GB2064116A

GB2064116A - North Seeking Gyro

Info

Publication number: GB2064116A
Application number: GB8038471A
Authority: GB
Original assignee: Teldix GmbH
Current assignee: Rockwell Collins Deutschland GmbH
Priority date: 1979-11-29
Filing date: 1980-12-01
Publication date: 1981-06-10
Also published as: FR2471582B1; IT8026322A0; GB2064116B; IT1134515B; FR2471582A1

Abstract

A north seeking gyro comprises a gyro rotor (22) with a horizontally alignable spinning axis (21), a first frame (20) in which the gyro rotor (22) is mounted, a second frame or pendulum (4) in which the first frame (20) is pivotally mounted about an axis (13), the second frame or pendulum (4) being in turn pivotally mounted from a Cardan joint (3) in a casing (1) and whose axis of rotation (13) is vertically alignable, a vertical alignment magnetic device (16, 17) for vertical alignment of the second frame or pendulum (4) a follow up device (30) with a pick off (31) for the angular position of the first frame (20) relative to the second frame or pendulum (4) and a stepping motor (5) for follow up of the second frame or pendulum (4) as a function of the pick off signal, the stepping motor (5) serving at the same time as an angular position sensor for the second frame or pendulum (4). <IMAGE>

Description

SPECIFICATION North Seeking Gyro The invention relates to a north-seeking gyro comprising a gyro motor with a horizontally alignable spinning axis, a first frame in which the gyro motor is mounted, a second frame or pendulum in which the first frame is pivotally mounted, which second frame or pendulum is in turn mounted pivotally suspended in a casing and whose axis of rotation is vertically alignable, a vertical alignment device for vertical alignment of the second frame or pendulum, a follow up device with a pick off for the angular position of the first frame relative to the second frame or pendulum and a servo drive for follow up of the said frame or pendulum relative to the casing as a function of the pick off signal. This type of gyro is known from German Auslegeschrift No. 14 98 042 and is formed basically as a rate gyro with one degree of freedom.The knonw gyro contains a frame pivotal about an axis of rotation which is aligned as vertically as possible, having a gyro rotor, the spin axis of which is orthogonal to the said axis of rotation or vertical axis and thus lies in a horizontal plane. The frame is arranged in a so-called "pendulum" so as to be pivotal with the aid of gas bearings, while the pendulum itself is suspended within a casing by means of a cardan joint such that the axis of rotation of the pendulum may be aligned vertically. The vertical alignment device so formed also contains an electrically actuable vertical magnet at the lower end of the pendulum, its armature being supported on a circular part of the casing, once vertical alignment has been carried out. The frame is restrained to a zero position in relation to the casing.Moreover, a follow-up circuit is provided containing a pick-off for the angular position of the frame, a suitable amplifier device and a servo drive between the casing and pendulum in order to being about follow-up of the pendulum in the sense of a reduction in the frame pick-off signal. With a northseeking gyro of this type, the horizontal component of the rotary speed of the earth acts, as is known, to provide a northward moment which depends on the sine of the deflection angle of the gyro vector axis from the north direction, which moment causes a corresponding deflection of the frame. The pendulum is therefore followed with the aid of the follow-up circuit until the deflection of the frame reaches zero. The angular position of the pendulum with respect to the casing, detected with the aid of an angle sensor formed as a resolver, corresponds largely to the north direction.Besides the northward moment, however, the slave moment and the interference moments caused in particular by gyro drifts, have to be taken into account as well as the frame bearings, current supply lines and vibrations from outside so that north errors arise which depend on the quality of the gyro. Moreover, there is a need for north alignment to be carried out in a short time since northing has to be carried out when the gyro carrier, a vehicle for example, is stationary and a fairly long period in which the vehicle is stationary is usually undesirable for tactical reasons.

Furthermore, a north-seeking gyro of the type stated is known from German Offenlegungsschrift No. 23 36 956, in which initially there is rapid follow-up into a first position in which the gyro spin axis is aligned roughly into the north direction. Then the follow-up circuit is isolated and a torque generator arranged between the frame and the pendulum is operated in dependence on the frame pick-off signal.

The current of the torque generator corresponds to the slave moment in the slave circuit thus formed so that, when the gyro vector is taken into account, the input angular velocity and thus the deflection from north can be ascertained.

Suitable filters are provided to limit the external disruptions which are to be expected.

Furthermore, in order to detect the angular position of the pendulum as compared to the casing, an angle sensor is provided in which the north direction is apparent from its signal and the north deflection. By switching on slave circuit, the dominant natural frequency of the gyro is increased if the transmission factor is predetermined appropriately so that the north error caused by detection moments and/or sensitivity of the gyro to disruptive movement is reduced. However, on the other hand, since the slave moment depends on the gyro drifts which change, more particularly, with temperature or time in a manner which cannot be foreseen or preselected, there may be not inconsiderable errors in the output north angle.

