GB2056062A

GB2056062A - Attitude and Reading Reference System and Inertial Navigation System Based on Gyroscopic Pendulums

Info

Publication number: GB2056062A
Application number: GB8022739A
Authority: GB
Original assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 1979-07-17
Filing date: 1980-07-11
Publication date: 1981-03-11
Also published as: JPS5619406A; GB2056062B; FR2461926A1; DE2928817C2; DE2928817A1; FR2461926B1

Abstract

An attitude and heading reference and inertial navigation system of the kind including, as sensors, two gyroscopic pendulums having upwardly and downwardly directed gyroscopic vectors, each gyroscopic pendulum being a two-degree-of- freedom gyroscope having gimbal suspension, and two accelerometers connected rigidly to the gyroscopes by mechanical means and operable in the two input axes of the gyroscopes, the output signals from the accelerometers being connected directly, via an amplifying circuit, to a gyroscope torquer situated orthogonally with respect to the gyroscopes, the system also including a computer to determine ground speed, position and north direction, in which the gyroscopic pendulums are arranged, in their null settings, along the vector of the earth's gravitational force. <IMAGE>

Description

SPECIFICATION Attitude and Heading Reference System and Inertial Navigation System Based on Gyroscopic Pendulums The invention reiates to an attitude and heading reference and inertial navigation system including, as sensors, two gyroscopic pendulums having upwardly and downwardly directed spin vectors, each gyroscopic pendulum comprising a displacement (two-degree-of-freedom) gyroscope having gimbal suspension, and two accelerometers connected rigidly to the gyroscopes by mechanical means and operable in the two input axes of the gyroscopes, the output signals from the accelerometers being connected directly, via an amplification circuit, to a gyroscopic torquer situated orthogonally with respect to the gyroscopes, the system also including computer means to determine ground speed, position and north'direction.

When a gyroscopic pendulum is used on a moving vehicle, the pendulum must be tuned, that is the amplification (F) must have a predetermined relation to the gyroscopic spin (H).

It is an object of the invention to find a tuning condition for the gyroscopic pendulum which is constant, that is independent of the flight position, since otherwise it is necessary to determine new values of the factor F/H continuously, with the consequent consumption of computer load.

This object is solved, according to the invention, in that the gyroscopic pendulums are directed, in their null settings, along the vector of the earth's gravitational force, G.

The earth's gravitational force G is inclined, relative to the vector g of the acceleration due to gravity, to the vertical, at an angle y with positive direction of rotation in the easterly direction.

The magnitude of this angle y is given by the formula:

(where Q=the earth rate, R=radius of the earth, g=value of the acceleration due to gravity, s=geographic latitude).

By this means is achieved that the tuning relationship is given by the following simple relation:

where K=the scale factor.

If this angle y is not taken into consideration, that is if the vertical is chosen as the null setting, then the tuning must satisfy the following relation:

(where VEe=east-west velocity plus the local value of the earth's velocity of rotation, 0=the geographic latitude and + for the upward and downward directed spin vectors respectively).

In the above formula, the east-west velocity plus the local value of the earth's velocity of rotation and the geographic latitude are parameters which vary continuously during movement of the vehicle. This, however, means that the factor F/H must be continuously redetermined with the consequent consumption of computer load.

In the attitude and heading reference and inertial navigation system according to this invention as described herein it is possible, in addition to determine the north direction from the gimbal angles of the gyroscopic pendulums and the ground speed, in coordinates fixed with respect to the vehicle. The difference of corresponding synchro-signals is formed, this being proportional to the inertial velocity of the vehicle. The velocity of the earth's circumference is determined in coordinates fixed with respect to the vehicle from the difference between the inertial velocity of the vehicle and the externally or internally determined ground speed, the north direction being determined from the earth's circumferential velocity after division by the known value of the earth's circumferential velocity at any point.

It is also possible, according to a feature of the invention, to determine the ground speed inertially, by calculating the arithmetic mean of corresponding acceleration signals, resolving the arithmetic mean with the aid of the calculated north direction, into north and east components, which are integrated to give the ground speed in geographic coordinates and transformed by means of the calculated north direction into horizontal ground speed in coordinates fixed with respect to the vehicle.

The invention is illustrated by a number of diagrams and is described in the following with reference to these diagrams which are:- Figure 1. A diagram showing comparatively, the null setting Zn of the gyroscopic pendulum which leads to a tuning condition dependent on manoeuvres and the null setting Zn which leads, according to the invention, to a constant tuning condition.

Figure 2. A block circuit diagram showing the use of a gyroscopic pendulum as attitude reference.

Figure 3. A block circuit diagram showing dead reckoning navigation and determination of attitude and heading based on externally measured speed together with the synchron output signals of two gyroscopic pendulums.

