Theory and Operation of the Gyroscope and the Sperry Gyroscopic Compass.
By Ensign R.E. Gillmor, U. S. Navy.
Note.—A short article in the Proceedings Of The Naval Institute of September, 1911, by Lieut.-Commander Dinger, gave a general description of the Sperry Gyroscopic Compass. Inasmuch as this instrument has now become an important part of the navigational equipment of our most modern battleships, it is thought that accurate knowledge of its principles, and instructions for its care and operation, should be placed before the service.
Historical.
In the present day of the steel ship and submarine, on which the magnetic compass cannot be considered an accurate and dependable instrument, the development of the gyroscopic compass to its present practical stage represents as important an advance in navigational science as the step the ancients made hundreds of years ago when they discovered the property of the magnetic needle and applied it to directing their ships instead of depending upon observations of the sun, moon and stars. Especially in submarines, and to a certain extent in battleships, with their movable masses of magnetic material, the use of the magnetic compass is becoming almost as much of an approximation as were the observations of celestial bodies the ancients used before the introduction of the magnetic needle.
As an indication of the rapid strides the present generation is making in science, it is interesting to compare the time taken to develop the magnetic compass to the time in which the gyroscopic compass has been developed. The original discovery of the properties of the magnetic needle is variously ascribed to the Chinese, Arabs, Greeks, Etruscans, Finns and Italians; Chinese historians claim the existence of an authentic record, showing that the properties of the magnetic needle were known in China 2634 years B. C, but there is little definite proof that these properties were applied to nautical use until the beginning of the thirteenth century. Supposing the first intelligent and general application of the magnetic needle to have been about 1300, it required approximately 600 years to develop the compensated magnetic compass. Even the principles of the gyroscope, on which the gyroscopic compass depends, were not deduced until about 1851, so that from that time to the finished and accurate compass a period of only 60 years has elapsed; actually, only about eight of these years have been taken up in the real development of the compass.
Theory.
The French scientist Foucault was the first to analyze the phenomena of the gyroscope, and to deduce any laws covering its action. This he did in 1851, when he used the gyroscope for demonstrating the rotation of the earth. Inasmuch as there was no means at that time of driving the gyroscope continuously at high rates of speed, great practical difficulties were encountered, and it was chiefly due to his powers of deduction that Foucault was enabled to obtain his theories, which were far beyond the points reached experimentally.
Before stating these laws, which were originally deduced by Foucault, such general terms as "degrees of freedom" and "precession" will be defined. "Degrees of freedom" refers to the manner in which the gyro is suspended; "three degrees of freedom" means suspension allowing freedom of motion about all three axes. Fig. 1 shows a typical gyroscope with three degrees of freedom, i.e., freedom about the axis of rotation B, freedom about the axis EE and freedom about the axis GG. If the band J be securely clamped about the wheel I, attached to the ring C, freedom about the axis EE would be suppressed and the gyro would then be said to have a suspension with "two degrees of freedom." The term "gyroscope" is generally taken to mean a rotating mass suspended with "three degrees of freedom" and the term "gyrostat" is generally taken to mean a rotating mass suspended with "two degrees of freedom."
Suppose the clamp J in Fig. 1 is released and the wheel A is rotating rapidly with the side nearest the observer moving from right to left. If a force be impressed on the near side of ring D, tending- to push that side downward about the axis GG, this force will be resisted and movement of the gyro will take place about the axis EE, the top of ring C and axle B moving toward the left. Movement of the gyro is thus seen to take place at right angles to the impressed force. This movement normal to the impressed force is called "precession."
Throughout the study of any gyroscopic apparatus it must be remembered that movement of the gyroscope does not take place in the direction of the impressed force but that this tendency to move is transferred ninety degrees in the direction of rotation before it is manifested as motion.
Nothing but force caused by angular motion is resisted or causes precession; this angular motion may be about an axis within the gyro or remote from it, but must be angular motion. If linear motion be impressed, no gyroscopic reaction will result, even though the force so impressed be of large magnitude. As angular motion approaches linear motion, i.e., as its radius of action is increased, the less will be its gyroscopic effect.
If the precessional ring C, Fig. 1, be clamped to the ring D so that precession is not allowed, then forces impressed on the ring D will not be resisted, even though they be due to angular motion, thus showing that precession must be allowed or the gyroscope will not have the property of resisting forces impressed upon it. It is for this reason that fly wheels and turbine wheels have no real gyroscopic effect, precession being suppressed by the bearings in which the shaft is held.
