CHAPTER XVII

THE ANSCHÜTZ (1912) COMPASS

A plan and sectional elevation of the modern form of Anschütz compass are given in Figs. 50 and 51. The casings of the three gyros K L M hang by vertical stalks below a triangular spider A at the centre of which is affixed a float B immersed in a bowl C containing mercury. After the manner followed in the 1910 form, the float and all attached to it are centralised relatively to the bowl by means of a rod D fixed centrally to the cover of the compass. As before, this rod D is composed of a central core and a liner insulated from the core. The ends of the core and liner dip into two concentric mercury cups carried within the float in order that two phases of the three-phase current driving the gyro-motors may be transmitted through the core and liner of the rod D. The third phase is transmitted through the mercury and float by earthing the bowl C.

Matters are so arranged that the centre of flotation of the float B and its attached parts is above the centre of gravity of the floating parts. These floating parts--the spider A, the float B, the three gyros, and other items not yet mentioned--constitute the sensitive element, so that this element, as in the 1910 form, does not need the addition of a separate weight to provide the required pendulum action about a horizontal axis through the centre of flotation. Instead of there being but one such horizontal axis--the east and west axis E F of our models and diagrams--it is clear that the support of the sensitive element by means of a float provides the element with an east and west horizontal axis and an infinite number of other horizontal axes.

Attached to the spider A--and therefore forming a portion of the sensitive element--is a sheet metal annular casing of the section shown at E E or F F. The gyros are enclosed within this casing. Ventilating tubes and baffles G are provided at points on the casing between each pair of gyros. The compass card--in the form really of a ring--is attached to the casing at H. Regarding the casing and the spider A as the equivalent of the horizontal ring shown in our diagram (Fig. 41), it will be noticed that the gyro casings are not fixed rigidly to it, but are really mounted on ball bearings surrounding their stalks, so that they may rotate about a vertical axis relatively to the rest of the sensitive element. As shown in the plan view, however, the casing of the gyro K is connected by two springs J to the rest of the sensitive element, so that in whichever way the gyro turns on the ball bearings it applies through one or other of the springs a force in the same direction to the annular casing, etc. The gyro is therefore substantially, though not actually, rigidly connected to the rest of the sensitive element, the springs being introduced to provide a yielding connection which will prevent the full force of a sudden turn of the ship from being thrown all at once on to the gyro. The gyros L M are similarly connected to the rest of the sensitive element, but in their case one pair of springs is made to serve both gyros by employing links and a bell-crank lever. The springs, it may be remarked, undoubtedly do play a part in the transmission of the directive force from the gyros to the card and in the avoidance of the quadrantal error. But their presence is not essential to the fundamental principle of action of the compass.

The damping system of the 1912 design is of a very simple nature, and represents a great improvement over the air blast method previously used. Although the wheels are not required to act as blowers, their casings are not exhausted of air. The casings, in fact, are perforated with four large holes on each side, the cooling effect of the circulating air being regarded as of more value in practice than the saving of power which would result if the wheels were run in an exhausted atmosphere.

The damping force is supplied by the weight of a body of oil contained within a trough N extending right round the foot of the annular casing containing the gyros. This trough, a circle as seen in plan, is blocked by eight bulkheads, one each below the north and south points of the compass card, and the others equally spaced round the trough. Through each bulkhead a short pipe passes, so that the oil in the trough may flow from one compartment to another. With the exception, however, of the north and south bulkhead pipes, which are quite free in the bore, the passage of the oil is restricted by means of a wire partially filling the bores of the pipes. By varying the size of wire used, the restriction to the flow of the oil from one compartment to the others, and therefore the rate at which the oil will flow when the sensitive element tilts, can be regulated to give the degree of damping required or to suit any change of viscosity in a fresh supply of oil.

