GB2040450A

GB2040450A - Gyroscope apparatus

Info

Publication number: GB2040450A
Application number: GB7929977A
Authority: GB
Original assignee: Bodenseewerk Geratetechnik GmbH
Current assignee: Bodenseewerk Geratetechnik GmbH
Priority date: 1979-01-29
Filing date: 1979-08-29
Publication date: 1980-08-28
Also published as: DE2903282B1; FR2447538A2; DE2903282C2; FR2447538B2; GB2040450B

Abstract

An instrument for finding the north direction includes a two-axis X, Y electrically-restrained gyro (40) with a substantially vertical spin axis Z and having a position pickoff (24, 30) and a torquer (26, 32) on each of the two mutually perpendicular input X, Y axes, each pickoff output being cross- connected to the torquer of the other input axis; and a north deviation computer (56) which comprises a memory for storing the two signals applied to the torquers (26, 32). The gyro (40) is rotatable through 180 DEG about a horizontal axis (46) by a servo mechanism (44) after these signals have been stored. Signals then applied to the torquers are fed to the north deviation computer (56) where they are compared with those stored for the elimination of drift and other errors and then computed to provide a north-deviation signal. <IMAGE>

Description

SPECIFICATION Gyroscope apparatus This invention relates to an instrument for the automatic determination of north direction by means of a gyro which is affected by the rotation of the earth, the instrument being of the kind including a two-axis, electrically-restrained gyro, the spin axis of which is substantially vertical, the gyro having a position pickoff and a torquer on each of its two input axes which are mutually perpendicular and are perpendicular to the spin axis, the signal of each position pickoff associated with an input axis being applied crosswise to the torquer of the respective other input axis; and including a north deviation computer to which the signals applied to the two torquers are simultaneously applied and which provides, from the ratio of the signals, a signal representing the deviation of an instrument-fixed reference direction from north.

An instrument of this kind is described in our co-pending Patent Application No. 14781/78. In that instrument, signals Ux and Uy applied to the torquers and restraining the gyro are related to components Qv and #x of the rotational speed of the earth by the following relations UxKTx Mx = = #v (1) H H Uv # KTy My = = - #x, (2) H H wherein Mx is the torque which occurs at the torquer acting about the x-axis My its the torque which occurs at the torquer acting about the y-axis, and H is the angular momentum of the gyro.

The north deviation # can be derived from the ratio KTx # Ux #y = = tan # (3) KTy # Uy #x in accordance with the relation Km Ux # = arc tan (4) KTv # Uy This relation is based on idealized conditions, which are not present in actual practice. In particular, the following sources of error appear: (a) Due to assembly tolerances, the gyro input axes x and y do not coincide exactly with a housing-fixed reference system Xe, ye, z,, on which the signal processing is based.These errors can be characterized by angles aU, wherein the indices i and j represent variables from x, y and z or Xe, Ye Ze, respectively, and i represents an input axis and j represents an axis of the reference system. Thus axy is, for example, the deflection of the input axis x about the coordinate axis yg.

(b) Accelerations may act on the system: These accelerations can be represented by the components ax, ay and a2 in the housing-fixed coordinate system Xe, yG, Ze.

(c) A mass unbalance, represented by a mass unbalance coefficient m may occur.

(d) Anisoelasticity, represented by an anisoelasticity coefficient n, may occur.

(e) There is a mass anisotropy, as axial and polar moments of inertia of the gyro rotor are not equal.

This is represented by a mass anisotropy coefficient.

(C-A) 1 # # H o/h (f) An uncompensated, fixed gyro drift Bx, By [o/h] about the x-and y-axes, respectively, may occur.

Eventually a quadrature coefficient q is to be taken into account.

Due to these errors, the gyro measures the following signals by means of its torquers instead of equations (1) and (2) Mx(1) = TX(1) = #y + α yx #z - αyz #x - may + q ax (5) + n ay az + #z #y + By H -My(1) = - Ty(1) = #x +αxz #y - αxy #z - m ax - q ay (6) H + n ax.az + C-A #z #x + Bx Therefore the measurement of the north deviation # in accordance with equation (4) is falsified by the disturbing influences mentioned.

