is an accelerometer?
is it used?
is used mainly on Navigation or Guidance Systems called Inertial Navigation
System (INS), or Inertial Reference System (IRS), or Inertial Reference Unit (IRU) or Air Data Inertial Reference Unit
(ADIRU). These systems use
Accelerometers Gyroscopes and electronic circuitry to provide accurate Guidance,
Navigation, Position calculation, Speed measurement and Attitude determination,
factors absolutely necessary for the auto pilot operation on sea and air
vehicles. On airplanes the Flight Data Recorder use Accelerometers to measure
and record acceleration forces on the three axis of the vehicle, and on cars the
accelerometers are used to deploy the air bags at the right time.
do accelerometers work?
accelerometer usually uses a concentrated mass (seismic mass), which resists
movement because of its inertia. The displacement of the seismic mass relative
to its supporting frame or container is used as a measure of acceleration.
There are many different ways to make an accelerometer. Some accelerometers use the piezoelectric effect - they contain microscopic crystal structures that get stressed by accelerative forces, which cause a voltage to be generated. Another way to do it is by sensing changes in capacitance. If you have two microstructures next to each other, they have a certain capacitance between them. If an accelerative force moves one of the structures, then the capacitance will change. Add some circuitry to convert from capacitance to voltage, and you will get an accelerometer. There are even more methods, including use of the piezoresistive effect, hot air bubbles, and light.
we are going to describe the operation of an accelerometer using the capacitance
effect, which is part of an Inertial Navigation System or Inertial Reference
Unit (IRS) used on passenger airplanes.
at the schematic diagram above, we can see the proof mass in orange colour
placed between the magnets. Around the proof mass there is a torquer coil.
The suspension metal blade of the proof mass in yellow colour, comprise
together with two plates above and below it, two capacitors of the same value
when the proof mass is null.
Reference : Boeing Aircraft Maintenance Manuals