Precession can be very easily and clearly explained by analyzing the actions of the rapidly rotating particles. A circle may be defined as a polygon of an infinite number of sides, and circular motion may be considered as motion about the sides of a polygon having an infinite number of sides. For purposes of analysis and illustration, we may reduce the number of sides to four, as shown in Fig. 2. We can disregard deviations from the straight path, as these deviations are equal and opposite on the sides of the square. Considering the infinite number of particles in the wheel passing first from A to B, then from B to C, from C to D, and from D back to A, suppose a force is impressed on the end of the axis nearest the observer, tending to push it to the right, that is, tending to force the right-hand side of the wheel downward. In impressing this force there is a tendency to move the paths BC and DA parallel to themselves, and the particles in passing from B to C, and from D to A, have no tendency to be deflected from their original path. However, when these particles move from A to B, they find there is a force being impressed, tending to make them flow downward, and when passing from C to D they find a force tending to make them flow upward. Newton's first law of motion states that all matter resists being deflected from its path when once it has assumed a line of motion; the particles in the wheel resist the tendency to make them flow downward when passing from A to B, and upward when passing from C to D, and in resisting they react upward on the upper side of the wheel, and downward on the lower side of the wheel, causing motion of the wheel about the axis XX; this motion is termed "precession." The motion of precession causes the particles flowing from B to C and from D to A to be deflected from their paths; they resist this deflection, and this resistance is manifested by a stress resisting the impressed force and exactly equal to it. It will be noted that the force impressed has been deflected 90° in the direction of rotation, transferred into motion, and by this motion deflected another 90° in the direction of rotation, where it is manifested in a stress opposing the force originally impressed. No motion will take place about the axis YY', because if this motion should take place it would immediately remove its own cause. It will be seen from the above that resistance to impressed force is entirely due to precession, so that if precession is suppressed the impressed force will not be resisted.
The phenomena of precession can be very well illustrated by using the analogy of a rotating mass of air, set up by two fans, as shown in Fig. 3. If the air passing through the paper tubes be deflected by moving the left-hand side of the tubes downward, as shown in Fig. 4, these particles in passing through the tube will react, causing precession, as shown in Fig. 5. If the force is impressed, as shown in Fig. 6, precession will result, as shown in Fig. 7.
Foucault's first law states that any gyroscope possessing three degrees of freedom, that is, free to move in all three planes and unaffected by the force of gravity (as it will be if the axis of rotation in the three planes coincide with each other and with the center of gravity), must indicate the rotation of the earth since the gyro will continue with its plane of rotation fixed in space, while the earth turns around under it." His second law states that any gyrostat free to move in two planes only, will, at any place on the earth's surface, other than the two poles, tend to set itself with its axis of rotation in the plane of the axis of the earth itself by reason of the relative rotation of the two bodies.
This second law is illustrated in Fig. 8. Suppose a gyro, suspended so that freedom about the horizontal axis is partly suppressed by gravity, this suspension being so devised that the force of gravity will always tend to place the axis in a horizontal position. Consider this gyro rotating at full speed with axis E and W on the earth's surface, as shown at A. Since the gyro will tend to maintain its plane of rotation in space, the earth in rotating will rotate "out from under" the gyro, leaving it suspended out of the vertical with one end of its axis inclined up from the horizontal, as shown at A', which is the position to which A moves in about 3 hours. The manner in which it is suspended will cause the force of gravity to impress forces on this gyroscope, tending to bring its axis to the horizontal. These forces will cause precession as indicated by the arrow at A'. As the gyro precesses the plane of its rotation will become nearer and nearer coincident with that of the earth, and, consequently, the earth will have less and less tendency to rotate "out from under" it. Now if the axle if the gyro could be brought to a horizontal position when on the meridian it would persist in that position since its plane of rotation would be coincident with that of the earth and the earth would have no tendency to turn " out from under" it. However, where freedom of motion is suppressed about the horizontal axis only, as in Fig. 8, there exists no force which will bring the axle of the gyro to the horizontal, once it has attained an inclined position, inasmuch as the force to accomplish this must be impressed about the vertical axis, causing precession about the horizontal axis. The axle of the gyro will, therefore, precess across the meridian with its north end (the dark end in Fig. 8) inclined downward. After crossing the meridian, the effect of the rotation of the earth will be to incline the north end upward and the south end downward. This will, eventually, bring the axle again horizontal on the opposite side of the meridian from which it started, and at the same angular distance from the meridian as it was originally. As the earth continues to rotate the north end of the axle of the gyro will be inclined up from its horizontal position, gravity will again impress a force about the horizontal axis causing precession of the gyro back toward the meridian. It will again cross the meridian, this time with its north end inclined upward. It can be seen from the above that oscillation of the gyro across the meridian will continue indefinitely, the ends of the axle describing ellipses as they oscillate back and forth.
To make the use of the gyro practicable as a compass these oscillations across the meridian must be damped in such manner as to bring the gyro to a resting position with its plane of rotation coincident with that of the earth and with its axle horizontal. This is done by impressing a constantly decreasing force about the vertical axis to cause precession of gyro axle toward the horizontal as it approaches the meridian, the ends of the axle describing a spiral as they come to rest on the meridian. The manner in which this is done will be described later.
