The ships main gyro compass gives the heading of the vessel with respect to true north.
A Gyro compass is a form of gyroscope, used widely on ships employing an electrically powered, fast spinning gyroscope wheel and frictional forces among other factors utilizing the basic physical laws, influences of gravity and the earth’s rotation to find the true north.
The gyro compass is an electromechanical instrument that works on the gyroscopic inertia of a wheel spinning at a very high rate. It is unaffected by magnetic influence.
When in proper running order, the gyrocompass points constantly to true north.
Because it is powered by electricity, it would be useless in a power failure.
The main unit is called the Master Gyro and other units that gets inputs from it are called repeaters.
Master gyro compass sends ship’s heading to repeaters located at different places on the ship for the convenience of navigation. This has been shown in a block diagram below.
Gyroscope is a spinning wheel or rotor in which the axle is free to assume any orientation.
A gyroscope with spinning rotor is shown
Its bearings are mounted in metal frame called a Gimbal, this Gimbal is mounted within another gimbal whose bearing are at 90° to the inner gimbal. There may be three or four gimbals.
The final, outer gimbal is fixed to the vessel such that when the vessel turns to port or starboard, the spinning rotor is allowed, by the gimbals, to continue pointing to a fixed point in space.
To make the spinning rotor point to true north, as opposed to a fixed point in space, we rely upon a property of spinning disks called precession.
Precession causes the spinning rotor to turn to the left or right if we put vertical pressure on the end of the spinning gyro axis.
Basic Properties of Gyroscope
Gyroscopes have two basic properties:
- Rigidity in space
The spin axis of a gyroscope tends to remain in space in the direction in which it is started.
A freely spinning gyroscope will maintain its axis of spin in same direction in space regardless of how its supporting base is turned.
It resists any force attempting to turn its axis of spin in new direction.
This property is termed as “rigidity in space”
Precession causes the spinning rotor to twist the spin axis in anti cloclkwise direction when upward vertical tipping force is applied on the end of the spinning gyro axis, as shown in the figure
The gyro spin axis will precess in the other direction (clockwise) if applied tipping force is downwards.
THE GYRO WILL ALWAYS PRECESS AT RIGHT ANGLES TO THE DIRECTION OF THE APPLIED FORCE.
This property is called precession.
Precession causes the gyro to point in different directions depending upon the force applied to either end of the axis of spin.
Because of precession, we can control the direction that the spin axis points.
This enables us to aim the spin axis where we want it to point. Without precession, the rigidity of the gyro would be useless.
By mechanically or electrically applying forces to precess the gyroscope this gyroscope become north seeking so that it can be used as compass.
Principle of North seeking Gyroscope
The Principle of operation of the gyrocompass is based on the utilization of the properties of
- A gyroscope
- Of the earths daily rotation
The single rotor gyrocompass utilizes a free gyroscope whose centre of gravity shifts in its equatorial plane below the suspension point that is, a positional gyroscope.
In the pendulum gyrocompass, the rotor is enclosed in a chamber, with a weight attached to its lower part.
The chamber is mounted in the outer gimbal suspension ring, whose axis of rotation is vertical.
When the axis AB of the rotor is not in the plane of the meridian (when it is deflected to the east or west), it will deviate from the plane of the horizon because of the earths rotation, since the rotor, in accordance with the properties of a free gyroscope, tends to maintain its orientation relative to the stars.
For example, if its end B deviated toward the east, it will rise, as if following the rise of the stars. Together with the axis AB, the chamber and weight will deviate with respect to the plane of the horizon.
As a result, a moment of the force of gravity will be generated with respect to the suspension point, which will cause the precession of the axis AB toward the plane of the meridian.
Oscillation of spin axis
In its motion, the axis AB passes through the plane of the meridian, whereupon it begins to precess in the opposite direction under the influence of the gravity moment.
This axis AB swings (oscillates) east and west of the plane of meridian.
Oscillation damping mechanism
For a gyrocompass to point north, it is necessary that the oscillation be damped out so that the unit can settle on the meridian and not keep passing through it. Damping an oscillating axis AB involves changing its energy state by opposing the velocity of the body.
Two principle methods for damping have been used.
- Applying an antipendulous torque caused by the restricted flow of a viscous fluid responding to the tilt of the gyroscopic element.
- With a power-driven follow up system, known as a phantom ring, which is a type of servomechanism that serves to damp the spin axis AB toward the meridian.
After these oscillations have damped out, the axis AB decomes fixed in the plane of the meridian in North-south direction.
Brief description of gyrocompass
Gyrocompass works on principle of gyroscope. A gyroscope is an equipment where a heavy wheel is made to rotate at high speed.
This makes the axis wheel to seek a direction in space and stay at that direction till disturbed.
The spin axis of a gyroscope tends to remain fixed in space in thr direction in which it is started.
By mechanically or electrically applying forces to precess the gyroscope, this gyroscope become north seeking so that it can be used as a compass.
Gyrocompass is used to ascertain heading of vessel and to find bearings of vessels or other crafts which are approaching or in vicinity of our vessel.
Gyrocompass is also used to take visual bearings of objects which are in close vicinity of vessel to plot position of vessel.
Since gyrocompass is electrical equipment it can be used to give feed of heading to other equipments like RADAR, ECDIS, Course recorder and to equipment as there is need.
Usage and Errors
Gyro compasses are pre-eminently used in most ships in order to detect true north, steer and find position and recors courses.
But due to the ships (i) course (ii) speed and (iii) latitude, there could appear some errors in gyrocompass readings.
It has been found that:
- On northerly courses the gyro compass north is slightly deflected to the west of true meridian.
- On southerly courses it is deflected to the east.
Modern ships use a GPS system or other navigational aids feed data to the gyrocompass for correcting the error.
Construction features of gyrocompass system
Gyrocompass has become one indispensable instrument in almost all merchant ships or naval vessels for its ability to detect the direction of true north and not the magnetic north. It is comprised of the following units:
Discovers and maintains the true north reading with the help of gyroscope
Receive and indicate the true direction transmitted electrically from the master compass.
Makes a continuous record of the maneuvering on a moving strip of paper.
Governs the electrical operation of the system and ascertains the running condition by means of a suitable meter.
Maintains constant supply of the ship to the motor-gnerator
Indicates failure of the ship’ supply
Controls the follow up system
Converts the ship’s DC supply to AC and energizes the compass equipment.