The system in principle has much in common with the Brown method of damping. Should the compass card suffer an easterly deflection, the north point of the card will, as we know, tend to rise under the influence of the earth’s rotation, and will continue to rise until the turning moment applied by the deflected pendulum weight precesses the card back to the meridian, whereafter the north point of the card passing over towards the west will begin to fall towards the horizontal plane, and then, descending still farther, it will once more come back to the meridian. During this compound motion the oil in the trough flows backwards and forwards, accumulating below the south point of the card when the north point of the card is rising and gathering below the north point when the north point is falling. In other words, there is an excess weight of oil below the southern point of the card throughout the complete half-swing from east to west, the maximum excess occurring when the card is crossing the meridian. On the half-swing from west to east the excess weight of oil is below the northern point of the card, the maximum excess occurring, as before, when the card is crossing the meridian. The excess weight of oil at all times thus tends to increase the rise or dip of the north point of the card above or below the horizontal plane, whereas the pendulum weight at all times tends to diminish such rise or dip. Hence the excess weight of oil tries to precess the card in the direction opposed to that in which the pendulum weight is precessing it. The vibration of the card in this compass, as in the Brown design, is therefore damped by the generation of a counter-precessional tendency, and not, as in the early Anschütz and the Sperry designs, by precessing the sensitive element in the direction required to reduce the angle by which the pendulum weight is tilted away from the plumb line.

From what we have already said regarding the Brown system of damping, it will readily be inferred that there is no latitude error in the Anschütz 1912 compass. The damping force is equivalent to a reduction in the weight of the pendulum “bob,” and is not applied directly to reduce the tilt of the bob. The tilt of the pendulum weight required in north or south latitudes to provide the appropriate rate of westerly or easterly precession is not opposed by the damping force called into play by such tilt. Instead, the damping force merely makes the bob lighter, so that the tilt has to be carried farther before the _effective_ weight of the bob can balance the tilting action of the earth’s rotation. The balance will be automatically struck when the moment of the effective weight of the bob is just sufficient to generate the required rate of westerly or easterly precession appropriate to the latitude.

The arrangement of the three gyros at the corners of an equilateral triangle and the general form given to the annular casing and the rest of the sensitive element results in the distribution of the mass of the sensitive element in a very uniform manner around the vertical axis. There is no excessive concentration of the mass towards the east and west plane, and as a result it is unnecessary to add compensator weights to this compass in order to avoid the effects of centrifugal force during quadrantal rolling.

The gyro-wheels are made of a special quality of nickel steel, and are mounted on axles of the de Laval type--that is to say, they are tapered and made of very small diameter (about 0.15 in. at the parallel ends)--in order that they may yield a little should the centre of gravity of the wheel not be truly coincident with the centre line of the shaft. The wheels are 5 in. in diameter, weigh 5 lb. 2 oz., and run at 20,000 revolutions per minute. The motors are of the squirrel-cage type with the rotor windings fixed to the wheels inside a recess concentric with the axle. The field coils are fixed relatively to the gyro casing. It is of interest to note that when the gyro-wheel is being run up to its full speed--an operation taking about five minutes to complete--the axle passes through three critical speeds. These speeds are approximately 7000, 11,000, and 14,000 revolutions, and are believed to be associated, the first with one end of the axle, the second with the other end, and the third with a combined action at both ends. During the period of running up the gyros the starting current is, of course, heavier than the current taken to drive the wheels at the top speed, and as a result a considerable temperature is developed in the wheels and their casings. When, however, the gyros have been running for some time at the top speed the temperature drops, and throughout the compass remains fairly constant at about 150 deg. Fahr. The viscosity of the damping oil, on the constancy of which the constancy of the damping force depends, is therefore less affected by external atmospheric changes of temperature than might be expected. The oil used is a mineral one. It serves not only to damp the vibrations of the card, but also to lubricate the gyro-axles. To this end, as shown in Fig. 51, pipes are led down from each end of each axle to dip into the oil trough, the flow of oil being induced by means of wicks inside the pipes.

The method adopted for the transmission of the readings from the master compass to the repeaters is of considerable interest. The bowl C containing the mercury and the float is surrounded by two semi-cylindrical strips P Q of silver-plated brass. At one of the gaps between these strips the two abutting edges are faced with platinum. The gap between these platinum faces is 0.11 in. in width, and into it there is inserted, as shown in the plan view, a platinum-iridium ball R, measuring 0.095 in. in diameter.