When the gyro is anywhere but on the equator, another difficulty is encountered, due to the fact that the earth's rotation has two components, one tangential to the surface of the earth, which, as already explained, tends to cause the gyro to precess toward the north and south meridian, and one normal to the surface of the earth, which tends to leave the north end of the gyro on one side of the meridian.
Problems in Construction.
The deduction of Foucault's laws immediately suggested the use of the gyroscope as a compass, either by using it with three degrees of freedom for establishing certain fixed lines in space with which bearings might be compared or by which courses already definitely known might be maintained, or by using a gyroscope with two degrees of freedom, which would place itself in the plane of the earth's rotation, thus indicating the north and south mechanically by the position of its axis. The first idea had to be abandoned because of the difficulty of constructing a gyroscope with its centers of rotation coincident with the center of gravity. The second idea was abandoned at first because of the apparently insurmountable difficulty of driving the gyroscope wheel. As the use of steel in ship construction increased and the necessity of placing the compass in a protected position below decks on men-of-war began to be realized, the importance of developing some device for indicating direction, which would be unaffected by the varying magnetic influences, became more and more apparent.
Magnetic compasses are very markedly affected by such varying conditions as fires in boilers, positions of turrets, cranes, and shock of gun fire, etc. Not only are ships endangered by reason of incorrect or wrongly applied deviations, but there are several instances of peculiar and unaccountable changes in variations, which, in at least one instance, have resulted in running a ship on the rocks. The Carnegie, a ship built for scientific research, is now engaged in a trip around the world, for the purpose of checking up and verifying variations as shown on Admiralty, Coast Survey and Hydrographic survey charts. Several errors have already been found and it is reasonable to believe that there are errors which will not be found and that variations will change from year to year, making navigation by the magnetic compass in the vicinity of rocks and islands somewhat hazardous. In view of these facts, as brought out by the development of steel ships and submarines, the investigations of the world's magnetic properties and the increasing need for placing the standard compass in some protected position, so that it can be used after an action, several inventors have for the past six or eight years been endeavoring to apply the gyroscope for use as a compass.
The problems which confronted the inventors having as their starting point Foucault's second law, as explained before, may be stated as follows:
When these first four problems were solved, the result would have been an instrument which would accurately indicate the north and south meridian, if such instrument could be used only in a fixed position on land. If the instrument were to be placed on a ship, several further problems remained to be solved. These were:
- To suspend the gyro with perfect freedom about the vertical axis so that it is free to precess toward the meridian.
- To damp the oscillations of the gyro across the meridian so that it will quickly settle down with its axis north and south.
- To constantly impress just exactly the right degree of force to keep it precessing to follow the component of the earth's rotation, which is normal to the earth's surface.
- To drive the gyro at a high rate of speed in order to furnish the directive force required.
- To so suspend the gyro that forces of acceleration and deceleration introduced by movements of the ship will not cause it to oscillate off the meridian, making its readings incorrect. These forces of acceleration and deceleration introduced by the ship are:
- Acceleration and deceleration arising from starting and stopping the ship and changing its speed.
- Centrifugal forces introduced by the ship in turning.
- Forces due to non-coincidence of the center of the gyroscope with the center of oscillation of the ship, that is, forces introduced by rolling and pitching of the ship.
- To correct the compass for the deflection, due to northerly and southerly components of the ship's movement, which, of course, act on the compass just as the movement due to the earth's rotation, except, of course, in a very much less degree.
Mr. Edison says that when in the development of machines and apparatus you find yourself proceeding toward greater and greater complexity, you can be almost certain that your efforts are along the wrong line, and that you will not be very successful, but that when you find your developments tending toward greater and greater simplicity, you can be sure that you are proceeding along correct lines, and that your efforts will most probably be successful.
In the development of the gyroscopic compass this has been especially true. At first the development led to much complication, but the apparatus finally evolved is extremely simple and easily understood.