It is the object of the invention to eliminate, at least partially, the falsification of the measurement by disturbing influences with an instrument of the type hereinbefore described.

In accordance with one aspect of the invention the north deviation computer comprises a memory for storing the two signals Tv(1), Tx(1) applied to the torquers; the gyro is arranged to be rotated through 180 about a horizontal axis bv a servo mechanism, after these sianais have been stored; the sianals Ty(2), Tx(2) then applied to the torquers are fed to the north deviation computer; the north deviation computer is adapted to provide signals # Ty = Ty(1) - Ty(2) # Tx = Tx(1) + Tx(2) wherein M@ (1) T (1) = Y H M (2) T (2) = y H Tx (1) = Mx/H Mx (2) Tx (2) = H are the stored signals and the signals applied to the Mx (1) and Mx (2) torquer which acts about one axis, after the 180 rotation, respectively, M (1) and My (2) are the stored signals and the signals applied to the and H is the angular momentum of the gyro, and, furthermore, the north deviation computer is adapted to provide a signal #Tx #1 = arc tan @T ATy as north deviation signal.

After the 1 800 rotation about yG-axis, the following signals are measured at the torquers: Mx(2) = Tx (2) = #y - αyx #z + α yz #x - m ay - q ax (7) C-A -n ay az - #z #y + By My(2) - = - Ty (2) = - #x + αxz #y + αxy #z + m ax - q ay (8) H C-A + n zx az + #z #x + Bx H Forming the difference and thue sum, respectively, of the signals achieved with the two angular positions yields:: #Ty = Ty(1) - Ty(2) = -2#x + 2αxy #z + 2max (9) #Tx = Tx(1) + Tx(2) = 2#y - 2 m ay + 2 By (10) Thus it is found that, during the measurement of the north deviation # the major portion of the disturbing measuring effects can be eliminated, when in equation (4) KTY Uy is replaced by # Ty H and KTx Ux is replaced by # Tx H In an instrument in which the housing of the gyro is rotatable through 900 about one of the input axes of the gyro by means of a servo mechanism, whereby the gyro may be used as heading reference instrument, the same servo mechanism may be adapted to optionally rotate the housing of the gyro through 1 800. Thereby the 1800 rotatability of the housing requires only small additional expenditure.

According to another aspect, applicable particularly in those cases where the fixed gyro drift (By) contributes the dominating disturbing influence, the north deviation computer comprises a memory for storing the two signals applied to the torquers; the gyro is adapted to be rotated by a servo mechanism through 1800 about a vertical axis coincident with the gyro spin axis; the signals Tx(3), Ty(3) then supplied to the torquers are fed to the north deviation computer; the north deviation computer is adapted to provide signals DTx = Tx (1) - Tx (3) DTy = Ty (3) 5 (1) wherein My (1) Ty (1) = My (3) Ty (3) = (1J Tx (1) = @@/H Mx (3) Tx (3) = , H are the stored signals and the signals applied to the Mx (1) and Mx (3) torquer, which acts about one axis, after the 180 rotation, respectively, are the stored signals and the signakls applied to the My (1) and My (3) other torquer after the 180 rotation, respectively, and H is the angular momentum of the gyro, and furthermore, the north deviation computer is adapted to provide a signal DTx #= arc tan -DTy as north deviation signal.

According to a further aspect, the north deviation computer comprises a memory for storing the two signals Ty(1), Tx(1) applied to the torquers; the gyro is adapted to be rotated by a servo mechanism through 1800 about a vertical axis coincident with the gyro spin axis, after these signals Tx(1), Tv(1) have been stored; the north deviation computer comprises a memory for storing two signals Tx(3), Tv(3) then applied to the torquers; the gyro is adapted to be rotated by a servo mechanism through 1 800 about a horizontal input axis, after these latter signals Tx(3 Ty(3) have been stored and the gyro has been rotated back into its initial position; the signals Tx(2), Ty(2) then applied to the torquers are fed to the north deviation computer; the north deviation computer is adapted to provide signals #Txc = tx(2) - Tx(3) #Ty = Ty(1) - Ty(2), wherein Tx2 is the signal which, after the gyro has been rotated about said one input axis y, is supplied to the torquer which acts on the other input axis, Tx(3) is the signal which, after the gyro has been rotated about the vertical axis, is supplied to the torquer which acts on said other input axis, Ty(1) is the signal which, in the initial position prior to the rotation about the vertical axis, is supplied to said one input axis, and Ty(2) is the signal which, after rotation about the vertical axis (y), is supplied to the torquer, which acts on said one input axis, and the north deviation computer is adapted to provide a signal

as north deviation signal.

