Page 5 - Electronic Navigation Cyber Book

CHAPTER 1 MARINE GYROCOMPASS

1.1 Introduction
Of all the navigation instruments in use today, the master compass is the
oldest and probably the one that most navigators feel happiest with.
However, even the humble compass has not escaped the advance of
microelectronics. Although modern gyrocompasses are computerized
the principles upon which they work remain unchanged.

1.2 Gyroscopic principles
At the heart of a marine gyrocompass assembly is a modern gyroscope
consisting of a perfectly balanced wheel arranged to spin symmetrically
at high speed about an axis or axle. The wheel, or rotor, spins about its
own axis and, by suspending the mass in a precisely designed gimbals
assembly, the unit is free to move in two planes each at right angles to
the plane of spin. There are therefore three axes in which the gyroscope
is free to move.

• the spin axis
• the horizontal axis
• the vertical axis.

In a free gyroscope, none of the three freedoms is restricted in any way.
Such a gyroscope is almost universally used in the construction of marine
gyrocompass mechanisms. Two other types of the gyroscope, the
constrained and the spring-restrained are now rarely seen.

In order to understand the basic operation of a free gyroscope, reference
must be made to some of the first principles of physics. A free gyroscope
possesses certain inherent properties, one of which is inertia, a
phenomenon that can be directly related to one of the basic laws of
motion documented by Sir Isaac Newton. Newton's first law of motion
states that 'a body will remain in its state of rest or uniform motion in a
straight line unless a force is applied to change that state'. Therefore a
spinning mass will remain in its plane of rotation unless acted upon by an
external force. Consequently, the spinning mass offers opposition to an
external force. This is called 'gyroscopic inertia'. A gyroscope rotor
maintains the direction of its plane of rotation unless an external force of
sufficient amplitude to overcome inertia is applied to alter that direction.
In addition, a rapidly spinning free gyroscope will maintain its position in
free space irrespective of any movement of its supporting gimbals Also
from the laws of physics it is known that the linear momentum of a body

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