Navis.gr - Gyrocompass

GYROCOMPASS

How the GyrocompassSteers Ships 

In 1911 Elmer A.Sperry, an American, patented his now famous gyrocompass.This device harnesses the force of gravity to a gyrowheelso that the axis of the wheel will always seek thenorth-to-south line.

Almost every naval vessel and merchant ship today carriesat least one master gyrocompass, installed in its owngyro room. A transmission system links the mastergyrocompass to "repeaters." These are used onthe ship for such purposes as steering, position finding,and course recording. Some ships have a gyropilot, or"metal Mike," that steers the shipautomatically more accurately than a human helmsman.

The master gyrocompass rotor may weigh 1 1/4 pounds or as much as 55 pounds. An electric motor drivesthe rotor thousands of revolutions per minute. Itsframework, called the "phantom," is drivenindependently. When the ship alters its course thephantom moves with it, but the rotor system goes onpointing northward. Thus the phantom and the rotor systemare no longer in step. This lack of alignment is used tomake the phantom send a signal to its driving motor,which now moves the phantom until it is again in stepwith the rotor system. In thus moving, the phantom maytraverse several degrees of the compass circle or only afraction of a degree. As it moves back into step thephantom sends out electrical impulses to the repeaters somany for each degree it traverses.

The essential part of a gyro compassconsists of a spinning wheel, so mounted that it hasfreedom of movement about three mutually perpendicularaxes. The wheel in its mounting, known as a gyroscope, isso controlled that its axis of spin settles parallel withthe true meridian, under the influences of the Earth'srotation and gravity. It then becomes known as a gyrocompass.

All the practical applications of the gyroscope are basedupon two fundamental characteristics, namely 'GyroscopicInertia' and 'Precession'. The first, 'GyroscopicInertia', or 'rigidity in space' as it is sometimesknown, is the tendency of any rotating body to preserveits plane of rotation. The second, 'Precession', is thatproperty which, when a couple is applied, causes thegyroscope to move, not in the direction of the couple,but in a direction at right angles to the axis of theapplied couple, and also at right angles to the axis ofthe spinning wheel.

With these two properties, and by the utilization of theEarth's two natural properties, rotation and gravity, thegyroscope can be made 'north-seeking', and once it hassettled on the true meridian it will remain thereindefinitely, so long as the ship's electrical supplyremains constant, and no external forces are permitted todisturb it. 

Steaming ErrorThe gyro compass settles with its North-South axis atright angles to the resultant movement of the Earth'srotation and the ship's movement round the curve of theglobe. When the ship's course is East or West there is nosteaming error, but on Northerly courses the compassNorth is deflected slightly to the West of the truemeridian and on Southerly courses it is deflected to theEast of the true Meridian. The amount of error depends onthe ship's course, speed and latitude.
The error can be corrected automatically by a mechanismwhich moves the lubber line by an amount equal to theerror, or it can be found from correction tables or froma portable correction calculator and then applied asnecessary. 

Repeater CompassesThe indications of the master gyro compass can betransmitted to almost any number of repeater compasses.The average merchant vessel equipment includes a steeringcompass having an enlarged scale dial, at least onebearing repeater, a repeater mounted alongside the RadioDirection Finder, and repeater motor incorporated in theRadar PPI to provide true North Stabilisation. A courserecorder which plots a continuous graphic record of theexact course steered may be fitted.