GB2083910A

GB2083910A - Gyro Damping Apparatus

Info

Publication number: GB2083910A
Application number: GB8124908A
Authority: GB
Original assignee: Bodenseewerk Geratetechnik GmbH
Current assignee: Bodenseewerk Geratetechnik GmbH
Priority date: 1980-09-04
Filing date: 1981-08-14
Publication date: 1982-03-31
Also published as: FR2489504A1; GB2083910B; DE3033281C2; DE3033281A1; FR2489504B1

Abstract

A two-axes dynamically-tuned displacement gyro (30) is mounted in gimbals to pivot about mutually perpendicular x and y axes, and the gyro rotor spins about a Z axis. Movements of the gyro housing (44) relative to inertial space are measured by pick-offs (54A, 54B). Torquers (56A, 56B) are arranged on the x and y axes, respectively. Known types of such gyros may suffer from nutation oscillation which causes them to drift further and further from their correct attitude. The present invention damps such oscillation by providing a differentiating circuit between the x- axis pick-off and the x-axis torquer. <IMAGE>

Description

SPECIFICATION Gyro Damping Apparatus This invention relates to apparatus for damping nutation oscillation of a dynamically-tuned twoaxes displacement gyro, of the type including first and second mutually perpendicular input axes, first and second displacement pick-offs responding to relative movements, about the first and second input axes, respectively, between a gyro rotor and a housing, a further including a torquer acting about the first input axis.

In the absence of outside torques, a two-axes displacement gyro tries to maintain its orientation in space. Movements of the gyro housing with respect to the gyro rotor, i.e. with respect to inertial space, may be measured by pick-offs. The pick-off signals may, for example, be applied to servomotors to stabilise a platform. It is a requirement for such displacement gyros that, as far as possible, there are no torques acting on the gyro rotor through the bearing. It is known to construct such two-axes displacement gyros as so-called "dynamically-tuned gyros". Such gyros include a rotor connected to the drive shaft by means of a gimbal, which is connected to the drive shaft by means of a leaf spring joint, and which is also connected to the rotor by means of a leaf spring joint, extending perpendicular with respect thereto.Upon deflection of the gyro housing with respect to the gyro rotor, the leaf spring joints would normally apply a torque to the gyro rotor, which would cause the gyro rotor to change its reference attitude in space. With a dynamically-tuned gyro, the tuning is chosen such that the torques applied by the leaf spring joints are compensated for by dynamic torques. These dynamic torques are caused by an oscillating movement of the gimbal, which oscillating movement occurs with deflection of the gyro housing with respect to the gyro rotor which is fixed in space.

British Patent Specification No. 1,551,309 (German Offenlegungsschrift 25 45 025) discloses a navigational instrument for the navigation of land vehicles which has a northseeking meridian gyro to determine north direction, and a free gyro as a heading reference unit which may be aligned with the meridian gyro.

A speed sensor produced a signal proportional to the driving speed. A computer determines the position of the vehicle from the heading angle of the heading reference unit and the signal produced by the speed sensor. A corrective signal depending on geographical latitude is produced from the positional signals and is applied to the free gyro or is taken into account by the computer, the corrective signal compensating for the drift of the free gyro relative to the earth-fixed coordinate system caused by the rotation of the earth.

In a practical embodiment of such a navigational instrument, a dynamically-tuned gyro was employed as a two-axes displacement gyro.

It was found that, whilst the navigational instrument operated satisfactorily in general, the displacement gyro began to drift from its correct position and ran against the stop at a certain speed. Similar phenomena have also been observed with other applications of dynamicallytuned displacement gyros.

It is an object of the present invention to overcome such drifting of the displacement gyro.

According to the invention, in a dynamicallytuned displacement gyro of the type described, the signal of the first pick-off is applied to the torquer acting about the first input axis through a differentiating input network.

The invention is based on an examination of the causes of the described disturbing effects, and on the discovery that undamped nutation oscillations being excited to resonance are responsible therefor. The invention provides for a displacement gyro circuit which damps nutation oscillation but does not affect the features of the displacement gyro which are important for its function.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of the construction of two-axes displacement gyro, Fig. 2 is an idealized block diagram demonstrating the interaction between torques and deflections about the two input axes, Fig. 3 is a schematic pictorial view of a dynamically tuned two-axes displacement gyro, Fig. 4 is a longitudinal section through a.

dynamically-tuned two-axes displacement gyro, Fig. 5 is a pictorial view of a displacement gyro in a heading reference unit.

