Low cost self aligning strapdown attitude and heading reference system

• US 4,318,300 A
• Filed: 01/14/1980
• Issued: 03/09/1982
• Est. Priority Date: 11/30/1978
• Status: Expired due to Term

• Current Assignee: Litton Systems Incorporated
• Original Assignee: Litton Systems Incorporated
• Inventors: Maughmer, Robert W.
• Primary Examiner(s): WOODIEL, DONALD
• Application Number:
  US06/112,247
• Time in Patent Office: 785 Days
• Field of Search: 73/178 R, 33/326, 364/453, 364/454
• US Class Current: 73/178.00R
• CPC Class Codes: G01C 21/166 Mechanical, construction or arrangement details of inertial navigation systems
• CPC Class Codes: G01C 21/185 for gravity
• CPC Class Codes: G01C 21/188 for accumulated errors, e.g. by coupling inertial systems with absolute positioning systems

Abstract

Apparatus including two two-degree-of-freedom gyroscopes and at least two accelerometers mounted on a turntable on a vehicle. The turntable can be positioned about a vehicle vertical axis in one or the other of two positions 180 degrees apart for alignment. After alignment, the turntable is caged into its zero degree position, and the instruments thereafter operate in a strap-down mode with the yaw, roll and pitch angles of the vehicle computed by computer mechanisms which are sensitive to signals from the gyroscopes and accelerometers.

Claims

1. A heading and attitude reference system for use in a vehicle having pitch, roll and yaw axes designated x, y and z, respectively, comprising:

a turntable mounted for rotation about the z axis of said vehicle including means for positioning said turntable in a predetermined zero degree position and means for turning said turntable from said zero position into its 180 degree position;

at least two angular rate sensors mounted on said turntable with their sensing axes parallel to x, y and z;

at least two accelerometers mounted on said turntable with their sensing axes parallel to x and y;

means for initially storing signals from said rate sensors and accelerometers when said vehicle is substantially stationary and said turntable is in its zero position;

means for controlling the turning of said turntable from its zero degree into its 180 degree position;

means for combining signals from said rate sensors and accelerometers when said turntable is in its 180 degree position with said stored signals to produce sum and difference signals; and

means for combining said sum and difference signals to produce accelerometer and rate sensor initial bias signals, rate sensor scale factor signals, and signals indicative of the initial attitude angles between said x, y and z axes and a reference set of coordinates.

2. Apparatus as recited in claim 1 in which said means for combining said sum and difference signals comprises:

means for producing a signal which is a measure of the initial pitch angle in response to the said difference signal of the said y axis accelerometer;

means for producing a signal which is a measure of the initial roll angle in response to said pitch angle signal and said difference signal of said x axis accelerometer;

means for producing a signal which is a measure of x axis accelerometer bias in response to said sum signal of said x axis accelerometer;

means for producing a signal which is a measure of y axis accelerometer bias in response to said sum signal of said y axis accelerometer;

means for producing a signal which is a measure of the initial drift parameters about the x axis of a first said rate sensor in response to said sum signal of rotation rate of said first rate sensor about the x axis and to signals which are a measure of earth rate;

means for producing a signal which is a measure of the initial drift parameters about the y axis of a second said rate sensor in response to said sum signal of that second said rate sensor about the y axis and to said earth rate signals;

means for producing a signal which is a measure of the initial yaw angle in response to said roll and pitch angle signals, to said difference signals of the angular rate about the x axis of one said rate sensor and of the angular rate about the y axis of the other said rate sensor, to a signal which is a measure of a component of angular velocity due to earth'"'"'s rotation, and to signals which are measures of the acceleration sensitive drift coefficient of both said rate sensors; and

means for producing a signal which is a measure of the initial drift rate of said rate sensors about the z axis in response to said pitch, roll and yaw signals, to said component of earth'"'"'s rate signal, and to said sum signals of the angular rate of each respective rate sensor about its z axis.

3. Apparatus as recited in claim 2 and further comprising means for producing a signal which is a measure of the scale factors about the z axis of said rate sensors in response to signals which measure the rotation of said turntable, and said sum signals about the z axis of rate sensors.

4. Apparatus as recited in claim 3 in which said signals are responsive to instrument temperature and further comprising means for generating a temperature signal and for modifying said other signals in response to said temperature signal.

5. Apparatus recited in claim 4 and further comprising means for storing said signals for use during the operative mode of said apparatus.

6. Apparatus as recited in claim 5 and further comprising:

means for continuously modifying the angular rate signal about said x axis from said first rate sensor to remove instrument drift rate errors;

means for continuously modifying the angular rate signal about said y axis from said second rate sensor to remove instrument drift rate errors;

means for resolving earth'"'"'s rate signals into components about said x, y, and z axes;

means for subtracting said earth'"'"'s rate signal components from the respective modified angular rate signals; and

means for producing direction signals which are measures of the rotation of said instruments about said reference coordinate axes including means responsive to said initial pitch, roll and yaw angle signals, continuously to update said means for producing direction signals, and means for resolving said updated signals to produce updated pitch, roll and yaw angle signals.

7. Apparatus as recited in claim 6 and further comprising feedback means for bounding errors in the pitch and roll angle signals.

8. Apparatus as recited in claim 7 in which said bounding signals are pitch and roll signals obtained in response to said accelerometer signals.

9. Apparatus as recited in claim 8 and further comprising logic means for selectively disconnecting and reconnecting said bounding signals in response to the magnitude of said pitch and roll angles and to vehicle acceleration signals.

10. Apparatus as recited in claim 9 in which said bounding means is disconnected in said x channel in response to either the pitch angle signal or the y axis vehicle acceleration signal exceeding predetermined thresholds;

said bounding means is disconnected in said y channel in response to either the roll angle signal or the x axis vehicle acceleration signal exceeding predetermined thresholds;

said x channel bounding means is reconnected in response to both the pitch angle signal and the y axis vehicle acceleration signal being below predetermined thresholds; and

said y channel bounding means is reconnected in response to both the roll angle signal and the x axis vehicle acceleration signal being below predetermined thresholds.

11. A method for determining the initial angular orientation of a first Cartesian set of rate sensor and accelerometer instrument axes relative to a second set of reference axes and for determining the bias errors of said rate sensors and accelerometers, comprising:

positioning the sensing axes of said rate sensors and accelerometers in a first predetermined position about a predetermined first axis of said first set of axes, designated zero degrees, and reading the output signals of said gyroscopes and accelerometers;

turning said rate sensors and accelerometers 180 degrees about said first instrument axis;

re-reading the output signals of said rate sensors and accelerometers; and

adding and subtracting said output signals in the 180 degree position to and from said signals in the zero degree position to obtain sum and difference signals which are functionally related to the angles between the two said sets of axes and to the bias errors and scale factors of said rate sensors and accelerometers.