The invention therefore seeks to provide north-seeking gyro at a low cost in which the north direction can be ascertained accurately and rapidly and reliably.

According to the invention, there is provided a north seeking gyro comprising a gyro motor with a horizontally alignable spinning axis, a first frame in which the gyro motor is mounted, a second frame or pendulum in which the first frame is pivotally mounted, which second frame or pendulum is in turn mounted pivotally suspended in a casing and whose axis of rotation is vertically alignable, a vertical alignment device for vertical alignment of the second frame or pendulum, a follow up device with a pick off for the angular position of the first frame relative to the second frame or pendulum and a servo drive for follow up of the said frame or pendulum relative to the casing as a function of the pick off signal wherein the servo drive comprises a stepping motor having a high degree of step accuracy and serving at the same time as an angular position sensor for the second frame or pendulum.

The north-seeking gyro in accordance with the invention has a comparatively simple and lowcost construction due to integrating the stepping motor and the angle sensor which today's requirements for accuracy require and enable angle sensor systems which would otherwise be required and are expensive to be dispensed with. It is advisable to provide a flat wobble plate stepping motor provided with gear rims which have a high calibration accuracy. Thus the bearing between the stator and the rotor of the stepping motor may at the same time, form the bearing for the pendulum, thus considerably simplifying the design. In order to avoid Cardan errors, the stepping motor or angle sensor may be arranged between the Cardan joint fixed to the casing and the pendulum.In addition, the external axis of the Cardan joint may also be aligned very simply to the roll axis of the vehicle if the gyro is used in a vehicle. If necessary, however, a stepping motor of conventional construction may be provided, to the output of which a helical gearing is connected. In all cases it is particularly advantageous if the angular position of the pendulum can be ascertained from a predetermined zero position simply from control signals for the step motor, i.e. by counting the steps.

The pendulum is aligned vertically and the axis of rotation of the pendulum is vertically aligned, preferably with the aid of an electromagnet arranged on the lower end, the armature of which is supported on a spherical part of the casing. Of course, different vertical sensors may be provided for this purpose and may be used to control adjusting elements.

In a preferred embodiment in which a slave circuit is provided for the frame in known manner, the slave moments or input angular velocities are measured in two positions, preferably rotated with respect to each other, by a predetermined angle of 900 and the deflection from north is ascertained approximately from these moments and velocities. The pendulum is subsequently reset in accordance with the deflection from north with the aid of the step motor and therefore is aligned roughly towards the north direction. The north direction is then ascertained in a fine measurement. This extremely advantageous procedure provides a first approximate value for the north direction after a short time with an accuracy of the order of magnitude of 1 degree, the accuracy of which is improved by one order of magnitude by the fine measurement which takes slightly longer.It is advisable to carry out several angle measurements in order to suppress interference and to form mean values or carry out filtering of interference values. The above northing operation is controlled with the aid of a microprocessor or a computer device which also controls the determination of the angle and the filtering. In order to compensate for the gyro drift, in a preferred embodiment it is proposed to rotate the gyro after one fine measurement by a fixed angle, e.g. 1 800, with the aid of the stepping motor and then to carry out the second fine measurement. The north direction can then be ascertained to a very high degree of accuracy and the gyro drift can also be ascertained from the slave moments or input angular velocities which have been measured. Other advantages are apparent from the dependent claims and the example of embodiment.

The invention will now be described in greater detail, by way of example, with reference to the drawings, in which: Fig. 1 shows a general view of the north-seeking gyro partially in longitudinal section; Fig. 2 shows a block circuit diagram of the gyro in accordance with Fig. 1; Fig. 3 is a general sketch of the different axes of the north seeking gyro; Fig. 4 is a general sketch of the axes of the directional and the north seeking gyro; Fig. 5 is a block circuit diagram of the directional and north seeking gyro, and Fig. 6 is a general sketch of the axes of the north seeking gyro during measurement with 1 800 envelope.

The north-seeking gyro in accordance with Fig. 1 contains a pendulum 4 suspended from a baseplate 2 by a Cardan joint 3 in a casing 1. A stepping motor 5 formed as wobble plate motor is arranged between the Cardan joint 3, which is only shown schematically, and the pendulum 4. The stepping motor 5 contains a stator 6 fixed to the Cardan joint and a rotor 8 rotatably in a ball bearing 7.

At the same time, the ball bearing 7 acts as the bearing for the pendulum 4 which is connected to the rotor 8. A number of electrical windings 9 are arranged distributed over the periphery of the stator 6.