Fig. 9 is a diagrammatic section of the compass. The gyro wheel itself is mounted with its shaft in ball bearings in the vacuum casing, indicated at B, in Fig. 9. This casing is carried in trunnions in the ring D called the vertical cardian ring. This ring is suspended in the frame H by the suspension wire E, the top of which is held in the frame and the bottom of which is made fast to the vertical cardian ring. Attached to the frame and integral with it is a ring G, which surrounds the vertical cardian ring; this is termed the "phantom ring," because it is made to follow all tendencies of the vertical cardian ring to move about the vertical axis. Swung in trunnions in the phantom ring and passing through the vertical cardian ring, but not touching it, is the bail R, which is attached to the gyro casing at the point marked S. The vertical cardian ring is of such shape at the bottom as to permit the bail to swing freely through it. Attached to the vertical cardian ring, and projecting through, but not touching the phantom ring, are two posts, not shown in Fig. 9. On these posts are carried small silver wheels, which rest on a flat commutator, which is carried on the movable frame of the compass, of which the phantom ring and suspension head are integral parts. This commutator has two silver segments, insulated from each other by a narrow piece of hard rubber. Normally, the silver trolley wheel attached to the vertical cardian ring rests on this insulating strip. If the gyro tends to move about the vertical axis, this trolley is moved off of the insulating segment and on to one of the silver segments, thus energizing a small motor M, termed the azimuth motor. The armature of this azimuth motor is geared to the frame of the compass and when energized will turn this frame and consequently the top of the suspension wire in such manner as to bring the insulated segment under the trolley wheel and stop the motor. It can be seen that the frame, phantom ring and suspension head will always turn to follow the gyro in azimuth, the azimuth motor assisting the gyro by taking all the work of turning on itself. This is the solution of the first problem, as before stated, that is to say, the gyro is perfectly free to move about the vertical axis in precessing toward the meridian, being suspended from what is, in effect, a torsionless wire, inasmuch as the top of the wire is constantly moving to follow the bottom.
The bail R, being attached to the lower side of the casing, suppresses the freedom of the gyro about the horizontal axis, tending always to bring its axis of rotation horizontal. Thus, if the gyro is rotating at full speed with its axis East and West, the earth, in turning, will rotate "out from under" it, as shown before, leaving its axis suspended out of the horizontal. The bail will exert a force through the point of attachment S, tending to bring the axis to the horizontal and in so doing will cause the axis of the gyro to precess toward the meridian.
It will be noticed that the bail is attached to the casing eccentrically, so that when the axis of the gyro is out of the horizontal the bail acts not only about the horizontal axis, but also about the vertical axis. This force impressed about the vertical axis opposes the oscillation of the gyro by processing its axle toward a horizontal position, thus damping the precession about the vertical axis as it approaches the meridian. The amount of this damping constantly decreases as the axle of the gyro approaches the meridian, and comes nearer and nearer to the horizontal, until, when the axle of the gyro is on the meridian and horizontal, the weight of the bail ceases to act about either axis the weight being then entirely supported by the trunnions in the phantom ring. When on the meridian with axle horizontal the gyro may be said to be suspended with three degrees of freedom and is statically balanced about all axes. In effect, the action of the bail is exactly as if you damped the swing of a pendulum with your hand and constantly lessened the amount of restraining force as the pendulum approached the vertical. This is the solution of the second problem, as before stated, that is to say, the oscillations of the gyro across the meridian are damped so that it will quickly settle down with its axis north and south.
Fig. 10 shows that the angular velocity of the earth at any point 0 can be resolved into two components, ? cos ?, about a tangent to the surface of the earth at the point 0 and ? sin ? about the normal to the surface of the earth at that point; ? being the latitude of point 0. If the plane of rotation of the gyro is not coincident with the plane of rotation of the earth's tangential component, the earth moves "out from under" the gyro, leaving its axis suspended out of the horizontal. The component of the earth's rotation about the normal to its surface leaves the north end of the axis behind on the east side of the meridian, inasmuch as the gyro tends to maintain its plane of rotation in space.
Considering the gyro with its axis on the meridian, and analyzing the effects of the vertical component of the earth's rotation, we see that the earth, in turning from west to east under the compass, which tends to maintain its direction in space, will leave the north end of the axis behind on the east side of the meridian. The moment the axis is left behind the meridian, a portion of the tangential component of the earth's rotation causes a tilt, which introduces the gravity couple of the pendulum. Simultaneously with this, the damping couple of the pendulum is also introduced, tending to destroy the gravity couple. There will be some position of lag behind the meridian at which the damping couple just maintains the axis at a tilt such that the gravity couple can cause the necessary precession to follow the vertical component of the earth's rotation. This is the solution of the third problem. Since the component about the normal varies with the latitude, this position of lag is different for different latitudes.
The fourth problem, that of driving the gyro continuously at a high rate of speed, is solved by using the three-phase induction motor.
If the axle of the gyro is north and south, easterly and westerly movements of the ship are merged in the movement due to the earth's rotation and are negligible in amount. Northerly and southerly components of the ship's speed, however, will cause the ship to move out from under the gyro, deflecting it to one side of the meridian, inasmuch as the gyro tends to maintain its plane of rotation in space. The point at which the bail is attached to the gyro casing is very accurately determined, so that in accelerating and decelerating the ship in starting and stopping, or changing speed, the inertia of the bail acting about the horizontal axis will impress just the right amount of force to orient the gyro to the proper position on one side of the meridian to fulfill the new condition of speed and its damping couple will prevent any oscillation.