Fig. 6 is a block diagram of an input network for the gyro, and Fig. 7 is a block diagram showing the arrangement of the input network in a heading reference unit.

Fig. 1 illustrates schematically a two-axes displacement gyro. A gyro rotor 10 rotates about its spin axis 12. The gyro rotor 10 is mounted in a gimbal 1 6 and pivots about an axis 14 extending perpendicularly to the spin axis 12. The gimbal 16 is mounted in a gyro housing 20 and pivots about an axis 18 extending perpendicularly with respect to the axis 14.

The axis 1 8 forms the first input axis, or x-axis, of the displacement gyro, and the axis 14 forms its second input axis, or y-axis. A first displacement pick-off 22 and a first torquer 24 are arranged on the first input axis 18. A second displacement pick-off 26 and a second torquer 28 are arranged on the second input axis 14.

Referring also to Fig. 2, MTX and M, designate the torques acting about the first input axis 18 and the second input axis 14, respectively. a and are the deflections of the housing 20 with respect to inertial space about the first and seond input axes 1 8 and 14, respectively. Pick-off signals a and DA, respectively, are obtained from the pick-offs 22 and 26.The deflections of the gyro rotor 10 with respect to inertial space about the first and second input axes 18 and 14, respectively, are designated by a:R and JBRT resPeCtiVelV Fig. 2 shows the interrelation between the torques MTx, MTy and the deflections a= P as well as a ,BR and aCA, ssA A deflection XBR of the gyro rotor 10 about the second input axis, or y-axis, causes a torque about the first input axis, or xaxis, with a transfer function Hs, and in turn a deflection α;R of gyro rotor 10 about the first input axis, or x-axis, causes a torque about the second input axis, or y-axis, with the same transfer function, wherein the spin is designated by H and the Laplace-operator is designated by s. Torques Mtx and MTy cause deflections atR and X13RT respectively, about the same input axis, having the transfer functions

a lxs2 and , respectively, 1 52 wherein Ix and Iy designate the moments of inertia about the x- and y-axes, respectively.The pick-off signals provide the differences α-αR and ss-ssR respectively, of the rotating movement of the housing 20 and the gyro rotor 10 relative to inertial space.

If there are no torques MT,, MTy acting about the input axes 18 and 14, sgR=,BR=Of and the pickoff signals (ZA and ,BA immediately provide the rotary movements a and p, respectively, of the gyro housing 20 relative to inertial space.

However, disturbing torques always occur because of continually present deficiencies of the displacement gyro and because of environmental influences, which disturbing torques excite an undamped characteristic oscillation of the twoaxes displacement gyro, the so-called nutation oscillation. This nutation oscillation has a frequency of

These oscillation components occurring due to the nutation oscillation in the pick-off signals of the displacement gyro are generally filtered out by comb filters. However, the system consisting of the gyro rotor 10 and the gimbal 16 makes these oscillations.

Fig. 3 is a schematic pictorial view of a dynamically-tuned two-axes displacement gyro 30. The gyro includes a housing-fixed motor 32 having an elongate drive shaft 34. A gimbal 36 is connected to the drive shaft 34 by means of an inner leaf spring joint 38. In turn, a gyro rotor 40 is connected to the gimbal 36 by means of an outer leaf spring joint 42 extending perpendicularly to the inner leaf spring joint 38.

With respect to the kinematic conditions, the inner leaf spring joint 38 corresponds to the first input axis 18 of Fig. 1, the gimbal 36 corresponds to the gimbal 16 and the outer leaf spring joint 42 corresponds to the second input axis 14. The drive shaft 34 of the motor 32 is mounted housing-fixed thereto and fulfils the functions of the "gyro housing" 20 in the schematic illustration of Fig. 1 and that of a rotor drive at the same time. The system is tuned in a known manner such that the torques exerted on the gyro rotor 40 by the leaf spring joints 38 and 42 due to their spring restraint are compensated for by dynamic torques produced by an oscillating movement of the gimbal 36 during the deflection of the gyro housing 44 relative to the gyro rotor 40 fixed in space.