A very considerable compensation of the disturbing influences can be achieved by this latter aspect of the invention.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a schematic perspective view of a gyro for finding the north direction, Fig. 2 is a schematic perspective view showing a rotatable mounting of the gyro housing and also showing control and signal processing apparatus, Fig. 3 shows schematically the arrangement of the gyro housing and of the associated coordinate axes in a first position for finding the north direction, Fig. 4 shows the gyro housing in a second position rotated through 1 800 for finding the north direction, Fig. 5 is a schematic block diagram of signal processing apparatus, Fig. 6 is a schematic pictorial view of another embodiment of the invention, in which the gyro is rotated about a vertical axis, in its initial position, Fig. 7 shows the embodiment of Fig. 6, after the gyro has been rotated through 1800 about the vertical axis, Fig. 8 is a schematic block diagram of signal processing apparatus, similar to Fig. 5, for the embodiment of Figs. 6 and 7.

Fig. 9 is a schematic pictorial view of a third embodiment of the invention, and Fig.10 is a schematic block diagram of associated signal processing apparatus.

Referring to Fig. 1, a housing 20 of a gyro has a spin axis z which is vertical. The angular momentum of the gyro is designated by H. The housing 20 is pivoted in an inner gimbal 22 about a first input axis designated by x and perpendicular to the spin axis z. A pickoff 24 is provided on the gimbal 22 and responds to deflection of the housing 20 about the second input axis y. A torquer 26 is attached to the gimbal 22 on the opposite side and is arranged to exert a torque on the housing 20 about the second input axis y. The inner gimbal 22 is pivoted in an outer gimbal 28 about the first input axis x, which is perpendicular to the second input axis y. A pickoff 30 is attached to the outer gimbal 28 and responds to deflection of the inner gimbal 22 relative to the outer gimbal 28 about the first input axis x.

A torquer 32 is attached to the outer gimbal 28 on the opposite side and is arranged to exert a torque on the inner gimbal 22 about the first input axis x. The pickoffs 24 and 30 and the torquers 26 and 32 are interconnected crosswise, i.e. the pickoff 24 on the input axis y is connected through a frequency dependent amplifier 34 to the torquer 32 on the input axis x, and the pickoff 30 on the input axis x is connected through a frequency-dependent amplifier 36 to the torquer 26 on the input axis y. The gains of the amplifiers 34 and 36 are selected so high that the gimbals 22 and 28 are substantially restrained electrically to the relative positions illustrated in fig. 1.

It is assumed in fig. 1, that the spin axis H of the gyro is exactly vertical. The first input axis x of the gyro, which axis serves as reference direction, forms an angle # with the geographic north direction 38, which angle is called "north deviation" and is to be measured. A component #@ cos # of the rotation ot the earth falls into the geographic north direction 38, WE being the angular speed of the rotation of the earth and being geographic latitude.Of this component, in turn, a component Qx = cos COS cos (11) twill act on the first input axis Xe, and a component Qy = -- w, COS cup sin # (12) will act on the second input axis y. The pickoff signals from the pickoffs 24 and 30, amplified by the amplifiers 34 and 36 generate such input signals Uy and Ux, respectively, to the torquers 26 and 32 that the precession torques caused by the angular speed components Qx and Qyt respectively, are just balanced. This yields, with disturbing torques neglected: Ux = H/KTX # y (13) U@ = #/K@@ # y, (14) -KTy wherein 7 [voits3 is the voltage applied to the torquer 26, Ux [volts] is the voltage applied to the torquer 32,

sTy om cm volt 3 KTx p cm volt 21 is the constant of the torquer 26, and is the constant of the torquer 32.

Therefore the north deviation # can be derived from these two voltages Ux and Uy with known constants of the torquers: KTY##E cos # sin # Ux/Uy = = KTy/KTx tan # (15).