Only the northerly or southerly components of the centrifugal forces introduced by the ship in turning- need be considered and these forces are exactly comparable to the northerly or southerly components of ship's speed. That is, if the ship, steaming due east, were to swing about a quadrant to a course due north, the effect of oscillation would be equal to that of the ship starting from rest and attaining speed all on a northerly course. All are, therefore, cases of the same acceleration and are compensated for by the ballistic properties of the bail.
Exact compensation for acceleration of the ship is only obtained theoretically at one given latitude, but the amount of oscillation produced at other latitudes is comparatively slight in all cases, and disappears entirely within a short interval after the ship comes upon a steady course.
Forces introduced by rolling and pitching of the ship are eliminated by mounting the compass in ordinary gimbal rings or cardian mounting, so that all these forces which reach the compass are linear and have no gyroscopic reaction.
As stated before, the gyro is, when on the meridian, suspended with three degrees of freedom and is statically balanced about all axes, so that extraneous forces have no effect on it except by reason of the inertia of the bail acting through the point of attachment to the gyro casing.
It is seen from the above that the fifth problem is solved by the method of suspension and by accurately calculating the point of attachment of the bail to the gyro casing so that the inertia of the bail acting through this point will orient the gyro to its correct resting position to harmonize with the attained speed condition due to latitude, speed and course of the ship, and at the same time will, by its action about the vertical axis, effectually prevent any oscillation from the correct resting position.
It will also be seen from the above that the axis of the gyroscopic compass on a moving ship does not point exactly north because:
- It must lag a certain distance behind the meridian in order to introduce the necessary tilt for following the vertical component of the earth's rotation, the amount of this lag depending upon the latitude.
- The northerly or southerly component of the ship's speed causes a certain deflection from the meridian. This deflection is, for a given latitude and speed, proportional to the cosine of the angle between the ship's course and north.
In the Sperry gyroscopic compass a mechanical device is used, which, when set approximately for latitude and speed of the ship, will automatically apply all corrections to the lubbers' point, making the readings of the compass card referred to the lubbers' point absolutely true. This device will also, at all times, show the amount of the correction that is being applied. It consists of an arrangement of cams, as shown in Figs. 19 and 20. This is the solution of the sixth problem.
Description of Compass and Accessories.
The gyro is driven by a three-phase induction motor the stator of which is mounted on the side of the gyro casing and projects inside of the gyro wheel, and the internal periphery of which carries the short-circuited bars of the squirrel-cage induction motor. The gyro wheel is shown in Fig. 11, which also shows the short-circuited bars and the ball bearings.
Fig. 12 shows the stator of the motor, which is fixed to the side of the casing. This motor is capable of driving the gyro at 10,000 r. p. m.; the normal speed is about 8600.
Fig. 13 is a side view of the vertical cardian ring, gyro casing and suspension wire. This shows the two posts on which are carried the silver trolley wheels. Levels are provided as shown. Since the axle of the gyro is always inclined out of the horizontal when it is off the meridian and precessing toward it, the position of the bubbles in the levels will tell whether or not the compass readings are to be depended upon. In starting the compass off the meridian, if it is seen by the position of the bubble in the level that it is not on the meridian, it may be oriented by hand by impressing a force on the upper end of the axis. The manner in which this is done will be described in detail later under Installation, Care and Operation. Just below the level will be seen the hole through which one of the pendulum arms passes.
Fig. 14 is another view of the vertical cardian ring and gyro. This shows how the casing is mounted in the vertical cardian ring. The window on lower right side is for the purpose of getting the speed of the gyro wheel by means of the stroboscope.
Fig. 15 is another view showing the space allowed for the pendulum and the point at which the pendulum is attached. The small pipe shown just below the center line on the right is for equalizing the amount of oil on both bearings. The condition of oil and bearings can be seen through the windows on the ends of the journals. The gage at the top shows the degree of vacuum on the casing. The casing is kept under vacuum so that less power is required in driving gyro, there is less heating, and the bearings are kept in better condition. With the new compass no loss of vacuum has occurred for several months. The pendulum is swung in the space shown between casing and vertical cardian ring, its arms go through the vertical cardian ring without touching it, and are secured in trunnions in the phantom ring which is shown in Fig. 16.
At the top of the structure on the phantom ring is seen the suspension head. Through the slip rings shown just below the suspension head, current is conveyed to the moving system for driving the gyro and for energizing the azimuth motor. The inclined ring shown just below the compass card is the cam which, acting in combination with cams set for latitude and speed, automatically applies the corrections to the lubbers' point, according to the heading of the ship. If heading east or west, no correction is applied. If heading north or south, the maximum correction for the particular speed and latitude is applied.
