3 Basic schemes

3.1 Gyrostabilized platforms

Some systems place the linear accelerometers on a gimballed gyrostabilized platform. The gimbals are a set of three rings, each with a pair of bearings at right angles. They let the platform twist in any rotational axis. There are two gyroscopes (usually) on the platform.

Why do the gyros hold the platform still? Gyroscopes try to twist at right angles to the angle at which they are twisted (an effect called precessionPrecession (also called gyroscopic precession is the phenomenon by which the axis of a spinning object (e. a part of a gyroscope) "wobbles" when a torque is applied to it. The phenomenon is commonly seen in a spinning toy top, but all rotating objects can). When gyroscopes are mounted at right angles and spin at the same speed, their precessions cancel, and the platform they're on will resist twisting.

This system allowed a vehicle's roll, pitch and yaw angles to be measured directly at the bearings of the gimbals. Relatively simple electronic circuits could add up the linear accelerations, because the directions of the linear accelerometers do not change.

The big disadvantage of this scheme is that it has a lot of precision mechanical parts that are expensive. It also has moving parts that can wear out or jam, and is vulnerable to gimbal lockGimbal lock in gyroscopic devices controlled by Euler mechanics or Euler angles, is caused by the alignment of two of the three gimbals together so that one of the rotation references (pitch/yaw/roll, often yaw) is cancelled. This would require a reset of. The primary guidance systemThe Apollo Primary Guidance, Navigation and Control System (PGNCS (pronounced pings was a self-contained inertial guidance system that allowed Apollo spacecraft to carry out their missions when communications with Earth were interrupted, either as expecte of the Apollo spacecraftSpacecraft Modules The Apollo spacecraft was designed as part of the Apollo Program, by the United States in the early 1960's to land people on the moon before 1970 and return them safely to earth. It was made up of multiple units or modules that worked t used a 3-axis gyrostabilized platform, feeding data to the Apollo Guidance Computer. Maneuvers had to be carefully planned to aviod gimbal lock.

3.2 Strapdown systems

Lightweight digital computers permit the system to eliminate the gimbals, creating "strapdown" systems, so-called because their sensors are simply strapped to the vehicle. This reduces the cost and increases the reliability by eliminating some of the moving parts. Angular accelerometers called "rate gyros" measure how the angular velocity of the vehicle changes. The trigonometry involved is too complex to be accurately performed except by digital electronics. However, digital computers are now so cheap that rate gyro systems are now practical. The Apollo lunar module used a strapdown system in its backup Abort Guidance System (AGS).

4 Types of sensors

4.1 Laser gyros

Laser gyroscopes were supposed to eliminate the bearings in the gyroscopes, and thus the last bastion of precision machining and moving parts.

A laser gyro moves laser light in the two opposite directions around a circular path. As the vehicle twists, the light has a doppler effect. The different frequencies of light are mixed, and the difference frequency (the beat frequency) is extractable as an electromagnetic signal whose frequency is proportional to the speed of rotation ( Sagnac effect ).

In practice, the electromagnetic peaks and valleys of the light lock together. The result is that there's no difference of frequencies, and therefore no measurement.

To unlock the counter-rotating light beams, laser gyros either have independent light paths for the two direction (usually in fiber optic gyros), or the laser gyro is mounted on a sort of audio speaker that rapidly shakes the gyro back and forth to decouple the light waves.

Alas, the shaker is the most accurate, because both light beams use exactly the same path. Thus laser gyros retain moving parts, but they don't move as much.

4.2 Vibrating gyros

Less expensive navigation systems intended for use in automobiles, may use a Vibrating structure gyroscope to detect changes in heading, and the odometer pickup to measure distance covered along the vehicle's track. This type of system is much less accurate than a higher-end INS, but is adequate for the typical automobile application where GPS is the primary navigation system, and dead-reckoning is only needed to fill gaps in GPS coverage when buildings or terrain block the satellite signals.

4.3 Brandy snifter gyros

If a standing wave is induced in a globular brandy snifter, and then the snifter is tilted, the waves continue in the same plane of movement. They don't tilt with the snifter. This trick is used to measure angles. Instead of brandy snifters, the system uses hollow globes machined from piezoelectric matierals such as quartz. The electrodes to start and sense the waves are evaporated directly onto the quartz.

This system almost has no moving parts, and it's very accurate. It's still expensive, though, because precision ground and polished hollow quartz spheres just aren't cheap.

4.4 Quartz rate sensors

This system is usually integrated on a silicon chip. It has two mass-balanced quartz tuning forks, arranged "handle-to-handle" so forces cancel. Electrodes of aluminum evaporated on the forks and the underying chip both drive and sense the motion. The system is both manufacturable and inexpensive. Since quartz is dimensionally stable, the system has a good possibility of accuracy.

As the forks are twisted about the axis of the handle, the vibration of the tines tends to continue in the same plane of motion. This motion has to be resisted by electrostatic forces from the electrodes under the tines. By measuring the difference in capacitance between the two tines of a fork, the system can determine the rate of angular motion.

4.5 Pendular accelerometers

The basic accelerometer is just a mass on a spring with a ruler attached. The ruler may be an exotic electromagnetic sensor, but it still senses distance. When the vehicle accelerates, the mass moves, and ruler measures the movement. The bad thing about this scheme is that it needs calibrated springs, and springs are nearly impossible to make consistent.

A trickier system is to measure the force needed to keep the mass from moving. In this scheme, there's still a ruler, but whenever the mass moves, an electric coil pulls on the mass, cancelling the motion. The stronger the pull, the more acceleration there is. The bad thing about this is that very high accelerations, say from explosions, impacts or gunfire, can exceed the capacity of the electronics to cancel. The sensor then loses track of where the vehicle is.

Both sorts of accelerometers have been manufactured as integrated micromachinery on silicon chips.

4.6 Accelerometer-only systems

Some systems use four pendular accelerometers to measure all the possible movements and rotations. Usually, these are mounted with the weights in the corners of a tetrahedron. Thus, these are called "tetrahedral inertial platforms", or TIPs.

When the vehicle rolls, the masses on opposite sides will be accelerated in opposite directions. When the vehicle has linear acceleration, the masses are accelerated in the same direction. The computer keeps track.

TIPs are cheap, lightweight and small, especially when they use micromachined integrated accelerometers. However as of 2002 they are not very accurate. When they're used, they're used in small missiles.

5 See also

aircraft, spacecraft, attitude control, Kalman filter, Schuler tuning

6 External links

• A history of inertial navigation systems (.pdf format)

Missile guidance

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