KTx##E cos # cos # which leads to equation (4) given above.

As can be seen from figure 1, the angle # is the angle between the instrument-fixed axis x and the north direction 38.

In figure 2, of the drawings, a gyro unit 40 contains the gyro and the gimbals similar to figure 1.

The spin axis z of the gyro 20 is substantially vertical. The two input sxes of the gyro 20 are designated by x and y. The pickoff signals from the pickoffs (not shown) are connected crosswise to the torquers (not shown) through the amplifiers 34 and 36, as has been described with reference to figure 1.

Numeral 42 designates a vehicle-fixed instrument housing. Coordinate axes xG and ye are defined in the instrument housing and, with the mode of operation of figure 2, are in the ideal case, parallel to the input axes x and y, respectively, of the gyro 20.

The gyro unit 40 is arranged to be rotated by a servo motor 44 about axis 46 parallel to the coordinate axis yG, An angle sensor 48 is also located on this axis 46. The output signal of the angle sensor 48 is applied through a change-over switch 50 and a follow-up servo system 52 to the servo motor 44. In the "0 "-position illustrated in figure 2, the servo motor 44 is controlled such that the gyro unit 40 is rotated into the position illustrated in figures 2 and 3 and is locked there. In this position, the coordinate axes x, y, z of the coordinate system associated with the gyro unit are parallel to the coordinate axes Xe, ye, zG of the coordinate system which is associated with the instrument housing 42.

In the "1 80 "-position shown in dashed lines of the change-over switch 50, the gyro unit 40 is rotated by the servo motor 44 through 1800 into the position illustrated in figure 4. In this position, the coordinate axis zG is directed to the top, thus anti-parallel to the coordinate axis z, of the coordinate system reference to the instrument housing 42. The coordinate axis x is directed to the front in figure 4, thus anti-parallel to the coordinate axis xG of the coordinate system reference to the instrument housing 42.

In the "900"-position of the change-over switch 50, the gyro unit is rotated through 900 and can then be used as heading reference instrument or as heading attitude reference instrument, as described in Application No. 14781/78. The same angle sensor 48 and servomotor 44 which are used for the latter change-over in the arrangement of that application can be utilized also for the 1 800-rotation.

In addition, the output signals Ux and Uv of the amplifiers 34 and 36 are applied to a north deviation computer 56. The north deviation computer is illustrated in simplified and schematic form in figure 5.

In the position of the gyro unit 40 illustrated in figures 2 and 3, the output signals Ux and Uy or the signals Tx(1) and Tylt) proportional thereto are stored in a memory 58. After the gyro unit 40 has been rotated through 1800 into the position illustrated in figure 4, the signals Tx(2? and Ty(2) are applied. The difference ATy and the sum STx in accordance with equations (9) and (10) are formed in the difference and summing points 60 and 62, respectively. A quotient former 64 forms the quotient STx ATy which, as described above, represents tan &num;. An inverse tangent function generator 66 provides the angle # therefrom.

In practice, the north deviation computer 56 will be constructed as a digital computer.

The arrangement described can also be used in combination with the other measures described in Application No. 14781/78 for taking the vehicle attitude into account.

In the embodiment of figure 6 and 7,the gyro unit 70 is mounted in the vehicle-fixed instrument housing 72 for rotation about a vertical axis coincident with the gyro spin axis z. The angular position of the gyro unit 70 about this axis z is picked off by means of an angle sensor 74. The output signal of the angle sensor 74 is applied to a follow-up servo system 78 through a change-over switch 76. A servombtor 80, which acts about the axis z is controlled by the follow-up servo system 78. The gyro unit 70 is rotated by the servomotor 80 into selected fixed positions relative to the instrument housing 72, depending on the position of the change-over switch 76. In the "initial position" illustrated in figure 6, the first input axis x of the gyro extends to the rear in the figure, and the second input axis y extends to the right.The other position of the gyro unit 70, which is reached after a 1 800-rotation about the axis z, is illustrated in figure 7. In this position the first input axis x of the gyro points to the front in the figure, and the second input axis points to the left. As in the other embodiment, the gyro unit 70 comprises a gyro arrangement of the type shown in figure 1, and corresponding components are provided with the same reference numerals in figures 6 and 7 as in Figure 1.

