Inertial Instruments and Inertial Navigation
Gimbals
Gimbals are essentially hinges that allow freedom of rotation about one axis.
Gimbals often have superb bearings and motors to help achieve virtually
frictionless behavior. Sensors in the bearings provide measurements of
gimbal angles. Three gimbals allow freedom of rotation of a vehicle about
three axes while a central platform remains stationary with respect to inertial
space.
Gvros
A
gyro is a spinning mass with relatively large angular momentum. We
know that the rate of change of angular momentum is equal to the applied
moment.
A


H
=M
6Itr
If no torque is applied then the angular momentum vector remains stationary
with respect to inertial space. Gimbals allow a vehicle to rotate iieely about
a gyro so the gyro spin axis can provide a single axis direction that is
stationary with respect to inertial space.
Restraining a gyro about an axis perpendicular to the angular momentum
vector provides a means for measuring angular velocity with respect to
inertial space. This device is called a rate gyro and is a common sensor for
aiding in rate stabilization of vehicles (e.g., the
D
in a
PD
controller).
Inertial Platforms
A
gyro mounted on a platform can be used
as
a sensor in a feedback loop to
stabilize the platform with respect to inertial space. This is called an
inertially stabilized platform.
AS
we will see, the inertially stabilized
platform is an essential element of inertial navigation.
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View Full DocumentApplying torque to the gyro causes its spin vector (i.e., angular momentum
vector) to move with respect to inertial space. Thus the inertially stabilized
platform can be reoriented with respect to inertial space.
Accelerometers
A second important inertial sensor is the accelerometer. A simplified
diagram of an accelerometer is as follows
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dehic\e
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 Fall '09
 widnall
 Dynamics, Angular Momentum, Rotation, Inertial navigation system, inertial systems

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