The Evolution of Compasses

Chinese and Mediterranean navigators must have used magnetic compasses to guide their ships in about 1100’s. These compasses were simple pieces of magnetic iron, usually floated on straw or cork in a bowl of water. About the 1300’s, the compass card was marked off into 32 points of direction. During the following years, navigators learned more about deviation or variation of compasses in various parts of the world, and came to use magnetic compasses with greater accuracy.

When iron and steel vessels appeared in the late 1800’s, it became more difficult to make accurate magnetic compass readings on board a ship. The effect of the local magnetic variation of steel has been noted by the navigators. Shifts in cargo, such as fish tubs or iron ore and billets, also greatly affect the heading of a magnetic compass.

After World War II (1939-1945), scientists developed special gyroscopes for compass use in the polar region. The first gyroscope that resembled modern mechanical gyroscopes was made by G. C. Bohnenberger, a German inventor, in 1810. In 1852, Jean Foucault, a French physicist, built a gyroscope to demonstrate that the earth rotates on its axis. Foucault named the instrument from two Greek words-gyros , meaning circle or ring, and skopien, meaning to view- because the gyroscope had enabled him to see how earth revolves on its axis.

With modern technologies and equipment , the gyrocompass was invented in 1908 by Hermann Anschutz-Kampfe, a German engineer. In 1911, an American scientist, inventor, and manufacturer, developed and improved gyrocompasses for ships. For years now, navigators had learned to rely on gyrocompass.

A GYROCOMPASS is essentially a gyroscope which seek and continuously align itself with the meridian and point to true north. Basically, the gyrocompass depends upon the properties of a gyroscope- the gyroscopic inertia (or rigidity in space ) and precession to seek the true north direction using the earth’s rotation and the earth’s gravitation as reference.

Inertia is the property that causes the gyroscope to resist any force that tends to deflect the spin axis from its direction. Precession is the property that when a force is applied to a gyroscope , a torque is created and the spin axis will be deflected from the position to which it has been set. With these properties, the gyrosphere maintains its absolute position in space and earth’s rotation from east to west becomes the variation to this absolute position. Thus, the compass will continuously compensate by pointing north.

The advantages of using a gyrocompass compared to a magnetic compass is that it detects and points to the true north after three to four hours of running. Unlike a magnetic compass, a gyro can reliably acquire bearing data even when there exists a magnetic deviation. In a magnetic compass , there is usually an error in obtaining the bearing so there is always a need to compensate the difference by using a magnetic corrector to each side of the unit. These errors are acquired due to the magnetic variations such as locality or area.

Gyrocompasses need no magnetic corrections because it gives the actual and accurate "true north". Another advantage of using a gyro is that it is capable of driving repeaters located in both starboard and the port side. This will enable the captain of the ship to know anytime the bearing that will not only save time but Also more convenient in steering and docking the ship.

The best feature of the gyrocompass is that it can be interfaced with almost all other navigational equipment such as radar, global positioning system (GPS), autopilot and sonar. Therefore, these navigational equipments will, too, have an accurate and reliable true north bearing.

The evolution of compasses from magnetic compass to gyroscope to gyrocompass, has helped navigators find their way when there were no landmarks to guide them. For 100 years, TOKIMEC has been working hard to make the oceans of the world safer and more comfortable for everyone through a combination of technologies designed to provide greater assurance and reliability.

GYROCOMPASS

A Gyrocompass system is consist of four major assemblies : master compass, static inverter, transmission unit and power adapter.

The static inverter is a bulkhead mounted type panel and provides the correct power and frequency necessary for the operation of the master compass. This unit operates on the 24 volts DC output of the power adapter or 24 VDC ship’s power supply directly.

The master compass is consist of a shock-mounted, binnacle which houses the sensitive element and latitude corrector. The compass card has dark-adapted illumination and it’s brightness is adjustable at the static inverter.

The transmission unit is a bulkhead mounting type panel and consist of a three switching amplifiers and seven repeater switches and fuses. This unit is used to transmit the heading data of the master compass to the repeater compasses and other components that may be in the ship’s system.

Power adapter is used for converting the ship’s power supply into a power necessary for operating the static inverter and transmission unit.

Repeater compass indicates the ship’s heading specified by the master compass by means of a signal from a transmission unit, the gyrocompass equipment makes use of repeater compasses each incorporating a motor which depends on a power supply for each operation.

In order to have the gyro move freely regardless of the movement of a rolling and pitching ship, we must understand the following.

Firstly, it is necessary to understand the construction of a free gyroscope. It is consists of a wheel having the greater part of its mass distributed toward its periphery and rotating at high speed. The wheel is mounted so that it is free to spin and also pivot about two mutually perpendicular planes , such that its support is free to turn in any plane without applying a force to the wheel. The whole system must be perfectly balanced so that its center of gravity lies at the point of intersection of the three axes. Furthermore, all its bearings must be as free from friction as possible.

A gyroscopic instrument possesses two fundamental properties: the inertia and precession . Note that the spin axis of the gyroscope will continue to point to the same direction because of speed.

Inertia is the property of which causes axis will be deflected from the position to which it has been set. It will deflect toward a point 90 degrees away from it. The rate of the precession will be proportional to the applied torque.

To make a gyroscope into a gyrocompass, it is made to seek and maintain a true north indication. First, it must make the spin axis of the gyro seek the plane of the meridian and to make it maintain this position once it has reach it. Therefore, the first step is to make the gyro spin seek the meridian . To do this, a weight is added, which is a ballistic oil.

With the gyro at the equator, it is pointing E-W. As the earth rotates, the spin axis and vertical ring become inclined, the weight (ballistic) acted upon by force of gravity causes a torque about horizontal. That means, as the axle tilted, the oil container will also be tilted. The gravitational force acts to generate torque around the horizontal axis to start precession and will start to face north . However, at the meridian, the angle of the tilt, the torque caused by the weight and the speed of precession are all maximum and causes the spin axis to precess past the meridian.

As the earth continues to rotate, the north end of the spin axis start to dip. The weight will now act at the opposite side. At this point, the cycle repeated and will go on indefinitely. If the spin axis were pointing E-W at the start, precession will take place at 180 degrees E to W and gyro will never come to rest because there is no force tending to restore to the horizontal position.

If the weight is increased, he rate of precession is increased. If the weight is decreased, the rate of the precession is decreased and the ellipse will broaden. The period of oscillation for undamned gyro is 113 minutes, and as the spin axis seek the meridian some means is required to suppress this oscillation so that it will come to rest in north direction.