Gyro-Compass or Gyroscopic Compass

GYRO-COMPASS or GYROSCOPIC COMPASS is an instrument invented and developed in comparatively recent years. It was the outcome of an increasing need for a reliable compass which would not be subject to the troubles caused by the larger and larger masses of steel in ships. As gyroscopes and gyroscopic reactions became better understood, experiments along these lines resulted, finally, in the successful application of the gyroscope to obtain a compass which would indicate geographic north, and at the same time be immune from the magnetic disturbances inevi table on all steel ships. In 1911 the compass which now has had the widest adoption in shipping successfully completed its trials, first on a merchant ship operating between New York and Nor folk, Va., and then in the powder magazine of a torpedo boat destroyer. It was installed on a U.S. battleship and two "re peater" compasses were electrically connected to it, one used for steering and the other for taking bearings. It was soon found that the gyro-compass, on account of its integrity and its refusal to take up any of the yawing movements of the ship, afforded a base line from which graphic course records could be made and turret guns controlled in azimuth. Eventually the master gyro-compass became a most important factor of the gun control system. After the first successful trials, gyro-compasses were rapidly installed on battleships and submarines throughout the U.S. navy, and were further taken up by the British, Japanese, French, Italian and Russian navies in the order named; in the years following the war, the gyro-compass was also adopted by a majority of the impor tant merchant fleets throughout the world.

In addition to the compass mentioned above, which is manu factured by the Sperry Gyroscope company in the borough of Brooklyn, New York city, a gyro-compass is also manufactured by the Anschutz company in Germany. Still later there has ap peared a compass manufactured by S. G. Brown in Great Britain, and still more recently one made by the Arma company in New York. With each, the operation is based upon the same funda mental principles, the method in which the gyroscopic element (from which the compass obtains its directive force) is suspended being the chief difference. The Anschutz compass employs the flotation method, the Brown the so-called "oil pump" method, and the Arma the flotation method; while with the Sperry compass the weight of the sensitive element is supported by a mechanical method.

The Component Parts.

The compass first mentioned above consists, essentially, of a gyro wheel driven at moderately high speed by electricity, its auxiliary parts and compass card all sup ported in gimbals in a suitable housing or binnacle. The compass is in reality built up around the gyro wheel. When the compass is functioning normally, its spinning axis will lie in line with the true north-south meridian. The gyro wheel and shaft are sup ported in a housing called the rotor-case, which is fitted with the necessary oil reservoirs to supply lubrication to its main bear ings for many months. The weight of the rotor-case is supported in turn in two ball bearings, one on either side, from a surround ing ring called the vertical ring, which lies in a vertical plane at right-angles to the gyro axis. The vertical ring is supported by means of a stranded wire suspension from the head of the corn pass at the centre of the compass card, and is suitably guided. In order to prevent lag due to friction, however, between the corn pass card and the gyroscopic element which is suspended from it, the vertical ring is suspended within an outer or "phantom" ring which is rigidly attached at the upper end to the compass card itself. The phantom element, including the compass card, is kept in line with the sensitive element by a reversible motor (called the azimuth motor) which is operated by means of a suitable control system and applies a small oscillation or "hunt" to the compass card. The weight of the card and phantom ring is supported on thrust bearings by the main frame. Surrounding the phantom ring is a frame called the ballistic, which is supported on ballhbearings carried on either side of the phantom ring. This ballistic frame contains reservoirs which are connected by tubes, so that the liquid is free to flow from one side to the other in a direction parallel to the gyro axis. The ballistic frame is connected to the rotor-case by means of an arm called the link, which fits over a bearing on the bottom of the case. It is through this connection that the force of gravity generated by the ballistic is applied to the sensitive element, causing the gyro axis to seek and main tain the meridian. Although the gyro-compass is absolutely free from magnetic variation and deviation, correction must be made for latitude and speed. The reason for this is that the progress of a ship over the earth's surface creates an effect on the gyrocom pass similar to that resulting from the earth's rotation itself. If this movement of the ship is east or west, the effect upon the compass is nil, but if the ship moves north or south or in a direc tion which is a component of a north or south heading, the axis of the gyro is triflingly displaced from the axis of the earth by an amount depending upon the latitude, the speed of the ship and the direction of the heading. The error resulting from this displace ment is automatically eliminated in the Sperry compass by means of a corrector so contrived as to utilize the very disturbing ele ments themselves to introduce the proper correction, so that all readings of the compass together with all repeaters and other instruments driven therefrom are true to the meridian.

The main frame of the master compass is provided with a lubber-ring concentric with the compass card and rigid except for the small movement applied to it by the corrector mechanism described above. A lubber-line or base-line inscribed on this ring provides the necessary references, so that when the compass has been installed with the lubber-line parallel to the fore and aft line of the ship, the angle between the ship's head and the true geographic meridian may be read in degrees and fractions from the compass card. A transmitter forming a part of the compass transmits correct azimuth to all repeaters and other auxiliaries. The master compass as described above, together with its subordi nate parts, is supported in gimbal rings in a hollow, cylindrical body or binnacle. One form of the binnacle is provided with two sliding doors which can be unlatched from the top and lowered to provide access to the compass. The binnacle has a hinged cover with a large glass window so that the operation of the compass may be viewed with the dust-proof cover closed.

