the z-axis accelerometer would show a negative value. An important consideration here is the frame of reference.