The rotor comprises a gear rim 11 connected to the pendulum by an axially resilient and radially rigid wobble plate or by a membrane 10. A gear rim 12 is associated with the gear rim 11, lying opposite thereto on the stator. The gear rims 11, 12 have a large number of teeth, the difference in the numbers of teeth preferably being equal to one. In a stepping motor of this type, the step width may be relatively coarsely selected in accordance with the number of teeth and the number of windings 9, while despite this, a very high division accuracy can be achieved. The pendulum may be rotated through the whole of its angular range about the axis of rotation 13 and set with a high degree of accuracy. The step motor 5 also serves as an angle sensor for sensing the position of the pendulum 4 with respect to the casing 1.

the control signals for the windings 9 being counted out starting from a predetermined zero position. A simple zero position pick-off not shown here is provided for this. An electrically controllable vertical magnet 1 6 is provided at the lower end of the pendulum 4, the armature 1 7 of which is constructed as a clamping foot and is supported on a spherical part 1 8 of the casing 1. It is apparent that a vertical alignment device is achieved for the cardanically suspended pendulum 4. If, in contrast to Fig. 1 , the vertical axis 1 9 is not vertically aligned, then by controlling the vertical magnet 16 with a pulse operation, the armature 17 can be raised away from the spherical portion with the result that the pendulum 4 becomes precisely vertically aligned. With this vertical alignment, which takes a very short time, approximately five seconds, it is advisable to select the control frequency of the vertical magnet 16 to be approximately four times as great as the natural frequency of the pendulum device. In some cases vertical alignment may be undertaken by means of vertical sensors and control adjusting elements such as servomotors or torque generators. The precision frame 20 is located within the pendulum 4, this frame containing a gyro rotor 22 which can be rotated about a spin axis 21.The frame 20 is mounted so as to be pivotable about the output or rotation axis 1 3 in relation to the pendulum 4 or follow-up frame with the aid of a static gas bearing containing annular axial and radial air gaps 24, 25. A schematically indicated compressor 27 is provided in the lower part of the pendulum 4 in order to feed the bearing. The compressor 27 is supported in the pendulum 4 through annular resonance damping elements 28, 29. Furthermore, an angular position pick-off and a torque generator 30, 31 are provided. On the one hand, the angular position of the frame 20 or the spin axis 22 with respect to the pendulum 4 is taken off and, on the other hand, a slave moment can be produced about the axis of rotation 1 3 in order to maintain the frame 20 in a predetermined zero position.A motor 32, 33 is provided as the drive for the gyro rotor 22 and is formed preferably as a brushless direct current motor. This motor comprises a multi-phase stator winding 32 connected to the frame 20 on two longitudinal sides, and permanent magnets 33, polarized alternately on the gyro rotor 22 towards the spin axis 21 and lying opposite the stator winding 32. The required electronic commutation unit 34 is arranged above the gyro rotor 22 in the frame 20. Current supply springs 36 which are shown schematically are provided between the pendulum 4 and the frame 20 moreover. The whole of the electronic unit 37, required for carrying out northing, including a computer, microprocessor and current supply, is arranged in the upper part of the gyro on the baseplate 22 or may be housed in a separate casing.

The general construction of the electronic unit of the gyro will now be described with reference to the block circuit diagram shown in Fig. 2 in which the same reference symbols are used for components which have already been described above. A current supply 40 is provided for the vertical magnet 16, compressor 27, d.c. motor 32, pick-off 30 and for an electronic control unit 41 of the stepping motor 5 and a computer or microprocessor 42. Besides serving to control the northing process and calculate the various angle values, the microprocessor 42 also serves to filter out the effects of interference movements, described in greater detail below, which occur, for example, in the form of vibrations acting on the casing. It can be seen that, in some cases, a conventional electronic computer or control device may be used.A simple zero-position pick-off 43 for indicating a fixed zero position for the pendulum 4 is associated with the stepping motor 5. The zero-position pick-off 43 is connected to the reset input of a forwards/backwards counter 44, signals being fed to its inputs from the electronic control unit 41, in accordance with the step number and the direction of rotation. Taking the step width of the stepping motor into account, the counter state corresponds to the angular position of the pendulum. The frame 20 is restrained into a defined zero position with respect to the pendulum 4 or follow-up frame by a slave circuit 45. In the stationary condition, the current of the torque generator 31 is a measure of the slave moment which is required in order to compensate the northward moment and interference moments.A signal proportional to the current is supplied, in a preferred manner, to the microprocessor 42 via an analog/digital converter 46. The clock pulses for the microprocessor 42 and the remaining parts of the electronic unit are provided by a clock generator 47 which may be quartz-stabilized.