Fig. 18 is a view of the small reversible motor called the azimuth motor. The armature of this motor is slightly offset from its field so that when it is not energized the armature drops and disconnects itself mechanically, by means of the clutch shown, from the gearing driving the compass frame. The motor is driven by direct current of 20 volts. It will be replaced in later compasses by a three-phase A. C. motor for the reason that the problem of reversal is simpler with the A. C. motor, and brushes will be eliminated thus making the clutch unnecessary, as without brushes the motor will not act as a drag on the moving system.
Fig. 19 shows the front view of the automatic correction device.
Fig. 20 is a back view of this device showing the cams which, when connected to the cosine cam under the compass card, automatically solve the equations of speed and latitude.
Fig. 21 is a top view of the compass showing a correction of a little over six degrees applied to lubbers' point. This also shows method of suspension in gimbal rings. On the upper and lower sides of the inner circle can be seen the silver segments carried on the phantom ring and with which the trolley wheel attached to the vertical cardian ring makes contact in order to drive the frame of the compass to follow the sensitive or gyroscopic element.
Fig. 23 shows the compass assembled with casing removed.
Fig. 24 shows compass with casing on, cover removed.
Fig. 25 shows it completely covered by its casing. It can be locked in this condition to prevent tampering.
The apparatus which has just been described is called the master compass. Its position in the ship should be below the protective deck, behind the heaviest armor of the ship and as near the meta-centric line of the ship as possible. This location would of course remove it from the view of the helmsman and navigator. For the purpose of indicating, to distant stations, the heading of the ship, as shown on the master compass, an auxiliary system is used, called the compass repeater system. These repeaters are placed on the bridge, in the conning tower, in the steering engine room, in the chart house, in the captain's cabin, and wherever else desired. They have a dial similar to the ordinary compass dial controlled electrically by the movements of the master compass, so that the repeaters at all times indicate the same readings as the master compass. This distant control of repeater dials from master compass dials is accomplished by a commutator geared to the moving frame of the master compass, and controlling the flow of current to the poles of a small six-pole motor in the repeaters, a diagram of which is shown in Fig. .26. The armature of this motor is geared to the repeater dial so that one complete revolution of this armature turns the dials through two degrees. The brushes of the controlling commutator are geared to the master compass dial so that they make one complete revolution with two degrees movement of the compass. Four wires connect the segments of the commutator to the motor; one of these wires is common to all poles, the other three each go to one of the pairs of poles, as shown in Fig. 26. Suppose the brush of the master compass commutator to be turned into such position as to supply current over wire 1 returning it by wire 3, then poles A and D would be energized and the armature would be drawn into the position shown by the sketch. Now suppose that the brush is turned so as to connect wires 3 and 4 to the negative source of supply as will be the case if the master compass moves 1/6°, then the armature is drawn half way between poles A and B, and poles E and D, or it has moved through 1/12 of a revolution, and the dial through 1/6 of a degree. Since the move takes place half way in the sixth of a degree movement of the master compass, the maximum error of the repeaters is 1/12 of a degree or 5'. These repeaters are portable and may be placed in any position; they are made in two types at present, the binnacle type, which is mounted in gimbal rings, and the wall type, which may be fixed to the wall. Another type is being developed, called the recording type, which is so designed with clock-work attachment as to keep a complete record of all courses steered. This will be of considerable value to the navigator, both for checking up courses used in dead reckoning and for checking the work of the helmsman and officer of the deck.
Fig. 27 is a view of the wall type of repeater, such as is used in conning tower, captain's cabin, etc.
Fig. 28 is a view of the new card which it is proposed to use on repeaters. It will be noticed that the degrees are marked in heavy characters on the outside of the circle. A design for a repeater having an integrating dial in the center has been proposed and will probably be tried in the near future. The center or integrating dial will be so geared to the outer dial as to make one complete revolution for each ten degrees movement of the outer dial. The inner dial will be graduated by degrees and 1/12 degrees so that the heading can be determined and the course steered very accurately. The transmitting commutator is so mounted on the master compass that corrections which are applied to the lubbers' point by the automatic correcting device are also applied to all repeaters. All wires to repeaters pass through cut-out switches on the switchboard shown in Fig. 22. Each wire is fused so that in case of short circuit on any repeater its fuses will blow, thus isolating it so that its troubles will not affect the other repeaters. In the common wire to each repeater is a small incandescent lamp which can be seen in the center of the switches. When repeaters are in operation and current is flowing to their motors these lamps are lighted. If anything should occur to open a repeater circuit this state of affairs would be indicated at the switchboard by the lamp for that repeater failing to glow.
It will be noticed that the cut-out switches are all double-throw; this permits of connecting any repeater to the master compass or to a device termed the synchronizer (shown at bottom of board) with which the repeaters may be turned by hand to any desired reading. The synchronizer is used to bring the repeaters to the same reading and to the reading of the master compass.