Figure 4. A block circuit diagram showing the inertial navigation and determination of attitude and heading based on the signals of two gyroscopic pendulums.

Figure 5. A block circuit diagram of another version of the inertial navigation and the determination of attitude and heading based on the accelerometer signals and the synchro-signals of two gyroscopic pendulums.

Figure 6. A diagram illustrating the principles of the calculation of the flight attitude and of the velocity relative to inertial space.

The attitude and heading reference or dead reckoning navigation and inertial navigation system according to the invention is constructed on the basis of two gyroscopic pendulums, the spin vectors of which point upwardly and downwardly respectively, and which consist, in each case, of a displacement (two-degree-offreedom) gyroscope having gimbal suspension, together with two accelerometers connected rigidly thereto by mechanical means and which are operative in the two gyroscope input axes, the output signals being connected via an amplifying circuit directly to a gyroscope torquer situated orthogonally with respect to the gyroscopes.

In inertial navigation the two gyroscopic pendulums are the cnly sensors for course, bearing and for the navigation. In dead-reckoning navigation they serve as attitude and heading references for an external sensor of velocity such as Doppler radar. Doppler sonar or log.

The illustration in Figure 1 shows the known null setting of a gyroscopic pendulum such that the gyroscopic axis is in the direction of the vertical (i.e., acceleration due to gravity g). In this arrangement tuning of the gyrosopic pendulum depends on the earth rate plus the transport rate.

The tuning F/H is therefore, like the period of the characteristic motion, dependent on the geographic latitude and the east-west velocity, as described hereinbefore. In accordance with the invention, the null setting of the gyroscopic pendulum in the direction of the gravitational force G (earth's gravitational force) leads to a constant relation of F/H for the tuning condition and to a simplification of the further calculation of the measurement signals.

This novel null setting, provided in accordance with the invention, is achieved by a slight tilt , about the east-west axis which is less then 6 min reiative to the known setting. It is self-adjusting during operation of the tuned gyroscopic pendulum when the measured acceleration is connected directly, that is without compensation of the Coriolis acceleration, via the amplifying circuit to the gyroscopic torquer.

On a vehicle at rest or when moving, the tuned gyroscopic pendulum indicates by the inclination cgx V about the horizontal x- and y-axes the velocity of of the vehicle relative to inertial space:

where The ground speed+the value of the earth's velocity of rotation and +indicates upwardly or downwardly directed gyroscopic vector.

In the vehicle, the angle a;, and the angle of roll 0 and of pitch 8 are superimposed on the gimbal angle of a gyroscopic pendulum. As can be seen from Figure 6, the separation of the roll and pitch angles and of the angle of tilt takes place by the sum and difference formation of the gimbal angle of the two gyroscopic pendulums with upwardly and downwardly directed gyroscopic vectors or opposite gyroscopic vectors, which will be termed briefly counter-rotating gyroscopic pendulums in the following.

A gyroscopic pendulum can be used as an attitude reference system when, as shown in Figure 2, an external velocity reference V* (e.g., pressure tube, log, Doppler radar, Doppler sonar) and a heading reference * (e.g., magnetic compass) are available. It is then possible by means of the earth's velocity of rotation, which can be taken as constant over a wide range, to compensate the above mentioned angle a for magnitude and direction at the gimbal angles of the gyroscopic pendulums. The angles of roll and pitch can then be determined with high accuracy.

According to Figure 3, two counter-rotating gyroscopic pendulums can be used as attitude and heading reference, where, as stated above, the sum of corresponding gimbal angles give the angles of roll and pitch. For the determination of heading, the share which results from the externally measured ground speed is compensated in the angle a,. There is then available a signal proportional to the earth's velocity of rotation Vie* according to magnitude and direction, which is evaluated for the determination of the north direction and the geographic latitude.

Inertial navigation makes possible an autonomous determination of the ground speed and the determination of heading can also take place autonomously with inclusion of the signals of the accelerometers of the two counter rotating gyroscopic pendulums. A corresponding system is illustrated by the block circuit diagram according to Figure 4.

In order to compensate for the effect of vertical acceleration on the inclined gyroscopic pendulum, the arithmetic mean of the corresponding accelerometer signals of the counter-rotating gyroscopic pendulums is calculated in each case and this is further processed for navigation.