On the switchboard serving the compass there is a reversible motor, two of the windings of which are constantly connected to a generator--the same generator as serves the gyro-motors. The contact ball R is connected to the third phase of the generator, while the two strips P Q are connected to the third winding of the reversible motor, this winding being duplicated in such a way that the motor revolves in one direction or the other, according as the circuit is completed at the ball R through the strip P or the strip Q. A commutator is mounted on the axle of the reversible motor, and from it current is distributed to the motors operating the repeaters and to the “follow-up” motor S (Fig. 51). The latter motor is geared to the shaft carrying the bowl C, and when started up by the reversible motor turns the bowl in the direction required to restore the ball R to the middle of its slot, and so break the connection with the strip P or Q. Thus when the ship’s course is altered the bowl tends to rotate with the ship, but the ball R is mounted on the sensitive element, and therefore maintains its position. Contact is thus established between the ball and one of the strips P Q, the reversible motor is set rotating in the appropriate direction, and current is distributed to the “follow-up” motor S to rotate the bowl relatively to the ship until the ball R is once more lying midway in the gap. The tendency of the bowl to rotate with the ship is thus counteracted; the action of the “follow-up” motor practically results in the bowl being held in constant relationship to the sensitive element substantially as though it were part thereof. Simultaneously the cards of the repeaters are prevented from rotating with the ship, so that virtually they, too, act as if rigidly connected to the sensitive element, without, however, any frictional drag being thrown from them on to the sensitive element.

As illustrating the refined construction of the entire compass, the design of the ball contact may be noticed. The ball is carried at the end of a tapered spiral spring. It is free to rotate on the spring end, but is prevented from moving axially thereon. The spring end is provided with a button. The ball is drilled out and beaded over the button. To ensure good electrical contact at all times between the ball and the spring a drop of mercury is carried inside the ball between it and the button. Should the ship turn very suddenly the ball may spring out of the gap and be dragged across the face of one or other of the strips P Q. It is for this reason that these strips are silver-plated.

The repeater compasses are provided not only with an ordinary card graduated from 0 deg. to 160 deg., but also with an inner dial which makes one revolution for an alteration of 10 deg. in the ship’s course. This dial is graduated to 1/10 deg., and permits very small departures from the set course to be immediately noticed and corrected. An elaboration of the same idea is provided in the multiple repeater of the Brown compass. In this repeater the inner dial is the ordinary 360 deg. card. The outer annular dial makes four revolutions for every complete turn of the ship. With the ship sailing due north the graduations on the outer dial are numbered from 0 to 45 round the east half of the dial, and from 360 to 315 round the west half. The numbers, however, are not marked on the dial itself, but on the edges of discs seen through slots in the dial. As the ship turns from the north towards the east, the discs on the west side of the dial are successively rotated one stage as the south end of the lubber line passes over them, so as to exhibit numbers forming a continuation of the numbers on the east side of the dial. The outer magnified dial is thus of itself sufficient for navigational purposes.

In the Anschütz equipment arrangements are made for attaching an azimuth mirror to the repeater dial for the purpose of providing an artificial horizon during the taking of bearings. A separate gyroscopically stabilised artificial horizon device, such as is sometimes to be found on board ships, is thus rendered unnecessary.

INDEX

Absence of latitude error in Anschütz (1912) compass, 158

Absence of latitude error in Brown compass, 66

Action of excentric pin in Sperry compass, 55 _et seq._

Air-blast pressure, Brown compass, 61

Anschütz (1910) compass-- Air-blast damping system, 42 _et seq._ Damping curve, 50 Latitude error, cause of, 65 -- -- correction for, 67 -- -- value of, 68 Magnitude of directive force at equator, 23 Non-gyroscopic details, 138 Period of vibration of axle, 33, 37 Peripheral speed of wheel, 47 Quadrantal error, 96, 107, 120 Weight, diameter, and speed of wheel, 23 Weight of sensitive element, 30 Wheel tested to destruction, 140

Anschütz (1912) compass-- Absence of latitude error, 158 Directive force, value of, 125 Fourth gyro type, 128 Gyro-wheels, details of, 159 Non-gyroscopic details, 154 Oil damping system, 156 Quadrantal error, 107, 120, 126 Repeater compasses, 162 Temperature rise, 160 Weight, diameter, and speed of wheels, 123