To synchronize: (1) Throw switch of the repeater or repeaters to be synchronized to the left thus disconnecting them from the master compass transmitter and connecting them to a similar transmitter within the synchronizer. This operation also turns out small lamps which illuminate the repeater dials and which, when lighted, indicate to the helmsman or other person using the repeater, that it is connected to the master compass. (2) Throw lower right hand switch to the left thus energizing a device within the repeater termed the stop magnet which will stop the repeater dial with its zero mark at the lubbers' point. (3) Throw lower left hand switch to the right thus energizing the synchronizer transmitter with direct current at 20 volts. (4) Turn synchronizer dial through a complete revolution. This operation turns repeater dials until their zero marks arrive at the lubbers' mark at which point they will be stopped and held by the stop magnet. (5) Turn synchronizer dial to zero and open stop magnet switch. Repeaters are now at the same reading as the synchronizer and are free to turn with it. (6) Turn synchronizer (and consequently the repeaters) to the reading of the master compass and throw repeater switches to the right thus connecting them to the master compass. (7) Open lower left hand switch.
It will be observed that the cut-out switches have seven legs. One of these is the common wire to repeater motor and stop magnet. Three are section wires to the three pairs of poles in repeater motor, one is to stop magnet and the two remaining go to the binnacle lamp.
The switchboard in Fig. 29 supplies 125 volts D. C. for driving the motor generator, and supplies the three-phase A. C. from this motor generator to the induction motor of the compass. This switchboard also supplies 20 volts direct current to the repeater system and the azimuth motor of the compass. The instruments at the top of the board are A. C. ammeter and voltmeter for showing the current being applied to the gyro motor and the voltage at which it is being supplied. The voltmeter is provided with a switch shown between the two instruments by means of which the voltage of any of the three phases may be determined. The three lamps at the top of the board are in series with each phase wire and indicate by their glow when the three phases are all in operation. The starting and speed control box for motor generator is shown, at bottom of switchboard. The repeaters are provided with two means of 20 volt supply, one from motor generator and one from storage battery. The solenoid operated switch, shown just above starting box, is used for automatically disconnecting the generator in case of failure of its voltage so that battery may remain connected and not discharge back through generator.
A reversing switch is provided with which two of the phase wires to the gyro motor may be reversed, thus reversing the motor to assist in stopping gyro when shutting down the compass. Another switch is provided for short circuiting the phase indicating lamps in case any of them burn out. The shorter arm on the starting box is used for energizing the motor field. The longer arm is for the purpose of controlling motor-armature resistance and motor field resistance; i. e., for starting the motor and controlling its speed. The usual no load and overload releases are provided which reset starting arms when circuit supplying motor generator is opened or overloaded.
Fig. 17 is a view of the motor generator. It has a capacity at the generator end of J4 K. W. with unity power factor and is about 10 inches high and 14 inches long.
Installation.
When installing the gyro compass outfit the master compass and the two switchboards should be placed together below the protective deck behind heavy armor, and as near the meta-centric line of the ship as possible. The central station on board battleships has usually been found to be the best location.
The navy standard practice should be followed as to conduit, cable and connection boxes in putting in the wiring for repeaters.
The installation should be placed under the care of a good electrician who is not a watch stander and who can give most of his time to keeping the outfit in good condition. One of the electricians in the fire control division will be found to be a good man for this. On the Delaware the electrician who cared for the compass also looked out for the condition of the ship's service and fire control telephones, but as he was an exceptionally good man, he had no difficulty in doing both.
The switchboards and motor generator should be given the ordinary care usually given such electrical appliances, that is, they should be kept clean, binding posts tightened, etc.
The master compass and repeaters should be kept clean and free from dust. Master compass casings should be kept on at all times except when adjusting, starting or making observations of compass. Level of oil in bearing cases should be frequently noted—it should never be filled above center of the glass and may be allowed to fall quite low before refilling. A special oil which is furnished with the compass should be used.
Starting (preliminary adjustment).
Before starting it is well to remove the silver trolley wheels and examine contacts carefully. Replace trolleys with a small piece of paper inserted in trolley arm in such manner as to hold it out of physical contact with silver segments—note position of sensitive element—it should be in an exact central position and when moved away from that position should return to it by oscillations with period of from 10 to 20 seconds. If sensitive element does not return to a central position relative to frame, or phantom ring, the head of suspension wire should be shifted until it does. This is done by means of a tangential screw found under the cap over the suspension head. Note positions of bubbles in levels—levels should be adjusted so that when compass is at rest the bubbles are at the southerly end—when running in 40 N. Lat. the bubbles will be in the center. All compasses are adjusted to run with level in 40 N. Lat.