A gyro-compass equipment usually includes a motor generator for converting the ship's supply current to alternating current of the proper characteristics to drive the gyro wheel, a control panel with suitable switches and meters for operating the equipment, and a number of repeater compasses for indicating the compass readings wherever required. To these more or less fundamental units other equipment is frequently added, such as a course re corder for automatically recording the ship's course, and the gyro-pilot which steers the ship automatically from the gyro compass.

Method of Application.

On merchant vessels the master gyro-compass is usually placed below decks in a clean and dry compartment. A steering repeater fitted with a reading glass is located beside the ship's wheel and adjusted to suit the conve nience of the helmsman. Bearing repeaters are mounted on the port and starboard wings of the bridge, and a repeater is often placed aft at the emergency steering station. Repeaters with bulk head mounting may be used in the captain's stateroom and in the wireless room, and if the ship is equipped with a wireless direc tion-finder, a repeater compass is almost always used in con junction with it to obtain accurate wireless bearings. The course recorder is located in the chartroom, where its record is available for inspection at all times by the captain and officers of the ship. While a magnetic compass is still standard equipment on every vessel, it is fast coming to be considered simply as a standby in case anything should go wrong with the gyro. The gyro-com pass is an instrument capable of withstanding considerable rough treatment. It is not affected by the vibration of the ship's engines on account of the resilient support of the gyro. The gyro is like wise carefully balanced in such a way that it is not affected by the rolling and pitching of the ship.

Operating Principles.

The principles upon which the opera tion of the gyro-compass is based depend upon the fundamental characteristics of gyroscopic action, and may readily be observed with the aid of a small gyroscope model. A typical gyroscope may briefly be described as a spinning wheel, universally mounted; i.e., the wheel shaft is so mounted in supporting rings that its axis is free to take up any position in space. On spinning the wheel in either direction it will be found to have assumed a rigidity of direction of its axis and plane of rotation relative to space. It can be carried about on its support in any manner without changing the direction of its axis relative to space. With the wheel still spin ning, the gyro will exhibit another interest ing trait. Apply a pressure against its sup porting ring in a direction calculated to move the wheel about the horizontal axis; instead of moving in the direction of the applied force, the supporting ring will re sist the pressure and move about the verti cal axis—at right angles to the applied force. Similarly, if a pressure be applied about the vertical axis of the gyroscope, it will resist the pressure in this direction and move about the horizontal axis. This phenomenon is known as precession.

The gyrocompass exhibits the same characteristics as the simple model de scribed above, within practical limits, only in the case of the gyro-compass the precessional movement is controlled by the force of gravity in the form of the ballistic previously alluded to. Otherwise it would merely be a gyroscope, with out any north-seeking quality whatsoever. Returning now to the elementary gyroscope model, imagine it placed on the earth's surface at the equator with its spinning axis in an east west direction. Because of the model's rigidity-in-space charac teristic it will maintain its axis in the same plane with regard to space, though to an observer standing beside it on the earth's surface the end toward the east appears to tilt up as the earth revolves from west to east, moving the base of the gyroscope just as if it were moved by hand as described above. If left alone, the gyro axis would apparently make a complete revolu tion every 24 hours and, disregarding the effect of friction in the bearings, it would always have its axis in the original east-west plane.

Recalling the effect of precession on the revolving wheel, the natural force of gravity may now be applied by strapping a semicircular tube of liquid to the frame in which the wheel of the model is suspended (fig. i ). With the model placed in the same position as described above, with its axis in the east-west direc tion, as the earth revolves and the axis tilts in regard to the earth the liquid flows to the low side, which we will designate as S, thus applying a force about the horizontal axis of the gyro (fig. 2). Owing to the law of precession, however, this pressure causes the gyro to turn about the vertical axis, which it proceeds to do, until the opposite end of the axis, N, reaches and crosses the north-south meridian of the earth. As soon as it has crossed the meridian, the other end of the axis S commences to tilt up, the liquid flows to the low side N and reverses the precessional force, causing the gyro to precess back across the meridian. Thus the gyro axis will continue to oscillate across the meridian in definitely, with the end of the axis designated as the north end N pointing toward the North Pole as it crosses the meridian.

In the case of the gyro-compass these oscillations are damped out and the gyro axis is caused to settle permanently on the meridian. The precessional force then, due to gravity, instead of acting only about the horizontal axis of the gyroscope also acts, in a much lesser extent, about the vertical axis. This small force about the vertical axis introduces (by the law of preces sion) a tilting movement about the horizontal which is counter to the natural tilt of the gyro axis as it approaches the meridian, and which in turn slows down or damps the precession of the gyro axis as it approaches the meridian. Thus its oscillations are damped on both sides of the meridian, and its axis will come to rest in a state of equilibrium on the meridian without outside aid, no matter where it happens to point when it is started up. (See