Aircraft Maintenance

By Jim Sparks

Gyroscopes and accelerometers

Inertia shows its presence in a variety of ways on a daily basis. Being pressed back into your seat as you go down the runway or having your coffee cup spill over as the pilot applies maximum braking. Inertia is almost always the property that contributes to the fact that any time a tool slips out of hand; it will end up in the most inconvenient place possible. Sir Isaac Newton’s first law of motion states that an object set in motion tends to stay in motion unless acted upon by external forces.

Sensors can be used to measure different types of motion and by applying some mathematics accurate position calculations can be made from a specific starting point. The two most conclusive types of motion are acceleration and rotation. Gyroscopic sensors are used for measuring degree of rotation, while rate gyros measure the speed of rotary motion.

Accelerometers on the other hand are devices used to measure acceleration. The "G" force is the main unit for measurement of acceleration and is calculated as 1G = 32 feet per second per second.

Both gyroscopes and accelerometers use an inertial reference frame, which is a means of providing a fixed point from which measurements can be made. An accelerometer in free fall has no detectable input therefore no measurement can be made. The input axis of an inertial sensing device defines what it can measure and inertial navigation uses gyros and accelerometers to maintain an estimate of position. These inertial navigation systems (INS) not only provide a reliable means of position sensing for aircraft but have also found uses in spacecraft, missiles, ships, submarines, and even surface vehicles. An Inertial Navigation System is comprised of some type of inertial measuring unit (IMU) or inertial reference unit (IRU).

This type of navigational system includes a group of sensors including gyroscopes and accelerometers. All sensing devices contained within are secured to a common base which ensures all sensing devices have the same reference orientation. These sensing units supply acquired information to a microprocessor where integration of all available data is initiated and only then an estimated position is formulated based on applied motions and initial position.

"Gimbaled" and "strap down"

Even though there are numerous designs, INS with different capabilities fall into two groups, "gimbaled" and "strap down."