The northing function and the control and computing steps to be carried out will now be described. In the first instance, the gyro is switched on by means of an external command for northseeking. In order to align the pendulum 4 vertically, the vertical magnet 1 6 is energized by signals having four times the natural frequency of the pendulum and after a few seconds, the output axis or axis of rotation 1 3 is exactly aligned vertically. The gyro rotor 21 reaches rated speed and at the same time the stepping motor 5 is rapidly rotated so that, when the zero position is exceeded, the counter 44 is reset and from then on the counter state corresponds to the angle position of the pendulum. It is advisable to shut off the slave circuit during start-up.As soon as the gyro rotor 22 has reached its rated speed, a first coarse measurement of the slave moment M, is carried out or the input angular velocity o, is measured taking the gyro vector H into account, in accordance with the relationship Ma H.

The slave moment M applied by the torque generator 31 is proportional to the current iwhich in turn produces a proportional number value N at the output of the A/D converter. The magnitude N is accordingly proportional to the input angular velocity w. The characteristic value # K= N is the overall amplification of the gyro. It is approximate constant. However, slight changes arising, for example, from a change in temperature or ageing of components, can be detected automatically and taken into account by a calibration process described below.

The following relationship applies to 01: (1) 1=H 'sin Q:o+D+S where: cos . cos ep=horizontal component of the earth's rotation at a particular geographic latitude: O=random azimuth angle towards geographic north; D=gyro drift; So,=error in measurement due to internal and external interference values.

Then, the gyro is rotated by a fixed angle of 900 with the aid of the stepping motor 5 and a second coarse measurement is carried out.

(2) Q)2=QH . sin (a0+900)+D+2 With coarse measurements, the measured values supplied to the micro-processor 42 via the analog/digital converter 46 are interrogated regularly over a period of time of a few seconds in each case and a mean value is formed from the individual measured values of WI and W2 which have been obtained in this way. If the individual measured values, more particularly as a result of external interference movements, have a high noise factor, it is advisable to lengthen the periods of measurement.

If the stepping motor 5 can be rotated through an angle range of more than 3600, the second coarse measurement may be carried out after either a positive or a negative rotation, but of course the sign must be taken into account.

If the angle range is exactly 3600, then it is advisable to carry out the two coarse measurements such that the second coarse measurement is at least approximately in the centre of the rotation range.

Without taking account of the errors in measurement be,)1 and SG92 but taking into account the sign or direction of rotation with the aid of the microprocessor 42, a first coarse estimated value aO for the azimuth angle can be found, i.e. in accordance with the relationship: #1-D* (3) #o=arc tan #2-D* An estimated value D is inserted for the gyro drift here, this value being zero in the case of the first northing carried out and in subsequent northing processes, as described below, the value is improved repeatedly. If at this point in time, northing has to be interrupted, there is at least a first coarse approximate value o of the azimuth angle.

The stepping motor 5 and the pendulum 4 is then rotated about the angle a0t900 by means of the electronic control unit 41 controlled by the microprocessor 42 so that the spin axis 21 is roughly aligned to the north direction. In a preferred refinement, the rotation is for the shortest path and rotation to north or to south takes place according to the position which is closer. It should be noted at this point that coarse alignment of the gyro may be carried out, if necessary, in a different manner, for example with the aid of a different north direction reference.

When coarse alignment is carried out, a first fine measurement of the angular velocity is carried out and the following relationship applies: (4) 03=QH Aa+D+co3 The deflection from north which remains due to inaccuracy of the calculation of a0 and due to the final step width of the stepping motor is designated Aa taking into account the fact that the value of the sin corresponds to the angle for small angles and the error in measurement is designated ##3. In the interests of optimally suppressing interference, individual measured values are interrogated with the aid of the microprocessor over a fairly long period of time and are averaged out within a period of, say, one minute.The individual values are smoothed, preferably by low-pass filters, in the microprocessor 42 while the time constant rises continuously. Moreover, it is highly advantageous not to take into account individual measured values in which the difference from the filtered mean value of the preceding values exceeds a preselectable tolerance limit, the tolerance limit dropping preferably over the period of measurement. Moreover, it is advisable to determine the tolerance limit for each individual measured value or for a group of individual measured values from the spread of preceding individual measurements and/or to shorten or lengthen the measurement period depending on the spread.

Higher frequency interference is filtered out of individual measured values and individual measured values which are far removed from the mean value are suppressed with the result that errors occurring only once are effectively suppressed. On the other hand, if necessary, northing can be interrupted during the first fine measurement and the north direction can be ascertained, of course with a lower degree of accuracy, from the mean angular velocity 3 obtained up to that time.