Ballistic deflection and error, 81 _et seq._

Ballistic deflection, dead-beat, 87

Ballistic deflection, tests by British Admiralty, 89

Ballistic gyro, Sperry compass, 111

Brown compass-- Absence of latitude error, 66 Air-blast pressure, 61 Compensator weights, 134 Damping bottles, 61 -- system, 59 _et seq._ Generation of directive force, 116 Magnitude of directive force at equator, 23 Non-gyroscopic details, 148 Oil control bottles, 113 On due west course, 117 Quadrantal error, 113, 126 Repeater compasses, 154, 162 Weight, diameter, and speed of wheel, 23 Weight of sensitive element, 30

Centrifugal forces during quadrantal rolling, 130 _et seq._

Clock, gyroscopic, 16

Compensator weights, 134

Correction mechanism for latitude and north steaming errors, 75

Course correction or cosine ring, 77

Damped and free motion of gyro-axle, 49

Damped and undamped vibrations, 35 _et seq._

Damping curve, 50

-- systems, 42, 43, 52, 59, 156

-- vibrations of gyro-compass, 29

Dead-beat ballistic deflection, 87

Details of gyro-wheels, 23, 159

Directive force, value of, at equator, 23, 125

Directive force, effective, 28

Effect of rolling on due west course, 93, 98, 99

Effect of rolling on due north course, 94

Effect of rolling on north-west course, 101

Elementary gyro-compass, 18

-- -- at equator, 20, 26

-- -- at 55 deg. N. lat., 24

-- -- near North Pole, 26

Elementary gyroscope at equator, 15

Elementary gyroscopic phenomena, 4 _et seq._

Elimination of the quadrantal error, 107

Excentric pin, action of, 55 _et seq._

-- -- stabilisation of, 110, 112

External gimbal mounting, 13, 97

Fourth gyro type of Anschütz compass, 128

Free and damped motion of gyro-axle, 49

Friction, solid and fluid, 39, 40

Generation of directive force, 18, 116

German submarines, 96, 120, 128

Gimbal mounting, external, 13, 97

Gyro-axle, free and damped motion of, 49

Gyroscope at equator, 15

-- with three degrees of freedom, 4 _et seq._

Gyroscope and rotation of the earth, 15

Gyroscopic compass at equator, 20, 26

Gyroscopic compass at 55 deg. N. lat., 24

Gyroscopic compass near North Pole, 26

Gyroscopic phenomena, elementary, 4 _et seq._

Gyro-wheels, details of, 23, 159

Latitude bail corrector, 69

-- corrector dial, 76

-- error, 65

-- -- absence of, 66, 158

-- -- cause of, 65, 66

-- -- correction of, 67, 75

-- -- value of, 68, 69, 75

Magnetic compass, 1, 23, 30

Magnitude of directive force at equator, 23, 125

North steaming error, 70

-- -- -- correction mechanism, 75

-- -- -- value of, 74, 79

Oil control bottles, 113

-- damping system, 156

Pendulum, simple, 31, 81

Period of rolling of a ship, 95

-- -- vibration of gyro-axle, 33, 37, 88

Peripheral speed of gyro-wheel, 47

Phantom ring, 53

Quadrantal error, 91, 96, 107, 120

Quadrantal error, elimination of, 107, 108, 113, 120, 126

Quadrantal error, value of, 107

-- rolling, centrifugal forces during, 130 _et seq._

Repeater compasses, 79, 80, 146, 152, 162

Rolling of a ship, period of, 95

-- on due north course, 93, 98, 99

-- -- -- west course, 94

-- -- north-west course, 101

-- quadrantal, centrifugal forces during, 130 _et seq._

Rotation of the earth and the gyroscope, 15

Simple pendulum, 31, 81

Sperry compass-- Bail, 54 Ballistic gyro, 111 Compensator weights, 134 Course corrector or cosine ring, 77 Damping system, 52 Directive force, magnitude at equator, 23 Excentric pin, action of, 55 _et seq._ Excentric pin, stabilisation of, 110, 112 Latitude bail corrector, 69 -- corrector dial, 76 -- error, cause of, 66 -- -- correction of, 67, 75 Latitude error, value of, 69 Non-gyroscopic details, 142 North steaming error, correction of, 75, _et seq._ On north-west course, 108 Phantom ring, 53 Quadrantal error, 108, 126 Repeater compasses, 146 Speed correction dial, 77 Vacuum in wheel casing, 52 Weight, diameter, and speed of wheel, 23 Weight of sensitive element, 30