The restraining torques, which are exerted on the gyro by the torquer acting about the x-axis due to the signals from the pickoff on the y-axis, are designated by Mx(1) in the initial position of figure 6. The restraining torques, which are exerted on the gyro by the torquer acting about the y-axis due to the signals from the pickoff on the x-axis, are designated by My(1) in the initial position of figure 6. In the position rotated through 1 800 relative to this initial position, the corresponding restraining torques are Mx(3) and My(3), respectively. The pickoffs and torquers provide signals.

Mx(1) My(1) Mx(3) My(3) Tx(1)= ,Ty(1)= ,Tx(3)= , and Tv(3)= H H H H The signals Tx(1) and Ty(1) result from equations (5) and (6). The signals obtained after the 180 -rotation are Tx(3) = - #y + αyx #z + α yx #x + m ay - (16) C-A - q ax - n ay az - #z #y + By H - Ty (3) = - #x - α xz #y - α xy #z + m ax + q ay- (17) C-A - n ax az - #z #x + Bx H The differences of the signals with the initial position of figure 6 and with the position of figure 7 rotated through 1800 become: DTx = Tx(1) - Tx(3) = 2 (#y - α yz #x - m ay + q ax + n ay az + C-A + #z #y ) (18) H DTy = Ty(3)- Ty (1) = C-A = - 2 (#x + α xz #y + m ax - q ay + n ax az + #z #x).

H It will be noted that the uncompensated gyro drifts Bx and By are not contained in these signals. With many gyros, however, these uncompensated gyro drifts are the essential source of error, which have greater weight than all other sources of error together. With such gyros the arrangement of figures 6 and 7 is to be given preference to the arrangement of figs. 3 and 4, for which the equations (9) and (10) are valid.

The north deviation computer pertaining to the arrangement of figs. 6 and 7 is illustrated in fig. 8.

The north deviation computer 82 comprises a memory 84, in which the signals Tx(1) and Ty(1) provided by the torquers in the initial position are stored. After the 180 -rotation, the signals Tx(3) and Ty(3) are applied to the north deviation computer 82. These signals are superposed with-opposite polarity to the stored signals Tx(1) and Ty(1), respectively, at summing points 86 and 88, respectively, whereby the signals DTx and DTy are provided. Dividing means 90 form the quotient DTx -DTY, This quotient is applied to an inverse tangent function generator 92, which provides the north deviation angle DT, = = arc tan -DTy Dex and DTy contain the useful signals #y and - x' respectively, and the various sensor errors.Apart from these sensor errors, equation (20) is identical with equation (4). The gyro drifts Bx and By are, however, eliminated.

A further modification, wherein a gyro unit 94 is arranged to be rotated through 1800 from the initial position corresponding to figure 6 both about a vertical axis, which in the initial position coincides with the gyro spin axis z, and about a horizontal axis, which coincides with the second input axis (y-axis), is illustrated in figures 9 and 10.

The gyro unit 94 is mounted in an intermediate housing 96 for rotation about a horizontal axis coincident with the second input axis (y-axis) of the gyro. An angle sensor 98 is located on this horizontal axis and provides a signal indicative of the angular position of the gyro unit 94 relative to the intermediate housing 96. This signal is applied to a follow-up servo system 102 through a change-over switch 100. A servomotor 104 is controlled by this follow-up servo system 102 and rotates the gyro unit 94 into the initial position illustrated or into a position rotated relative thereto by 1 800 about a horizontal axis (identical to fig. 4), depending on the position of the change-over switch 100.

The intermediate housing 96, in turn, is mounted with a shaft 106 for rotation relative to a vehiclefixed instrument housing about a vertical axis, which in the initial position illustrated coincides with the gyro spin axis. An angle sensor 110 and a servomotor 112 are arranged on the shaft 106. The angle sensor 110 provides a signal indicative of the angular position of the intermediate housing 96 relative to the vehicle-fixed instrument housing 108. This signal is applied to a follow-up servo system 116 through a change-over switch 114. The servomotor 112 is controlled by the follow-up servo system 116 such that it rotates the intermediate housing into the initial position illustrated or into a position rotated relative thereto by 1800 about the vertical axis, depending on the position of the change-over switch 114.In the latter position, the orientation of the gyro unit is identical to that of fig. 7.