The remaining mechanical deflection Aa* from north is ascertained from the mean value w", of the angular velocity w3, determined in this way, with the aid of the microprocessor: w3-D* (5) Aa*= #H The north direction is determined, in accordance with the angle position aO from the north deflection Aa* together with the signal from the stepping motor 5.

Thus there is an error: D*-D-##3 (6) #α3=#α-#α*= #H The error in measurement AC93 is relatively small in view of the filtering which has been described above and the mean value formation during the first fine measurement. Since, however, the actual gyro drift D usually differs from the estimated value D* which was previously used, because of changes in temperature, ageing, etc., the systematic error portion AD=D*-D is largely suppressed by subsequent calibration in a preferred further embodiment. The gyro is rotated as exactly as possible about 1 800 by means of the stepping motor 5.In this position, a second fine measurement of the angular velocity W4 is carried out in which the mean value is formed as described above and the individual measured values are filtered.

(7) W4=H . Aa+D+a;4 The north deflection Aa is now ascertained from the angular velocities of the two fine measurements, determined in this way, with the precondition that the errors in measurement 3 and are are substantially smaller than the error proportion AD, so that the following relationship applies: W3-W4 (8) Aa= 2QH The north direction or the north angle a is now determined from the north deflection Aa or the angular position of the pendulum and in fact has a very high degree of accuracy. In other words: the constant error proportion as a result of the interference moment in terms of time is calculated from the measured values of the two fine measurements and is compensated for by the computer.

Furthermore, the gyro drift is ascertained from the relationships (4) and (7), i.e. from the filtered measured values W3 and 04, at least approximately, namely according to the relationship: 3+4 (9) 3 The mean value ascertained is stored in the microprocessor 42 as the new estimated value D* of the gyro drift. In the next northing process, the north deflection is ascertained by this estimated value after the first fine measurement in accordance with equation (5).

Consequently, particularly in the case where there is no time for a second fine measurement, the north deflection is very accurately determined after the first fine measurement in accordance with equation (5).

The horizontal component of the earth's rotation n,=a . cos y is required in the above-mentioned relationships (5) and (8). As is known, for small north deflections it is sufficient to calculate with a mean value which is valid for any application and is accurate to a few percent, this mean value being fed externally into the computer or microprocessor. In a preferred embodiment, however, it is proposed to calculate the horizontal component S2w approximately from the measured values G91 2 of the two rough measurements.

The value obtained according to equation (9) is taken into account advantageously for the gyro drift D. Moreover, an approximate value P of the geographical latitude is obtained from the horizontal component of the earth's rotation DH calculated in this way and in fact in accordance with the following relationship: (11) =arccos QH/Q Since in the case of the measurements described above the angular velocities W are not directly available but rather the resultant digital values N are available as initial magnitudes, the total amplification K of the device must be known. From time to time, in conjunction with coarse measurement, the spin axis is rotated into two positions, rotated preferably by +90 with respect to north.Thus, the measured values: (12) 1 Ns=(QH sin (At+90 )+D+86)s) K N6=(QH sin (Aa-900)+D+6) K are obtained. From this, the amplification can be calculated approximately in accordance with the relationship: 2QH (13) K=- N5-N6 since sin (Aa+900)=cos A,a1, and sin (Aa-900)=-cos Aa""'-1.

The northing process of the gyro takes place in the above mentioned process steps while the pendulum 4 and thus the restrained spin axis 21 of the gyro are set with the aid of the stepping motor 5 into the position required, in each case, with a high degree of accuracy. In addition, the microprocessor 42 preferably serves to control the individual process steps and/or to control the stepping motor 5 and/or to calculate the stated angle values and filter the individual measured values obtained during coarse and fine measurement of the slave moment or angular velocity. When northing has already been carried out, then in subsequent northing processes, coarse northing may be dispensed with and a value for north obtained and stored during a preceding northing process may be used as the estimated value for coarse alignment of the gyro.

In the general sketch according to Fig. 3, a coarse north alignment N9 of the spin axis of the gyro rotor is shown graphically. Using a value which is related to north and is present or stored away in a store or some other means, e.g. the directional value display of a directional gyro, electrical pulses are supplied to the stepping motor 5 which cause the pendulum 4 to pivot in relation to the casing 1 and thus bring the spin axis 21 of the gyro rotor 22 and also the direction of the twist vector into a position Ng which is near north. Then the north deflection angle Aa* is ascertained with the aid of one or more fine measurements and the exact north direction N is determined.