Submarines, German, 96, 120, 128

Temperature rise in Anschütz compass, 160

Test of Anschütz wheel to destruction, 140

Vacuum in Sperry casing, 52

Vibration period of Anschütz compass, 33, 37

Vibration period of gyro-axle, standard, 88

Vibrations, damped and undamped, 35 _et seq._

Weight, diameter, and speed of gyro-wheel, 23, 123

Weight of sensitive element, 30

Printed in Great Britain at _The Mayflower Press, Plymouth_. William Brendon & Son, Ltd.

FOOTNOTES

[1] Earlier form as in use in 1910.

[2] Approximate. Differs in different compasses.

[3] The value of the latitude error is _b_ tan L, where L is the angle of latitude and _b_ a constant dependent upon the design of the compass. In the Sperry correction mechanism the value of the quantity _b_ is represented by the distance between the centre of the dial B and the centre of the pin C when the radial slot in the dial is lying at right angles to the bar D E.

[4] The value of the north steaming error is numerically equal to (_a_ K cos C)/cos L, where K is the ship’s speed, L the angle of latitude, C the angle between the course and the north and south direction, and _a_ a constant involving the speed of rotation of the earth. In the Sperry correction mechanism the value of the quantity _a_ is represented by the radius at which the pin L lies from the centre of the latitude dial.

[5] The ballistic deflection is dependent upon the _rate_ at which the northerly component of the ship’s speed is being changed. The difference in the north steaming errors is dependent upon the _initial_ and _final_ values of the speed towards the north, and would not appear to be affected by the length of time occupied in changing the speed. It would seem, therefore, that the ballistic deflection can only be dead-beat in the chosen latitude if the speed towards the north is reduced or increased at one particular rate. Thus if the ship is steaming due north at 20 knots and changes its speed to 10 knots in (_a_) 10 minutes or (_b_) 5 minutes, the initial and final values of the north steaming error will be the same in both cases, since the initial and final speeds are the same, but the ballistic deflection will be greater in the second case than in the first, since the rate at which the speed is changed is greater. If, then, the ballistic deflection is dead-beat in the first case it cannot be so in the second. The statement that the ballistic deflection in the chosen latitude is dead-beat thus appears, it may be suggested, to require qualification. On the other hand, the British Admiralty in connection with the use of gyro-compasses on destroyers and similar fast-manœuvring vessels wherein the ballistic deflection is a factor of very great importance has carried out lengthy experiments on the matter, and, although the results have not been divulged, it is understood that the ballistic deflection was found to be dead-beat independently of the rate at which the speed was changed.

[6] The wheels in the two designs being similar in form, their moments of inertia--the real factors determining the magnitude of the directive forces--are proportional to the fourth powers of their diameters. The fourth power of 5 is to the fourth power of 6 as 1 is to 2 approximately.

[7] The directive force is proportional to the sine of the angle of deflection. The gyros K L M therefore supply at 30 deg. deflection a total directive force of D sin 30 deg. + D sin 60 deg. + D sin 0 deg., or 1.366 D. For a single-gyro compass, having a wheel of twice the inertia of K L or M separately, the directive force at 30 deg. deflection would be 2 D sin 30 deg. or D.

[8] More precisely, the moment of inertia of the body must have a constant value about the line J K, Fig. 43, at all angles of setting.

Transcriber’s Notes

Simple typographical errors were corrected.

When necessary, illustrations have been moved between paragraphs, so some of the page references in the List of Illustrations are off by a page or two. Links to those illustrations are correct.

Index not checked for proper alphabetization or correct page references.

End of Project Gutenberg's The Gyroscopic Compass, by T. W. Chalmers