Fig. 4 shows the construction of such a dynamically-tuned displacement gyro 30 by means of a longitudinal sectional view. The motor 32 having an outer stator 46 and an inner rotor 48 is disposed on the gyro housing 44. The rotor 48 with its drive shaft 34 is mounted in the housing 44 by means of bearings 50 and 52. The gyro rotor 40 is mounted at the end of the drive shaft remote from the motor by means of the above-described leaf spring joint and gimbal arrangement (not shown in Fig. 4). The position of the gyro rotor 40 with respect to the gyro housing 44 is picked off in two planes extending perpendicularly to each other by pick-offs 54 which correspond to the pick-offs 22 and 26 of Fig. 1 as regards their functions.Furthermore, torquers 56 are provided through which torques can be exerted, in known manner, about the two input axes extending perpendicularly with respect to each other, and which torquers correspond to the torquers 24 and 28 shown in Fig. 1.

In the torque equations for the two input axes, the x-axis and the y-axis, shown in the housingfixed system, the following terms appear amongst others in equation (2) with reference to the x-axis:

wherein "c" designates a residual spring constant of the leaf spring joint 38 (Fig. 3) caused by inexact dynamic tuning of the displacement gyro and the gimbal 36, and "o" designates the cyclic frequency of the gyro rotor 40.

If it is assumed that due to angular vibration a sinusoidal housing deflection appears such that

this would lead to the following torque component in equation (2):

The nutation oscillation of the displacement gyro 30 is undamped. Each cyclic excitation from outside which has the frequency Ct)N of the nutation oscillations increases their amplitude.

The nutation oscillation of the displacement gyro is then excited in its inherent resonance, whereby the deflection of the gyro rotor 40 increases continuously because of the lack of damping, until the gyro rotor runs against the stops.

The nutation frequency w, of the dynamicallytuned displacement gyro considered here is smaller than twice the cyclic frequency e.) of the gyro rotor 40. Therefore, this nutation frequency can be excited by a torque component having a frequency of (2s9Q).

then yields

If, therefore, the gyro housing 44 makes angular oscillations with the frequency Q given by equation (5), disturbing torques appear about the x-axis, the frequency of which equals the nutation frequency N The undamped nutation oscillation is excited to resonance, which has the abovementioned results.

The order of magnitude of frequency Q will be illustrated by means of a numerical example for a two-axes displacement gyro in practical use esN=27r.461 per sec., a)=27t.240 per sec. or 20=27d.480 per sec.

From this we have 91)=20--WN=276. 19 per sec.

As excitation frequencies of 19 Hertz may certainly be expected in practice, instruments including such displacement gyros as sensors suffer from dynamic problems in many cases. Drifting of the displacement gyro was observed in a vehicle navigational instrument as shown in the abovementioned specification with the instrument in use, for example, in a track-laying vehicle, when the frequency resulting from the length of a track member and the speed of the vehicle reached 1 9 Hertz.

As the described phenomenon is a systematic effect, this effect cannot be eliminated in dynamically-tuned displacement gyros having one individual gimbal only. The only counter-measure could be to reduce the consequence of this effect by damping the nutation frequency by means of outside circuitry. Care must be taken, however, that the gyro will, to a large extent, maintain its displacement gyro characteristics with the transfer function "1" between a and CtA as well as p and pAt respectively (Figure 2).It must also be ensured that the displacement gyro in a vehicle navigation system of the type shown, for example, in the above-mentioned patent specification shows the correct follow-up performance, if it is to be maintained in the "grid-north" direction by means of a follow-up signal, as described above, for taking the rotation of the earth into account.

It has become evident that such circuitry is possible (only one, however). This circuitry is shown in Figures 5 to 7. Figure 5 shows a schematic pictorial view of a heading reference unit including a two-axes dynamically-tuned displacement gyro of the type shown in Figure 4.

The heading reference unit comprises an outer gimbal 58 disposed within an instrument housing 62 and rotatable about an aXis 60. An inner gimbal 64 is disposed within the outer gimbal 58, the inner gimbal being rotatable about an axis 66 which is perpendicular to the axis 60. A platform 68 is disposed within the inner gimbal 64 and is rotatable about a substantially vertical azimuth axis 70 which is perpendicular to the axes 60 and 66. The platform 68 is rotatable about the azimuth axis 70 by means of an azimuth servomotor 72.