Fig. 4 shows the principle of a coarse north alignment with the aid of a directional value drawn from a directional gyro. In a first north seeking process, the north direction N was ascertained with the aid of a fine measurement. The spin axis of the direction gyro lies in any deisred direction a in relation to north and includes an angle y with the longitudinal axis F1 of the vehicle. In order to simplify the representation, the spin axes of the directional and the north seeking gyro are projected into a horizontal plane and shown with a common intersection point, even if the spin axes are spatially separated in a practical embodiment. The angle a between the spin axis KK 1 and the north direction N is established as a reference value and stored by the directional gyro.The instantaneous course a of the vehicle is determined by adding the angles a and y.

During the vehicle's journey the longitudinal axis F of the vehicle and of the north seeking gyro (which is not in operation) change their position relative to the north direction, while the spin axis of the directional gyro retains its position with the exception of its drift. As a result of this drift however, the directional gyro spin axis eventually moves out of its position so that the course information is falsified by the integral of this drift, the error angle p. After a certain time the vehicle longitudinal axis takes up the position F2, for example, the spin axis of the directional gyro takes up the position KK2. By renewed northing with the aid of the north seeking gyro and forming an angle aneu, this error is corrected.The spin axis of the north seeking gyro is initially aligned such that it includes the reference value a of the preceding northing together with the spin axis and thus is pre-northed up to an angle error of'3. The spin axis of the north seeking gyro is north aligned roughly and is located in position Ng and in a following fine measurement the north deflection angle Aa* and the exact north direction is ascertained and the angle aneu between north and the instantaneous course gyro position is stored.

In Fig. 5 the arrangement of the gyro and a part of the peripheral devices on a vehicle is shown in a block circuit diagram. During a journey, the course and the position of the vehicle is ascertained and displayed with the aid of the course gyro KK and a distance counter not shown in Fig. 5. If the vehicle is stationary at the end of the journey, they at the same time the directional gyro is switched off. The last course value i.e. the course direction a of the longitudinal axis of the stationary vehicle in relation to north is stored away in a store Sp. When setting the vehicle in operation again the exact north direction must be ascertained with the aid of the north seeking gyro.A computer determines the required pivot angle AS for the pendulum from the stored course information a and the position SAS of the spin axis of the north seeking gyro, in order to align the spin axis of the north seeking gyro into a position which is close to north. After this process, fine measurement is carried out. Of course, the computer itself may serve as the store Sp.

Another north seeking process to be carried out after a certain time may be carried out with the method described in relation to Fig. 4.

If, after a measurement process has ended with coarse alignment or measurement in order to determine the north deflection is required in relation to the north direction, then a further fine measurement may follow directly without switching off the north seeking gyro, i.e. with the aid of a suitable operating element another start command is given during the first measurement.

Fig. 6 shows the graphic representation of this method in which the north value Aa* of the spin axis, ascertained during the preceding fine measurement, is used directly as a coarse north value Ng here.

For further fine measurement the spin axis is pivoted about the angle Aa*, so that the remaining deflection of the spin axis from north and therefore errors depending on the angle are reduced.

Furthermore, certain gyro errors may be eliminated by pivoting the spin axis about an angle 1 800-Aa*, in which Aa* corresponds to the north value of the preceding fine measurement and a further fine measurement takes place in the position NF.

Claims

1. A north-seeking gyro comprising a gyro motor with a horizontally alignable spinning axis, a first frame in which the gyro motor is mounted, a second frame or pendulum in which the first frame is pivotally mounted, which second frame or pendulum is in turn mounted pivotally suspended in a casing and whose axis of rotation is vertically alignable, a vertical alignment device for vertical alignment of the second frame or pendulum, a follow up device with a pick-off for the angular position of the first frame relative to the second frame or pendulum and a servo-drive for follow up of the second frame or pendulum relative to the casing as a function of the pick-off signal wherein the servo drive comprises a stepping motor having a high degree of step accuracy and serving at the same time as an angular position sensor for the second frame or pendulum.

2. A north-seeking gyro according to claim 1, wherein the second frame or pendulum is suspended in the casing by a cardon joint.

3. A north-seeking gyro according to Claim 2, wherein the cardan joint is fixed to the casing and the stepping motor is arranged between the cardan joint and the second frame or pendulum.

4. A north-seeking gyro according to Claim 2, or 3, wherein the stepping motor is a stepping motor which is flat in the direction of the axis of its rotation and the stator of which is connected to the cardan joint the second frame or pendulum being fixed to the rotor of the stepping motor, and a bearing for the rotor serving at the same time as a bearing for second frame or pendulum.

5. A north-seeking gyro according to any one of the preceding claims, wherein the stepping motor has cooperating gear rims on its stator and rotor with a relatively large number of teeth, the difference in the number of teeth being at least one and the gear rim of the rotor being resiliently arranged in relation to its axis of rotation and rigidity arranged in the radial.