The north deviation computer 118 in fig. 10 comprises a first memory 120, a second memory 122 and a third memory 124. In the initial position of figure 9, at first the signals TX1(1) and Ty1(1) of the torquers are stored in the first memory 120, said signals resulting from equations (5) and (6). (The lower index "1" indicates the first measurement of these signals). Thereafter the intermediate housing 96 is rotated through 1800 about the vertical axis by means of the servomotor 112. The signals Tx(3) and then obtained from the torquers are stored in the memory 124. The intermediate housing 96 is rotated back into its initial position. The signals Tx2(1) and Ty2(1) of the torquers are again picked off in this initial position and are stored in the memory 122. Thereafter the gyro unit 94 is rotated through 1800 about the horizontal axis by the servomotor 104.The signals Tx(2) and Ty(2) obtained from the torquers and corresponding to equations (7) and (8) are also supplied to the north deviation computer 11 8, as indicated at 126, 128.

A mean value former 130 forms the mean values from the signals stored in the memories 120 and 122 Tx(1) 2 21 (Tx1(1) + Tx2 (1)) and (21) T (1) - I (Ty1 (1) + T (1)) (22) y(1) # (Ty1 (1) + Ty2 The difference # Ty = Ty(1) - Ty(2) (23) is formed at a summing point 132 (ses also equation (9)). The formation of the difference #Txc = Tx(2) - Tx(3) = 2(#y - m.ay - αyx.#z) (24) is effected at a further summing point 134.The two difference signals ATy and 2:TXC are supplied to dividing means 136, which form the quotient STxc ATy This quotient is applied to an inverse tangent function generator 138, which forms the north deviation signal #Txc # = arc tan (25) -# Ty- For self-testing purposes the sum STx = Tx(1) + Tx(3) (26) may be formed at a summing point 140 and the sum STx = -(Ty(1) + Tv(3)) (27) may be formed at a summing point 142, which, as can be shown, are substantially proportional to the gyro drifts.

From equations (5) and (16) follows: STx = Tx(1) + Tx(3) = 2(αyx #z + By) (28) and STv = -Ty(1) - Ty(2) = 2(-αxy #z + Bx). (29) With negligible deviations ayx and axy respectively, of the gyro input axes relative to the housing axes, which can be achieved by appropriate precision of manufacture, the following is valid with good approximation: 2By = STx@ (30) 2Bx=STy, (31) wherein Bx and By are estimated values for Bx and By. Therefore the expression z Tx from equation (10), which enters the equation for the north deviation, can be corrected for the gyro drift By, after STx has been determined.The expression z TXC thus corrected is A A #Txc = #Tx - s By = Tx(1) + Tx(2) - 2By (32) A and, if 2 By from equation (30) is substituted: #Txc = Tx(1) + Tx(2) - Tx(1) - Tx(3) = Tx(2) - tx(3) (33) Equations (7) and (16) then yield: #Txc = 2#y - 2may - 2αyx . #z (34) Furthermore, as has already been stated above, the following is true #Ty = Ty(1) - Ty(2) = -2#x + 2αxy#z + 2max (9) independently of Bx. With disturbing terms neglected, this yields #Txc = #y/#x = tan #, -#Ty this term being compensated for the gyro drifts Bx and By.

The change-over switches 50, 76, 100 and 114 and the application of the signals to the various memories and inputs are controlled in the correct sequence by a program control unit (not shown).

Claims

1. An instrument for the automatic finding of the north direction of the kind described, wherein the north deviation computer comprises a memory for storing the two signals Ty(1), Tx(1) applied to the torquers; the gyro is arranged to be rotated through 1800 about a horizontal axis by a servo mechanism after the signals Ty(1), Tx(1) have been stored; the signals Ty(2), Tx2 then applied to the torquers are fed to the north deviation computer; the north deviation computer is adapted to provide signals d Ty = T (1) - T (2) y #Tx = Tx(1) + Tx(2) wherein M (1) T (1) - Z y H M (2) Y H Mx(1) Tx(1) = H Mx(2) Tx(2) = H are the stored signals and the signals applied to the Mx(1) and Mx(2) torquer which acts about one axis, after the 180 rotation, respectively are the stored signals and the signals applied to the My(1) and My(2) other torquer after the 180 rotation, respectively, and H is the angular momentum of the gyro; and the north deviation computer is adapted to provide a signal #1 = arc tan # Tx ATy as a north deviation signal.