The displacement gyro 30 is disposed on the platform 68 such that the spin axis z extends horizontally and substantially radially with respect to the azimuth axis 70, the x-axis is vertically disposed, and the y-axis extends horizontally, perpendicularly to the spin- and x-axes. A servomotor 74 is mounted on the platform 68, so that the housing 44 of the displacement gyro 30 is rotatable by the servomotor about the y-axis.

As can be seen from Figures 5 and 7, the signal from the x-pick-off 54A is applied to the azimuth servomotor 72 by means of a pre-amplifier 76, a demodulator 78 and a suitable network 80, all of these items being indicated by an amplifier 82 in Fig. 5. The signal from the y-pick-off 54B is applied to the servomotor 74 by means of a preamplifier 84, a demodulator 86 and a network 88. In that way, the housing 44 is continuously caused to follow up the gyro rotor 40. The gyro rotor 40 remains fixed in space with respect to intertial space.The spin axis Z can be deflected at an angular speed with respect to the inertial space around the x-axis by means of a torque exerted on the torquer 56B acting about the y axis, which angular speed compensates for the rotation of the earth and retains the spin axis Z in a fixed relation with the "grid-north" direction of a UTM grid, for example. A signal from a vertical direction sensor 90 (Fig. 7), for example a clinometer, which is sensitive about the y-axis, is applied by means of a demodulator 92 and a filter 94 to the torquer 56A which acts about the xaxis. It will be ensured, thereby, that the spin axis of the displacement gyro 30 is continuously horizontal. The construction of the heading reference unit described up to this point is known per se.

The signal from the pick-off 54A acting about the x-axis is now also applied by means of an input network 96 to the torquer 56A which acts about the x-axis. This damps the nutation oscillations.

As shown in Fig. 7, the signal from the pick-off 54A, pre-amplified by the pre-amplifier 76 and demodulated by the demodulator 78, is supplied to a summing circuit 98 through an input network 96 and is added to the signal of a vertical direction sensor 90, whereby the sum is applied to the torquer 56A.

Fig. 6 is a block diagram of the input network 96. An input voltage UE to the input network 96 is applied to an input of an operational amplifier 100 by means of a capacitor C and an ohmic resistor R,. The output voltage UA of the input network 96 at the output of the operational amplifier 100 is fed back to the input of the operational amplifier by means of a negative feedback resistor RR The transfer function of this input network is UA (S) RRC . S (7) UE(S) 1+RECS s again being the Laplace operator.

If we assume that (8) RECS 1,.

this results in UA(S) (9) SRK-C-S, UE(S) which means that the network 96 has differentiating characteristics and operates as an ideal differentiator and thus an ideal damping element, if the cut-off frequency of the denominator is sufficiently high.

As already mentioned above, the normal behaviour of the displacement gyro 30 must not be falsified too much through the choice of the input network. Damping factors of 0.1 to 0.2 for nutation frequencies have proved to be suitable.

For damping factor of 0.1 the associated gain V of the damping circuit formed by the input network and the torquer 56A results in V=36p cm sec. By the derived value of V the value of the product RKC is also determined, if a sufficiently small time constant RE.C is selected. Thereby the circuitry of Fig. 6 is dimensioned.

Claims

1. Apparatus for damping nutation oscillation of a dynamically-tuned two-axes displacement gyro including first and second mutually perpendicular input axes, first and second displacement pick-offs responsive to relative movements about the first and second input axes, respectively, between a gyro rotor and a housing, and a torquer acting about the first input axis, wherein the signal of the first pick-off is applied to the torquer which acts about the first input axis, through a differentiating input network.

2. Apparatus as claimed in Claim 1, wherein the displacement gyro is a heading gyro having a horizontal spin axis; and wherein the first input axis extends vertically.

3. Apparatus as claimed in Claim 1 or Claim 2, wherein the input network includes an operational amplifier to the input of which an input voltage is applied via a series connection of capacitance means and resistance means, and the output voltage of which, forming the output voltage of the input network, is fed back to the input of the operational amplifier through a negative feedback resistor.

4. Apparatus as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.