The levels should never require adjustment. If on examination they are found to be out of adjustment, one should look carefully over the gyro casing to see if any weights have been shifted or heavy objects allowed to accumulate thereon. Remove paper from trolleys and test azimuth motor with D. C. power on. Motor should respond freely in either direction and should not cause an oscillating motion of frame when gyro is brought to rest.
To Start (see Fig. 29).
In accelerating to full speed the gyro may pass through two critical stages at which the period of revolution is synchronous with the characteristic of the shaft thus causing vibration of the sensitive or gyroscopic element. It is well to steady the sensitive element with the hand in passing through these stages although no harm will result.
About twenty to twenty-five minutes are required to accelerate the gyro to full speed.
If, in starting the compass, there is any need to get correct readings immediately, the axis may be placed on the meridian by hand in the following manner: When the gyro is running at full speed bring its axis level by exerting a slight torque on the trolley posts in the direction in which it is desired to make the bubble go. After leveling the gyro watch the bubble and time its departure as it moves from the center of the level. The number of divisions on the level over which the bubble moves per minute, multiplied by five, gives approximately, the displacement in degrees between the axis of the gyro and the north and south meridian. The direction of north is shown by the direction in which the bubble moves. The compass may be set approximately, in accordance with the displacement ascertained from the action of the bubble, by applying moderate vertical presure on the end of the axis which is highest. Pressure on the north end of the axis causes westerly movement and vice versa, but it is only necessary to remember to exert a force on the highest end tending to force it to a normal position and in that way assist the action of the bail. When the gyro has attained its normal north and south position the bubble will remain stationary or will vibrate equally on both sides of its normal position if the ship is rolling.
- See that all switches on gyro energy board are open starting arms back. Tell distribution room to throw power on gyro system.
- Throw power on azimuth motor.
- Throw power on motor generator.
- Connect gyro motor to motor generator being careful to see that double-throw switch is thrown into position marked running and not position marked stopping.
- Both starting arm and generator field control arm being thrown to left, the motor armature circuit is open and all resistance is cut out of generator field and motor field.
- Throw the shorter arm all the way over to the right, thus energizing the motor field. Move the longer arm to the right, thus gradually accelerating the motor-generator until the A. C. ammeter shows a current of three amperes flowing to the gyro motor. Voltage will then be about 40. As gyro gradually accelerates, current will drop and voltage will rise. As current tends to drop keep it at 3 amperes by moving starting arm to the right cutting out more and mere armature resistance until gyro is nearly up to full speed, as will be shown by voltage of about 120 with current of 3 amperes. Starting arm by this time will have cut out all armature resistance and will be on speed regulating points to the right which weaken field of motor by cutting in field resistance, thus increasing the speed (shunt motor). Leave starting arm in this position and as gyro gradually accelerates up to full speed current will drop to one ampere and voltage will rise to 130 which is the proper criterion for correct speed.
- After the gyro has been brought to full speed and is on the meridian, synchronize all repeaters and connect them to the master compass.
When the gyro is running careful attention should be paid to all switchboard instruments and indicators to see that circuits are in normal condition.
In case of failure of the direct current supply to the motor generator the gyro will continue to run with sufficient speed to maintain its directive force for about two hours. In case of momentary failure of direct current, such as would be caused by a fuse blowing, the gyro motor should be disconnected from the generator. When supply is re-established the motor generator should be brought to normal speed before connecting it to the gyro motor.
The phase indicating lamps at the top of the board in Fig. 29 are normally short circuited by the three-pole switch below the ammeter. To ascertain whether or not all phases are in operation open the three-pole switch; a glow in all of the three lamps indicates that current is flowing in all phases. In case of failure of one phase speed of the motor-generator should be reduced to about two thirds of normal running speed. Gyro motor will continue to run on two phases but if shut down will not start unless all three phases are operative.
The compass may be left running for months without any appreciable wear on the bearings, but it is best to shut down when the ship is to be in port for more than two or three days.
To shut down: (1) Stop motor generator, (2) disconnect gyro motor, (3) throw gyro motor switch to position marked "stopping," (4) start motor generator and run at a sufficient speed to maintain a reversing current of three amperes on the gyro motor, (5) disconnect repeaters, (6) open twenty volt supply to azimuth motor and repeaters, (7) stop motor generator. Gyro wheel will come to rest in about twenty minutes. Azimuth motor should be kept energized until gyro wheel has come to rest.
Remember (a) to keep the compass covered, (b) to start gyro at least two hours before getting underway, (c) to turn current on azimuth motor before starting gyro, (d) to accelerate gyro by maintaining a flow of three amperes, (e) that a correct running speed is indicated by a flow of one ampere at one hundred and thirty volts, (f) that compass will come to the meridian automatically by reason of the earth's rotation but may be placed on the meridian by hand in much less time, (g) that the azimuth motor should be kept energized while stopping the gyro.
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