2. An instrument as claimed in Claim 1, including a servo mechanism to rotate the housing of the gyro through 900 about one of the input axes of the gyro such that the gyro is suitable for use as heading reference instrument, said servo mechanism being adapted to rotate the housing of the gyro optionally through 1800.

3. An instrument of the kind described, wherein the north deviation computer comprises a memory for storing the two signals applied to the torquers; the gyro is adapted to be rotated by a servo mechanism through 180 about a vertical axis coincident with the gyro spin axis; the signals (Tx(3), Ty3)) then supplied to the torquers are fed to the north devlation computer; the north devlation computer is adapted to provide signals DTx = Tx - Tx(3) DTy = Ty(3) - Ty(1) wherein My(1) Ty(1) = My(3) Ty(3) = H Mx(1) Tx(1) = H Mx(3) Tx(3) = , H are the stored signals and the signals applied to the Mx(1) and Mx(3) torquer, which acts about one axis, after the 180 rotation, respectively, are the stored signals and the signals applied to the My(1) and My(3).

other torquer after the 180 rotation, respectively, and H is the angular momentum of the gyro; and wherein the north deviation computer is adapted to provide a signal #= = arc tan DTx DTy as a north deviation signal.

4. An instrument of the kind described, wherein the north deviation computer comprises a memory for storing the two signals Tyltl, Tx(1) applied to the torquers: the gyro is adapted to be rotated by a servo mechanism through 1800 about a vertical axis coincident with the gyro spin axis, after the signals (T)x(1), Ty(1)) have been stored; the north deviation computer comprises a memory for storing the two signals (Tx(3), Ty3)) then applied to the torquers; the gyro is adapted to be rotated by a servo mechanism through 1800 about a horizontal input axis, after these latter signals (Tx(3), Ty(3) have been stored and the gyro has been rotated back into its initial position; the signals (Tx(2), Ty(2)) then applied to the torquers are fed to the north deviation computer; the north deviation computer is adapted to provide signals #Txc = Tx(2) - Tx(3) #Ty = Ty(1) - Ty(2) wherein Tx(2) is the signal which, after the gyro has been rotated about said one input axis y, is supplied to the torquer which acts on the other input axis.

Tx(3) is the signal which, after the gyro has been rotated about the vertical axis, is supplied to the torquer which acts on said other input axis, Ty(1) is the signal which, in the initial position prior to the rotation about the vertical axis, is supplied to said one input axis, and Ty(2) is the signal which, after rotation about the vertical axis (y), is supplied to the torquer, which acts on said one input axis, and the north deviation computer is adapted to provide a signal = = arc tan z Txc ATy as north deviation signal.

5. An instrument as claimed in Claim 4, wherein the north deviation detector is adapted to provide signals STx = Tx(1) + Tx(3) and STy =Ty)l)Ty)3) which represent the components of the gyro drift, wherein Tx(1) is the signal which, in the initial position prior to the rotation about the vertical axis, is applied to the torquer, which acts on said other input axis, and Ty(3) is the signal which, after rotation about the vertical axis, is applied to the torquer, which acts on said one input axis.

6. An instrument as claimed in Claim 4 or Claim 5, wherein the north deviation computer comprises an additional memory for storing the two signals (Tx2(1) Ty2(1)) applied to the torquers after the gyro has been rotated back about the vertical axis into its initial position; and wherein the north deviation computer is adapted to provide the mean values Tx(1) = (Tx1(1) + Tx2(2)) and Ty(1) = (Ty1(1) + Ty2(2)), wherein the index 1 designates the signal prior to the rotation about the vertical axis and the index 2 designates the signal after rotation back into the initial position.

7. An instrument as claimed in Claim 1, Claim 3 or Claim 4 and substantially as hereinbefore described with reference to the accompanying drawings.