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Inertial navigation systems use information provided by accelerometers and gyroscopes to provide the position and orientation of an object relative to a predetermined origin point, orientation and speed.
Inertial measurement units (IMUs) normally contain three gyroscopes and three accelerometers, to gauge angular velocity and linear acceleration respectively. The signals from these devices allow for inertial navigation to be used in a wide range of applications, including the navigation of aircraft, missiles, spacecraft, ships, and submarines. Development in micro-machined electromechanical system (MEMS) devices have allowed for the manufacture extremely small and light inertial navigation systems. These systems broadened the range of potential applications to areas like video motion capture.
Accelerometers have an inertial reference frame that establishes a fixed point from which measurements can be made. The input axis of an inertial sensor specifies what it can gauge, and inertial navigation systems use gyros and accelerometers to sustain a determination of position.
All sensor devices in an inertial navigation system are secured to the same base to ensure they all have the identical reference orientation. The sensing units send the data they generate to a microprocessor that integrates of all available data to develop an estimated position based on applied motions and primary position.
Types of Inertial Navigation System
Inertial navigation systems generally fall into two categories: stable platform systems and strap down systems.
Stable platform systems have all sensing devices attached to a platform that is removed from as much outside rotational action as possible with the use of rotating bearings. In an ideal scenario, the bearings are frictionless, and the frames are balanced perfectly, resulting in the platform being totally free of rotational inertia. Because this ideal is not possible, a gyroscope mounted in the frame can sense torque due to friction or instability. This system also includes feedback to compensate for forces not having a relevant effect on the gyroscope. A minimum of three rotational bearing, or gimbals, are necessary to isolate a supported platform and are called roll, pitch, and yaw axes.
Strap down systems are mounted to the object being tracked. In this type system, a computer performs calculations to satisfy all six degrees of motion. Based on the same principles as stable platform systems, strap down systems have less mechanical complexity and as a result, tend to be physically smaller.
Strapdown platforms are however more vulnerable to vibrations than stable platform units. It is essential to take specific safeguards when setting up these devices in locations where vibration may have an impact on the system.
Types of Accelerometer
An accelerometer can be generally classified as a mechanical, solid-state or MEMS device
Mechanical
A mechanical accelerometer is made up of mass hanging by springs. The displacement of the mass is assessed by using a displacement pick-off, which sends a signal that is proportionate to the force (F) on the mass on the input axis. The rate of acceleration acting on the device can then be determined using Newton’s second law (Acceleration = Force/mass).
Solid-State
Solid-state accelerometers can be divided into several sub-groups, including surface acoustic wave (SAW), vibratory and quartz units. Solid-state accelerometers are small, dependable and rugged.
A SAW accelerometer is comprised of a cantilever beam that is resonated at a specific frequency. A mass is mounted on one end of the beam is able to move, and the other end is connected to the case. When an acceleration is used along the input axis, the beam flexes. This will cause the frequency of the surface acoustic wave to shift proportionally to the applied stress. By calculating this change in frequency, acceleration can be established.
MEMS Accelerometers
Micro-machined silicon accelerometers are based on the same principles as mechanical and solid-state sensors. There are two fundamental classes of MEMS accelerometer. One consists of mechanical accelerometers, like those which gauge the movement of an attached mass. The second class includes devices that evaluate the change in frequency of a vibrating part brought on by a difference of tension, as seen in SAW accelerometers.
MEMS units are more compact, lighter, require less power and have faster start-up times than conventional accelerometer units.
Sources and Further Reading
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