6. A north-seeking gyro according to any one of the preceding Claims, wherein the vertical alignment device comprises an electromagnet controllable electrically and arranged at the lower end of the second frame or pendulum, the armature of the electromagnet being engageable with a spherical part of the casing.

7. A north-seeking gyro according to any one of Claims 1 to 5 wherein the gyro or pendulum can be set in fixed positions by the stepping motor in the course of which the angular position is detected; a slave circuit is provided to restrain the first frame into a predetermined zero position, the slave circuit having a frame pick-off and a torque generator acting about the output axis or axis of rotation of the first frame the current of the torque generator constituting a measure of the input angular velocity w and wherein the north direction is determined from the measured value or values of the input angular velocity.

8. A north-seeking gyro according to Claim 7, wherein a computer unit or a microprocessor is provided, measured values related to the angular speed or the current of the torque generator or the angular position of the pendulum being supplied thereto and the north direction is determined from these measured values.

9. A north-seeking gyro according to Claim 7 or 8, wherein the step width of the pendulum which can be achieved with the aid of the stepping motor is relatively large while the accuracy of the division or the reproducible setting accuracy is of the order of magnitude of the desired measurement accuracy of the gyro.

10. A north-seeking gyro according to any one of the preceding Claims, wherein an electronic control unit is provided for the stepping motor and a zero position pick-off is provided, the angular position of the gyro being determined from the signals of the electronic control unit and the zero position pick-off.

11. A north-seeking gyro according to any one of the preceding Claims, wherein angular velocities o,, W2 are measured in at least two positions of the spin axis while the setting process is carried out with the aid of the stepping motor; a first estimated value a""0 for the azimuth angle is determined from the measured values of two course measurements; and subsequently the gyro or the pendulum is rotated with the aid of the stepping motor in accordance with the estimated value ci O of the azimuth angle such that the spin axis is coarsely aligned to north.

1 2. A north seeking gyro according to claim 11, wherein the two positions of the spin axis are rotated with respect to each other by 900.

13. A north-seeking gyro according to Claim 11 or 12, wherein the two coarse measurements are carried out in two positions rotated by exactly 900 and the estimated value a""0 of the azimuth angle is determined from the arc tan of the measured values " W2 of the angular velocities which have been inserted, with the correct sign, in accordance with the relationship: W1-D* aO=arc tan W2-D* where D* is a value previously known and/or calculated from other measurements, for the gyro drift.

14. A north-seeking gyro according to Claim 12 or 13, wherein the stepping motor is rotatable through an angular region equal to or greater than 3600 and the second coarse measurement is carried out at least approximately in the vicinity of the centre of the angular region.

1 5. A north-seeking gyro according to any one of Claims 12 to 14, wherein the coarse measurements are carried out in each case for a preselectable time of a few seconds while, in each case, a number of individual measured values are regularly interrogated and the measured values el), and W2 of the angular velocities are determined by filtering and/or mean value formation.

1 6. A north-seeking gyro according to any one of Claims 11 to 1 5, wherein an approximate value of the horizontal component of the earth's rotation ?2H is produced from the angular velocities &commat;1, W1, W2 in accordance with the relationship.

where D* is a value for gyro drift.

1 7. A north-seeking gyro according to claim 7 and any one of Claims 8 to 13, when appendent directly or indirectly to claim 7, wherein the spin axis of the gyro is held in its aligned position by the slave circuit after a coarse alignment; the angular velocity (03) is measured in a first fine measurement; and from this measurement the deflection Aa* from the north position is determined according to the relationship: W3-D* At*= #H where D* is a value, previously known and/or calculated from other measurements, for gyro drift and QH is the horizontal component of the earth's rotation.

1 8. A north-seeking gyro according to Claim 1 7 wherein the fine measurement is carried over a fairly long and preselectable period of time, a number of individual measured values is interrogated regularly and a mean measured value W3 is determined from these by filtering and/or mean value formation.

1 9. A north-seeking gyro according to Claim 18, wherein the individual measured values are smoothed by means of a low-pass filter.

20. A north-seeking gyro according to Claim 1 9, wherein the time constant of the low-pass filter increases continuously or in step.

21. A north-seeking gyro according to any one of Claims 18 to 20, wherein individual measured values are not taken into account when the difference between them and the mean value from the preceding values exceeds a preselectable tolerance limit.

22. A north-seeking gyro according to Claim 21, wherein the tolerance limit is reduced during the period of measurement.

23. A north-seeking gyro according to Claim 21 or 22 wherein the tolerance limit is determined for each of the individual measured values or for a group of individual measured values in each case from the spread of the preceding individual measured values and/or the period of measurement is changed as a function of the spread.