According to Figure 4, a navigation computer is employed which is programmed in such a way that the vector of the ground speed Vn* of the vehicle is calculated directly in geographic coordinates. In Figure 4 the central branch represents the calculation of the value of the earth's velocity of rotation in terms of magnitude and direction for the determination of the north direction and for the redundant calculation of the geographic latitude. For the calculation of the ground speed, the mean or averaged signal of the accelerometers is transformed by means of the known north direction-transformation matrix Cna (#*) into the geographic coordinate system. After correction for the apparent acceleration, integration to north and east-west velocity takes place.The horizontal ground speed Va* with axes fixed relative to the vehicle is obtained by a renewed rotation by means of the transposed value of Cna (*), The block circuit diagram of Figure 5 differs from that of Figure 4 by another mode of inertial navigation. While, according to Figure 4, the ground speed Vn* is determined first, according to Figure 5, the increment of the inertial velocity Vn* is determined first and is fed to the central branch for the determination of course. After this, the direction of the earth's velocity of rotation at the starting point is used for the determination of the north direction.The method of treatment of the signal according to Figure 5 is advantageous for vehicles which performs strong vertical manoeuvres since in this case, the signal C1* calculated from the velocity in a known manner for compensation of the apparent acceleration, does not need to include any percentage proportional to the vertical velocity.

In the two systems of Figure 4 and Figure 5, the back-coupled and intermeshed signal flux leads to dynamic properties which correspond to those of a conventional inertial navigation system.

Special advantages of the invention are to be found in the fact that the gyroscopic pendulums operate without a feedback signal from the computer. In the absence of a computer, the two gyroscopic pendulums could still be used as an attitude and heading reference system for an external sensor of velocity. In the absence of one of the two gyroscopic pendulums, the other may still be used as a highly accurate attitude reference.

Claims

1. An attitude and heading reference and inertial navigation system including, as sensors, two gyroscopic pendulums having upwardly and downwardly directed spin vectors, each gyroscopic pendulum comprising a displacement (two-degree-of-freedom) gyroscope having gimbal suspension, and two accelerometers connected rigidly to the gyroscopes by mechanical means and operable in the two input axes of the gyroscopes, the output signals from the accelerometers being connected directly, via an amplifying circuit, to a gyroscopic torquer situated orthogonally with respect to the gyroscopes, the system also including a computer to determine ground speed, position and north direction, the gyroscopic pendulums being arranged, in their null settings, along the vector of the earth's gravitational force.

2. A system according to Claim 1 in which the north direction is determined, from the gimbal angles of the gyroscopic pendulums and the ground speed, in coordinates fixed with respect to the vehicle, by forming the difference of corresponding synchro signals, this being proportional to the inertial velocity of the vehicle, and the earth's velocity of rotation is determined in coordinates fixed with respect to the vehicle from the difference between the inertial velocity of the vehicle and the externally or internally determined ground speed, the north direction being determined from the vector of the earth's velocity of rotation, after division by the known value of the earth's velocity of rotation at any point

3.A system according to Claim 1 or 2, in which the ground speed is determined inertially by calculating the arithmetic mean of corresponding acceleration signals, said arithmetic mean being resolved, with the aid of the calculated north direction, into north and east components, integrated to give the ground speed in geographic coordinates and transformed by means of the calculated north direction to give the horizontal ground speed in coordinates fixed with respect to the vehicle.

4. The method of determining the position of a vehicle by means of an attitude and heading reference and inertial navigation system including, as sensors, two gyroscopic pendulums having upwardly and downwardly directed spin vectors, each gyroscopic pendulum comprising a displacement (two-degree-of-freedom) gyroscope having gimbal suspension, and two accelerometers connected rigidly to the gyroscopes by mechanical means and operable in the two input axes of the gyroscopes, the output signals from the accelerometers being connected directly, via an amplifying circuit, to a gyroscopic torquer situated orthogonally with respect to the gyroscopes and by positioning the gyroscopic pendulums, in their null settings, along the vector of the earth's gravitational force, the system also including a computer to determine ground speed, position and north direction.

5. The method according to Claim 4 in which the north direction is determined, from the cardan angles of the gyroscopic pendulums and the ground speed in coordinates fixed with respect to the vehicle by forming the difference of corresponding synchro signals, this being proportional to the inertial velocity of the vehicle, and the velocity of the earth's rotation is determined in coordinates fixed with respect to the vehicle from the difference between the inertial velocity of the vehicle and the externally or internally determined ground speed, the north direction being determined from the vector of the earth's velocity of rotation, after division by the known value of the earth's velocity of rotation at any point.

6. The method according to Claim 5 in which the ground speed is determined inertially by calculating the arithmetic mean of corresponding acceleration signals, said arithmetic mean being resolved, with the aid of the calculated north direction, into north and east components, integrated to give the ground speed in geographic coordinates and transformed by means of the calculated north direction to give the horizontal ground speed in coordinates fixed with respect to the vehicle.

7. An attitude and heading reference and inertial navigation system constructed and arranged substantially as described herein with reference to the accompanying drawings.