24. A north-seeking gyro according to Claim 17, wherein after carrying out the first fine measurement, the spin axis of the gyro or the pendulum is rotated through a defined angle, with the aid of the stepping motor; subsequently the angular velocity W4 is measured in a second fine measurement; and from the measured values of the second fine measurement the proportion of the disruptive moments, which is largely constant in terms of time, is ascertained and stored.

25. A north-seeking gyro according to Claim 24, wherein the spin axis of the gyro or the pendulum is rotated through 1800.

26. a north-seeking gyro according to Claim 24 when appendant to any one of Claims 18 to 23 wherein filtering and/or formation of the mean value which corresponds to that of the first fine measurement is carried out during the second fine measurement.

27. A north-seeking gyro according to Claim 24, 25 or 26, wherein the deflection Aa from the north position is determined from the angular velocities W3, W4 in accordance with the relationship: W3-W4 Aa=

2.

in which QH is the horizontal component of the earth's rotation.

28. A north-seeking gyro according to Claim 22 or 23, wherein the gyro drift D is determined at least approximately from the angular velocities W3, s94 in accordance with the relationship: #3+#4 D= 2 and stored, in a subsequent north-seeking process, the stored gyro drift D is used both during coarse measurement and fine measurement as an aprioriestimated value D*.

29. A north-seeking gyro according to Claim 7 or any claim appendent directly or indirectly thereto; wherein the microprocessor controls the northing operation and/or controls of the stepping motor and/or makes the various calculations and/or filters individual measured values.

30. A north-seeking gyro according to Claim 11 or any claim appendent directly or indirectly thereto, wherein the spin axis of the gyro is rotated, after calculation of the azimuth angle aO into one or more directions successively and the change AN in the output value N delivered by an A/D converter responsive to the slave circuit output is ascertained.

31. A north-seeking gyro according to claim 30, wherein the spin axis of the gyro is displaced by +900.

32. A north-seeking gyro according to Claim 30 or 31, wherein the entire amplification # K= N of the gyro is calculated from the measured value AN and the change AW in the component of the earth's rotation which acts on the gyro, this amplification giving the relationship between an angular velocity W acting from outside and the digital value N which arises from the resultant current i of the torque generator after A/D conversion, in accordance with the relationship: AW K= AN

33.A north-seeking gyro according to Claim 30, 31 or 32 wherein the rotation relative to the coarse measurement takes place exactly into the successive positions a5=900-a0 and a6=a511 800; output magnitudes N5 and N6 corresponding to these positions are measured; and the characteristic value K is calculated according to the relationship: 2QH K= N5-N6

34. A north-seeking gyro according to Claim 17, wherein with a subsequent north seeking process, the rough north alignment of the spin axis of the north seeking gyro takes place for a further fine measurement using the north direction ascertained during the preceding north seeking process.

35. A north-seeking gyro according to Claim 34, wherein a directional gyro connected to the north seeking gyro, serves for coarse north alignment of the spin axis, alignment (a) of the directional gyro with respect to north having been ascertained during the preceding north seeking process.

36. A north-seeking gyro according to Claim 35, wherein the coarse north alignment of the north seeking gyro takes place with the aid of the directional gyro only within a predetermined period after the last north seeking process.

37. A north-seeking gyro according to Claim 34, wherein the alignment of a directional gyro with respect to north is communicated to the directional gyro with the aid of north seeking processes, wherein when the directional gyro is switched off the alignment S of the longitudinal axis of a vehicle carrying the gyro with respect to north is stored away and that in the case of a following north seeking process, the stored value serves as a north reference for coarse north alignment of the spin axis.

38. A north seeking gyro according to any one of Claims 34 to 37, wherein selection is made between a normal north seeking process with coarse and fine measurement and a coarse north alignment based on the north direction ascertained during the last north seeking process with the aid of an operating element.

39. A north seeking gyro according to Claim 34, wherein the north direction ascertained during the last fine measurement is used as coarse north alignment of the spin axis during further measurement required subsequently to a fine measurement without switching off the gyro.

40. A north seeking gyro according to Claim 38, wherein the further measurement after rotatiny the pendulum about an angle of a5=. Aa* takes place where Aa* is the north deflection ascertained during the last fine measurement.

41. A north-seeking gyro according to Claim 38, wherein the further measurement takes place after a rotation of the pendulum about an angle a5=1 8O0-Aa* where A* is the north deflection ascertained during the last fine measurement.

42. A north-seeking gyro substantially as described herein with reference to the drawings.